C-Glycosylidene derivatives (exo-glycals):: their synthesis by reaction of protected sugar lactones with tributylphosphonium ylids, conformational analysis and stereoselective reduction

Stabilised tributylphosphonium ylids Bu3PCH=CH(EWG), where EWG is CO2Me, (CO2Bu)-Bu-t or CN, react with protected sugar lactones under mild conditions to give high yields of glycosylidene derivatives (4 and 5) with good Z/E selectivity. X-Ray crystallography shows that in the solid state the tetra-O-benzyl protected (Z)-glucosylideneacetonitrile (Z)-4c adopts a conformation intermediate between a boat and a twist-boat, whereas the isomeric galactose derivative (Z)-5c exists as a distorted chair. NMR data suggest that in solution chair-like conformations are again more favoured for galactosylidene derivatives than for their glucosylidene analogues. Solution phase NMR studies and molecular modelling show that the (E)-double bond geometry disfavours the chair-like geometry of the ring, even in the galactose series; this is consistent with the avoidance of allylic 1,3-strain. Reduction of the glycosylidene double bond to give stereoselective formation of beta-C-glycoside derivatives may be achieved by using Et3SiH-CF3CO2H or Et3SiH-BF3.Et2O. (C) 2003 Elsevier Ltd. All rights reserved.