Stereoselective Synthesis of the Trisaccharide Moiety of Ganglioside HLG‑2

The glycan portion of ganglioside HLG-2, which was identified in the extracts of the sea cucumber Holothuria leucospilota, was synthesized in a highly efficient and stereoselective manner. The unusual sequence of the trisaccharide moiety, α-N-glycolylsialyl-(2,4)-α-N-acetylsialyl-(2,6)-glucoside, was assembled by stereoselective coupling of a 5-N,4-O-carbonyl-protected sialyl phosphate donor, a N-2,2,2-trichloroethoxycarbonyl (Troc)-protected sialyl acceptor, and a (trimethylsilyl)­ethyl-β-glucosyl acceptor in high yield. The synthesis featured the high-yielding construction of two α-sialyl linkages.