Synthetic Studies on Tumor-Associated Antigens: Efficient Syntheses of Le^a and Sialyl-Le^a Oligosaccharides, and Their Deaminated Analogs¹

ABSTRACT

Efficient syntheses of tumor-associated Le^a, sialyl Le^a and their deaminated analogs are described. The suitably protected D-glucosamine (4) or D-glucose (5) derivative was successively coupled with the methyl-1-thioglycosides (glycosyl donors) of D-galactose (6) and L-fucose (11) in high yields by using N-iodosuccinimide/trifluoromethanesulfonic acid (NIS/TfOH) as the glycosyl promoter. The resulting trisaccharides (12 and 13) were each converted, by deprotections, to the Le^a determinant (17) and its deaminated analog (18), and by further glycosylation with the phenyl-2-thioglycoside of N-acetylneuraminic acid (25), to the sialyl-Le^a determinant (30) and its deaminated analog (31), respectively.