| Abstract: | A concise approach to a Neu5Ac‐α‐2,3‐LacNPhth trisaccharide derivative was developed. First, the regio/stereoselective glycosylation between glycoside donors and glucoNPhth diol acceptors was investigated. It was found that the regioselectivity depends not only on the steric hindrance of the C2‐NPhth group and the C6‐OH protecting group of the glucosamine acceptors, but also on the leaving group and protecting group of the glycoside donors. Under optimized conditions, LacNPhth derivatives were synthesized in up to 92 % yield through a regio/stereoselective glycosylation between peracetylated‐α‐galactopyranosyl trichloroacetimidate and p‐methoxyphenyl 6‐O‐tert‐butyldiphenylsilyl‐2‐deoxy‐2‐phthalimido‐β‐d ‐glucopyranoside, avoiding the formation of glycosylated orthoesters and anomeric aglycon transfer. Then, the LacNPhth derivative was deacylated and then protected on the primary position by TBDPS to form a LacNPhth polyol acceptor. Finally, the Neu5Ac‐α‐2,3‐LacNPhth derivative was synthesized in 48 % yield through the regio/stereoselective glycosylation between the LacNPhth polyol acceptor and a sialyl phosphite donor. Starting from d ‐glucosamine hydrochloride, the target Neu5Ac‐α‐2,3‐LacNPhth derivative was synthesized in a total yield of 18.5 % over only 10 steps. |
