Stereoselective Synthesis of (<i>E</i>)-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-<i>O-</i>benzylmannono-1,5-lactone giving high yields of mannosylidene derivatives; in contrast to the glucose and galactose analogues, the (<i>E</i>)-mannosylidenes are predominant (<i>E</i>:<i>Z</i> > 9:1), thus minimizing dipole−dipole repulsions in the Wittig reactions. NMR indicates chair-like conformations for solutions of the (<i>E</i>)-mannopyranosylidenes, but not for those (<i>Z</i>)-isomers where data are available (EWG = CN or CO₂Et). X-ray crystallography shows an approximately twist-boat conformation for the tetra-<i>O</i>-benzyl-protected (<i>Z</i>)-mannosylideneacetonitrile.