Pre-activation based, highly alpha-selective O-sialylation with N-acetyl-5-N,4-O-carbonyl-protected p-tolyl thiosialoside donor

A new method for pre-activation of N-acetyl-5-N,4-O-carbonyl-protected p-tolyl thiosialoside donor with AgOTf and p-toluenesulfenyl chloride followed by coupling with other p-tolyl thioglycosides is reported. Excellent α-sialylation yields were achieved for these coupling reactions, as high as 91% yield of only α-isomer.