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  • 1
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Carboxonium Compounds in Carbohydrate Chemistry, XI1. Acyloxonium Rearrangement of D-Xylose and D-Arabinose, of 6-Deoxy-D-glucose and D-Glucuronic Acid, of D-Galactose and D-AltroseThe equilibrium of the pentopyranose acetoxonium ions exists of 28 % xylo-ion 4, 10% lyxo-ion 5 and 57% arabino-ion 6 + 7 (25°CH3NO2). Anomerisation reactions of carbohydrate acetates are generally catalyzed by acetoxonium salts. In the series of pentofuranoses and C-6 substituted glucopyranoses allways the 1,2-acetoxonium ion 9 or 15, respectively, is the most stable one. In the equilibrium of the galacto-ion 18 and the talo-ion 19 the latter predominates, in the equilibrium of the altro-ion 23 and the allo-ion 24 the first is prefered.
    Notes: Im Gleichgewicht der Pentopyranose-Acetoxonium-Ionen liegen 28% xylo-Ion 4, 10% lyxo-Ion 5 und 57% arabino-Ion 6 + 7 (25°/CH3NO2) vor. Anomerisierungsreaktionen von Kohlenhydrat-Acetaten werden durch Acetoxoniumsalze generell katalysiert. Bei Pentofuranose- und am C-6 substituierten Glucopyranose-Derivaten besitzt jeweils das 1.2-Acet-oxonium-Ion 9 bzw. 15 erhöhte Stabilität. In den Gleichgewichten zwischen dem galakto-Ion 18 und -talo-Ion 19 wird 19, zwischen dem altro-Ion 23 und allo-Ion 24 dagegen 23 bevorzugt.
    Additional Material: 4 Tab.
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  • 2
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 103 (1970), S. 2450-2462 
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Carboxonium Compounds in Carbohydrate Chemistry, X. Investigations on Acetoxonium Rearrangement of D -Glucose to D-IdoseEquilibration experiments in nitromethane show that in equilibrium of the four acetoxonium ions the gluco-ion 5 is present in 60%, the manno-ion 6 in 12%, the altro-ion 7 in 7% and the ido-ion 8 in 21 %. Pure idose-4,6-acetoxonium salt 8 crystallizes from CH2Cl2 as it is formed from the equilibrium 5⇋6⇋7⇋8. The glucose-1,2-acetoxonium salt 5 is precipitated in CCl4 at -10°C. Penta-O-benzoyl-α-D-idopyranose (15) is obtained by benzoxonium rearrangement from glucose. Tetra-O-acetyl-6-bromo-6-deoxy-α-D-idopyranose (16) is formed by „trans“-opening of the idose-4,6-acetoxonium salt 8 with LiBr.
    Notes: Äquilibrierungsversuche in Nitromethan zeigen, daß von den vier Acetoxonium-Ionen das gluco-Ion 5 zu 60%, das manno-Ion 6 zu 12%, das altro-Ion 7 zu 7% und das ido-Ion 8 zu 21 % im Gleichgewicht vorliegen. Einheitliches Idose-4.6-Acetoxonium-Salz 8 kristallisiert aus CH2Cl2, da es über das Gleichgewicht 5⇋6⇋7⇋8 ständig nachgebildet wird. Das Glucose-1.2-Acetoxonium-Salz 5 kann durch Fällung aus CCl4 bei -10° erhalten werden. Penta-O-benzoyl-α-D-idopyranose (15) ist durch Benzoxonium-Umlagerung aus Glucose gewinnbar. Tetra-O-acetyl-6-brom-6-desoxy-α-D-idopyranose (16) wird durch „trans“ Öffnung des Idose-4.6-Acetoxonium-Salzes 8 mit LiBr dargestellt.
    Additional Material: 5 Tab.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 67 (1984), S. 1625-1629 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two new syntheses of verrucarinic acid (2S, 3R-dihydroxy-3-methylpentanoic acid) and its derivatives, suitably protected for the further conversion to macrocyclic trichothecenes, are described. The first one makes use of a diastereoselective alkylation of a (-)-(S)-malic acid ester and the regioselective reductin of one carboxyl function toa methyl group. The second approach involves a stereoselective addition of an allylsilane to a chiral glyoxylate.
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  • 4
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The title compounds have been synthesized starting from verrucarol and diacetoxyscripenol (anguidine), (E, Z)-muconic half ester and a derivative of verrucarinic acid. The latter has been prepared in optically active form from dimethyl 3-methylglutarate.
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  • 5
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chloro(cyclopentadienyl)bis(1,2:5,6-di-O-isopropylidene-α-D-glucofuranos-3-O-yl)titanium (1) is used for the transmetallation of Li-enolates obtained from propionyl derivatives. While such Ti-enolates of ketones and hydrazones appear to be unreactive, the (E)enolate 13 of 2,6-dimethylphenyl propionate (11) adds to the re-side of aldehydes, affording various syn-aldols 14 with high dia- and enantioselectivity (92-97% ds, 91-97% ee, cf. Scheme 2 and Table 1). Racemic syn-aldols (±)-14 are obtained analogously from the achiral bis(2-propyloxy)-Ti-enolate 15 (Scheme 2 and Table 2). In contrast to the unstable Li-enolate 10, the Ti-enolates 13 and 15 isomerize at -30°, presumably to the thermodynamically more stable (Z)-enolates (Scheme 4), While the diastereoselectivity of the achiral enolate 15 is lost upon this equilibration, the chiral (Z)-enolate 27 quite unexpectedly affords anti-aldols 12 of high optical purity (94-98% ec) and, in most cases, with acceptable-to-good diastereoselectivity (82-90% ds). Notable exceptions are branched unsaturated and aromatic aldehydes which form a greater proportion of synepimers of moderate optical purity (Scheme 5 and Table 3). Consistent with these findings, re-facial-and ami -selective aldol-addition is also exhibited by the (Z)-configurated Ti-enolate 22 of N-propionyl-oxazolidi-none 19 (Scheme 3).
