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  • 1
    ISSN: 1434-193X
    Keywords: Chromophores ; Photochemistry ; Heterocycles ; Diazenes ; Imines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The chances for intramolecular imine/ene (→ azetidines), diazene/ene (→ 1,2-diazetidines), diazeneoxy/ene (→ 1,2-diazetidine oxides) and diazenedioxiene (→ 1,2-diazetidine dioxides) [2+2]photocycloadditions and for the isolation of the respective photoproducts, have been probed with specifically designed substrates. Upon direct or sensitized excitation, [2+2]cycloaddition was found to be the exclusive or at least dominant chemical process for the C=N/C=C, N=N/C=C and ON=N(O)/C=C systems featuring very small π,π-distances of 2.8-3.0 Å and large π,π-interorbital angles of 160-170° (7 → 51, 17 → 55, 33 → 58 (competing N2 elimination), 22 → 62). This is not the case, however, in ON=NO/C=C (23, where electron transfer is a possibility), or in the more flexible, less “proximate” C=N/C=N (57) and C=NO/C=N (63) systems (π,π-distances of 〉3.8 Å). While the corseted 1,2-diazetidine photoadducts (55, 58) proved to be thermally stable, their N-oxides (62, 65) were thermally too labile to be directly observable above -65 °C. For the latter's only fleeting existence, electronic rather than strain effects are held responsible (B3LYP/6-31G* calculations). Very facile C=NO/C=C (12 → 13) and N=NO/C=C (22 → 24) [3+2]cycloadditions, homoconjugate addition of H2 and of dienophiles ([2+2+2]) to the diazene/ene 17 (→ 39, 41, 45) are manifestations of “proximity” in these bichromophoric skeletons.
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  • 2
    ISSN: 1434-193X
    Keywords: Bisdiazenes ; Homoconjugation ; Photochemistry ; Heterocycles ; Diazenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Rigid N=N/N=N (diazene/diazene) systems (F) consisting of more or less alkylated DBH and DBO chromophoric units (1, 2, X-ray structures), with very short π,π distances [d = 2.849 (1a, av.), 2.822 Å (2)] and almost perfect syn-periplanar π,π alignments [ω = 168.6 (1a), 174.2° (2)] as well as the more flexible, less “proximate” metathesis isomers (3a,c, 27a,c, d 〉4.6 Å, ω = 90-100°) have been synthesized. Homoconjugate π,π interaction (in 1, 2, not in 3, 27) is deduced from UV spectroscopic measurements [π → π* maxima at 239 (234) nm (sh, 260)], while PE analyses furnished only small interaction parameters (1a: 〈0.3 eV). The potential of the novel syn-periplanar N=N/N=N motif in 1 and 2 for the synthesis of somewhat exotic polyheterocycles has been explored by calculation (B3LYP) as well as experimentally: i.a. kinetically stabilized, all-cis-peralkylated tetrazolidines (38, 44) and perhydro-1,2,4,5-tetrazines (41, 47) have become accessible (i.a. via novel azomethine/diazene and azomethine/azomethine cycloadditions). In 1a with its unreactive DBO chromophoric subunits, in the “buttressed” derivatives 1b-d, as well as in the DBH/DBO combination 2, and likewise in more ‘distant’ 27 (differently from the analogous C=C/C=C and N=N/C=C systems), irrespective of the excitation conditions employed (light of λ ≥≥ 280, 254 nm, low temperature matrix irradiation, acetone sensitization) no [2+2]photocycloaddition was observed. Instead exclusively N2-elimination took place. It is argued that unproductive N=N/N=N photocycloaddition would have become observable through metathesis isomerization of the respective tetrazetidines.
