Stereoselective synthesis of (E)-mannosylidene derivatives using the Wittig reaction |
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Authors: | Coumbarides Gregory S Motevalli Majid Muse Warda A Wyatt Peter B |
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Institution: | School of Biological and Chemical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK. |
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Abstract: | Stabilized ylides Bu(3)P=CH(EWG), where EWG is an ester or nitrile group, react with 2,3,4,6-tetra-O-benzylmannono-1,5-lactone giving high yields of mannosylidene derivatives; in contrast to the glucose and galactose analogues, the (E)-mannosylidenes are predominant (E:Z > 9:1), thus minimizing dipole-dipole repulsions in the Wittig reactions. NMR indicates chair-like conformations for solutions of the (E)-mannopyranosylidenes, but not for those (Z)-isomers where data are available (EWG = CN or CO(2)Et). X-ray crystallography shows an approximately twist-boat conformation for the tetra-O-benzyl-protected (Z)-mannosylideneacetonitrile. |
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