Stereoselective alpha-sialylation with sialyl xanthate and phenylsulfenyl triflate as a promotor

Reaction alpha- and beta-xanthates 2 and 3 of sialic acid with glycosyl accepters 5-8 in the presence phenylsulfenyl triflate (PST) as a promotor in a 2:1 mixture of CH3CN/CH2Cl2 at low temperature affords alpha-sialosides in good yield and stereoselectivity. PST is prepared in situ by reacting benzenesulfenyl chloride with silver triflate. Less reactive accepters 5 and 6 give a higher alp ratio than more reactive allylic alcohol 7 and primary alcohol 8; alpha-stereoselectivity is increased in a dilute solution. A possible mechanism of the reaction that involves intermediate alpha- and beta-nitrilium cations 16 and 17 is discussed.