The development of synthetic polysialic acid vaccines with high immunogenicity and understand the optimal epitope has attracted much attention in recent years.Convergent block synthesis strategy was considered the best method to synthesize oligosialic acid but wasnt realized because the c-selectivity decreased with the increasing of sialyl donors length in conjugation reaction until we found a new sialylation reagents which combine 5-N,4-O-carbonyl protections and anomeric dibutyl phosphate as a leaving group.