Sialic acids are acidic 9 monosaccharides having a carboxylic acid and play a very important biological role. In addition, they are important and attractive targets in synthetic carbohydrate chemistry due to the difficulty of formation of α-sialyl linkage. We found that sialic acid units having a 5N,4O-carbonyl protecting group acted not only as an efficient glycosyl donor undergoing α-silylation without use of nitrile effects but also as glycosyl acceptor for sialylation of hydroxyl groups at C8 and 9 positions. By using it, we successfully prepared α(2,8)oligosialic acids and an epitope of of ganglioside (GP1c) containing five sialic acids. Furthermore, we successfully developed an efficient method for preparation of oligosialic acid based on a combination of one pot glycosylation and solid-phase assisted deprotection. We also found that a sialic acid sugar donor having free hydroxyl groups except for that at the 9 position underwent α-selective glycosidation in dichloromethane. These results indicated that the remote neighboring participation of the carbonyl group on protecting groups reduced the α-selectivity of sialylation.
