arsenic ylides with pyranose and furanose carbohydrates mainly leads to E-olefinic derivatives. Cyclisation of the olefinic derivatives obtained from 2,3,4,6-tetra-O-benzyl-D-glucose and D-galactose, promoted by ZnBr2, gives C-glycosyl compounds with high stereoselectivity. One the other hand, reaction of the corresponding arsonium salt, in presence of zinc, with a furanosederivative gives directly and
C-Glycosylidene derivatives (exo-glycals): their synthesis by reaction of protected sugar lactones with tributylphosphonium ylids, conformational analysis and stereoselective reduction
作者:Miguel Gascón-López、Majid Motevalli、George Paloumbis、Peter Bladon、Peter B Wyatt
DOI:10.1016/j.tet.2003.09.081
日期:2003.11
Stabilised tributylphosphonium ylids Bu3PCHCH(EWG), where EWG is CO2Me, CO2tBu 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
cyclization leading to the expected C-glycosyl compound. The first step is a Wittig reaction with protected furanose and pyranose hemiacetals. When stabilized ylides are used4, subsequent cyclization of the unsaturated intermediate may occur by treatment with bases or, sometimes, spontaneously. But when the Wittig reaction takes place with unstabilized ylides, the cyclization can be obtained by the iodoor