The β-selectivity of mannosy-lation has been found to be dependent on the addition rate of the mannosyl trichloroacetimidate donor in an inverse-addition (I-A) procedure. This rate dependent I-A procedure can improve the selectivity of direct β-mannosylation and is applicable to orthogonal glycosylations of thioglycoside acceptors. Further elaboration of this novel procedure enables the development of the contiguous sequential glycosylation strategy, which streamlines the preparation of oligosaccharides invoking β-mannosidic bond formation. The synthetic utility of the contiguous glycosy-lation strategy was demonstrated by the preparation of the trisaccharide core of human N-linked glycoproteins and the trisaccharide repeating unit of the O-specific polysaccharide found in the cellular capsule of Salmonelle bacteria.
- Inverse addition