Stereoselective Synthesis of (E)-Mannosylidene Derivatives Using the Wittig Reaction

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.