Convergent synthesis of tetra- and penta-saccharide fragments of dimeric Lewis X.

The convergent synthesis of tetra- and penta-saccharide fragments of the TACA dimeric Le is described. The synthetic strategy relied on the preparation of a protected GlcNTCA-(1，3)-Gal-(1，4)-GlcNAc trisaccharide diol free at O-3 of both glucosamine residues. Key steps in the preparation of this diol involved glycosylation at O-4 of N-acetylglucosamine using activation of a trichloroacetimidate with BF·EtO at 40 °C， removal of the non-reducing end O-3' chloroacetate with thiourea， and glycosylation with a N-trichloroacetamido glucosamine trichloroacetimidate donor. After conversion to the diol acceptor， the trisaccharide was selectively fucosylated at the nonreducing end under NIS/TMSOTf activation， or di-fucosylated under CuBr/BuNBr activation. The protected tetra- and pentasaccharides were then efficiently deprotected under dissolving metal conditions and the nonreducing end glucosamine residues were N-acetylated during the reaction work up. The deprotected compounds will be used as soluble competitors to characterize the epitopes recognized by anti-polymeric Le antibodies.