Variations on the SnCl 4 and CF 3CO 2Ag-promoted glycosidation of sugar acetates: a direct, versatile and apparently simple method with either α or β stereocontrol

Abstract

Glycosidation of sugar peracetates ( d- gluco, d- galacto) with SnCl 4 and CF 3CO 2Ag led to either 1,2- cis-, or 1,2- trans-glycosides, depending primarily on the alcohols used. In particular, 1,2- trans-glycosides, expected from acyl-protected glycosyl donors, were formed in high yields with alcohols sharing specific features such as bulkiness, presence of electron-withdrawing groups or polyethoxy motifs. In contrast, simple alcohols afforded ∼1:1 mixtures of 2,3,4,6-tetra- O-acetyl, and 3,4,6-tri- O-acetyl 1,2- cis-glycosides due to anomerization and/or acid-catalyzed fragmentation of 1,2-orthoester intermediates. After reacetylation or deacetylation, acetylated or fully deprotected 1,2- cis-glycosides (α- d- gluco, α- d- galacto) were obtained in ∼90% yields by a simple and direct method.