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  • Wiley-Blackwell  (14)
  • Springer Nature  (4)
  • National Academy of Sciences  (3)
  • 2000-2004
  • 1995-1999  (12)
  • 1990-1994  (9)
  • 1996  (12)
  • 1994  (9)
  • 1
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We used semi-empirical and ab initio calculations to investigate the nucleophilic attack of the OH- ion on the β-lactam carbonyl group. Both allowed us to detect reaction intermediates pertaining to proton-transfer reactions rather than the studied reaction. We also used the PM3 semi-empirical method to investigate the influence of the solvent on the process. The AMSOL method predicts the occurrence of a potential barrier of 20.7 kcal/mol due to the desolvation of the OH- ion in approaching the β-lactam carbonyl group. Using the supermolecular approach and a H2O solvation sphere of 20 molecules around the solute, the potential barrier is lowered to 17.5 kcal/mol, which is very close to the experimental value (16.7 kcal/mol).
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1793-1802 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The degradation of cefaclor (1), an oral cephalosporin antibiotic, was studied at 37° in a neutral aqueous medium by HPLC and 1H-NMR. Under these conditions, 1 underwent intramolecular aminolysis by the 7-side-chain NH2 group on the β-lactam moiety to give a piperazine-2,5-dione. The most prominent peak in the HPLC profile of a degradation solution from 1 was isolated by prep. HPLC. Mechanistically, the formation of this degradation product cis-11 from 1 involves the contraction from a six-membered cephem ring to a five-membered ring, which presumably takes place via a common episulfonium ion intermediate 9 (see Scheme). Loss of the Cl-atom from 3-chloro-3-cephem is a general reaction subsequent to β-lactam ring opening.
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  • 3
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The gas-phase basic hydrolysis of clavulanic acid (a) was studied by using the AM1 semi-empirical method. The results obtained show that the hydroxyethylidene side chain at C(2) is pivotal to the stability of the different reaction products involved. The products with an open oxazolidine ring are more stable than those with a closed ring fused to the β-lactam ring. This behaviour differs from that of penicillins and cephalosporins where the most stable degradation products are those with an intact thiazolidine or dihydrothiazine ring, respectively, fused to the β-lactam ring. The different chemical reactivity of clavulanic acid relative to penicillins and cephalosporins could explain the disparate behaviour of the latter two types of compound towards β-lactamases. Once the acyl-enzyme intermediate of clavulanic acid has been formed, it can evolve with cleavage of the oxazolidine ring to form a difficult to deacylate compound.
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  • 4
    Electronic Resource
    Electronic Resource
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 34 (1996), S. 1959-1968 
    ISSN: 0887-624X
    Keywords: optically active polyamides ; helical nylons ; nylon-3 ; poly(β-amide)s ; poly(β-aspartate)s ; poly(α-isobutyl-β-L-aspartate) ; stereoselective polymerization ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Different polymerization methods were used for the preparation of poly(α-isobutyl-β-D,L-aspartate)s containing variable ratios of D- to L-aspartic units and the microstructure of the resulting stereocopolymers was examined by NMR spectroscopy. Anionic ring-opening polymerization in solution of enantiomeric mixtures of α-isobutyl-β-D- and L-aspartalactams was found to proceed stereoselectively rendering block copolymers composed of right- and left-handed helical sequences. Configurationally statistical copolymers were obtained instead when the enantiomeric lactam mixtures were polymerized in the bulk. Random stereocopolymers could be prepared also by polycondensation in solution of mixtures of pentachlorophenyl α-isobutyl-β-D and -L-aspartates. The conformation in solution and the crystal structure of the resulting copolymers were investigated in connection with their stereochemical configuration and these features compared with those displayed by optically pure poly(α-isobutyl-β-L-aspartate). © 1996 John Wiley & Sons, Inc.
