A Highly α-Stereoselective Sialylation Method Using 4-O-4-Nitropicoloyl Thiosialoside Donor

An efficient alpha-sialylation methodology for various primary, secondary, and tertiary alcohol acceptors has been developed. The sialic acid ethyl thioglycoside 1b, bearing a 4-nitropicoloyl moiety as the stereodirecting group at the C4 position, was utilized as the glycosyl donor, and the sialylation reaction was activated with N-iodosuccinimide and catalytic triflic acid in a 1:1 mixture of dichloromethane/acetonitrile as solvent. The method was successfully applied to the stereocontrolled synthesis of protected trisaccharide 11 found in the repeating unit of serotype Ia Group B Streptococcus capsular polysaccharide.