Novel glycosylation reactions using glycosyl thioimidates of N-acetylneuraminic acid as sialyl donors

Novel sialyl donors 4 bearing a thioimidolyl moiety as the leaving group were successfully prepared from the corresponding arylthio derivatives 3 and a peracetylated chloro derivative of Neu5Ac 2 in the presence of N,N-di-isopropylethylamine with moderate yields. The reaction of 4 with various alcohols 5 was effectively activated by AgOTf as the promoter to give the corresponding O-sialosides 6 with good yields. Selective activation of 4a over 4-pentenyl 2-glycoside of Neu5Ac 7 with AgOTf was also achieved.