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  • 5
    Electronic Resource
    Electronic Resource
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 32 (1994), S. 3147-3151 
    ISSN: 0887-624X
    Keywords: polymerization of methyl methacrylate ; diphenylbutadiynes ; ESR spectra ; radical stabilization ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The free radical polymerization of methyl methacrylate (MMA) in the presence of p,p′- disubstituted diphenylbutadiynes was studied. Both the rate and degree of polymerization are somewhat lowered by the presence of the diynes, but the propagating radicals were stabilized giving clear ESR signals of the interacted polyMMA radicals at the polymerization temperature of 70°C. The magnitude of the interaction depended on the electron density of the diynes; in the cases of diphenylbutadiyne and dimethoxycarbonyldiphenylbutadiyne, the intoraction was more enhanced showing ESR signals with smaller spectra widths and increasing the number of radicals with the polymerization time, while in the cases of electron donor-substituted diynes the interaction was weaker and the radical concentration remained constant during the polymerization. These systems are considered to be examples of the stabilization of transient radicals by the direct interaction of radicals with additives without formation or breaking of chemical bonds. No diacetylenic group was found in the polyMMA obtained. © 1994 John Wiley & Sons, Inc.
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Electroanalysis 8 (1996), S. 1072-1074 
    ISSN: 1040-0397
    Keywords: Molecular recognition ; Phosphate ; Cyclodextrin ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The complexation of phosphate ion by β-amino-cyclodextrin (host 1) was detected by voltammetric methods through the competitive displacement of water-soluble ferrocene derivatives from their complexes with host 1 in neutral aqueous media. The half-wave potential and the anodic peak current for oxidation of the ferrocene derivative were found to be sensitive to the phosphate concentration throughout a range which is determined, among other factors, by the concentration of host 1 and the binding constant between the selected ferrocene derivative and the β-amino-cyclodextrin host.
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  • 7
    Electronic Resource
    Electronic Resource
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 34 (1996), S. 963-973 
    ISSN: 0887-6266
    Keywords: poly(ethylene terephthalate) ; crystal modulus ; mechanical properties ; force-field parametrization ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A force-field suitable for the calculation of the mechanical properties of poly(ethylene terephthalate) and their relation with the molecular structure of the polymer has been developed. The force-field parameters were derived from quantum mechanical AM1 calculations and tested against thermodynamic and vibrational spectroscopy data available for a set of closely related small molecules. The crystal moduli of the two solid phases known for poly(ethylene terephthalate) were estimated by means of this new force-field considering both the isolated chain and the chain within the unit cell. Results were qualitatively consistent with reported x-ray data showing that the triclinic crystal form is stiffer than the mesomorphic phase provided that sample heterogeneities were taken into account. Although overestimated moduli resulted for both cases, divergences with experimental values were found to be slighter than those obtained by other theoretical methods. © 1996 John Wiley & Sons, Inc.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    Polymers for Advanced Technologies 5 (1994), S. 423-427 
    ISSN: 1042-7147
    Keywords: Liquid crystalline polymers ; Blends ; Viscosity reduction ; Reinforcing action ; Microfibrillar morphology ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Blends of polyarylate of bisphenol A, PAr, with two commercial main chain liquid crystalline polymers, Vectra A950 and Vectra B950, are studied. From dynamic viscoelastic measurements it is deduced that both systems (PAr/Vectra A950 and PAr/Vectra B950) are immiscible and scanning electron microscopy (SEM) micrographs show the presence of spherical domains of the liquid crystalline polymer when PAr constitutes the matrix. Extrusion capillary measurements reveal that, under conditions of temperature and shear rate similar to those of processing, the viscosity is reduced to approximately 10% of its value when the content of liquid crystalline polymer is only 20%. This great improvement of the rheological properties is observed in both PAr/Vectra A950 and PAr/Vectra B950 blends. The effect of draw ratio on Young's modulus for different compositions is also analyzed, pointing out the reinforcing action of both liquid crystalline polymers on polyarylate: for instance, 20% of Vectra B950 in the blend gives rise to a 700% increase of the modulus of fibres prepared at a draw ratio of 50. SEM of the extrudates reveals that the spherical domains are elongated at the entrance of the capillary giving rise to a microfibrillar morphology which is related to the excellent rheological and mechanical properties of the blends.
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  • 9
    ISSN: 0044-8249
    Keywords: Komplexe mit Stickstoffliganden ; Magnetische Eigenschaften ; Nickelverbindungen ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 3 Ill.
