Stereoselective α-Sialylation with Sialyl Xanthate and Phenylsulfenyl Triflate as a Promotor

摘要

Reaction α- and β-xanthates 2 and 3 of sialic acid with glycosyl acceptors 5-8 in the presence phenylsulfenyl triflate (PST) as a promotor in a 2:1 mixture of CH_3CN/CH_2Cl_ at low temperature affords α-sialosides in good yield and stereoselectivity. PST is prepared in situ by reacting benzenesulfenyl chloride with silver triflate. Less reactive acceptors 5 and 6 give a higher α/β ratio than more reactive allylic alcohol 7 and primary alcohol 8; α-stereoselectivity is increased in a dilute solution. A possible mechanism of the reaction that involves intermediate α- and β-nitrilium cations 16 and 17 is discussed.