Study for stereoselective synthesis of oligosaccharides by new glycosylation methods

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Highly p-selective mannnosylations of glycosyl acceptors with an α-mannosyl 6-nitro-2-benzothiazoate donor were carried out smoothly in the presence of a catalytic amount of tetrakis(pentafluorophenyl)boric acid to afford the corresponding disaccharides in good to high yields.A mild and highly a-selective glycosylation of several glycosyl acceptors was performed with an in situ formed glycosyl donor, benzyl-protected glycosyl methyldiphenylphosphonium iodide, to afford the corresponding α-disaccharides in high yields in CH2Cl2 at room temperature without any assistance of acid promoters.A convergent total synthesis of F1a antigen, a member of the tumor-associated O-linked mucin glycosyl amino acid, was tried by one-pot sequential glycosylation. Highly a-selective glycosylation of amino acid with thioglycoside was successfully carried out by combining trityl trifluoromethanesulfonate and N-iodosuccinimide which gave glycosyl amino acid in high yield. Next, the glycosylation of thioglycoside with galactosyl phenyl carbonate or fluoride was tried by the promotion of trityl tetrakis(pentafluorophenyl)borate or trifluoromethanesulfonic acid ; protected F1a was afforded in 80 or 89% overall yield, respectively, by the further addition of glycosyl aminoacid and NIS. The desired trisaccharide was obtained in high yield after removal of the protecting groups.