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

Abstract

Stabilised tributylphosphonium ylids Bu 3PCHCH(EWG), where EWG is CO 2Me, CO 2 t Bu 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 β- C-glycoside derivatives may be achieved by using Et 3SiH–CF 3CO 2H or Et 3SiH–BF 3·Et 2O.