Synthesis of the trisaccharide moiety of gangliotriosylceramide (asialo GM 2)

Abstract

The synthesis of the trisaccharide methyl glycoside β- d-GalNAc-(1→4)-β- d-Gal-(1→4)-β- d-Glc-OMe, which corresponds to the carbohydrate portion of gangliotriosylceramide (asialo GM 2), was accomplished by the reaction of 4- O-acetyl-3,6-di- O-benzoyl-2-deoxy-2-phthalimido- d-galactopyranosyl bromide ( 18) with a benzylated derivative of methyl 4- O-β- d-galactopyranosyl-β- d-glucopyranoside. Comparative studies with a 6′-benzyl ether and a 6′-benzoate revealed that the substituent at O-6′ is crucial to the outcome of glycosylations at O-4′, the ether derivative being much the more reactive. tert-Butyl 4- O-acetyl-3,6-di- O-benzoyl-2-deoxy-2-phthalimido- d-galactopyranoside, which was readily converted into the corresponding bromide 18, was obtained from the gluco derivative via a single-step, crown ether-assisted epimerization.