α-Selective sialylationreactions were carried out using novel sialicacidbuildingblocks that possess a thioester auxiliary. In contrast to other arylthio- and benzylthioester derivatives, sialyl phosphite 1a (with the phenylthioester moiety) was employed as the α-selective buildingblock, and was reacted with various primary alcohols, including the C6–OH group of galactose and glucose, with moderate