Chemical Synthesis of Oligosialic Acid by Using 5-N,4-O-Carbonyl-Protected Sialyl Donors

Abstract

Synthesis of oligosialic acids using 5-N,4-O-carbonyl-protected sialyl donors is described. The cyclic protection of sialyl donor enables an efficient a-sialylation of various acceptors in non-coordinating solvents such as CH2Cl2. The protection is also effective for improving the reactivity of the hydroxy group at the eight position. In addition, protecting groups of the alcohol in exocyclic side chain affected not only their reactivity towards sialylation but also selectivity in glycosidation. The 7,8 dichloroacetyl-protected donor was effective for glycosylation of the primary alcohols and the C8 hydroxy group of sialic acid. Cyclic acetal protection of the 7,8 dihydroxy groups is effective for a-sialylation of galactoside at the three position. The sialyl phosphonate underwent chemoselective glycosylation of thiosialoside and would be effective for convergent synthesis of complex oligosaccharides.