Stereoselective α-Glycosylation with 3-O-Acetylated d-Gluco Donors

The effect of a 3-O-acetyl group on the stereoselectivity of α-glycosylation with 2-O-benzylated d-gluco glycosyl donors was studied. It was shown that 3-O-acetylated donors gave α-anomers predominantly or exclusively, whereas glycosylation with the corresponding per-O-benzylated donors afforded mixtures of comparable amounts of α- and β-anomers. The higher α-stereoselectivity in the first case was accounted for by the remote anchimeric assistance of the 3-O-acetyl group, which was confirmed by theoretical calculations.