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  • Chemistry  (9,407)
  • Life and Medical Sciences  (1,516)
  • Inorganic Chemistry  (773)
  • 1995-1999
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  • 1960-1964
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  • 1991  (10,923)
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  • 1995-1999
  • 1990-1994  (10,923)
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  • 101
    Electronic Resource
    Electronic Resource
    Dibenzoyl peroxide as an initiator of radical polymerizations: Reactions of the benzoyloxy radical with monomersDedicated to Prof. Dr. D. Braun on the occasion of his 60th birthday (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 185 (1991), S. 1-10 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Experimental methods for comparing the reactivities of unsaturated substances towards the benzoyloxy radical are considered. Results for various monomers are discussed. It is shown that some substances show high reactivity towards the radical although they do not ordinarily give homopolymers or act as comonomers; they include β-methylstyrene, stilbene and related substances, and 1.4-diphenylbuta-1, 3-diene.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051850101
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  • 102
    Electronic Resource
    Electronic Resource
    GEORDNETE EPOXIDNETZWERKE AUS 4-GLYCIDYLPHENYL-4-GLYCIDYL-BENZOAT und 3- BZW. 4-AMINOPHENYL-4-AMINOBENZOAT Herrn Prof. Dr. D. Braun zum 60. Geburtstag gewidmet (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 185 (1991), S. 33-42 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The introduction of mesogenic groups in main- and sidechains of epoxy thermosets results in an ordered multiphase network with cellular structure, if gelation occurs below the maximal cure temperature Tmax. Tmax is individual for a given combination of monomers. The multiphase network consists of relatively soft anisotropic cell nuclei and hard isotropic cell walls. If gelation occurs above Tmax single phase networks are obtained. The size of the cell nuclei strongly depends on the used monomers and varies up to two orders of magnitude. The multiphase structure was found to have no impact on the tensile elastic modulus.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051850104
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  • 103
    Electronic Resource
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    Radikalbildung aus Hydroperoxiden in Gegenwart starker SäurenHerrn Prof. Dr. D. Braun zum 60. Geburtstag gewidmet (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 185 (1991), S. 43-50 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The polymerisation of styrene and methylacrylate initiated by hydroperoxides is accelerated in the presence of strong acids such as sulfuric acid, p-toluene-sulfonic acid or perchloric acid. By copolymerising styrene and methylmethacrylate and by phenolic type inhibitors the radical nature of the catalytic effect of the acid was proved. Kinetic experiments demonstrated the acid involvement in the starting reaction.
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051850105
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  • 104
    Electronic Resource
    Electronic Resource
    Durch Matrixmodifizierung und Zusatzstoffe zu Polyesterverbunden mit speziellen EigenschaftenHerrn Prof. Dr. D. Braun zum 60. Geburtstag gewidmet (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 185 (1991), S. 11-21 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: By modification of the resin matrix and by additives unsaturated polyesters result in materials with a large variety of properties. The mechanical values can be tailor-made by chemical modification of the polymer chain or rubber-like additives. Antistatic properties and electrical conductivity can be achieved by addition of carbon black, graphite or metallic fillers. Furthermore the thermal conductivity can be influenced. Other fillers improve the friction and wear behaviour or lead to a significant reduction of weight when microspheres are added. The tailored modification of properties effectively contributes to meet higher security requirements.
    Notes: Ungesättigte Polyester ergeben durch Modifizierung der Harzkomponente oder durch Additive Werkstoffe mit einem breiten Eigenschaftsprofil. Die mechanischen Werte sind durch chemische Modifikation der Polymerkette oder durch kautschukartige Additive gezielt einstellbar. Antistatische Eigenschaften und elektrische Leitfahigkeit lassen sich durch Zugabe von Rußen, Graphit oder metallischen Füllstoffen erreichen. Gleichzeitig kann auch die Wärmeleitfähigkeit beeinflußt werden. Andere Füllstoffe verbessern das Reibungs- und Verschleißverhalten oder können bei Zugabe von Mikrohohlkugeln zu deutlicher Gewichtsverminderung führen. Die gezielte Veränderung der Eigenschaften trägt wesentlich zum Einhalten erhöhter Sicherheitsanforderungen bei.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051850102
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  • 105
    Electronic Resource
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    NEUE VERARBEITUNGSMETHODEN FÜR RIESELFÄHIGE DUROPLASTEHerrn Prof. Dr. D. Braun zum 60. Geburtstag Gewidmet (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 185 (1991), S. 23-32 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Duroplastische Formmassen werden nach konventionellen Verfahren der Kunststofformgebung wie Formpressen und Spritzgießen bzw. deren Varianten Spritzpressen und Spritzprägen verarbeitet. Wie in anderen Industriezweigen hat auch hier der wachsende Kostendruck zu einer steten Weiterentwicklung dieser Verfahren geführt. Über erfolgreiche Verfahrensverbesserungen zur Reduzierung der Herstellkosten wird im folgenden berichtet.Die Einführung der Spritzgießverarbeitung für Duroplaste in den 60er Jahren bedeutete gegenüber der bis dahin ausschließlich betriebenen Preßverarbeitung einen erheblichen Rationalisierungsschritt. Die Zykluszeiten ließen sich durch das Aufschmelzen der Formmasse in der Plastifiziereinheit sowie durch das Einbringen von Friktionswärme beim Einspritzvorgang erheblich reduzieren. Heute erlebt die Preßverarbeitung eine Renaissance insbesondere dann, wenn es gilt, hochpräzise Formteile mit minimaler Schwindung und geringstem Verzug herzustellen. Durch Rundtischautomaten, Vorplastifiziergeräte und Entnahmegeräte ließen sich die wirtschaftlichen Nachteile der klassischen Preßverarbeitung weitgehend kompensieren.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051850103
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  • 106
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    Dimensions- und Dichteänderung von Polymidformteilen durch WasseraufnahmeHerrn Prof. Dr. D. Braun zum 60. Geburtstag Gewidmet (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 185 (1991), S. 51-60 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: The crystallization and solidification behaviour of polyamides are influenced by additives such as nucleating agents or fillers. During processing morphological structures are obtained which affect the shrinkage, linear thermal expansion and dimensional changes due to water absorption. By studying the change of volume after water absorption, the dimensional change in length, breadth and thickness are easily understood.
    Notes: Additive in Polyamiden wie Nukleierungsmittel oder Verstärkungsmittel beeinflussen das Kristallisations- und Erstarrungsverhalten. Während der Herstellung von Probekörpern bilden sich morphologische Strukturen, die die Verarbeitungsschwindung, den linearen Ausdehnungskoeffizienten und die Dimensionsänderung während der Wasseraufnahme beeinflussen. Anhand der Volumenänderungen wird der Zusammenhang zwischen der Wasseraufnahme und der Dimensionsänderung in allen drei Raumrichtungen verständlich dargestellt.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051850106
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  • 107
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    Synthesis of macromonomers of poly(2,6-dimethyl-1,4-phenylene oxide)Dedicated to Prof. D. Braun on the occasion of his 60th birthday. (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 185 (1991), S. 75-86 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Poly(2,6-dimethyl-1,4-phenylene oxide) with exactly one functional endgroup was synthesized by oxidative polymerization, halogen substitution polymerization, and by phase transfer catalysis. Reaction with 4-vinylbenzoyl chloride resulted in a quantitative conversion to a macromonomer as shown by GPC and endgroup analysis. The macromonomer is a glassy material with a Tg from 150 to 190°C depending on the molecular weight.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051850108
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  • 108
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    Poly[1-(2-hydroxyethyl) aziridine] as polychelatogen for liquid-phase polymer-based retention (LPR)Dedicated to Prof. Ernst Bayer on the occasion of his 65th birthday (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 193 (1991), S. 195-203 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Die Eigenschaften von Poly[1-(2-hydroxyethyl) aziridin] wurden im Hinblick auf die Komplexbildung für die Metalle Co(II), Zn(II), Ni(II), Cu(II), Cr(III), Fe(III), Cd(II), Pb(II) und Sr(II) in wäßriger Lösung untersucht. Die LPR-Methode beruht auf der Retention von anorganischen Ionen durch lösliche Polymere in einer Membran-Filtrationszelle und nachfolgender Abtrennung der niedermolekularen Anteile vom gebildeten Polymerkomplex. Die Wechselwirkung des hydrophilen Polymeren wurde als Funktion von pH und Filtrationsfaktor bestimmt. Nach dem erhaltenen Wechselwirkungsmuster bildet Cu(II) die stabilsten Komplexe mit dem Polyaziridin innerhalb des pH-Bereiches von 3 bis 7. Bei pH 3 ist es möglich, Cu(II) von allen anderen Metallionen zu trennen. Sr(II) wird im ganzen untersuchten pH-Bereich nur schwach retentiert. Bei pH 7 traten alle Metalle, außer Sr(II), mit dem Polymeren in Wechselwirkung und wurden nahezu quantitativ retentiert.
    Notes: The metal complexing properties of poly[1-(2-hydroxyethyl) aziridine] for the metals Co(II), Zn(II), Ni(II), Cu(II), Cr(III), Fe(III), Cd(II), Pb(II), and Sr(II) in aqueous phase were investigated. The LPR method is based on the retention of inorganic ions by soluble polymers in a membrane filtration cell and subsequent separation of low-molecular species from the polymer complex formed. The metal interaction with the hydrophilic polymer was determined as a function of pH and filtration factor. According to the interaction pattern obtained, Cu(II) forms the most stable complexes with the polyaziridine within the pH range 3 to 7. At pH 3, it is possible to separate Cu(II) from all other metal ions. Sr(II) is only slightly retained at the pH range investigated. At pH 7, all elements, except Sr(II), did interact with the polymer and were almost quantitatively retained.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051930118
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  • 109
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    Synthesis and characterization of new urethane-acrylic polymers (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 187 (1991), S. 1-10 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Eine neue Klasse von Urethan-Acrylat-Monomeren wurde durch Umsetzung von Diisocyanaten wie Toluylendiisocyanat (TDI) und Hexamethylendiisocyanat (HMDI) mit Hydroxyethylmethacrylat oder Hydroxypropylmethacrylat und gesättigten Alkoholen wie Stearylalkohol oder Decanol hergestellt und durch IR- und NMR-Spektroskopie charakterisiert. Durch radikalische Polymerisation dieser Monomeren wurden Acrylatpolymere mit Urethanseitengruppen erhalten, die IR-spektroskopisch, viskosimetrisch und thermisch charakterisiert wurden.Die thermodynamischen Parameter des Polymerabbaus wurden berechnet; ein möglicher Abbaumechanismus wird vorgeschlagen.
    Notes: A new class of urethane-acrylic monomers was synthesized from diisocyanates such as toluylene diisocyanate (TDI) and hexamethylene diisocyanate (HMDI), unsaturated alcohols like hydroxyethyl methacrylate (HEMA), hydroxypropyl methacrylate (HPMA), and saturated alcohols such as stearly alcohol and decanol. The monomers were characterized by IR and NMR spectroscopy. Urethane pendent acrylic polymers, obtained by polymerizing these monomers, were characterized by IR, solubility viscosity, and thermal studies. The thermodynamic parameters for the degradation were calculated and a probable degradation mechanism is proposed.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051870101
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  • 110
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    Glass cloth reinforced composites of castor oil based polyurethanes and epoxy resin (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 187 (1991), S. 33-39 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Durch Umsetzung von Rizinusöl mit Toluylen-2,4-diisocyanat, Hexamethylendiisocyanat, Isophorondiisocyanat oder 4,4′-Diphenylmethandiisocyanat wurden Polyurethan-Präpolymere erhalten, die mit einem Epoxidharz aus Bisphenol-F und mit 1,3-Diaminopropan als Härter in Aceton homogen gemischt wurden. Mit diesen Mischungen getränkte Glasfasermatten wurden in einer Presse ausgehärtet; die Lösungsmittelbeständigkeit, elektrische Eigenschaften, Biegefestigkeit, Izod-Schlagfe-stigkeit und Rockwell-Härte der so erhaltenen Laminate wurde bestimmt. Die Abhängigkeit der elektrischen Leitfähigkeit σ von der Temperatur folgt der empirischen Gleichung σ = σ0, exp(-E/2kT).
    Notes: Prepolyurethanes were obtained by reacting the hydroxyl functionality of castor oil with diisocyanates, namely toluene-2,4-diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, and 4,4′-diphenylmethane diisocyanate, respectively. Homogeneous mixtures of prepolyurethanes and epoxy resin (prepared from bisphenol F in alkaline medium) were prepared in acetone containing 1,3-propanediamine as curring agent. Glass reinforced laminates were prepared by interacting PU/epoxy resin mixtures on glass cloth through compression moulding. The laminates were characterized by chemical resistance to solvents, flexural strength, izod impact strength, and Rockwell hardness. The variation of electrical conductivity σ with temperature was found to confirm with the empirical relation σ = σ0 exp(-E/2kT).
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051870104
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  • 111
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    Synthesis and characterization of some 2,4′-dibenzyldiisocyanate ionic polymers, 7Part 6: T. Buruianǎ, A. Caraculacu, Rev. Roum. Chim., in press.. Photochromic behaviour of ionenes with viologen units (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 187 (1991), S. 51-59 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Durch Menschutkin-Reaktion wurden Polyurethane mit kationischen Viologen-gruppen hergestellt. Die photochromen Eigenschaften dieser Polymeren in einer Poly(vinylalkohol)-Matrix wurden mit UV- und Laserlicht untersucht.
    Notes: The Photochromic properties by UV and laser irradiation of cationic polyurethanes with viologen groups introducted by Menschutkin reaction were studied and poly(vinylalcohol) was used as matrix.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051870106
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  • 112
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    Crystallinity of polyethylene measured by density, DSC, and raman spectroscopy. A comparison (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 187 (1991), S. 87-99 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Im Rahmen dieser Arbeit wurden drei verschiedene Meßmethoden für die Bestim-mung der Kristallinität von Polyethylen angewandt. HDPE, LLDPE und LDPE-Pro-ben wurden  -  nach verschiedener Vorbehandlung  -  durch Dichtebestimmung, DSC und Raman-Spektroskopie untersucht. Die mit verschiedenen Methoden gewonnenen Resultate wurden verglichen, und Korrelationen wurden bestimmt.