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  • 3
    ISSN: 1434-193X
    Keywords: Photochemistry ; Heterocycles ; Diazenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Of two very proximate syn-periplanar bisdiazenes (1,2) mono-, di-, tri- and tetra-N-oxides were prepared, representing six combinations of the individual N=N/N=NO/ON=NO chromophores. According to DFT calculations (B3LYP/6-31G*), [2+2]photocycloaddition to the respective oxidized tetrazetidines is significantly to moderately endothermic. The metathesis isomerization of the oxidized tetrazetidines is generally highly exothermic and kinetically increasingly favorable with increasing oxidation state. In practice, four out of the six bichromophoric combinations undergo selectively, in competition with N2 elimination from a DBH unit (13) still partially, metathesis isomerization upon π → π* excitation (monochromatic 254 nm light). In the case of the syn-N=NO/N=NO combinations (5/6, 14), the photoaddition is thermally reversed. For a ON=NO/N=N combination (30), internal electron transfer is responsible for a complex reaction pattern. The preparative value of the metathesis reactions, though, is limited: The metathesis-derived bis[diazene mono(di)oxides] undergo relatively fast secondary photoreactions, while the tri(tetra)oxides undergo rapid thermal transformations. For the N=N/N=NO systems (12), of three potential pathways for its metathesis isomerization, the one that takes place via σ-symmetric intermediates (63, 64) is excluded by virtue of the retention of optical purity in the photometathesis of a highly enriched enantiomer [(-)-12]. Matrix irradiation experiments (12 K, IR control) with 12 result in the appearance of a kinetically highly labile transient. Supported by DFT calculations it is concluded that in the metathesis reactions, the respective tetrazetidine oxides (increasingly destabilized by interactions between oxygen lone pairs and NNσ* orbitals) function as vibrationally excited transients. That thermal reversion of these transients might be a general, nonproductive competition, is suggested by the experimental verification of a “reversed photometathesis” (51 → 15) and by the generally low rates in product formation upon irradiation. The question remains to be answered why in structurally analogous molecular skeletons, [2+2]photocycloaddition occurs in the C=C/N=N and variously oxidized N=N/N=N, and not, however, in the parent N=N/N=N combinations.
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  • 4
    ISSN: 0947-3440
    Keywords: Antibiotics ; Sannamycin-type ; Mirror-images ; Syntheses ; Activity, biological ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Sets of variously protected purpurosamine/2-epi-purpurosamine-type glycosyl donors (O-acetates) and sannamine-type glycosyl acceptors were coupled as racemates and pure enantiomers, according to a modified Koenigs-Knorr procedure with TMSOTf as promotor. The α anomers isolated from the respective mixtures of glycosides (yields varying between 51% and 91%, α:β anomeric ratios between 1.5:1 and 13:1, α:α′ diastereomeric ratios between 1:1 and 1:1.5) were transformed through standard protection/deprotection and glycidation measures into 6′-des(N-methyl)sannamycins A (1, 1′, 2, 2′) and B (37, 37′), 2′-epi-6′-des(N-methyl)sannamycins (3, 3′), the enantiomeric sannamycins A and B (ent-4)/ (ent-40), the diastereomers (ent-4′)/(ent-40′), and the enantiomeric 2′-epi-sannamycin A (ent-5)/(ent-β-5). For one of the fluorinated glycosides (α-26′), featuring a somewhat unusual aglycon conformation, an X-ray structural analysis was performed. In explorative biological tests, the naturally configurated glycosides 1 and 3 showed limited, 2 a rather broad activity against a number of pathogenic microorganisms. Yet, none of the glycosides with non-natural configuration was found to be active.
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  • 5
    ISSN: 0075-4617
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Cyclic Cross-Conjugated Bond Systems, 29.  -  13C-Analyses of Cross-Conjugated π-Electron SystemsHerrn Prof. Dr. Arthur Lüttringhaus zum 70. Geburtstag gewidmet.13C-NMR spectra of derivatives of fulvene, triafulvene, heptafulvene, calicene, pentaful-valene, sesquifulvalene and fidecene (1-8) as well as of several model compounds were measured and for the most part assigned. The chemical shifts can be interpreted largely in terms of charge delocalisation and show a charge distribution in accord with known dipole moments.
    Notes: Die 13C-NMR-Spektren von Derivaten des Fulvens, Triafulvens, Heptafulvens, Calicens, Pentafulvalens, Sesquifulvalens und Fidecens (1-8) sowie von einigen Vergleichsverbindungen wurden gemessen und größtenteils zugeordnet. Die chemischen Verschiebungen sind weitgehend auf der Basis von Ladungsverschiebungen deutbar und zeigen eine Ladungsverteilung an, die mit den gemessenen Dipolmomenten in Einklang steht.
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  • 6
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 7
    ISSN: 0075-4617
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Die Synthese des 7.8;9.10-Dibenzo-sesquifulvalens (2) und des 7.8.9.10-Tetraphenyl-sesquifulvalens (3) sowie Versuche zur Darstellung des 7.8.9.10-Tetrachlor-sesquifulvalens (34) werden beschrieben. Die stabilen Derivate 2 und 3 des sehr unbeständigen Grundkörpers 1 sind durch große Reaktionsbereitschaft sowohl gegenüber elektrophilen als auch nucleophilen Reaktionspartnern ausgezeichnet. Mit Tetracyanäthylen bildet 2 ein 1.8-Addukt, 3 hingegen ein 1.4-Addukt. Die physikalischen Daten (UV, NMR) werden im Hinblick auf die π-Elektronendichteverteilung im Grundzustand des cyclischen gekreuzt-konjugierten Bindungssystems 1 diskutiert.
