Pii: 0040-4039(94)85307-x

In carbohydrate chemistry, glycosylation remains the most difficult transformation to effect.’ Very few methods combine both high stereoselectivity and high yield in the formation of the glycosidic bond. Competing Gl and S$ processes often make it difficult to predict the stereochemical course of a neattion. In addition, many glycosylation conditions require the use of toxic heavy metal salts or suffer from other practical shortcomings. Recently, the usefulness of 1,2-anhydro sugars, generated from the direct epoxidation of glycals with dimethyl dioxirane, as stereoselective glycosyl donors has been demonstrated.* These anhydm sugars, however, are not stable, and oxidation with dimethyl dioxirane is op erationally difficult on a large scale. It was our goal to find analogs of these epoxides that would be isolable, stable and accessible in large quantities. In recent years, the use of cyclic sulfates and cyclic sulfites as epoxide equivalents has become increasingly popular due to the availability of enantiomerically pure 1,2-dials via asymmetric dihydroxylation.3 However, the activation of cyclic sulfites or cyclic sulfates by Lewis acids has not been investigated. We report here a stereoselective glycosylation with glucose l+cyclic sulfites mediated by lanthanide(III) triflates (Scheme 1).