    Notes: In this work three different techniques were used to determine the crystallinity of polyethylene. After different pretreatment HDPE, LLDPE, and LPDE samples were investigated by density measurements, differential scanning calorimetry, and Raman spectroscopy. The results were compared and relationships were estabilished between the values measured by the different methods.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051870109
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  • 113
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    Alkylation of poly(n-vinylpyrrolidone-co-vinylamine) with esters of phosphorous acids (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 187 (1991), S. 135-142 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Poly(N-vinylpyrrolidon-co-vinylamin) wurde mit Estern verschiedener Phosphorsäuren (Dimethylphosphit, Methanphosphonsäuredimethylester, Trimethylphosphat und Bis-(2-chlorethyl)-phosphonat methyliert, um physiologisch aktive Polymere zu erhalten. Die alkylierten Produkte wurden mittels IR- und NMR-Spektroskopie sowie Elementaranalyse charakterisiert. Der Alkylierungsumsatz hängt vom verwendeten Alkylierungsmittel ab. Die methylierten phosphorhaltigen Polymeren weisen Herbizidaktivität auf und könnten als Pflanzenwachstumshemmer verwendet werden.
    Notes: Alkylation of amino groups bounded to a polymer chain with esters of phosphorous acids was studied in order to obtain physiologically active polymers. Poly(N-vinyl-pyrrolidone-co-vinylamine) was chosen as a polymer-carrier. The phosphorous acid esters used were dimethyl hydrogen phosphonate, dimethyl methanephosphonate, trimethyl phosphate and bis-(2-chloroethyl)-hydrogen phosphonate. The composition and structure of the isolated final products was proved by IR- and NMR-spectroscopy and elemental analysis. It is established that alkylation takes place to various extents.The obtained alkylated phosphorous-containing polymers exhibit herbicidal activity and could find application as plant growth retardants.
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051870112
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  • 114
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    Bulk grafting of decyl methacrylate onto poly(vinyl chloride) in the presence of a lead carboxylate stabilizer (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 187 (1991), S. 153-167 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Um die Weichmacherpermanenz in PVC zu verbessern, wurde Decylmethacrylat (DMA) in Gegenwart eines Bleistabilisators (LS) mittels radikalischer Polymerisation auf PVC gepfropft. Die Molekulargewichte und Glastemperaturen der erhaltenen Produkte bestätigten, daß homogene PVC-g-PDMA-Copolymere entstanden waren.
    Notes: In order to improve the permanence of the plasticizer in a poly(vinylchloride) (PVC) compound we have studied the free radical polymerization and grafting of decyl methacrylate (DMA) onto PVC in the presence of a lead stabilizer. The influence of different parameters has been investigated and homogeneous PVC-g-PDMA copolymers have been obtained. Their molecular weights and the glass transition temperatures were consistent with the grafted nature of the copolymers.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051870114
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  • 115
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    Zur charakterisierung von polymerfilmen und lichtleitfaserummantelungen aus additionsvernetzendem siliconkautschukHerrn Professor Dr. K. Hummel zum 60. Geburtstag gewidmet (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 187 (1991), S. 187-205 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: The crosslinking behaviour of addition-crosslinking Wacker SilGel 600 silicone was investigated in the temperature range of 50 to 90°C with respect to its dependence on heating time. Measurements of the swelling of thin polymer films on special paper (Pecozet Z10) allow the observation of the swelling process which reaches the swelling equilibrium within a few minutes, the selection of a suitable swelling agent, and the rapid determination of the temperature dependence of the degree of swelling Q. Aside from Q, the „solubility parameter“ of the polymer network, the rate constants, and the activation energy of the crosslinking process can be obtained. Measurements of the swelling combined with determination of Huggins' interaction parameter additionally provide the possibility of determining the density of crosslinking of the polymer films.On the basis of these investigations, a rapid method for characterizing the sheathing of optical fibres could be developed. The cylindrical polymer films detach from the glass core during swelling in toluene (25°C). After 5 min the degree of swelling of the sheathing can be measured.
    Notes: Das Vernetzungsverhalten des additionsvernetzenden Silicons Wacker SilGel 600 wurde im Bereich von 50 bis 90°C in Abhängigkeit von der Heizzeit untersucht. Quellungsmessungen an dünnen Polymerfilmen auf Spezialpapier (Pecozet Z10) gestatten eine Verfolgung des in wenigen Minuten bis zum Quellungsgleichgewicht ablaufenden Quellprozesses, die Auswahl eines geeigneten Quellmittels und die schnelle Ermittlung der Temperaturabhängigkeit des Quellungsgrades Q. Neben Q sind der „Löslichkeitsparameter“ des Polymernetzwerkes, die Geschwindigkeitskonstanten und die Aktivierungsenergie des Vernetzungsprozesses zugänglich. Bei der Kombination der Quellungsmessungen mit einer Bestimmung des Hugginsschen Wechselwirkungsparameters kann auch die Vernetzungsdichte der Polymerfilme ermittelt werden.Auf der Basis dieser Untersuchungen konnte eine schnelle Methode zur Charakterisierung der Ummantelung von Lichtleitfasern entwickelt werden. Die zylindrischen Polymerfilme lösen sich während der Quellung in Toluol(25°C) vom Glaskern. Nach 5 min kann der Quellungsgrad der Ummantelung bestimmt werden.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051870116
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  • 116
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    Synthesis and application of thermosensitive microcapsules containing azo groupsDedicated to Prof. Robert Kerber on the occasion of his 65th birthday (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 187 (1991), S. 207-224 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Die Synthese und die thermischen Eigenschaften von zwei reaktiven, symmetrischen Azomonomeren und Modellverbindungen wurden untersucht, um thermolabile Mikrokapseln mit thermosensitiven Azogruppen herzustellen. Die Zerfallskinetik dieser thermisch und photochemisch instabilen Verbindungen ist von erster Ordnung, vergleichbar mit AIBN.Durch Grenzflächenpolykondensation der Azomonomeren mit Ethylendiamin, 1,6-Hexamethylendiamin oder Diethylentriamin wurden azogruppenhaltige Polyamide und Polyharnstoffe erhalten, aus denen thermosensitive Mikrokapseln hergestellt wurden. Mit optimierten Herstellungsbedingungen wurden in hohen Ausbeuten Mikrokapseln einheitlicher Größe mit guten Thermo-Transfer-Druckeigenschaften erhalten.
    Notes: In view of applying thermolabile microcapsules with azo groups as thermosensitive sites it was necessary to study the preparation and the thermal behaviour of two reactive, symmetrical azo monomer-compounds and model compounds. They are thermally and photochemically instable and the kinetics of their decomposition is of first order, comparable to that of AIBN.Azo group-containing polyamides and polyureas were prepared by interfacial polycondensation reaction from ethylendiamine, 1,6-hexamethylenediamine, diethylenetriamine and azo monomer-compounds.These polymers were used to prepare thermosensitive microcapsules. Favourable conditions were found to obtain microcapsules of uniform size, in a high yield, and with good properties in thermo-transfer-print.
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    Halogenation of polymers by dichlorodifluoromethane (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 188 (1991), S. 27-40 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Die Fluorierung von Polyethylen und Polyvinylalkohol wurde mit Dichlordifluormethan CCl2F2 (Freon 12) unter Verwendung von Cobalt-60 γ-Strahlung bzw. elektrischer Lichtbogenentladung durchgeführt und der Einfluß von Reaktionstemperatur und -zeit, Katalysator, Verdünnung etc. untersucht. Dabei wurde bei Polyethylen sowohl die Fluorierung als auch die gleichzeitige Chlorierung beobachtet, wobei der Anteil an Fluorierung bei elektrischer Lichtbogenentladung gewöhnlich höher war als bei Verwendung von γ-Strahlung; in beiden Fällen war dennoch die Chlorierung deutlich bevorzugt. Bei Polyvinylalkohol konnten noch höhere Fluorierungs-bzw. Chlorierungsgrade ermittelt werden.
    Notes: Fluorination of polyethylene and polyvinyl alcohol was carried out by dichloro-difluoromethane under cobalt-60 gamma radiation and under electric spark discharge. The effect of temperature, reaction time, catalyst, inert diluent etc. was studied. Simultaneous chlorination of the polymer samples was observed along with fluorination. The extent of fluorination under electric spark discharge is usually higher than that under gamma radiation. Again, the extent of chlorination is higher than that of fluorination in both the cases. The extent of fluorination as well as chlorination is the highest in case of polyvinyl alcohol.
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    Studies on cellulose acetate phthalate: Estimation of mark-houwink-kuhn-sakurada constants and viscosity-temperature relationship (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 188 (1991), S. 41-49 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Celluloseacetatphthalat (CAP) mit einem Acetylgehalt von 19,5 Gew.-% und einem Phthalylgehalt von 21,4 Gew.-% wurde hergestellt und durch fraktionierte Fällung in fünf Fraktionen mit gleichmäßig abnehmendem Molekulargewicht aufgetrennt. Die Werte für das zahlenmittlere Molekulargewicht (M̄n) dieser Fraktionen wurden membranosmometrisch in 0,05 N NaHCO3 gemessen; die Werte für die Grenzviskosität ([η]) wurden ebenfalls von allen fünf Fraktionen bei verschiedenen Temperaturen bestimmt. Die Beziehung zwischen [η] und M̄n wurde nach der Mark-Houwink-Kuhn-Sakurada-Gleichung ausgewertet, und aus den dabei erhaltenen Werten für v zeigte sich, daß CAP offensichtlich ein sehr steifes Polymeres ist. Die Temperaturabhängigkeit der Viskosität verdünnter CAP-Lösungen wurde nach der Methode von Moore durch einen Arrhenius-Ausdruck beschrieben. Die Aktivierungsenergie der Durchflußgeschwindigkeit hängt sowohl von der Konzentration als auch vom Molekulargewicht ab; aus dem dabei erhaltenen empirischen Parameter Ke kann ebenfalls auf ein steifes Polymeres geschlossen werden.
    Notes: Cellulose acetate phthalate (CAP) having an acetyl content of 19.5 wt.-% and a phthalyl content of 21.4 wt.-% was prepared and fractionated into five fractions of regularly decreasing molecular weight. The values of the number average molecular weight (M̄n) of these fractions were measured in 0.05 N NaHCO3 by the help of high-speed membrane osmometer. The intrinsic viscosity [η] values for all the fractions of CAP in 0.05 N NaHCO3 were also determined at different temperatures. The Mark-Houwink-Kuhn-Sakurada equation was applied to the obtained results and the [η]-M̄n relationships for CAP have been established. The results of the values of v suggest that CAP is a stiff polymer. The temperature dependence of the viscosity of dilute CAP solutions has been described by an Arrhenius expression according to Moore's treatment. The activation energy of flow was found to depend on both concentration and molecular weight through an empirical parameter (Ke). The Ke value obtained for CAP also indicates that CAP is a rigid polymer.
    Additional Material: 6 Ill.
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    Studies on hydride terminated methylhydro-dimethylsiloxane copolymers (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 188 (1991), S. 63-72 
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    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Methylhydro-dimethylsiloxancopolymere wurden aus den Hydrolysaten von Dichlordimethylsilan mit 10, 20, 30, 50 und 70 Vol.-% Dichlormethylsilan hergestellt. Die Copolymeren wurden IR-, 1H-NMR-spectroskopisch und chemisch charakterisiert und als Härter für ein funktionalisiertes RTV-Siliconmaterial eingesetzt. Die Vulkanisateigenschaften wurden geprüft; die Copolymeren mit hohem Hydridgehalt verbesserten die mechanischen Eigenschaften und die thermische Stabilität der Vulkanisate.
    Notes: Synthesis of hydride terminated methylhydro-dimethylsiloxane copolymers from the hydrolyzates of 10, 20, 30, 50 and 70% vol.-% dichloromethylsilane with dichlorodimethylsilane have been carried out. The copolymers were characterized by IR-and 1H-NMR-spectroscopy and standard chemical analysis. They were used as curatives for a standard vinyl functional room temperature vulcanizing (RTV) silicone material based on addition cure, and the vulcanizate properties have been evaluated. The copolymers with high hydride content gave better mechanical properties and thermal stability to the vulcanizates.
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    Thermal behaviour of TiO2-encapsulating polymers (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 188 (1991), S. 73-84 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Die thermischen Eigenschaften von Polystyrol and Poly(methylmethacrylat) mit verkapseltem und dispergiertem TiO2 wurden verglichen. Die verkapselnden Polymeren zeigen zwei thermische Relaxationsbereiche, deren nach Wunderlich ermittelte Aktivierungsenergien denen von dynamischen Dispersionen ähnlich sind. Es wird vorgeschlagen, daß die Niedertemperaturrelaxation durch lokale Änderungen der Konformation der Makromoleküle hervorgerufen wird, während die Hochtemperaturrelaxation dem normalen Glasübergang einschließlich der Wechselwirkung mit TiO2 vergleichbar ist. Das thermische Verhalten nahe der Zersetzungstemperatur in verschiedenen Atmosphären deutet darauf hin, daß in den verkapselnden Polymeren spezifische Strukturen vorhanden sind, die eine große Menge Sauerstoff adsorbieren können.
    Notes: Comparing the thermal properties of TiO2 encapsulating polystyrene and poly(methyl methacrylate) with those of TiO2 dispersion polymers it was found that the encapsulating polymers have two thermal relaxation regions. The activation energy of those thermal relaxation regions was determined using the Wunderlich method and it was found that the values are similar to the activation energy for the dynamic dispersion. It is suggested that the low-temperature thermal relaxation is caused by the local change of conformation of molecular chains, while the high-temperature thermal relaxation is similar to that of the normal glass transition temperature including the interaction with TiO2. In addition, the thermal behaviour near the degradation point in different atmospheres indicates that the encapsulating polymer has a specific structure for adsorbing a large amount of oxygen.
