title: Stereoselective alpha-Sialylation with Sialyl Xanthate and Phenylsulfenyl Triflate as a Promotor.
abstract: Reaction alpha- and beta-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(3)CN/CH(2)Cl(2) 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 acceptors 5 and 6 give a higher alpha/beta 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.
authors.affiliation: Department of Chemistry, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138.
authors.forename: Valeri
authors.initials: V
authors.lastname: Martichonok
authors.forename: George M.
authors.initials: GM
authors.lastname: Whitesides