Stereoselective glycosylation using fully benzylated pyrimidin-2-yl 1-thio-β-d-glycopyranosides

Pyrimidin-2-yl 2,3,4,6-tetra-O-benzyl-1-thio-β-d-gluco- (5) and -β-d-galacto-pyranoside (6), and pyrimidin-2-yl 2,3,4-tri-O-benzyl-1-thio-β-d-xylo- (7), and -α-d-arabino-pyranoside (8) were readily prepared from the corresponding per-O-acetylated 1-thioglycopyranosides, which were in turn obtained from the relevant acetobromosugars and 2-mercaptopyrimidine under phase-transfer conditions. Glycosidic coupling reactions using 5 (or 6, 7, or 8) as the donor and methyl 2,4,6-tri-O-benzyl-α-d-mannopyranoside as the acceptor in the presence of trimethylsilyl triflate afforded 1,2-cis-configured, 1 → 3-linked disaccharides (α from 5, 6, and 7, β from 8) as the sole products in moderate to excellent yields. The coupling reaction of 6 (or 7 or 8) with 1,2:3,4-di-O-isopropylidene-α-d-galactopyranose in the presence of silver triflate furnished 1 → 6-linked disaccharides in high yield, with the 1,2-cis isomers predominant.