    Additional Material: 8 Ill.
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    Chemical synthesis of processible electrically conducting poly(3-dodecylthiophene) (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 188 (1991), S. 105-111 
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    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Poly(3-dodecylthiophen) (PDDT) wurde auf chemischem Weg synthetisiert und 1H-NMR-, IR- und UV-spektroskopisch charakterisiert. Das bei Raumtemperatur aufgenommene ESR-Spektrum von PDDT zeigt einen g-Wert von 2.0091 und eine Linienbreite von 6.60 Gauss. Aus der Lösung gegossene und mit Iod dotierte PDDT-Filme hatten eine elektrische Leitfähigkeit von 12 S/cm.
    Notes: The chemical synthesis of poly(3-dodecylthiophene) is reported. The polymer was characterized by 1H-NMR, IR and UV absorption spectroscopic techniques. The room temperature electron spin resonance spectrum yielded a g value of 2.0091 and a linewidth of 6.60 Gauss. The solution cast polymer films after doping with iodine exhibited an electrical conductivity of 12 S/cm.
    Additional Material: 4 Ill.
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    N-bromo-poly(styrene-co-divinylbenzene) sulphonamide metal salts. Synthesis and basic properties (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 188 (1991), S. 85-96 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Zwei Methoden zur Herstellung hochmolekularer poröser “Bromamine” (—SO2NBr-Na+) mit Poly (styrol-co-divinylbenzol)-Matrix wurden untersucht, und zwar (a) die Einwirkung von Hypobromiten auf das die entsprechenden Sulfonamidseitengruppen enthaltende Copolymere, und (b) die Einwirkung von Bromiden auf das entsprechende hochmolekulare “Chloramin”. In beiden Fällen wurden Metallsalze des hochmolekularen “Bromamins” erhalten. Das Natriumsalz wurde mit einer 100-proz. Ausbeute an funktionellen Gruppen erhalten; es enthielt 2.1 mmol · g-1 —SO2NBr-Na+ Gruppen, d. h. 4.2 meq · g-1 aktives Brom. Es wurde auch die Stabilität des Harzes getestet, und zwar im Bereich von 20-100°C sowohl im trockenen Zustand als auch in wäßrigen Medien. Darüber hinaus wurden thermoanalytische Messungen bis 1000°C durchgeführt. Es wurden auch Beispiele für die Anwendung des“Bromamins” als polymeres Reagenz für Bromierungen (Iodierungen), Oxidationen, Wasserdesinfektion und Kationenaustausch gegeben. Dieses sehr reaktive polymere Reagenz kann ohne merklichen Verlust seiner Eigenschaften wiederholt debromiert und bromiert werden.
    Notes: Macromolecular pendant group N-monobromosulphonamide metal salts —SO2NBr-M+ having a macroporous styrene-divinylbenzene matrix structure have been obtained by two methods:(a) by the action of aqueous metal hypobromites on the appropriate macromolecular pendant group sulphonamide, and(b) by the action of an aqueous metal bromide on the appropriate macromolecular N-chlorosulphonamide metal salt.In each case the sodium salt product was obtained with a 100% functional yield and it contained 2.1 mmol.g-1 —SO2NBr-Na+ groups, i. e., 4.2 meq · g-1 of active bromine. The stability of the resin was investigated in the range 20-100°C in the dry state as well as in aqueous media.A thermoanalysis (TG, DTG, and DTA curves) was carried out up to 1000°C and its course was interpreted. The resin has brominating, oxidative, microbiocidic, and ionexchanging properties. It reacts in a reversible way: after the loss of the active bromine, it can be reactivated again.
    Additional Material: 5 Ill.
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    Oligopurine · oligopyrimidine tracts do not have the same conformation as analogous polypurine · polypyrimidines (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 511-518 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ability of tracts of synthetic oligopurine · oligopyrimidines containing both adenosine and guanosine residues to approach the conformation of analogous polypurine · polypyrimidines has been examined as a function of tract length by CD spectroscopy. Tracts of up to 19 contiguous, alternating dA and dG residues yield CD spectra that are distinctly different from that of the analogous alternating polymer. Thus the structural changes reflected in the unusual CD spectrum of poly[d(AG)] · poly [d(CT)] must require even longer tract lengths. Tracts of contiguous adenosines flanked by guanosine residues were seen to approach the CD spectrum of poly[dA] · poly[dT] quite slowly as a function of tract length, requiring more than 24 contiguous adenosines to give CD spectra similar to the homopolymer. These results lead us to the conclusion that oligopurine tracts in vivo are not well modeled by synthetic polypurine · polypyrimidines with one or two base pair repeating units.
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    Conformations and time-resolved fluorescence of oligomers of (-)-epicatechin with 4β → 8 interflavan bonds (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 537-545 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oligomers of (-)-epicatechin with 4β → 8 interflavan bonds, and as many as five monomer units, have been studied by conformational analysis and time-resolved fluorescence. The conformational analysis yields 2x-1 conformations for each oligomer with x monomer units. There are two conformations accessible at each interflavan bond. These conformations are denoted by + and -. The dominant conformations for the trimer and higher oligomers have an interaction between the phenolic hydroxyl groups on monomers i and i+2. This interaction involves the hydroxyl group at C(5) on monomer i, and either C(13) or C(8) of monomer i+2, depending on whether the conformation of the two intervening interflavan bonds is + + or + -, respectively. Minor contributions to the ensemble for the tetramer and pentamer are made by conformations that contain the sequence of successive interflavan bonds denoted by -+ or --. In -+ the interaction between monomer units i and i + 2 involves an aliphatic hydroxyl with a phenolic hydroxyl, and there are no hydroxyl-hydroxyl interactions between units iand i + 2 in --. The onset of a different decay law for the fluorescence when x increases from 3 to 4 may be associated with the appearance of the -+ and -- conformations as minor constituents in the ensemble.
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    α-Helical small molecular size analogues of neuropeptide Y: Structure-activity relationships (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 613-619 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: C-terminal analogues of neuropeptide Y (NPY) of small molecular size have been synthesized. The influence of chain length, single or multiple amino acid substitution, and segment substitutions on receptor binding, pre- and postsynaptic biological activity, and conformational properties have been investigated. Receptor binding and in vivo assays revealed biological activity for NPY Ac-25-36 that increased with increasing α-helicity. In attempts to stabilize the α-helical content, three independent types of modified NPY Ac-25-36 analogues were synthesized. Strong agonistic activities could be detected in a series of discontinuous analogues, which are constructs of N-terminal parts linked via different spacer molecules to C-terminal segments. One of the most active molecules was NPY 1-4-Aca-25-36 (Aca, ε-aminocaproic acid). For the first time conformational properties of a series of small NPY analogues have been investigated by CD, and correlated with biological activity and receptor binding. A C-terminal dodecapeptide segment of NPY with an amount of 50% substitution to the native C-terminal sequence of NPY was found to exhibit significant receptor binding.
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    Conformational energy minimization by simulated annealing using molecular dynamics: Some improvements to the monitoring procedure (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 663-670 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The possibility of using molecular dynamics techniques as a tool for simulated annealing is tested in the case of the molecule of biological interest N-acetyl, N′-methyl amides of 3(S)-hydroxy, 4(S)-amino 6-methyl heptanoic acid [(3S, 4S)-statine] and of its (3R,4S) diastereomer. The approach is able to reach global minimum in the conformational space in the case of the mentioned molecule. A description of the method is given. The use of fractional fluctuations of the internal potential energy as an indicator able to point out transition from a given minimum region to a deeper one is investigated. The results about the lowest energy conformation of (3S,4S) diastereomer are in fairly good accordance with the results obtained in the case of search within the whole Conformational hyperspace.
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    Assignment of protein nmr spectra in the light of homonuclear 3D spectroscopy: An automatable procedure based on 3D TOCSY-TOCSY and 3D TOCSY-NOESYAbbreviations: 3D: three dimensional; TOCSY: total correlation Spectroscopy; NOESY: nuclear Overhauser effect spectroscopy; e. COSY: exclusive correlation Spectroscopy; z.COSY: z-filtered correlation Spectroscopy. (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 699-712 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Automated assignment of proteins is greatly simplified using data from 3D-nmr spectra. A strategy is presented which makes use of 3D-TOCSY-TOCSY and 3D-TOCSY-NOESY; its potential is demonstrated with the example of the spectra of BPTI.The discussion of the potential of 3D-nmr includes the introduction of a simple graph for the description of the information content of multidimensional nmr spectra.
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    Toward nonpeptidal substance P mimetic analogues: Design, synthesis, and biological activity (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 725-733 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1,4-Piperazine and 4-hydroxy proline, two small cyclic polyfunctional systems with defined stereochemistry, were introduced as “molecular scaffolds.” We define a “bioactive topology,” which is a derived putative low-energy conformation obtained through theoretical conformational analysis of substance P. Substitution of these molecular scaffolds by pharmacophors characteristic of the bioactive topology of the C-terminal hexapeptide of substance P resulted in active, partially nonpeptidal substance P mimetic agonists. The study discusses the concepts arid tools used to achieve this structural transformation, and points out the need to address flexibility-rigidity issues in an attempt to maintain sufficient molecular plasticity.
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    The solution structure of the lantibiotic gallidermin (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 803-811 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 21-peptide amide antibiotic gallidermin is a potential therapeutic against acne disease. It belongs to the class of polycyclic lanthionine and α,β-clidehydroamino acids containing polypeptides, which were named “lantibiotics.” The structural gene of the recently elucidated lantibiotic gallidermin encodes a precursor peptide containing Ser, Thr, and Cys residues in the Oterminal prolantibiotic part, and an unusually hydrophilic leader peptide. The ribosomally synthesized pregallidermin is posttranslationally modified and processed to a complex peptide antibiotic with four/sulfide rings and two unsaturated residues.The complete solution structure of gallidermin was determined in triffuoroethanol : water (95 : 5) and dimethylsulfoxide by two-dimensional 1H-nmr at 500 MHz, using a combination of double quantum filtered correlated spectroscopy, homonuclear Hartman-Hahn, and nuclear Overhauser enhancement spectroscopy experiments. Using a total number of 152 distance constraints from NOEs and 14 torsional constraints, derived from coupling constants, we obtained a screwlike solution structure of gallidermin. Restrained molecular dynamics simulations yielded a set of five converging structures with an atomic rms difference of 1.7 Å for the backbone atoms, not dependent on the starting structure. The spatial structure model is in excellent agreement with the amphiphilic and channel-forming properties of gallidermin on membranes and its tryptic cleavage at the exposed site between residues 13 and 14.
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    X-Ray conformational study of hydrazino peptide analogues (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 793-801 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have solved the crystal structures of nine pseudo-peptide analogues deriving from the hydrazino analogue of glycine or valine (NβH2-NαH-CαHR-CO2H, R = H or iPr) or proline (NβH2-Nα-CαH-CO2H) and containing the hydrazide (CO-NβH-Nα) or Nβ-Z-aminoamide peptidomimetic link. This study gives access to the average geometry of these two links, to their inter- and intramolecular interaction modes, and to their influence on the conformational properties of the molecules.
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    Condensation of DNA by multivalent cations: Considerations on mechanism (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1471-1481 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: DNA is generally found within viruses and cells in a tightly packaged state, typically occupying only 10-4-10-6 of the volume of the uncondensed DNA wormlike coil. Condensation can be induced in vitro at low salt by the naturally occurring polyamines spermidine3+ and spermine4+, by hexammine cobalt (III), and even by Mg2+ in methanol-water mixtures. These condensates generally have an orderly, toroidal, or rodlike shape and size similar to that of DNA gently lysed from phage heads. It is also striking that the condensate size distribution is independent of DNA molecular length from 400 to 40,000 base pairs (bp), but that shorter DNA molecules (e.g., 150-bp mononucleosomal DNA) cannot condense in this fashion. We have constructed a successive association equilibrium theory to attempt to explain these results, using an equation devised by Tanford for micelle formation. Most of the obvious attractive and repulsive free energy contributions (mixing, bending, hydration, and other nearest-neighbor interactions) are linear in the amount of DNA incorporated, but the net attractive ΔG° grows nonlinearly because of the increasing average number of nearest neighbors of each duplex as the particle grows. In order that the size distribution have a maximum, a quadratic repulsive free energy is also required, arising from the electrostatic self-energy of the incompletely neutralized particles. The net attractive free energy per base pair interaction is tiny, on the order of 10-3 kT. Despite the apparent generally correct order of magnitude of the various free energy terms, the calculated size distribution is smaller and narrower than observed experimentally. It appears that the size distribution of condensed particles is determined kinetically rather than thermodynamically. Very short DNA molecules cannot nucleate stable aggregates because they cannot develop adequate overlap, either internally or intermolecularly. A substantial fraction of rodlike condensates is observed in aqueous solutions only with a rather inefficient condensing agent, permethylated spermidine. This suggests that slow condensation kinetics may be required to overcome the high activation energy of highly distorted DNA bends or kinks at the turning points of rods. Evidence is reviewed that condensation may be associated with localized helix structure distortion provoked by condensing agents.
