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

A tetrasaccharide, α-l-Rhap-(1→3)-β-d-GalpNAc-(1→6)-α-d-Galp-(1→2)-α-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 α-d-Galp-(1→2)-d-Galp (5) and efficient coupling with the terminal disaccharide α-l-Rhap-(1→3)-d-GalpNAc building block, in which the N-phthalimido-protected trifluoroacetimidate 13 was proved to be an effective donor.