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  • 1
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 1-7 
    ISSN: 0947-3440
    Keywords: Diastereoselectivity ; Hetero Diels-Alder reactions ; Enamino ketones ; High-pressure kinetics ; Kinetics, high-pressure ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The hetero Diels-Alder reaction of the enamino ketones 1b and 1c with ethyl vinyl ether (2) in dichloromethane to give the dihydropyrans 3b/4b and 3c/4c is studied at high pressure up to 5 kbar. The kinetics is measured by on-line FT-IR spectroscopy up to 3 kbar. The cycloadditions show a relatively high pressure-dependent increase in diastereoselectivity in favor of the trans adducts 4b and 4c, respectively. The activation volumes at atmospheric pressure, ΔVo≠, are determined to be -(43.3 ± 2.1) and -(43.9 ± 2.9) cm3 mol-1 for the reactions of 1b and 1c at 100°C in dichloromethane solution. The activation enthalpies, ΔH≠ for the two cycloadditions at 1500 bar are (64.4 ± 0.4) kJ mol-1 and (64.0 ± 0.6) kJ mol-1. The ΔΔVo≠ values are (3.8 ± 0.3) and (4.6 ± 0.3) cm3 mol-1 and the ΔΔH≠ values (1.5 ± 0.2) and (2.1 ± 0.3) kJ mol-1, respectively.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 253-265 
    ISSN: 0947-3440
    Keywords: Bicyclo[4.2.0]octa-1,3,5-triene ; Chromium complexes ; [4+2] Cycloadditions ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of the tricarbonylchromium complexes 2 and 3 of 1-oxobenzocyclobutene and 1-hydroxybenzocyclobutene are described. Alcoholate 6 of 3 undergoes a ring opening to the corresponding ortho-quinodimethane intermediate between -30 and -20°C, which is trapped by dienophiles with high diastereoselectivity. While with most dienophiles products with a 1,2-cis configuration (7, 8, 10) are formed, vinyl sulfones react in an exo-selective cycloaddition with formation of 1,2-trans-configurated cycloadducts (12, 13). The configuration of the cycloadduct with phenyl vinyl sulfone is confirmed by an X-ray crystal structure analysis. As one of the important aspects of the chemistry of benzocyclobutene complexes is the chirality of compounds like 2 and 3, the enantiomeric resolution has been achieved via the semioxamazones of ketone 2.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 53-66 
    ISSN: 0947-3440
    Keywords: Carbohydrates ; Saccharides ; Glycosyltransferases ; Trichloroacetimidates ; Transferases ; Enzymes ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Building Units of Oligosaccharides, CIX.  -  Synthesis of Modified Oligosaccharides of N-Glycoproteins for Substrate Specificity Studies of N-Acetylglucosaminyltransferase IA series of modified derivatives of the trisaccharide octyl α-D-Man(1→3)-[α-D-Man(1→6)]-β-D-Man were synthesized for substrate specificity studies of N-acetylglucosaminyltransferase I (GlcNAc-T I). The hydroxyl groups at C-3, C-4 and C-6 of the α(1→3)-linked Mannose were replaced by hydrogen and methoxy groups. At the α(1→6)-linked mannose the hydroxyl group at C-2 was deoxygenated and the hydroxyl groups at C-3, C-4 and C-6 were replaced by methoxy groups. The syntheses of the ten trisaccharides were carried out by the interchangeable combination of modified building blocks. The trichloroacetimidate method was favourable for all glycosylation steps.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 77-80 
    ISSN: 0947-3440
    Keywords: Toadstools ; Tricholoma ; Trichaurantin ; Terpenoids ; Neodolastane, 2,3-seco- ; Dolastanes ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Novel Diterpenoids from the Toadstools Tricholoma aurantium and T. fracticum (Agaricales)Herrn Professor Dr. Heinz G. Floss zum 60. Geburtstag gewidmet.Two novel diterpenoids trichaurantin (1) and its 6-O-acetyl derivative 2 have been isolated from Tricholoma species. Their structures were established by spectroscopic methods and an X-ray crystal analysis, 1 and 2 appear to be biosynthetically related to the neodolabellane and dolastane group of diterpenes and can be designated as 2,3-seconeodolastane derivatives.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 81-85 
    ISSN: 0947-3440
    Keywords: Pyrones ; Pigments ; Polyketides ; Ceratiomyxa fruticulosa ; Myxomycetes ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Unsaturated 6-Alkylpyrones from the Slime Mould Ceratiomyxa fruticulosa (Myxomycetes)From plasmodia of the common slime mould Ceratiomyxa fruticulosa five 6-alkyl-4-methoxypyran-2-ones with varying degrees of unsaturation in the side chain have been isolated. Their structures have been determined by spectroscopic and chemical investigations.
    Additional Material: 2 Tab.
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 87-94 
    ISSN: 0947-3440
    Keywords: Antibiotics ; Benzoisochromanquinones ; Isokalafungins ; Isonanaomycins ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structurally Modified Isokalafungins and IsonanaomycinsThe total synthesis of structural analogues of the unnatural, C-6-hydroxylated isokalafungins and isonanaomycins is described. The new compounds show improved antibiotic activity and support theories for a mechanism of action which base on the formation of reactive quinone methides. Calculations of the molecular structures show that the chromophore of the most active 9-methyl derivatives 28 and 32 is bended off along its 5-CO-10-CO axis. This corresponds to an unusual shifting of characteristic 13C-NMR signals. Different experiments for the introduction of X-CH2- or sugar residues into 1 (e.g. isolactoquinomycins) demonstrate the limits of the general synthetic pathway. Finally a short overview on significant structure/activity relationships in the (iso)nanaomycin D field is given.
