New aspects of glycoside bond formation

Abstract

Glycoside bond formation generally requires activation of the sugar at the anomeric center. To this end, anomeric oxygen exchange reactions, resulting in the Koenigs-Knorr method and variations or, alternatively, activation through retention of the anomeric oxygen, resulting in the trichloroacetimidate method and in the phosphite method, have been proposed. The successful application of the trichloroacetimidate method to the total synthesis of GPI anchors is particularly worth mentioning. a(2-3)-Sialylation can be based on sialyl phosphites as glycosyl donors and on the nitrile effect for anomeric stereocontrol. This is exhibited for a preparative synthesis of ganglioside GM2 which is required for tumor vaccine studies. For the generation of a(2-8)-linkage between neuraminic acid residues anchimeric assistance by a 3-thiocarbonyloxy group is introduced. The required sialyl donor can be efficiently prepared and a-linkage to 8-O-unprotected neuraminic acid derivatives is almost quantitative. The limitations of chemical glycopeptide synthesis encourage to employ the protein biosynthesis machinery in combination with an expanded genetic code. This is exhibited for the synthesis of glycosylated hARF-1 protein.