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  • 10
    ISSN: 0044-2313
    Keywords: Stibocanes ; conformational analysis ; x-ray structures ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Die Beziehung zwischen starker transannularer sekundärer Bindung und Konformation in Diphenyldithiophosphinatstibocanen X(CH2CH2S)2SbS2PPh2 (X = O, S)Die beiden Stibocane 1-Oxa-4,6-dithia-5-stibocan-diphenyldithiophosphinat O(CH2CH2S)2SbS2PPh2 1 und 1,3,6-Trithia-2-stibocan-diphenyldithiophosphinat S(CH2CH2S)2SbS2PPh2 2 wurden aus den entsprechenden Chloro-oxa- und -thia-stibocanen 3 und 6 sowie dem Ammoniumsalz der Diphenyldithiophosphinsäure in CH2Cl2 dargestellt. 1 und 2 wurden durch IR, EI-MS und Multikern-NMR-(1H, 13C, 31P{1H}) Spektren charakterisiert. Die Kristallstruktur von 1 zeigt zwei Sb1 ⃛ S1 intermolekulare Wechselwirkungen [3.987(2) Å] so daß sich ein dimeres Molekül ergibt sowie eine endocyclische, transannulare Sb1 ⃛ O1 [2.555(6) Å] und eine exocyclische Sb1 ⃛ S4 sekundäre Wechselwirkung [3.327(2) Å]. Die Koordination am Antimon kann als AX4YE ψ-trigonal bipyramidale Geometrie beschrieben werden, mit A = Sb, X = S1, S2, S3, O1; Y = S4. S1, S2 und das freie Elektronenpaar liegen in der äquatorialen Ebene, O1 und S3 besetzen die axialen Positionen. Die sekundäre Bindung Sb1 ⃛ S4 liegt über einer Ebene, die von dem freien Elektronenpaar der trigonalen Bipyramide, S2 und S3 gebildet wird. Molekül 2 zeigt ebenfalls eine endocyclische, transannulare Sb1 ⃛ S2 [2.949(3) Å] und eine exocyclische Sb1 ⃛ S5 sekundäre Wechselwirkung [3.216(3) Å]. Antimon ist hier ebenfalls fünffach koordiniert, wodurch sich eine AX4YE ψ-trigonale bipyramidale Geometrie ergibt, mit S1, S3 und dem freien Elektronenpaar in der äquatorialen Ebene und mit S2 und S4 in den axialen Positionen. Die sekundäre Bindung Sb1 ⃛ S5 liegt auch hier über einer Ebene, die von dem freien Elektronenpaar der trigonalen Bipyramide, S3 und S4 gebildet wird.
    Notes: The two stibocanes 1-oxa-4,6-dithia-5-stibocane diphenyldithiophosphinate O(CH2CH2S)2SbS2PPh2 1 and 1,3,6-trithia-2-stibocane diphenyldithiophosphinate S(CH2CH2S)2 · SbS2PPh2 2 were prepared from the corresponding chloro oxa- and thia-stibocanes 3 and 6, and the ammonium salt of diphenyldithiophosphinic acid in CH2Cl2. 1 and 2 were characterized by IR, EI-MS and multinuclear NMR (1H, 13C, 31P{1H}). The crystalline state of 1 features two Sb1 ⃛ S1 intermolecular interactions [3.987(2) Å] that results in a dimer. Alongside 1 displays both an endocyclic, transannular Sb1 ⃛ O1 interaction [2.555(6) Å] and an exocyclic Sb1 ⃛ S4 secondary interaction [3.327(2) Å]. The coordination geometry at the antimony could be described as AX4YE ψ-trigonal bipyramid geometry with A = Sb, X = S1, S2, S3,O1; Y = S4; S1, S2 and the lone pair lays on the equatorial plane with O1 and S4 in axial positions. The Sb1 ⃛ S4 secondary bonding is face capping one of the planes form by the lone pair, S2 and S3 of the trigonal bipyramid. 2 also displays both an endocyclic, transannular Sb1 ⃛ S2 interaction [2.949(3) Å] and an exocyclic Sb1 ⃛ S5 secondary interaction [3.216(3) Å]. The antimony becomes five-coordinate, giving the AX4YE ψ-trigonal bipyramid geometry with S1, S3 and the lone pair laying on the equatorial plane with S2 and S4 in axial positions. The Sb1 ⃛ S5 also here is face capping the plane form by the lone pair, S3 and S4 of the trigonal bipyramid.The conformation of the eight membered ring in 2 is boat-chair. In 1 the main conformation is chair-planar.Die Konformation des Achtringes in 2 ist Wanne-Sessel. In 1 ist die Konformation des Achtringes Sessel-planar.
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