Synthesis of the H-type 1 and Lewis B antigens as 6-aminohexyl glycosides.

The Le b Le a heptasaccharide is a tumor-associated carbohydrate antigen that was isolated from the human colonic adenocarcinoma cell line Colo205 and is a good target for the development of anti-cancer immunotherapeutics. However, it displays on its reducing end the Le b tetrasaccharide: α-l-Fuc p-(1→2)-β-d-Gal p-(1→3)-[α-l-Fuc p-(1→4)]-d-GlcNAc p and the H-type 1 (H-1 antigen) trisaccharide: α-l-Fuc p-(1→2)-β-d-Gal p-(1→3)-d-GlcNAc p that are also found on noncancerous tissues. To discover analogues or fragments of Le b Le a that could be used as immunotherapeutics while not triggering immune responses against Le b and the H-1 antigen, we have synthesized the Le b tetrasaccharide hexyl glycoside and the Le b and H-1 antigens aminohexyl glycosides to be used in ELISA experiments. We describe an improved preparation of the 6- O-benzyl-2,3,4-tri- O-acetyl-α-d-galactopyranosyl bromide in neutral conditions and demonstrate the importance of appropriately “matching” the reactivity of acceptors with that of glycosyl donors. Mono- and di-fucosylation of a disaccharide diol acceptor with per-benzylated thioethyl fucoside activated in situ with bromine and under halide ion catalysis is described and our results are compared to literature reports. We observed that our fucosylation reactions required higher equivalents of fucosyl donor and extended reaction times than previously reported. We propose that the protecting groups on the galactosyl unit led to a reduced reactivity of the acceptor. The protected intermediates were converted to 6-azido hexyl glycosides and submitted to dissolving metal conditions to give 6-aminohexyl glycosides. We also prepared the n-hexyl tetrasaccharide glycoside Le b that will be used as a soluble antigen in competitive ELISA experiments.