Preparative route to per-O-acetylated N-acetyl- and N-(tert-butoxycarbonyl)neuraminyl-a-(2 -> 3)-galactosyl disaccharide glycosyl donors by regioselective acetolysis of sialyl-a-(2 -> 3')-lactose

摘要

Per-O-acetylated N-acetylneuraminyl-alpha-(2 -> 3)-galactopyranose was prepared in three steps in good overall yield from sialyl-alpha-(2 -> 3')-lactose by regioselective cleavage of the galactosyl-beta-(1 -> 4)-glucose linkage by acetolysis and then readily converted into the corresponding disaccharide 1-trichloroacetimidate or ethyl thioglycoside, valuable synthetic blocks for the preparation of complex sialylated oligosaccharides. The N-acetyl group in the disaccharide thioglycoside was replaced by an N-Boc one via intermediate formation of a mixed N-Ac-N-Boc imide followed by chemoselective de-N-acetylation with hydrazine hydrate in DMF. An example of application of the disaccharide thioglycoside for the preparation of a spacer-armed hexasaccharide SLe(x) is described.