Stereocontrolled α-Galactosylation under Cooperative Catalysis.

A recent discovery of a cooperative catalysis comprising a silver salt and an acid led to a dramatic improvement in the way glycosyl halides are glycosidated. Excellent yields have been achieved, but the stereoselectivity achieved with 2--benzylated donors was poor. Reported herein is our first attempt to refine the stereoselectivity of the cooperatively catalyzed galactosylation reaction. Careful optimization of the reaction conditions along with studying effects of the remote protecting groups led to excellent stereocontrol of α-galactosylation of a variety of glycosyl acceptors with differentially protected galactosyl donors.