Stereoselective (alpha)-Glycosylation with 3-O-Acetylated d-Gluco Donors

The effect of a 3-O-acetyl group on the stereoselectivity of (alpha)-glycosylation with 2-O-benzylated d-gluco glycosyl donors was studied. It was shown that 3-Oacetylated donors gave (alpha)-anomers predominantly or exclusively, whereas glycosylation with the corresponding per-O-benzylated donors afforded mixtures of comparable amounts of (alpha)- and (beta)-anomers. The higher (alpha)-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.