    Additional Material: 3 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 95-104 
    ISSN: 0947-3440
    Keywords: Thioketones and thioaldehydes, fluorinated ; 1,3-Dipoles, bis-stannylated-, bis-silylated- ; 1,2-Metallotropic migration ; NMR, 119Sn ; NMR, 29Si ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Partially and Perfluorinated Thioketones and Thioaldehydes: Chemical Storage, In Situ Generation and Surprising Reactivity towards Bis(trimethylstannyl)-diazomethane (BTSD) and C,N-Bis(triisopropylsilyl)nitrilimine (NI)The hetero-Diels-Alder anthracene adducts 1a-d are synthesized in a one-pot reaction from the corresponding partially and perfluorinated carbonyl compounds in good yields. By their thermolysis in solution in presence of BTSD or NI the 1,3-dipolar 1:1 cycloadducts 12a, d, 14a, b, d are obtained without any detectable by-products. The method can be of wide applicability predominantly for any aliphatic CS-unsaturated species, which are not accessible in substance due to tendency towards polymerisation, but can be stabilized as anthracene adducts. Other 1,3-dipolar cycloaddition products of BTSD and NI are obtained from reaction with iso-lable hexafluorothioacetone (6f) or bis(trifluoromethyl)sulfine (7f). Comparative studies were carried out with hexafluoroacetone (8), 12a, d, e, 13 are consecutive products from a 1,2-metallotropic migration of the primary addition compounds. 14a, b, d, 15, 16 are useful thiadiazolines with respect to the synthesis of active substances because of their seldom found peripheral functionality. Desilylation was achieved by fluoride support. An X-ray structure determination of 12a and 14d was carried out. In addition trifluoromethylsulfine (7c) was trapped at low temperatures. It dimerizes at ambient temperature to unsymmetrical 1,2-dithietane 11.
    Additional Material: 3 Tab.
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 365-384 
    ISSN: 0947-3440
    Keywords: syn β-Hydroxy ether ; γ-Lactones ; Polyethers ; Polyalcohols ; Polyol ; Rearrangements ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of two enantiopure skipped-chain pentaol building blocks 51 and 56 are described. They are based on a strategy which derives 1,3,7,9-tetraols from bis (γ-butyrolactones). Two equivalents of γ-lactone 17, readily available from L-glutamic acid, and one equivalent of the diiodoisobutene 22 furnished bis(γ-butyrolactone) trans,trans-21 stereoselectively in a single step. The same lactone, combined in a 2:1 ratio with the dibromoisobutene derivative 18, led in four steps and with good stereocontrol to the isomeric bis(γ-lactone) cis,trans-21. On the basis of the 1H- and 13C-NMR spectral data of these and other lactones it was possible to distinguish 1,3-cis- and 1,3-trans-disubstituted γ-lactones. Each of the bislactones was subject to a Criegee rearrangement of derived bis(peroxosulfonates) to give the diastereomerically pure 1,3,7,9-tetraols 26 and 31. The configuration of these tetraols was proven by the 13C-NMR shifts of the corresponding bis(acetonides) 27 and 32. Ozonolytic cleavage of the C=C bond of 27, reduction of the obtained ketone 28 to alcohol 29, acetonide rearrangement under thermodynamic control (→ 30), and fuctionalization of the liberated hydroxy group delivered the end group-differentiated 1,3,5,7,9-pentaol building block 51. It represents a segment of the polyol parts of the antibiotics roxaticin (52) and mycoticin (53). The oxidative cleavage of the C=C bond of bis(acetonide) 32 and chelation-controlled reduction of the resulting ketone 33 with Zn(BH4)2 furnished after acetonide migration and functionalization of the unprotected hydroxy group the all-syn-configured 1,3,5,7,9-pentaol building block 56. The latter compound should provide an entry into total syntheses of the naturally occurring poly(methyl ethers) 57 and 58.
    Additional Material: 1 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 401-406 
    ISSN: 0947-3440
    Keywords: Conjugate alkylation ; Organoaluminium ; Organotitanium ; Copper catalysis ; Nickel catalysis ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Regioselectivities in Michael-type reactions of organoaluminium and organotitanium reagents with sterically hindered carbonyl compounds 1 and 5 concerning 1,2- versus 1,4-addition were determined. Throughout this investigation Me3Al/cat. Ni(acac)2 was found to be the most useful reagent for strongly hindered systems.
    Additional Material: 5 Tab.
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  • 10
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 419-425 
    ISSN: 0947-3440
    Keywords: Basicity ; Imine protonation ; Solvent effect ; Steric shielding ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The protonation state of N-(1,1,3,3-tetramethyl-2-indanylidene)methylamine (5/6) is assessed on the basis of fully assigned 1H- and 13C-NMR spectra in [D8]toluene and in CDCl3. In either solvent the basicity of 5 remains intermediate between those of pyridine (weaker) and triethylamine (stronger). Nearly complete proton transfer to 5 in toluene occurs with trifluoroacetic and dichloroacetic acids, whereas partial protonation is achieved by chloroacetic acid or acetic acid and even by biphenyl-2,2′-diol (7). As a possible model for imine protonation in the hydrophobic region of proteins like the rhodopsins, the sterically shielded iminium cation 6 (or 2) does not require solvation by a polar solvent; rather it can be stabilized in toluene solution by a counteranion whose proton affinity is diminished by an assisting hydrogen bond.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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