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    Cyclic retro-inverso dipeptides with two aromatic side chains. I. Synthesis (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1503-1512 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of cyclic retro-inverso dipeptides - 2-[(4-hydroxy)benzyl]-5-benzyl-4,6(1H,2H,3H,5H)-pyrimidinedione (c[mPhe-gTyr]), 2-benzyl-5-[(4-hydroxy)benzyl]-4,6(1H,2H,3H,5H)-pyrimidinedione (c[mTyr-gPhe]), and 2-benzyl-5-amino-5-[(4-hydroxy)benzyl]-4,6(1H,2H,3H,5H)-pyrimidinedione {c[(α-amino)mTyr-gPhe]} - were synthesized in order to define the minimum structural requirements for binding affinity with opiate receptors and biological activity. Although the first two compounds lack a free amine proposed to be necessary for receptor recognition, the c[mPhe-gTyr] and c[mTyr-gPhe] analogues serve as model molecules in conformational studies of the target analogue, c[(α-amino)mTyr-gPhe]. The cis- and trans-c[(α-amino)mTyr-gPhe] contain all the functional groups such as the amine and phenolic groups in the tyrosine, and the aromatic group in the phenylalanine, necessary for opiate activity. In addition, the c[(α-amino)mTyr-gPhe] analogues possess similar geometries to the Tyr-Pro part of morphiceptin (Tyr-Pro-Phe-Pro-NH2) whose high μ-receptor activity is attributed to conformations with the Tyr-Pro amide bond in a cis conformation because the peptide bonds assume a cis conformation. However, both analogues are inactive in the guinea pig ileum and the mouse vas deferens assays. This may result from wrong orientation of the benzyl group of the gPhe residue with respect to the (α-amino)mTyr residue.Conformational studies of these molecules using 1H-nmr spectroscopy and molecular mechanics calculations will be reported in the following paper. Results of conformational analysis should provide information about backbone-side-chain interactions in the retro-inverso peptide chains since all the fundamental structural elements of the retro-inverso peptides are included in these model systems even though the peptide bonds must assume a cis conformation.
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    Conformational energy studies of β-sheets of model silk fibroin peptides. I. Sheets of poly(Ala-Gly) chainsPreliminary results were presented at the 1990 fall meeting of the Materials Research Society, Boston, November 1990. (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1529-1541 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new model structure is proposed for the silk I form of the crystalline domains of Bombyx mori silk fibroin and the corresponding crystal form of poly(L-Ala-Gly). It was deduced from conformational energy computations on stacked sheet structures of poly(L-Ala-Gly). The novel sheet structure contains interstrand hydrogen bonds but is composed of antiparallel polypeptide chains whose conformation differs from that of the antiparallel β-sheets that constitute the silk II structure. The strands of the new sheet have a two-residue repeat, in which the Ala residues adopt a right-handed and the Gly residues a left-handed sheet-like conformation. The computed unit cell is orthorhombic, with cell dimensions a = 8.94 Å, b = 6.46 Å, and c = 11.26 Å. The model accounts for most spacings in the observed fiber x-ray diffraction patterns of silk I and of the silk-I-like form of poly(L-Ala-Gly), and it is consistent with nmr and ir spectroscopic data. As a test of the computations, the well-established β-sheet structure of silk II and the corresponding form of poly(L-Ala-Gly) have been reproduced. The computed energies for the two forms of poly(L-Ala-Gly) indicate that the silk-II-like form is more stable, by about 1.0 kcal/mol per residue. The main difference between the two structures is the orientation of the Ala side chains of neighboring strands in each sheet. In the Pauling-Corey β-sheet and in the silk II form, referred to as an “in-register” structure, the Ala side chains of every strand point to the same side of a sheet. In the silk I structure, referred to as “out-of-register,” the side chains of Ala residues in adjacent strands point to opposite sides of the sheet.
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    Comparison of the conformational dynamics of the (1 → 4)- and (1 → 6)-linked α-D-glucans using 13C-NMR relaxationDedicated to Prof. Bruno H. Zimm on the occasion of his 70th birthday. (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1581-1592 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conformational dynamics of α- (1 → 4)- and α- (1 → 6)-glucan homooligomers in the nanosecond time domain have been compared by measuring the 13C-nmr longitudinal relaxation times T1 for carbons of the terminal and interior sugar residues. Measurements are reported on monomeric glucose and on oligomers containing up to ten glucose residues at room temperature in aqueous solution at concentrations of 3 and 20 g/dL. The carbons of terminal residues display longer relaxation times than do those of interior residues, presumably as a consequence of a greater degree of conformational mobility of the chain ends. The T1s of the reducing terminal residues of all oligomers are significantly longer than those of the corresponding nonreducing termini, a phenomenon that we associate tentatively with the anomeric equilibrium at the reducing end. Carbons of the reducing terminal residues in the β-anomeric form relax more slowly than their α-anomeric counterparts. At 20 g/dL the mean T1s for carbons of the terminal and interior residues attain asymptotic behavior with increasing chain length at a chain length of about six residues, and carbons of the α- (1 → 4)-linked maltooligomers relax significantly more slowly than those of the corresponding α-(1 → 6)-linked isomaltooligomers. The T1s of both glucan series increase with decreasing concentration. This concentration dependence disappears below 3 g/dL, where the T1s of the two series of homoligomers are no longer distinguishable. This suggests that in dilute aqueous solution at room temperature viscous damping effects predominate over contributions to the T1-sensitive conformational dynamics from structural differences in the glycosidic linkage region. At 3 g/dL the approach to long chain-length asymptotic behavior is more protracted than at 20 g/dL, and the T1s of carbons of interior oligomeric residues appear to match the corresponding high-polymer behavior at a chain length of eight and greater.
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    Masthead (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991) 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    URL: http://dx.doi.org/10.1002/bip.360311401
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    The three states of globular proteins: Acid denaturationWe dedicate this to Bruno Zimm on the occasion of his 70th birthday, with great admiration for his work and much appreciation for his thoughtful advice on many occasions. (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1631-1649 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We describe statistical mechanical theory that aims to predict protein stabilities as a function of temperature, pH, and salt concentration, from the physical properties of the constituent amino acids: (1) the number of nonpolar groups, (2) the chain length, (3) the temperature-dependent free energy of transfer, (4) the pKa's (including those in the native state) and their temperature dependencies. We calculate here the phase diagrams for apomyoglobin and hypothetical variant proteins. The theory captures essential features of protein stability including myoglobin's Tm vs pH as measured by P. L. Privalov [(1979) Advances in Protein Chemistry, Vol. 33, pp. 167-241] and its ionic strength vs pH phase diagram as measured by Y. Goto and A. L. Fink [(1990) Journal of Molecular Biology, Vol. 214, pp. 803-805].The main predictions here are the following: (1) There are three stable states, corresponding to native (N), compact denatured (C), and highly unfolded (U), with transitions between them. (2) In agreement with experiments, the compact denatured state is predicted to have enthalpy closer to U than N because even though there is considerable hydrophobic “clustering” in C, this nevertheless represents a major loss of hydrophobic contacts relative to configurations (N) that have a hydrophobic “core.” (3) C becomes more prominent in the phase diagram with increasing nonpolar content or decreasing chain length, perhaps thus accounting for (a) why lysozyme and α-lactalbumin differ in their denatured states, and (b) why shortened Staph nuclease molecules are compact. (4) Of major importance for protein calorimetry is Privalov's observation that the enthalpy of folding, ΔH (T, pH) is independent of pH. The theory accounts for this through the prediction that the main electrostatic contribution to stability is not enthalpic; the main contribution is the entropy, mainly due to the different distributions of protons and small ions in the native and denatured states.
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    The CD of ligand-DNA systems. I. Poly(dG-dC) B-DNA (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1709-1720 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A systematic theoretical study of the CD of double-stranded poly (dG-dC) and its complexes with small molecules is presented. The intrinsic CD of the polymer and the induced CD of a transition belonging to a molecule bound to DNA are calculated using the matrix method. The calculations show considerable differences between pyrimidine-purine and purine-pyrimidine binding sites, and we find that the induced CD of a groove bound molecule is one order of magnitude stronger than that of an intercalated molecule. The results form a sound basis for interpreting the CD of ligand-DNA systems in terms of molecular geometry, interactions, and spectroscopy.
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    Molecular dynamics simulation of Lewis blood groups and related oligosaccharides (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1737-1746 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Molecular dynamics simulations without explicit inclusion of solvent molecules have been performed to study the motions of Lewisa and Lewisb blood group oligosaccharides, and two blood group A tetrasaccharides having type I and type II core chains. The blood group H trisaccharide has also been studied and compared with the blood group A type II core chain. The potential energy surface developed by Rasmussen and co-workers was used with the molecular mechanics code CHARMM. The lowest energy minima of the component disaccharide fragments were obtained from conformational energy mapping. The lowest energy minima of these disaccharide fragments were used to build the tri- and tetrasaccharides that were further minimized before the actual heating/equilibration and dynamics simulations. The trajectories of the disaccharide fragments, e.g., Fuc α-(1 → 4) GlcNAc, Gal β-(1 → 4) GlcNAc, etc., show transitions among various minima. However, the oligosaccharides were found to be dynamically stable and no transitions to other minimum energy conformations were observed in the time series of the glycosidic dihedral angles even during trajectories as long as 300 ps. The stable conformations of the glycosidic linkages in the oligosaccharides are not necessarily the same as the minimum energy conformation of the corresponding isolated disaccharides. The average fluctuations of the glycosidic angles in the oligosaccharides were well within the range of ±15°. The results of these trajectory calculations were consistent with the relatively rigid single-conformation models derived for these oligosaccharides from 1H-nmr data.
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    The solution conformation of tubulin-β(422-434)-NH2 and its Nac-DATADEQG-NH2 fragment based on nmr (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 449-458 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The solution conformation of tubulin-β(422-434)-NH2 (YQQYQDATADEQG-NH2) and its Nac-DATADEQG-NH2 fragment has been studied by two-dimensional 1H-nmr spectroscopy in CD3OH/H2O (90/10 v/v) at neutral and low pH. The 13 amino acid peptide is a segment of the C-terminal region of tubulin, and is directly involved in the selective binding site with microtubule-associated proteins MAP-2 and the τ protein. Based on correlated spectroscopy, total correlation spectroscopy, and rotating frame nuclear Over-hauser effect spectroscopy experiments, a complete assignment of all proton resonances was achieved, and the conformation of the backbone could be deduced from coupling constants, NH temperature coefficients, and nuclear Overhauser effects. The spectroscopic evidence indicates that the T8-Q12 section of both molecules forms one complete α-helical turn, stabilized by a NH(Q12)-C=O(T8) hydrogen bond. Furthermore, strong pH-dependent backfolding of the E11 side chain to its own NH proton was found. In addition, close proximity between the aromatic side chains of Y1, Y4, and the α-helical part, resulting in some substantial chemical shift changes when comparing the entire 13-mer with the octamer, could be explained in terms of a nonclassical kink in the DATA section. The conformational space is dominated by extended structures and the nonextended conformers are only a minor, yet spectroscopically clearly discernible entity. The presence of the α-helical region at the C-terminus of the 13-mer is important because binding studies of this peptide with MAP-2 indicate that the D10-E11-Q12-G13 fragment is critical for the binding interaction.
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    Effects of proline ring conformation on theoretical π-π* absorption and CD spectra of helical poly(L-proline) forms I and II (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 529-535 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Absorption and CD spectra of the π-π* transition near 200 nm are calculated for helical (Pro)10 forms I and II with a variable proline ring conformation characterized by torsion angle χ2 in the range -60° to 60°. The spectra for poly (Pro) I are not sufficiently sensitive to χ2 to suggest a preferred ring conformation. The spectra for poly (Pro) II are more sensitive to χ2, and suggest preferred ring conformations near either or both of the χ2regions -50 ± 10° and 50 ± 10°.
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    Theoretical CD studies of polypeptide helices: Examination of important electronic and geometric factors (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 569-586 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An improved model for calculating the CD of polypeptides has been developed. Excited state wavefunctions were derived from CNDO/S (complete neglect of differential overlap, spectroscopic) calculations on N-methylacetamide. Four discrete peptide-localized transitions were employed: π0π*(NV1), π+π* (NV2), nπ*, and n′π*. Inclusion of the π+π*transition (λ0 = 140 nm) significantly improves the accuracy of the calculated CD spectra in the 180-250-nm region. Spectra were computed for various helical structures, including right-handed α-, αII-, ω-, π-, 310-, and poly(proline)I-helices, and the left-handed poly(proline)II-helix. Sensitivity to changes in the peptide backbone geometry and chain length are examined. Electronic factors such as ground-state charge distribution, hybridization effects, and basis set deorthogonalization have been investigated. The nonconservative nature of the poly (Pro) I and II CD spectra is reproduced, and the helix band present in earlier exciton calculations on the α-helix has been diminished.
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    A stable antiparallel cytosine-thymine base pair occurring only at the end of a duplex (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1087-1094 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ultraviolet hyperchromicity experiments indicate that in DNA duplex formation, a C-T mismatch is destabilizing in the center of a duplex, but behaves as a stable base pair at the terminus of a duplex. The C-T base pair is thought to contain two hydrogen bonds, but has thermodynamic parameters (ΔH° and ΔG° of dissociation) that are similar to a G-C base pair. AMBER molecular mechanics calculations were performed to study the possible structural properties of DNA duplexes with central and terminal C-T combinations. These calculations also indicate that a central C-T pair destabilizes a duplex, while terminal C-T forms a stable base pair. Hydrogen bonding between cytosine and thymine occurs only in the energy-minimized structures when the helix diameter decreases and the propeller twist angle between the bases increases. These changes are found to occur only at the end of a duplex in the calculations, which may explain the experimental results.
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    A molecular dynamics simulation of polyalanine: An analysis of equilibrium motions and helix-coil transitions (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1115-1134 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An understanding of helix dynamics can aid in interpreting the motions of proteins. The conformational transitions that occur also appear to play a role in protein folding. Structural studies of isolated peptides in solution are just becoming available. However, detailed analysis of the helix-coil transition is still not available and will be difficult to obtain experimentally. For these reasons, we performed a long molecular dynamics simulation of polyalanine at high temperature. Using this approach, we obtain a description of the overall structure and inherent flexibility of the chain as well as a structural picture of the conformational changes that occur. In this way, we can address both equilibrium properties of the peptide and the dynamics and mechanism of the structural transitions. Our results correlate fairly well with the available experimental data and previous simulations aimed at addressing α-helix dynamics. The peptide spends the bulk of its time fluctuating between different conformations with intermediate helix contents. Transitions between highly ordered and highly disordered structures were rare, but they occurred rapidly. Our distribution of conformations favored collapsed states. Hence, our transitions to structures with high helical content were from fluctuating compact structures. The conversion between helix and coil occurred sequentially on a residue-by-residue basis. However, there was local cooperativity; the transition of a residue to the coil state was facilitated after a neighboring group became non-helical. The relevance of our results to protein folding is also discussed.
