Synthesis of the tetrasaccharide residue of clarhamnoside, a novel glycosphingolipid isolated from the marine sponge Agelas clathrodes

A tetrasaccharide, alpha-l-Rhap-(1-->3)-beta-d-GalpNAc-(1-->6)-alpha-d-Galp-(1-->2)-alpha-d-Galp, the carbohydrate moiety of clarhamnoside isolated from the marine sponge Agelas clathrodes, was synthesized as its propyl glycoside via a convergent approach. The key steps to the synthetic strategy were the stereoselective construction of the reducing-end disaccharide alpha-d-Galp-(1-->2)-d-Galp (5) and efficient coupling with the terminal disaccharide alpha-l-Rhap-(1-->3)-d-GalpNAc building block, in which the N-phthalimido-protected trifluoroacetimidate 13 was proved to be an effective donor.