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  • 8
    ISSN: 0075-4617
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Cyclic Cross-Conjugated Bond Systems, 32.  -  Syntheses and Cycloaddition Reactions of 1,2,3,4-Tetraphenyl-, 1,2:3,4-Dibenzo-, and 2,3-DiphenylpentafulvaleneThe crystalline, isolable, 1,2,3,4-tetraphenyl- and 1,2:3,4-dibenzofulvalenes (9 and 10) and 2,3-diphenylfulvalene (11), which is stable only in solution, have been synthesized. The increasing ease of dimerisation via [4 + 2]-addition is responsible for the steeply diminishing thermal stability from 9 to 11. In model reactions, the fulvalenes behave both as dienes and dienophiles. The cyclopentadienylidenenorbornadiene derivatives 17, 24, 32a, b and the binorbornadienylidene derivatives 36a, b, 37a, b are accessible via 9, 10, and via 29b, resp., the “masked” form of 11.  -  On the basis of 1H- and 13C-NMR data pentafulvalene is placed within the series of cyclic cross-conjugated π-systems 1.
    Notes: Die kristallin isolierbaren 1,2,3,4-Tetraphenyl- und 1,2:3,4-Dibenzofulvalene (9 bzw. 10) und das nur in Lösung existenzfähige 2,3-Diphenylfulvalen (11) wurden hergestellt. Für die von 9 zu 11 rasch absinkende thermische Stabilität ist die zunehmend leichtere Dimerisierung durch [4 + 2]-Addition verantwortlich. In Modellreaktionen verhalten sich die Fulvalene sowohl wie Diene als auch wie Dienophile. Die Cyclopentadienylidennorbornadienderivate 17, 24, 32a, b und die Binorbornadienylidenderivate 36a, b, 37a, b sind über 9, 10 bzw. über 29b, die „maskierte“ Form von 11, zugänglich.  -  An Hand der 1H- und 13C-NMR-Daten wird das Pentafulvalen in die Reihe der cyclisch gekreuzt-konjugierten π-Systeme 1 eingeordnet.
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  • 9
    ISSN: 0570-0833
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 10
    ISSN: 0170-2041
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Aminoglycosid-Antibiotika - Fortamin-Aglyca Totalsynthese, Racemattrennung, chemischen ModifizierungenAusgehend von Dianhydro-epi-inosit 10 (letztlich erhältlich aus Benzol) wurde eine leistungsfähige Synthese für enantiomerenreine (+)/(-)-Des-O-methylfortamine (Derivate) entwickelt. Zentrale Schritte sind die regiospezifischen Epoxidöffnungen, intramolekular beim Diepoxybis(urethan) 11 und intermolekular mit (+)-1-Phenylethylamin bei den Epoxyurethanen rac-13. Die diastereomeren Addukte (30, 32) können verlustfrei durch Kristallisation/Chromatographie getrennt werden. Nach hydrierender Spaltung werden natürliche und nicht natürliche cis-1,4-Inosadiamine enantiomerenrein erhalten [z. B. Fortamin (-)-1, ent-Fortamin (+)-1, 3-O-Desmethylfortamin (-)-38, ent-3-O-Desmethylfortamin (+)-380]. Der in zahlreichen Modellreaktionen optimierte Syntheseweg erlaubt breite chemische Modifizerungen und führt u.a.zu 3-O-Desmethylfortamin- und ent-Fortamin-Derivaten, in denen nur die zur Glycosidierung gebrauchte (6)4-OH-Gruppe ungeschützt bleibt.
    Notes: Starting from the dianhydro-epi-inositol 10 (available ultimately from benzene) an expedient total synthesis of enantiomerically pure (+)/(-)-de-O-methylfortamines (derivatives) has been developed. Key steps are the regiospecific epoxide openings which occur intramolecularly in the diepoxybis(urethane) 11 and intermolecularly by (+)-1-phenylthylamine in the epoxyurethanes rac-13. The diastereomeric adducts (30, 32) are quantitatively separated by crystallization/chromatogrphy. Following hydrogenation, natural and nonnatural cis-1,4-inosadiamines are obtained optically pure [e.g. fortamine (-)-1, ent-fortamine (+)-1, 3-O-demethylfortamine (-)-38, ent-3-O-demethylfortamine (+)-38]. This approach, which was optimized by numerous model reactions, allows wide chemical modifications and leads among others to 3-O-demethylfortamine and ent-fortamine derivatives in which only the (6)4-OH-group, the one to be glycosidated, remains unprotected.
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