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    On the mechanism of dielectric relaxation in aqueous DNA solutions (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1171-1180 
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    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The complex dielectric response of calf thymus DNA in aqueous saline solutions has been measured from 1 MHz to 1 GHz. The results are presented in terms of the relaxation of the incremental contributions to the permittivity and conductivity from the condensed counterions surrounding the DNA molecules. Measurements of the low-frequency conductivity of the samples also lends support to the condensed counterion interpretation.
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    The cystine-stabilized α-helix: A common structural motif of ion-channel blocking neurotoxic peptides (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1213-1220 
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    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Neurotoxic peptides from venoms of scorpions and honey bees exhibit a consensus pattern in the two disulfide bridgings related to the sequence portions Cys-X-Cys and Cys-X-X-X-Cys. A revised three-dimensional structure of charybdotoxin, as determined by two-dimensional nmr spectroscopy, confirms that the consensus cystine dislocation generates in all these toxins a common structural element, i.e., the cystine-stabilized α-helical (CSH) motif, which may be correlated with their common ion channel blocking activity.
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    Valinomycin and its interaction with ions in organic solvents, detergents, and lipids studied by Fourier transform IR spectroscopy (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1205-1212 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structure of valinomycin in a range of organic solvents of varying polarity and in detergent and lipid dispersions has been studied by Fourier transform ir Spectroscopy. In solvents of low polarity such as chloroform, ir spectra of valinomycin are fully consistent with the bracelet structure proposed on the basis of nmr Spectroscopy, showing a single narrow amide I component attributable to the presence of β-turns and a single band arising from nonhydrogen-bonded ester C=O groups. K+ complexation results in a downward shift in the amide I band frequency, indicating an increase in the strength of the amide hydrogen bonds, along with a shift to lower frequencies of the ester C=O absorption due to a reduction in electron density in these bonds upon complexation. Identical results were obtained with NH4+, a finding not previously reported.In solvents of both medium (CHCl3/DMSO 3 : 1) and high (pure DMSO) polarity, we find evidence of significant disruption of the internal hydrogen-bonding network of the peptide and the appearance of a band suggesting the presence of free amide C=O groups. In such solvents, complexation with K+ and NH4+ was not observed.The structure of valinomycin in detergent micelles resembles that in nonpolar organic solvents. However, changes were found in the amide I and ester carbonyl maxima as 2H2O penetrated the micelle which suggest significant interaction between the solvent and peptide. Complexation with K+ was reduced in cationic detergent micelles as a result of a decrease in the effective K+ concentration due to charge repulsion at the micelle surface.In lipid bilayers the structure again appears identical to that found in chloroform. As in detergent micelles, the amide I and ester carbonyl bands exhibit shifts that indicate interactions with solvent. Complexation with both K+ and NH4+ is efficient, producing spectral changes similar to those seen in organic solvents.
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    Premelting thermal fluctuational interbase hydrogen-bond disrupted states of a B-DNA guanine-cytosine base pair: Significance for amino and imino proton exchange (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1273-1281 
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    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Modified self-consistent phonon theory when applied to the DNA double helix indicates the existence of fairly long-lived states in which single interbase H bonds are disrupted. One can then postulate a number of situations in which particular disrupted H bonds can enhance particular proton exchange. In this paper we postulate a number of such partially open states for a B-conformation GC base pair and calculate the probability of each of these states for a B-conformation poly(dG) · poly(dC). We compare these probabilities to those probabilities needed to explain various observed proton exchange rates. We propose that, for a GC base pair in B conformation, there are two amino proton exchangeable states - a cytosine amino proton exchangeable state and a guanine amino proton exchangeable state; both require the disruption of only the corresponding inter base H bond. The imino proton exchange, however, requires the disruption of all three inter base H bonds and this defines a third open state. Our calculated probabilities for a GC base pair in these three states are in fair agreement with available experimental estimates from measurements of amino and imino proton exchange.
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    Conformational correlatives of DNA band compression and bidirectional migration during field inversion gel electrophoresis, detected by quantitative video epifluorescence microscopy (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1297-1307 
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    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Individual DNA molecules in the Mb size range were monitored by epifluorescence video microscopy during field inversion gel electrophoresis (FIGE). DNA migrating in an agarose gel gives rise to characteristic V-conformational elements and when doing so exhibits a reduced mobility. When the V-conformational elements per DNA molecule are few, the degree of retardation appears proportional to the number of V's, and since larger DNA species exhibit more V's, to DNA size. For a particular pulse frequency, the proportionality breaks down progressively as the number of V-conformational elements per DNA molecule increases. The loss of proportionality between DNA length and migration rate is being correlated with the macroscopically observed loss of electrophoretic size discrimination known as band compression. For a particular pulsing frequency and size class of DNA, the loss of size discrimination is thought to be due to the different orientations of migration, caused by the asymmetric distribution of V-conformational elements when the number of these elements is moderate. Small and very large DNA by contrast migrate with the direction of the biased field. These events, analyzed by microscopic measurement, are consistent with the known macroscopically observed double-valued mobilities in FIGE.
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    A New General Synthesis of 2,2-Dialkyl-2,3-dihydro-4H-pyran-4-ones and Their Application for the in situ Preparation of Electron-Rich Dienes in Carbonyl-Alkyne Exchange Reactions with AcetylenesPresented in part at the autumn meeting of the Swiss Chemical Society in Bern on October 20, 1989. (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 27-46 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The substituted 2,2-dialkyl-2,3-dihydro-4H-pyran-4-ones of type II and III have been prepared by acid-catalyzed cyclization of the corresponding substituted acetylenic ketones I in good to excellent yields (Scheme 1). These 2,2-dialkyl-2,3-dihydro-4H-pyran-4-ones II and III have been used for the in situ preparation of highly reactive dienes of type IV-VI (Scheme 2) in carbonyl-alkyne exchange reactions with electron-poor alkynes VII to yield the highly substituted aromatic compounds VIII and IX. These reactions proceed in good yields and with excellent degree of regioselectivity. Aryl-substituted 2,2-dialkyl-2,3-dihydro-4H-pyran-4-ones III (R1 = Ar) subsequently yield highly substituted biaryls. Reaction mechanisms are presented for the formation of the 2,2-dialkyl-2,3-dihydro-4H-pyran-4-ones as well as for the carbonyl-alkyne exchange reactions with electron-poor acetylenes.
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    Structural and Luminescence Study of the 3:2 Complex between Europium Nitrate and the B Isomer of Dicyclohexyl-18-crown-6Part 40 of the series ‘Complexes of Lanthanoid Salts with Macrocyclic Ligands’; Part 39 [1]. (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 65-78 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The crystal and molecular structure of bis[dinitrato-(2,4,8,15,18,21-hexaoxatricyclo[20.4.0.09,14]hexacosane)europium(III)]pentakis(nitrato)europiate(III) [Eu(NO3)2LB]2([Eu(NO3)5]), has been determined at 170 K from single-crystal X-ray diffraction. The complex crystallizes in the monoclinic space group P21/n (ITC No. 14): a = 16.338(3) Å, b = 15.704(3) Å, c = 24.474(4) Å, β = 97.73(1)°, Z = 4. The structure was refined to a final R value of 0.058 (Rw = 0.060). The asymmetric unit contains three independent ions lying on general positions: [Eu(NO3)5]2- and two distinct [Eu(NO3)2LB]+ cations with the macrocyclic ligand in the cis-anti-cis conformation (B-isomer). The EuIIIions are 10-coordinate with the following mean bond lengths: Eu-O(nitrate) = 2.48(2) Å in the anion and 2.45(2) Å in the two cations, Eu-O(ether) = 2.56(8) and 2.55(5) Å. Small but significant differences are observed between the two complex cations, especially with respect to the positions of the cyclohexyl substituent. A conformational analysis performed on the six O-atoms of the complex cations confirms the predictions of a simple model. The metal ion sites of the complex have been probed by high-resolution excitation and emission spectra at 296 and 77 K. The 5D0←7F0 excitation spectrum displays two main bands along with several other minor components. A detailed analysis of the corresponding and selectively excited emission spectra leads to the observation of three types of spectra corresponding to the three crystallographically different EuIII ions. Moreover, three minor sites are identified, one anionic and two cationic, with a population equal to ca. 10% of the population of the main sites. We interpret this finding as reflecting the presence of molecules with slightly different conformations.
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    Asymmetric Diels-Alder Cycloadditions with C2-Symmetrical Chiral Carbamoylnitroso Dienophiles (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 103-109 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The C2-symmetrical chiral pyrrolidines 2 and 3 are of opposite helicity. The corresponding N-acylnitroso dienophiles 6 and 7 react in good yield with cyclohexadiene, leading thereby with excellent diastereoisomeric excess to the expected Diels-Alder cycloadducts (see Scheme). The [2.2.2] bicyclic moieties of the major diastereoisomers 9 and 11 proved to be of opposite configuration, as expected. Their configuration is best explained by assuming the acylnitroso dienophile to be in the s-cis conformation in the transition state, the approach of the diene being endo (see Fig.).
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    2,3- and 4,5-Secodopa, the Biosynthetic Intermediates Generated from L-Dopa by an Enzyme System Extracted from the Fly Agaric, Amanita muscaria L., and Their Spontaneous Conversion to Muscaflavin and Betalamic Acid, Respectively, and Betalains (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 124-140 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An enzyme is extracted from the red peel of Amanita muscaria which cleaves the C(2)-C(3) and the C(4)-C(5) bond of the aromatic ring of L-dopa (1) to form a mixture of 4,5-secodopa (= salt of 6-amino-2-hydroxy-4-(2′-oxoethylidene)hept-2-enedioic acid; 2) and 2,3-secodopa ( = salt of 7-amino-5-formyl-2-hydroxyocta-2,4-dienedioic acid; 3), two hitherto hypothetical biosynthetic intermediates (see Scheme). Though isolation of these products has not been possible, structural evidence is inferred from reaction products, kinetics, and spectroscopical characteristics in comparison with known compounds. Secodopas 2 and 3 are characterized in dilute solution by HPLC and UV/VIS spectroscopy (anions; λmax 424 and 414 nm, resp., ∊420 = 25500; on acidification, shift to 380 and 372 nm, resp.). They cyclize without enzyme catalysis, optimally at pH 4.5-5; 3 produces muscaflavin (5) and 2 betalamic acid (4). The products arc identified by direct comparison with authentic samples in HPLC, by 1H-NMR of 5, and by condensation of 4 with L-proline to form the well known betalain indicaxanthin (7). The enzymatic conversion of L-dopa (1) via 2 to betalamic acid (4; (S)) and its condensation with L-proline leads to pure natural indicaxanthin (7; (2S,115)); correspondingly, the enzymatic conversion of D-dopa to (R)-betalamic acid and its condensation with L-proline produces isoindicaxanthin ((2S,11R)) which is unknown in nature. Particularly relevant is the fact that the same enzyme cleaves pyrocatechol to produce a solution of the enolate form of the known 2-hydroxy-6-oxohexa-2,4-dienoate (secopyrocatechol; 9; see Fig. 5). Dissociation constants of the corresponding enolic functions in the cleavage products are determined by spectrometric titration and compared to those of known systems.
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    Photochemical Reactions of Tetrahydroquinoxalin-2(1H)-ones and Related Compound (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 225-231 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Photochemical behaviors of the pyrazinone derivatives 5,6,7,8-tetrahydroquinoxalin-2(1H)-ones 1a-c and 1,5,6,7,8,9-hexahydro-2H-cyclohepta[b]pyrazin-2-one 1d were investigated. Dye-sensitized photo-oxygenation of 1a-c gave the 1:1 adducts 5a-c of the corresponding 3,8a-epidioxy-3,5,6,7,8,8a-hexahydroquinoxalin-2(1H)-one 4 and H2O, whereas 1d gave 3,9a-epidioxy-1,3,5,6,7,8,9,9a-octahydro-2H-cyclohepta[b]pyrazin-2-one 4d (Scheme 2). The different kind of products was interpreted as being the result of the ring strain and steric hindrance of endoperoxides produced from 1a-d with singlet oxygen. Irradiation of 1a-b in the presence of alkenes gave tricyclic azetidine derivatives 9 by [2 + 2] cycloaddition of the C=N bond of 1 to the alkene.
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    Analogues of Sialic Acids as Potential Sialidase Inhibitors. Synthesis of C6 and C7 Analogues of N-Acetyl-6-amino-2,6-dideoxyneuraminic Acid (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 343-369 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The piperidines 12-18, piperidmose analogues of Neu5Ac (1) with a shortened side chain, were synthesized from N-acetyl-D-glucosamine via the azidoalkene 32 and tested as inhibitors of Vibrio cholerae sialidase. Deoxygenation at C(4) of the uronate 22, obtained from the known D-GlcNAc derivative 20, was effected by β-elimination (→ 23), exchange of the AcO at C(3) with a (t-Bu)Me2SiO group and hydrogenation (→ 26; Scheme 1). Chain extension of 26 by reaction with Me3SiCH2MgCl gave the D-ido-dihydroxysilane 28, which was transformed into the unsaturated L-xylo-mesylate 29 and further into the L-lyxo-alcohol 30, the mesylate 31, and the L-xylo-azide 32. The derivatives 29-31 prefer a sickle zig-zag and 32 mainly an extended zig-zag conformation (Fig. 2). The piperidinecarboxylate 15 was obtained from 32 by ozonolysis (→ 33), intramolecular reductive animation (→ 34), and deprotection, while reductive animation of 34 with glycolaldehyde (→ 35) and deprotection gave 16 (Scheme 2). An intramolecular azide-olefin cycloaddition of 32 yielded exclusively the fused dihydrotriazole 36, while the lactone 39 did not cyclize (Scheme 3). Treatment of 36 with AcOH (→ 37) followed by hydrolysis (→ 38) and deprotection led to the amino acid 18. To prepare the (hydroxymethyl)piperidinecarboxylates 12 and 17, 32 was first dihydroxylated (Scheme 4). The L-gluco-diol 40 was obtained as the major product, in agreement with Kishi's rule. Silylation of 40 (→ 42), oxidation with periodinane (→ 44), and reductive animation gave the L-gluco-piperidine 45. It was, on the one hand, deprotected to the amino acid 12 and, on the other hand, N-phenylated (→ 46) and deprotected to 17. While 45 and 12 adopt a 2C5 conformation, the analogous N-Ph derivatives 46 and 17 adopt a 5C2 and a B3,6 conformation, respectively, on account of the allylic 1,3-strain. The conformational effects of this 1,3-strain are also evident in the carbamate 47, obtained from 45 (Scheme 5), and in the C(2)-epimerized bicyclic ether 48, which was formed upon treatment of 47 with (diethylamino)sulfur trifluoride (DAST). Fluorination of 40 with DAST (→ 49) followed by treatment with AcOH led to the D-ido-fluorohydrin 50. Oxidation of 50 (→ 51) followed by a Staudinger reaction and reduction with NaBH3CN afforded the (fluoromethyl)piperidine 52, while reductive amination of 51 with H2/Pd led to the methylpiperidine 55, which was similarly obtained from the keto tosylate 54 and from the dihydrotriazole 36. Deprotection of 52 and 55 gave the amino acids 13 and 14, respectively. The aniline 17 does not inhibit V. cholerae sialidase; the piperidines 12-16 and 18 are weak inhibitors, evidencing the importance of an intact 1,2,3-trihydroxypropyl side chain.
