Highly stereoselective 1,2-cis-glycosylations employing the C-2 (S)-(phenylthiomethyl)benzyl ether as a chiral auxiliary

The principal challenge in the chemical synthesis of oligosaccharides remains the stereoselective introduction of the glycosidic bond. The stereoselective synthesis of 1,2-trans glycosidic linkages can be accomplished by employing neighboring group participation of a C-2 acyl functionality and is, therefore, relatively straightforward. The stereoselective synthesis of 1,2-cis linkages is much harder to achieve since no general strategy is available. Most commonly, a non-participating group at C-2 is needed in addition to a participating solvent system in order to achieve good 1,2-cis selectivity. In general, though, this approach still leads to the formation of mixtures of anomers.