Studies on the synthesis of Lewis-y oligosaccharides.

Lewis-y histo-blood group oligosaccharides are tumour-associated antigens prevalent in several different types of cancer, and they may also be secondary ligands for bacterial toxins from Escherichia coli and Vibrio cholerae. The key step in the synthesis of these sterically congested oligosaccharides involves difucosylation of partially protected lactosamine derivatives. Existing methods require either prolonged reaction times or elaborate glycosyl donors to ensure high stereoselectivity. Herein we report an optimised procedure for using a simple thioglycoside donor that leads to the desired products in high yield and excellent stereoselectivity. It is found that initial glycosylation of the 3'-hydroxy group of lactosamine derivatives in dichloromethane solution can inhibit subsequent glycosylation at the 2-position; however, reaction in toluene solution leads to Lewis-y oligosaccharides in high yield.