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    Synthese von Monomorin I, einem Spurpheromon der Pharao-Ameise (Monomorium pharaonis) (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 438-444 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of Monomorin I, a Trail Pheromone of the Pharao Ant (Monomorium pharaonis)Katalytic hydrogenation of the 4-nitroalkanone 5 yielded the cis-pyrrolidine ester 6 (Scheme). The preparation of 5 was accomplished by ethanolysis of the nitrocyclohexanone derivative 4. Iodination at C(α) of 6, followed by cyclization, gave a nearly 5:6 mixture of the two C(5)-epimers 10/9 of octahydroindolizine. Epimerisation of the undesired 10 to 9 and subsequent transformation of the latter gave racemic monomorine I (1; 34% yield from 5).
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    Masthead (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991) 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    Structure cristalline d'une thiazolorifamycine: rifamycine P (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 326-330 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Crystal Structure of a Thiazolorifamycine: Rifamycine PThe crystal structure of a natural thiazolorifamycin, rifamycin P, as an adduct with EtOH and H2O, has been determined by X-ray crystallography by a combination of direct methods and Patterson techniques. Block-diagonal least-squares refinement using 5996 independent reflections and 1475 parameters led to an R factor of 0.092 (two independent molecules of rifamycin P). The middle part of the ansa chain, essential for activity against the bacterial DNA-dependent RNA polymerase (DDRP) resembles that of active rifamycins. The four OH groups lie on the same side of the molecule and are almost in the same plane. Interatomic distances between the O-atoms agree well with a spatial model derived from X-ray studies, performed on semisynthetic active rifamycins.
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    Synthesis of 4-{{[(Isopropyloxy)methylphosphoryloxy]imino}methyl}-1-methylpyridinium Iodide and Its Characterisation (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 331-335 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oximes are used in the therapy of organophosphate poisoning. They form an interaction product with the organophosphate which is claimed to be more toxic than the organophosphate itself. We have synthesized one of these compounds which would be formed by the reaction of pyridine-4-carbaldehyde oxime methiodide with sarin (GB). Thus, reaction of oxime 1 with sarin chloridate (2), yielded 4-{{[(isopropyloxy)methylphosphoryloxy]-imino}methyl}-1-methylpyridinium iodide (4) which was characterized by NMR spectroscopy and X-ray crystallography.
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    Transformations in the Isoxazole Series: Synthesis of Substituted 2-AminothiazolesDedicated to Prof. Dr. Oskar Jeger (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 531-542 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Substitued N-(isoxazol-4-yl)thioureas 1 undergo a transformation in the presence of hexacarbonylmolybdenum and acid to yield functionalized thiazoles 3 in a one-pot reaction. In a few cases, 1,4,5-trisubstituted dihydroimidazolethiones 4 are also isolated as side products. Mechanistic considerations are outlined and scope and limitations of this new methodology discussed.
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    Biosynthesis of the Spermine Alkaloid Aphelandrine (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 565-571 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Aphelandrine (1) is shown to be biosynthesized in the root cells of Aphelandra tetragona (VAHL) NEES from labelled putrescine (4), spermidine (5), and cinnamic acid (3). Whether spermine (6) and the (p-hydroxycinnamoyl)spermidine 8 are precursors of 1 is uncertain, since the latter is hydrolysed to a large extent before incorporation, and the former is metabolized to 4 and 5. Methionine (7) is the source of the 3-aminopropyl unit of 5 and 6.
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    Glycosylidene Carbenes. Part 3. Synthesis of SpirocyclopropanesPresented at the ‘XVIth International Carbohydrate Symposium’, Yokohama, Japan, 12th-17th August 1990, Abstr. PL-05. (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 585-593 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thermolysis of the glycosylidene-derived O-benzylated diazirine 1 in the presence of N-phenylmaleimide (2), acrylonitrile (3), dimethyl fumarate (4), or dimethyl maleate (5) led in good yields to mixtures of the spirocyclopropanes 6/7, 8-11, 12/13, and 12/13/16/17. The diastereoselectivity depends upon the alkene. The cycloaddition of 1 to 5 is not diastereospecific, in keeping with previous results. Deprotection of 12, 13, 16, and 17 yielded the tetrols 14,15, 18, and 19, respectively.
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    Lithium-Salt Effects in Peptide Synthesis. Part II. Improvement of Degree of Resin Swelling and of Efficiency of Coupling in Solid-Phase Synthesis (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 628-643 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The course of solid-phase peptide-coupling reactions as well as the swelling properties of a peptide-resin are influenced by the addition of inorganic salts (LiCl, LiBr, LiClO4, KSCN). Used as additives, these salts can (i) improve coupling yields (e.g., for Fmoc-(Ala)5-Phe-resin → Fmoc-(AIa)6-Phe-resin in DMF/CH2Cl2 1:1 from 89.4 to 97.1% (for polyethylene oxide) on polystyrene (= PEO-PS) resin) or from 77.5 to 93.8% (for poly-(N,N′-dimethylacrylamide) on ‘Kieselgur’ (=PDMAA-KG) resin) without and with 0.4M LiCl, respectively), (ii) increase resin swelling (e.g. for Fmoc-(Ala)5-Phe-(polystyrene resin) from 2.42- to 5.73-fold in 1-methylpyrrolidin-2-one (=NMP) without and with LiCl, (respectively), and (iii) change coupling rates. Example;; of coupling reactions and swelling behaviour (degree and rate) in different solvents (DMF, DMF/CH2Cl2 1:1, THF, NMP, N,N-dimethylpropyleneurea (= DMPU) with and without salts) using different resins (polystyrene (PS); PEO-PS, and PDMAA-KG) and an improved analysis of alanine oligomers up to Ala12-Phe by HPLC and FAB-MS are reported.
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    On the Methyl-Transfer Reaction in Crystalline Methyl 2-(Methylthio)benzenesulfonate: a Thermally Induced Non-Topochemical Solid-State Reaction (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 662-669 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rearrangement of methyl 2-(methylthio)benzenesulfonate (1) to the zwitterionic 2-(dimethyl-sulfonium)benzenesulfonate (2) is known to proceed in solution by intermolecular Me transfers. The same rearrangement has been observed to occur in crystalline 1, but the crystal structure shows that the molecular packing is not conducive to intermolecular Me transfer. The reaction has been carried out with mixed crystals composed of 1 and deuteriomethylated (D6)-l. By fast-atom-bombardment mass spectroscopy, it has been shown that the product consists of a 1:2:1 mixture of the non-, tri-, and hexadeuterated species, the mixture expected, if the solid-state reaction proceeds by intermolecular Me transfers. From this result, together with the slower rates of conversion in the single crystal compared with the melt, it can be concluded that the reaction must occur not topochemically but rather at defects such as microcavities, surfaces, and other irregularities in the ordered crystal arrangement.
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    Syntheses and Crystal Structures of 4,5-Dihydro-2-(2′-hydroxyphenyl)oxazole-Containing Metal Complexes (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 717-726 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the complexes 3 of various metals ligated to chiral 4,5-dihydro-2-(2′-oxidophenyl-ϰO)oxazoles-ϰN is described (Scheme). Three of them, i.e. 3a, 3e, and 3f containing CuII, ZnII and NiII, respectively, were analyzed by X-ray diffraction studies. A series of CuII complexes (6a-d) with differently substituted dihydrooxazoles have been synthesized.
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    1-(2′-Deoxy-β-D-xylofuranosyl)thymine Building Blocks for Solid-Phase Synthesis and Properties of Oligo(2′-Deoxyxylonucleotides) (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 748-760 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1-(2′-Deoxy-β-D-threo-pentofuranosyl)thymine (= 1-(2′-deoxy-β-D-xylofuranosyl)thymine; xTd; 2) was converted into its phosphonate 3b as well as its 2-cyanoethyl phosphoramidite 3c. Both compounds were used for solid-phase synthesis of d[(xT)12-T] (5), representing the first DNA fragment build up from 3′-5′-linked 2′-deoxy--β-D-xylonucleosides. Moreover, xTd was introduced into the innermost part of the self-complementary dodecamer d(G-T-A-G-A-A-xT-xT-C-T-A-C)2 (9). The CD spectrum of d[(xT)12-T] (5) exhibits reversed Cotton effects compared to d(T12) (6; see Fig. 1), implying a left-handed single strand. With d(A12) (7) it could be hybridized to form a propably Left-handed double strand d(A12) · d[(xT)12-T] (7·5) which was confirmed by melting experiments in combination with temperature-dependent CD spectroscopy. While 5 was hydrolyzed by snake-venom phosphodiesterase, it was resistant towards calf-spleen phosphodiesterase. The modified, self-complementary duplex 9 was hydrolyzed completely by snake-venom phosphodiesterase, at a twelvefold slower rate compared to unmodified 8; calf-spleen phosphodiesterase hydrolyzed 9 only partially.
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    Conformational Analysis of Marine Polyhalogenated β-Chamigrenes Through Temperature-Dependent NMR SpectraPresented in part by G.G. at both CISCI 90, S. Benedetto del Tronto, 30 September-5 October 1990, and the Congresso Nazionale di Risonanze Magnetiche, Pisa, 22-24 October 1990. (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 774-786 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Temperature-dependent 1H- and 13C-NMR spectra reveal that polyhalogenated marine β-chamigrenes or synthetic derivatives thereof which are trans-diequatorially, substituted at C(8) and C(9), such as rogiolol ((-)-2), obtusol ((+)-3), and their acetates (+)-1 and (-)-4, undergo slow ring-A chair-chair inversion. Conformational equilibria and kinetics are investigated with the aid of synthetic model compounds and molecular-mechanics calculations. Thus, steric repulsions between Brax-C(2) and Heq-C(7) are seen to disfavour thermodynamically conformers 1b, 2b, 3b, and 4b, which can only be detected through cross-saturation transfer, while additional steric repulsions between Meax-C(1) and OHax-C(3) make conformer 8b of obtusol epimer so scarcely populated that it can not be detected. In agreement, with (+)-9 and (+)-10, which have a trigonal C(2), two conformers can be directly observed by NMR. The kinetic barriers, which are seen to arise mainly from steric repulsions between Hax-C(14) and the axial H or halogen atoms at C(8) and C(10), are calculated and discussed with respect to well documented exocycliemethylidene-substituted cyclohexane(ene) systems. This helps to rationalize why in rogiolol acetate ((+)-1) ring B is unusually inert towards Zn/Et2O/AcOH which causes bromohydrine-group elimination from ring A.
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    Cycloviolaxanthin (= (3S,5R,6R,3′S,5′R,6′R)-3,6:3′,6′-Diepoxy-5,6,5′,6′-tetrahydro-β,β-carotin-5,5′-diol), ein neues Carotinoid aus Paprika (Capsicum annuum) (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 819-824 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cycloviolaxanthin (= (3S,5R,6R,3′S,5′R,6′R)-3.6:3′,6′-Diepoxy-5,6,5′,6′-tetrahydro-β,β-carotene-5,5′-diol), a Novel Carotenoid from Red Paprika (Capsicum annuum)From red paprika (Capsicum annuum var. longum nigrum) cycloviolaxanthin was isolated as a minor carotenoid and, based on spectral data, assigned the symmetrical structure 8.
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  • 168
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    Studies Directed Towards the Biosynthesis of the C7N Unit of Rifamycin B: A New Synthesis of Quinic Acid from Shikimic Acid (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 807-818 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of quinic acid (4) via epoxide 13, starting from shikimic acid (5), is described (Scheme 1). Treatment of 13 with thiophenol yielded not only 17, but also the γ-lactones 18 and 19 as result of migration of silyl groups within a cis- and trans-diol system. The conversion provides a direct stereoselective epoxidation of a shikimic-acid derivative as well as an alternative pathway for the preparation of 4. A shorter approach via the disilylated epoxide 22 was unsuccessful because the γ-lactone 25 was obtained in place of the desired α-hydroxy ester 24 (Scheme 2).
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  • 169
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    Nucleotides. Part XXXVIPart XXXV:[1]. Syntheses and biological characterization of phosphorothioate analogues of (3′-5′)adenylate trimer (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 892-898 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The four protected diastereoisomcrs 7a/7b and 8a/8b P-thioadenylyl-(3′-5′)-P-thioadenylyI-(3′-5′)-adenosine were synthesized, separated, and deblocked to the free oligonucleotides (Scheme). Biochemical characterization of these (3′-5′)phosphorothioate analogues of adenyiate trimer indicate that these compounds, and the corresponding 5′-monophosphates, neither bind to nor activate RNase L, and are considered to be valuable control compounds in screening experiments.