    Additional Material: 5 Tab.
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  • 6
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stereoselective Synthesis of (2S,6S)-2,6-Diaminoheptanedioic Acid and of Unsymmetrical Derivatives of meso-2,6-Diaminoheptanedioic AcidSpecific inhibition of enzymes of the diaminopimelate pathway (L-lysine biosynthesis) should, in principle, lead to selective antibacterial agents or herbicides. For this purpose, enantioselective syntheses were devised for (2S, 6S)-2, 6-diaminoheptanedioic acid (L, L-diaminopimelate, 1), (2R,6S)-2,6-diamino-2-methylhept-3-enedioic acid (10), (2R, 6S)-2, 6-diaminohept-3-enedioic acid (9), (2R, 6S)-2, 6-diamino-4-fluorohept-3-enedioic acid (42), and (2S, 6S)-2, 6-diamino-3-chloroheptanedioic acid (5). The Schöllkopf bislactim-ether methodology was applied to control the configuration of C(2) and C(6) of 1, C(2) of 10, as well as C(6) of 9 and 42. Semialdehyde derivatives of L-glutamate afforded C(6) of 10 and 5, while the (R)-configurated C(2) of 9 and 42 were derived from L-serine. For this purpose, the synthesis of the Garner aldehyde 32 has been improved. As chromatographic purifications and the low temperatures for the reduction of the carboxylic acid are eliminated, this valuable intermediate can now be prepared in bulk quantities. An enantio- and diastereoselective aldol addition of a glycine titanium-enolate was applied for the construction of 5 (C(2) and C(3)). As all chiral building blocks and reagents used are available in both enantiomeric forms, these routes should also be suitable for the selective synthesis of the other stereoisomers of these bis(α-amino acids).
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 66 (1983), S. 744-754 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three syntheses of (2S, 3R)-2,5-dihydroxy-3-methylpentanoic acid (verrucarinic acid) and its derivatives suitably protected for the further transformation to macrocyclic trichothecenes are described. These involve an enantioselective ester hydrolysis by pig liver esterase, a Sharpless epoxidation and an asymmetric hydroboration.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 71 (1988), S. 354-362 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The protected serine aldehyde 10 was converted to the crystalline N-Boc-protected sphingosines 6-9 by a three-step reaction sequence. Compound 10 was transformed with high diastereoselectivity (95%) either to the erythro- or threo-alkynols, 17 and 18, respectively. The erythro-isomer 17 is formed by the addition to 10 of lithium pentadecyne 16 in THF/HMPT at -78°, whereas the corresponding threo-isomer 18 is produced in the presence of ZnBr2 in Et2O. Deprotection of the acetal moiety afforded 1,3-diols 19 and 20. These diols were selectively reduced with Red-Al to the (E)-sphingosines 6 and 8, or the (Z)-isomers 7 and 9 by partial hydrogenation over Lindlar's catalyst. Cleavage of the N-Boc group and further transformation to ceramides were readily achieved as demonstarted by the conversion of 6 to N-octadecanoyl-D-erythro-sphingosine 5.
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  • 9
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 104 (1971), S. 1311-1321 
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Carboxonium Compounds in Carbohydrate Chemistry, XV. Acyloxonium Rearrangements of Amino SugarsPyrano-oxazolinium ions and pyrano-dihydrooxazinium ions are much more stable than pyrano-acetoxonium ions. This effect is useful for shifting acyloxonium ion equilibria in the way that one ion is favoured. By this method there can be obtained the 2.3-oxazolino derivative 7 of 3-amino-3-deoxy-D-mannose starting from 3-amino-3-deoxy-D-glucose derivatives and the 5.6-dihydrooxazino derivative 21 of 6-amino-6-deoxy-D-idose from 6-amino-6-deoxy-D-glucose derivatives.
    Notes: Pyrano-oxazolinium-Ionen und Pyrano-dihydrooxazinium-Ionen sind erheblich stabiler als Pyrano-acetoxonium-Ionen. Man kann diesen Effekt ausnutzen, um Acyloxonium-Ionen-Gleichgewichte so zu verschieben, daß nur ein Ion bevorzugt vorliegt. Nach diesem Verfahren sind aus 3-Amino-3-desoxy-D-glucose-Derivaten das 2.3-Oxazolino-Derivat 7 der 3-Amino-3-desoxy-D-mannose und aus 6-Amino-6-desoxy-D-glucose-Derivaten das 5.6-Dihydro-oxazino-Derivat 21 der 6-Amino-6-desoxy-D-idose darstellbar.
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  • 10
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemie in unserer Zeit 23 (1989), S. 181-192 
    ISSN: 0009-2851
    Keywords: Chemistry ; Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In diesem Beitrag werden die allgemeinen Strukturkonzepte für temperaturbeständige Polymere vorgestellt und anschließend-am Beispiel der technisch wichtigen Vertreter-Synthesen, Eigenschaftsprofile und Anwendungen beschrieben, wobei der Schwerpunkt auf den unvernetzten Polymeren liegt.
    Additional Material: 18 Ill.
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