Stereoselective glycosylations using benzoylated glucosyl halides with inexpensive promoters.

Reactions of O-benzoylated glucopyranosyl halide (I, Br), isolated or generated in situ from per-benzoylated glucose (8a) and trimethylsilyl halide, with various alcohols were efficiently promoted by zinc halide (Cl, Br) or N-bromosuccinimide with a catalytic ZnI(2) to give the corresponding 1,2-trans-beta-glucosides in good to high yields. When the anomeric halogenation of 8a was carried out in the presence of reactive alcohols, 1,2-cis-alpha-glucosides were selectively formed.