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  • 170
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    Double-Diamond Inclusion Compounds of 2,6-Dimethylydeneadamantane-1,3,5,7-tetracarboxylic Acid (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 825-877 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The crystal structures of four inclusion compounds of 2,6-dimethylideneadamantane-1,3,5,7-tetracarboxylic acid (4) are described, which involve the following guest species: (a) a mixture of 4-methylpent-3-en-2-one, 2,6-dimethylhepta-2,5-dien-4-one, and mesitylene (condensation products of acetone); (b) mesitylene; (c) a mixture of 4-methylpent-3-en-2-one and mesitylene; (d) (tert-butyl)benzene. In all four cases, the host architectures consist of two interpenetrating super-diamond networks built up by the tetra-acid molecules via pairwise H-bonds between the tetrahedrally directed COOH groups. In the first three cases (tetragonal crystal symmetry), the two diamond-like host lattices interpenetrate symmetrically, in the fourth case (monoclinic) asymmetrically. This asymmetry is brought about by the increased steric bulk of the (tert-butyl)benzene guest molecules. Attempts to enforce an inclusion compound of 4 with a single, extremely hollow diamond-like host lattice by offering still bulkier guest molecules have as yet not been met with success. The generally very high propensity of 4 to form inclusion compounds was envisaged and designed beforehand by appropriate evaluation and modulation of the crystal structure of the parent adamantane-1,3,5,7-tetracarboxylic acid, which represents a fivefold diamond-like self-inclusion compound. Crystals of the free, uncomplexed 4 appear to be extremely unstable and could so far not be obtained. On the other hand, from aqueous solution a very stable monohydrate of 4 may be crystallized (4-H2O), which was also subjected to X-ray analysis. The (triclinic) crystal structure of 4·H2O involves an interesting dichotomy inasmuch its pattern of H-bonding may be rationalized either in terms of a double, cross-linked super-zincblende (sphalerite) architecture, or as a system of porous, puckered 4-connected sheets, which interpenetrate each other pairwise and are cross-linked by the H2O molecules. Various structure and (space group) symmetry characteristics of the supramolecular solid-state complexes reported here are highlighted by pointing out analogies with comparable structures retrieved from the literature.
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    Masthead (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991) 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    URL: http://dx.doi.org/10.1002/hlca.19910740501
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  • 172
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    Novel Antifungal Tetracyclic Compounds from Bauhinia rufescens LAM.Presented in part at the Meeting of the Swiss Chemical Society, Bern, 19th October 1990. (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 791-799 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: As the root bark CH2Cl2 extract of Bauhinia rufescens showed antifungal activity in a bioassay with the plant pathogenic fungus Cladosporium cucumerinum, a phytochemical investigation was undertaken on material collected in Niger. Activity-guided fractionation of this extract, using different preparative chromatographic methods, allowed the isolation of the four new antifungal tetracyclic compounds 1-4. Their structures were established by 1H- and 13C-NMR data, and single-crystal X-ray analyses were used to confirm the structures of 1 and 3. The isolated compounds seem to be biogenetically related to a stilbene.
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    Versatile Stereoselective Synthesis of Completely Protected Trifunctional α-Methylated α-Amino Acids Starting from Alanine (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 800-806 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new route to completely protected α-methylated α-amino acids starting from alanine is described (see Scheme). These derivatives, which are obtained via base-catalyzed opening of the oxazolidinones (2S,4R)- and (2R,4S)-2, can be directly employed in peptide synthesis. The synthesis of both enantiomers of Z-protected α-methylaspartic acid β-(tert-butyl)ester (O4-(tert-butyl) hydrogen 2-methylaspartates (R) or (S)-4a), α-methyl-glutamic acid γ-(tert-butyl) ester (O5-(tert-butyl) hydrogen 2-methylglutamate (R)- or (S)-4b), and of Nε-bis-Boc-protected α-methyllysine (N6,N6-bis[(tert-butyloxy)carbonyl]-2-methyllysine (R)- or (S)-4c) is described in full detail.
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  • 174
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    Nucleotides. Part XXXVPart XXXV:[1]. Synthesis of 3′-deoxyadenylyl-(2′-5′)-3′-deoxyadenyIyl-(2′-ω)-9-(ω-hydroxyalkyl)adenines (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 887-891 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Via the phosphotriester approach, new structural analogs of (2′-5′)oligoadenyiates, namely 3′-deoxyadenylyl-(2′-5′)-3′-dcoxyadenylyl-(2′-ω)-9-(ω-hydroxyalkyl)adenines 18-21, have been synthesized (see Scheme) which should preserve biological activity and show higher stability towards phosphodiesterases. The newly synthesized oligonucleotides 18-21 have been characterized by 1H-NMR spectra, TLC, and HPLC analysis.
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    Mitteilungen (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 905-906 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    URL: http://dx.doi.org/10.1002/hlca.19910740423
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  • 176
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    [5-Leucin]enkephalin-Related Glycoconjugates: Structurally Novel Agents Effective against HIV-1 (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 951-955 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of [Leu5]enkephalin-related glycoconjugates with an ester-, ether-, or amide-type linkage were synthesized and evaluated for antiviral activity against HIV-1 in a cell-culture system using peripheral blood lymphocytes. All tested glycoconjugates exhibited a certain antiviral activity which was significantly higher than the activity of the parent peptide compound itself. These results indicate that synthetic glycoconjugates of opioid peptides are good candidates for the development of anti-HIV agents.
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    Synthesis, Isolation, and Full Spectroscopic Characterization of Eleven (Z)-Isomers of (3R,3′R)-Zeaxanthin (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 969-982 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Developmental efforts to improve the yield of the chemical synthesis of (3R,3′R)-zeaxanthin resulted in the isolation, partly by chromatography from reaction mixtures, and full spectroscopic characterization by 1H-NMR, UV/VIS, and CD spectrosocpy of eleven (Z/E)-isomers of zeaxanthin: (7Z)-, (9Z), (13Z)-, (15Z)-, (7Z,7′Z)-, (9Z,9′Z)- (7Z,9Z,7Z)-, (7Z,11Z,7′Z)-, (9Z,13Z,9′Z)-, (7Z,9Z,7′Z,9′Z)-, and (7Z,9Z,11Z,7′Z,9′Z)-zeaxanthin. Five of these isomers were obtained by specific synthesis, namely the (7Z)-, (7Z,7′Z)-, (9Z,9′Z)-, (7Z,9Z,7′Z)-, and (7Z,9Z,7′Z-9Z)-isomers.
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    Ringerweiterung von 1,2-Thiazol-3(2H)-on-1,1-dioxiden und 3-Amino-2H-azirinen zu 4H-1,2,5-Thiadiazocin-6-on-1,1-dioxiden (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1002-1010 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ring Enlargement of 1,2-Thiazol-3(2H)-one-1,1-dioxides and 3-Amino-2H-azirines to 4H-1,2,5-Thiadiazocin-6-one-1,1-dioxidesReaction of 3-amino-2H-azirines 2 with the 1,1-dioxides 4 and 7 of 1,2-thiazol-3(2H)-ones and 1,2-thiazoli-din-3-ones, respectively, in i-PrOH at room temperature leads to 4H-1,2,5-thiadiazocin-6(5H)-one-1,1-dioxides 5 (Scheme 2, Table) and the corresponding 7,8-dihydro derivatives 8 (Scheme 4), respectively. The structure of some of the new 8-membered heterocycles as well as the structure of the minor by-product 6 (Scheme 3) have been established by X-ray crystallography (Chapt. 4). The proposed reaction mechanism for the ring expansion to 5 and 8 (Scheme 2) is in accordance with previously published results of reactions of 2 and NH-acidic heterocycles and is further supported by the results of the reaction of 4a and the (1-15N)-labelled aminoazirine 2a*.
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    Zur Oxidation von 1,2-Thiazolen: Ein einfacher Zugang zu 1,2-Thiazol-3(2H)-on-1,1-dioxidenHerrn Prof. Dr. Siegfried Hauptmann zum 60. Geburtstag gewidmet (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1059-1070 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidation of 1,2-Thiazoles; A Convenient Approach to 1,2-Thiazol-3(2H)-one 1,1-DioxidesThe 1,2-thiazoles obtained from 3-chloroalk-2-enals and ammonium thiocyanate (7 → 9, Scheme 1) are easily transformed to 1,2-thiazol-3(2H)-one 1,1-dioxidcs 10 on treatment with H2O2 in AcOH at 80°. Hydrogenation of 10 in AcOH yields the corresponding saturated 1,2-thiazolidin-3-one 1,1-dioxides 16 (Scheme 3). Cycloalka[c]-1,2-thiazoles 18 are prepared from 2-[(thiocyanato)methyliden]cycloalkan-1-ones and ammonia (Scheme 4). Surprisingly, oxidation of 18a with H2O2 in AcOH yields the tricyclic oxaziridine 19.
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    Zur Synthese von Derivaten des Pyrido[1,2-a]azepinsProf. Horst Prinzbach zum 60. Geburtstag gewidmet (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1095-1101 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On the Synthesis of Derivatives of Pyrido[1,2-a]lazepineThe conjugated pyridinium ylides of type 4 undergo 8π -electrocyclisation affording the heterobicyclic allenes 5 which, in the presence of H2O and H2O2, are subsequently transformed into the substituted pyrido[1,2-a]-azepinones 6, a novel heterocyclic system. The chemical behaviour of 6a,b is dominated by cleavage of the lactam bond and subsequent reaction to pyridinc and isoquinoline derivatives, respectively; e.g. reaction of 6a, b with 4,5-dihydro-4-methyl-3H-1,2,4-triazole-3,5-dione affords the cycloadducts 11 and 12 almost quantitatively. The successful transformation of 6b into 15 is described, the latter being the first representative of the presently unknown, potentially antiaromatic pyrido[1,2-a]azepine (1).
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    Mitteilungen (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1156-1156 
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    Regioselectivity, Stereoselectivity, and Isotopically Sensitive Branching in the Fe(I)-Mediated Dehydrogenation of Octane-1,8-diol in the Gas PhaseDedicated to Prof. Jürgen Sauer on the occasion of his 60th birthday (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1127-1134 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Regio- and stereospecific labeling experiments are conducted to unravel the mechanistic features of the Fe+-induced dehydrogenation of octane-1,8-diol in the gas phase. With regard to the regioselectivity, ca. 20% of molecular hydrogen originates from the C(3)/C(4) or the equivalent C(5)/C(6) positions. The remaining 80% are, provided by the C(4)/C(5) methylene units. The steps, preceding the reductive elimination of hydrogen, are irreversible, and the overall reaction follows a 1,2-elimination mode. The loss of HD from C(3)/C(4) is associated with a kinetic isotope effect KH2/KHD = 1.68. Formation of D2 from the positions C(4)/C(5) has an isotope effect of KH2/KD2 = 4.7; this figure is slightly dependent on the configuration at C(4)/C(5). Most interesting is the finding that the configuration at C(4)/C(5) in [4,5-D2]octane-1,8-diol, i.e. 5c vs. 5d, plays a pivotal role in the dehydrogenation of the central C(4)/C(5) part. This unexpected and unprecedented result is explained in terms of conformational analysis. A staggered-like conformation serves as a precursor to generate a trans-fused bicyclic intermediate 6. It is this very intermediate from which most of the molecular hydrogen is eliminated. Of minor importance is the as-fused chelate 7, which is formed from an eclipsed-like conformation of the octane-1,8-diol/Fe+ complex. The contribution of 6 and 7 to the product formation is controlled by the relative configuration at the labeled positions C(4)/C(5). For the D,L-form 5c, we estimate a ratio of ca. 9:1 for the contribution of 6 vs. 7; due to an isotope effect, this ratio drops to 1:85:1 for the meso-form 5d. This finding constitutes the first example for the existence of isotopically sensitive branching (‘metabolic switching’) in gas-phase organometallic chemistry.
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    Crystal Structures of the Lanthanum(III), Europium(III), and Terbium(III) Cryptates of Tris(bipyridine) Macrobicyclic Ligands (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1163-1170 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The crystal structures of the LaIII, EuIII, and TbIII complexes of macrobicyclic [bpy.bpy.bpy] ligands, [La3+ ⊂ 1]3 Cl- ( = 3-La), [Tb3+ ⊂ 1]3 Cl- ( = 3-Tb), and [Eu3+ ⊂ 2]3 C1- ( = 3-Eu), have been determined. They confirm the cryptate nature of these species, the cations being bound to the eight N-sites of the ligand. The macrobicycle presents two open faces, thus allowing additional coordination of two species, Cl- ions or H2O molecules, to the bound cations. These data provide structural support for the photophysical studies of the luminescent properties of the EuIII and TbIII cryptates, which indicated residual coordination of H2O molecules.
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    Biphenyltetrols and Dibenzofuranones from Oxidative Coupling of Resorcinols with 4-Alkylpyrocatechols: New Ciues to the Mechanism of Insect Cuticle Sclerotization (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1205-1212 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidation of 4-alkylpyrocatechols 2 by means of an insect diphenoloxidase (laccase) or K3 [Fe(CN)6] yields, in the presence of resorcinols 1 (R2 = H), complex mixtures of products from which biphenyltetrols 3 (R3 = H) and dibenzofuranones 5 and 6 were isolated. It is suggested that similar homo-coupling products are formed from pyrocatechols 2b and 2c in insects during cuticle sclerotization.
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    Quantitative Formation in Water of [Ru(CO)(H2O)5]2+ from Hexaaquaruthenium(II) and Carbon Monoxide (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1236-1238 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: High-pressure 17O- and 13C-NMR show that [Ru(H2O)6]2+ reacts quantitatively with carbon monoxide (50 bar) in water to form [Ru(CO)(H2O)5]2+.
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    Synthese von ‘Push-Pull’-Oligoacetylenen24. Miteilung über Aminoacryl-Derivate. 23. Mitteilung: [1]Kurzmitteilung: [2] (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1264-1272 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of ‘Push-Pull’-OligoAcetylenes‘Push-pull’ triacetylenes 11a, b, c, as well as ‘push-pull’ tetraacetylene 13b have been prepared by reaction of the corresponding trichloroene(oligoinyl)amines 9 and 10 with 2 mol-equiv. of BuLi followed by acylation. The sequences (Schemes 3 and 4) are very simple and straightforward, they could in principle be applied to the synthesis of ‘push-pull’ pentaAcetylenes 15 and hexaacetylenes 17 (Scheme 5). Main limitations are the moderate yields as well as the low thermal stability of push-pull oligoacetylenes.
