Catalytic and a-selective sialylation of several glycosyl acceptors with 5-azido sialyl fluoride 2 was successfully carried out in n-valeronitrile using a combined catalyst system of stannic chloride (SnCl4) and silver perchlorate (AgClO4) (1:2) in the coexistence of MS 5A and the corresponding sialosides were obtained in excellent yields with high alpha-selectivities.