STEREOSELECTIVE GLYCOSYLATION USING FULLY BENZYLATED PYRIMIDIN-2-YL 1-THIO-BETA-D-GLYCOPYRANOSIDES

Pyrimidin-2-yl 2,3,4,6-tetra-O-benzyl-1-thio-beta-D-gluco- (5) and -beta-D-galacto-pyranoside (6), and pyrimidin-2-yl 2,3,4-tri-O-benzyl-1-thio-beta-D-xylo- (7), and -alpha-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-alpha-D-mannopyranoside as the acceptor in the presence of trimethylsilyl triflate afforded 1,2-cis-configured, 1 --> 3-linked disaccharides (alpha from 5, 6, and 7, beta 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-alpha-D-galactopyranose in the presence of silver triflate furnished 1 --> 6-linked disaccharides in high yield, with the 1,2-cis isomers predominant.