    Additional Material: 1 Tab.
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    Metabolic products of microorganisms. Part 262Part 261: [1].. The absolute configuration of sphydrofuran, a widespread metabolite from streptomycetes (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1281-1286 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Sphydrofuran (1) was isolated from the culture filtrate of Streptomyces sp. (strain Gö 28 and Tü 3616) by chemical screening methods. Metabolite 1 is an anomeric and ring-chain tautomeric mixture and could easily be transformed into the stable furan derivative 2 under acidic conditions. The constitution and relative configuration of 1 was established by X-ray crystallography of its 8-O-methyl derivative 3a. The absolute configuration at C(4) of 1 and thus of the whole molecule (3R,4S,5S) could be determined starting from 2 using the Helmchen method.
    Additional Material: 1 Ill.
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  • 188
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    Structure and reactivity of xanthocorrinoids. Part VPart IV, [1].. Formation of trans-diol derivatives of 5,6-dihydrocobyrinic acid from xanthocorrinoids under acidic conditions (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1287-1295 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On treatment with H2SO4/MeOH, epimerization of hexamethyl cis-5,6-dihydroxycobyrinate c,8-lactam (3) takes place quantitatively at C(6), yielding the corresponding trans-diol 4. The corresponding lactone 7, whose structure has been established by X-ray analysis, is obtained from xanthocorrinoids 5 and 6 under similar conditions.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740618
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  • 189
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    Alternative Strategies for the Fmoc Solid-Phase Synthesis of O4-Phospho-L-tyrosine-Containing Peptides (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1314-1328 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A number of biologically relevant O4-phospho-L-tyrosine-containing peptides have been synthesized by either the global phosphorylation of the side-chain-unprotected L-tyrosine moiety in presynthesized resin-bound peptides or alternatively by the incorporation of suitably protected O4-phospho-L-tyrosine building blocks in the continuous-flow method of Fmoc solid-phase peptide synthesis. Different phosphate-protecting groups have been applied.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740621
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  • 190
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    Stereoselektive Synthese von Pyrrolizidin-Alkaloiden aus NitroalkanonenHerrn Professor Dr. C. H. Eugster zurn 70. Geburtstag gewidmet (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1352-1361 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stereoselective Synthesis von Pyrrolizidin Alkaloids from NitroalkanonesReduction of 5-nitropentadecane-2,8-dione (11), synthesized by a Michael reaction from nitromethane, methyl vinyl ketone, and dec-1-en-3-one, gave, depending on the conditions, two epimeric 3-heptyl-2,3,5,6,7,7a-hexahydro-5-methyl-1H-pyrrolizines as the main products: Catalytic hydrogenation (Pd/C) afforded the expected 7aα-hydro epimer 1b with cis-orieted H-atoms at C(3), C(5), and C(7a). NaBH3CN/NH4OAc reduction of the nitrodione 11 yielded all four diastereoisomers with the 7aβ-hydro epimer 1a as the predominant component; 1a is a pheromone of the cryptic thief ant Solenopsis sp. The N-atom of the pyrrolizidine ring stems from NH4OAc exclusively as shown by reduction of 11 with NaBH3CN/(15N)H4OAc.
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  • 191
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    Glycosylidene Carbenes. Part 4. Synthesis of Spirocyclopropanes from Acetamidoglycosylidene-Derived Diazirines (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1362-1372 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the first glycosylidene-derived 2-acetamido-2-deoxydiazirine 4 from N-acetylglucosamine 6 is described. Thus, 6 was transformed into the 3-O-mesylglucopyranoside 9 by glycosidation with allyl alcohol, benzylidenation, and mesylation (Scheme 2). Solvolysis of 9 gave the allopyranoside 10 which, upon benzylation and glycoside cleavage, yielded the hemiacetals 12. Using our established method (via the lactone oxime 14 and the diaziridines 16), 12 gave the diazirine 4. Thermolysis of this diazirine in the presence of i-PrOH gave the dihydro-1,3-oxazole 5 (Scheme 1); in the presence of acrylonitrile, the four diastereoisomeric spirocyclopropanes 17-20 and the acetamidoallal 21 were obtained and separated by prep. HPLC (Scheme 3). Assignment of the configuration of 17-20 is based on NOE measurements and on the effect of diamagnetic anisotropy of the CN group. The ratio of the four cyclopropanes, which is in keeping with earlier results, is rationalized.
    Additional Material: 4 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19910740625
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  • 192
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    Mammalian alkaloids: Synthesis and O-methylation of (S)- and (R)-3′-hydroxycoclaurine and their N-methylated analogues with S-adenosyl-L-[methyl-14C]methionine in presence of mammalian catechol O-methyltransferase (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1399-1411 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: O-Methylation of the optically active 3′-hydroxycoclaurines 3a and 3b and of the N-methylated analogs 5a,b with S-adenosyl-L-[methyl-14C]methionine in presence of mammalian COMT was investigated in vitro. The N-unsubstituted (1S)- and (1R )-isomers 3a and 3b, respectively, afforded almost equal amounts of the corresponding N-norreticuline 4 and N-nororientaline 19, besides two unknown by-products (see Fig. and Table 1). The N-methylated (1S)-isoquinoline 5a, on the other hand, afforded largely (S)-orientaline ((S)-19), while an almost equal mixture of (R)-reticuline (6b) and (R)-orientaline ((R)-19) was obtained from the (1R)-enantiomer 5b. The isoquinolines 3a,b and 5a,b were prepared by a Bischler-Napieralski cyclization yielding O-benzyl-protected isoquinoline 10 (Scheme 1). The optical resolution of 10 was accomplished with 2′-bromotartranilic acid. The N-methylated isoquinolines were prepared by N-formylation of 10a,b and reduction of the formamides 13a,b with diborane (→ 14a,b). Deblocking of the benzyl-ether moieties of 10a,b and 14a,b was accomplished by catalytic hydrogenation in presence of HCl, affording directly 3a,b·HCl and 5a,b·HCl, respectively.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19910740704
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  • 193
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    A Novel, Degraded Polyketidic Lactone, Leptosphaerolide, and Its Likely Diketone Precursor, Leptosphaerodione. Isolation from Cultures of the Marine Ascomycete Leptosphaeria oraemaris (LINDER)Presented in part by A. G. at the ‘Giornate di chimica delle Sostanze Naturali’ meeting, Maratea (Potenza), on June 3, 1991. (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1445-1450 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Acetone extraction of cultures of the marine ascomycete Leptosphaeria oraemaris (LINDER) on cornmeal disk gave the novel polyketide derivative leptosphaerolide ( = (+)-7-[(1E)-l,3-dimethylpent-1-enyl]-10-hydroxy-3-methoxybenzo[1,2-b:5,4-c′]dipyran-2(9H)-one; (4+)-8) besides the o-dihydroquinone 3-[(1E)-1,3-dimethylpent-1-euyl]-8,10-dihydroxy-7-methoxy-8-(2-oxopropyl)-1H-naphtho[2,3-c]pyran-9(8H)-one (1) as a 10:9 mixture of epimers. retro-Aldol reaction of 1 gave leptosphaerodione ( = (-)-3-[(1E)-1,3-dimethylpent-1-enyl]-10-hydroxy-7-methoxy-1H-naphtho[2,3-c]pyran-8,9(8H)-dione; (-)-6) which was also present in small amounts in the extracts and which gave 1 on reaction with acetone. It is thus likely that 1 is an artefact of the extraction by acetone. Biogenetically (+)-8 might derive from (-)-6 via an unusual oxidation with loss of CO2.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19910740707
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  • 194
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    1,2-Stereoinduction in Radical Reactions: Stereoselective Synthesis of 2-Alkyl-3-hydroxybutanoates (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1600-1607 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Radical addition to 2-cyclohexyl-5-methylidene-6-methyl-1,3-dioxan-4-one (2) affords stereoselectively 5,6-trans-products trans-3. The size of the radical has no influence on the selectivity. These trans-acetals are converted into threo-3-hydroxy-butanoates 4. Methyl 2-methylidene-3-[(tert-butyl)diphenylsilyloxy]butanoate (5), treated under the same conditions, leads mainly to the erythro-isomer of 4 after deprotection. An influence of the steric bulkyness of the radical is observed. The stereochemical course of the reactions is discussed.
    Additional Material: 1 Ill.
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  • 195
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    Gas-Phase Chemistry of Ti+ to Zn+ with Butyl Isothiocyanate (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1609-1621 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using a complete set of 2H-labeled isotopomers, the gas-phase reactions of Ti+ to Zn+ with butyl isothio-cyanate are studied. The main product for most of the metal ions is MHNCS+ formed by an ion/dipole mechanism. Exceptions are Cr+, which yields significant amounts of H2 and H2S loss, Mn+, which is the only ion that does not form MHNCS+ at all but produces mainly C2H4 and C3H6, and Zn+, which predominantly reacts by charge transfer.
    Additional Material: 6 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19910740802
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  • 196
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    1,3-Oxathiolan-Synthese: Spirocyclische 1,3-Oxathiolane aus der Lewis-Säure-katalysierten Umsetzung von cyclischen Trithiocarbonaten und OxiranenHerrn Professor Dr. C. H. Eugster zum 70. Geburtstag gewidmet (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1500-1510 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1,3-Oxathiolanc Synthesis: Spirocyclic 1,3-Oxathiolanes from the Lewis-Acid-Catalyzed Reaction of Cyclic Trithiocarbonates and OxiranesThe cyclic trithiocarbonates 1.3-dithiolane-2-thione (4) and 1,3-dithiole-2-thione (9) in 1,2-dichloroethane and MeCN, respectively, react with alkyl- and phenyl-substituted oxiranes 2 in the presence of Lewis acids to give 1-oxa-4,6,9-trithiaspiro[4.4]nonanes 5 and 6 (Scheme 2) and 1-oxa-4,6,9-trithiaspiro[4.4]non-7-enes 10 and 11 (Scheme 3), respectively. The reactions proceed regioselectively yielding 2-alkyl (5, 10) and 3-phenyl derivatives (6, 11) as the main products. From the reaction of 4 and 2-phenyloxirane (2e) with TiCl4, 2-phenyl-1,4,6,9-tetrathia-spiro[4.4]nonane (7) is isolated as a minor product. The molecular structures of 5a, 6e, and 7 are established by X-ray crystallography.
    Additional Material: 2 Ill.
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    2D-NMR ConformationaL Studies of ChiraL Functionalized Ferrocenylphosphine Ligands for the Gold(I)-Catalyzed Aldol Reaction (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1533-1543 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conformations in solution of six selected chiral 1,1′-bis (diphenylphosphino)ferrocenyl ligands bearing functionalized side chains, the diastercoisomers 4 and 5 and 6-9, have been elucidated by 2D-NMR methods (COSY, TOCSY, NOESY, and 1H,31P and 1H,13C correlations). The possible relationship between the preferred conformation and the stereoselectivity observed in the gold(I)-catalyzed asymmetric aldol condensation in the presence of these ligands is discussed.
    Additional Material: 5 Ill.
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    Crystal Structure and ESR Study of a Bimetallic Copper/Iron Crown Ether Inclusion Complex (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1591-1599 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Bimetallic inclusion complexes have been synthesized by a secondary coordination interaction between the guest complex [Fe(η5-C5H5)(CO)2(NH3)][PF6] and copper(II) complex 1a or nickel(II) complex lb containing crown-ether hosts. The X-ray crystal-structure analysis established that the Cu,Fe inclusion complex 2 crystallize as a centrosymmetric dimer with a Cu—Cu separation of 3.73 Å and a novel out-of-plane Cu—N interaction. The magnetic parameters for 2 were obtained by ESR and ENDOR spectroscopy. ESR susceptibility measurements down to 6 K exclude the presence of any antiferromagnetic coupling interaction between the CuIIcenters of the dimer.
    Additional Material: 5 Ill.
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  • 199
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    Synthesis of Aristotelia-type alkaloids. Part VIIIPart VII: [1].. Synthesis of (±)-aristolasicone (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1636-1642 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The revised structure of the indole alkaloid aristolasicone (2) was confirmed through a convergent total synthesis of the racemic form of this metabolite. The key step involves a one-pot condensation/cyclization reaction between 1-(4-methoxyphenylsulfonyl)-1H-indole-2-acetaldehyde (9) and (±)-trans-5-(2,6-difluorobenzyloxy)-p-menth-l-en-8-amine ((±)-7). The resulting allohobartine derivative (±)-13, obtained in 84% yield, was deprotected and oxidized to (±)-alloscrratenone ((±)-15) which cyclized smoothly to the target molecule (±)-2 upon exposure to BF3 · Et2O.
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    URL: http://dx.doi.org/10.1002/hlca.19910740804
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  • 200
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    From 1-(Silyloxy)Butadiene to 4-Amino-4-Deoxy-DL-Erythrose and to 1-Amino-1-Deoxy-DL-Erythritol Derivatives via Hetero-Diels-Alder Reactions with Acylnitroso Dienophiles (1991)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 74 (1991), S. 1653-1670 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Acylnitroso dienophiles 4 reacted instantly with 1-(silyloxy)butadiene 5α and led in good yield to the regioisomeric cycloadducts 6 (major) and 7 (minor; Scheme 2, Table 1). cis-Hydroxylation of these primary cycloadducts with OsO4 (catalyst) occurred stereospecifically and in high yield (→8 and 9, resp.; Scheme 2). It was followed by reductive ring cleavage to give either 1-amino-1-deoxy-DL-erythritol or 4-amino-4-deoxy-DL-erythrose derivatives 10 and 14, respectively, depending on the nature of the reducing agent (Schemes 3 and 4).
    Additional Material: 7 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19910740806
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