Stereoselective synthesis of (E)-mannosylidene derivatives using the Wittig reaction

(Chemical Equation Presented) Stabilized ylides Bu3P=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 CO2Et). X-ray crystallography shows an approximately twist-boat conformation for the tetra-O-benzyl-protected (Z)- mannosylideneacetonitrile. © 2006 American Chemical Society.