Synthesis of S-linked alpha(2-->9) octasialic acid via exclusive alpha S-glycosidic bond formation.

A new approach for the synthesis of S-linked alpha(2-->9) oligosialic acids was developed by using an asymmetric tert-butyl disulfide linkage as an anomeric thiol protecting group. Compared with conventional thiosialosides, the asymmetric disulfide sialosides can tolerate the conditions under which functional groups are modified without producing the undesired elimination and racemization products. In addition, the asymmetric tert-butyl disulfide protecting group can be efficiently removed to afford a thiol nucleophile at the alpha anomeric position without flipping the anomeric stereochemistry. By this strategy, the syntheses of alpha(2-->9) tetra-, hexa-, and octasialic acids were achieved.