Enhanced Sialylating Activity of O-Chloroacetylated 2-Thioethyl Sialosides

It has been shown that O-chloroacetyl protecting groups enhance significantly sialylating activity of 2-thioethyl sialosides in model reactions of (alpha)(2-8)-sialylation. Ethyl 4,7,8,9-tetra-O-chloroacetyl-3,5-dideoxy-2-thio-5-trifluoroacetamido-Dglycero-(alpha)-D-galacto-non-2-ulopyranoside that combines electron-withdrawing O-chloroacetyl and N-trifluoroacetyl protecting groups displayed the best reactivity and enabled the most efficient synthesis of the Neu5Ac(alpha)(2-8)Neu5Ac dimer. The GD3 tetrasaccharide was synthesized in stepwise manner using this sialyl donor in the key (2-8)-coupling, albeit the yield was noticeably lower than in the model sialylation of monosaccharide acceptors.