Selective defucosylation by mercaptolysis. A potential step in analyzing branched oligosaccharides

A method of stepwise chemical degradation was elaborated on a μg quantity of 3-O-α-l-fucosyllactose. The key step, TiCl4-catalysed dithioacetal formation from the permethylated N-4-nitrophenyl triosylamine (4) was accompanied by quantitative defucosylation. [14C]Acetylation of the dried mercaptalation mixture gave radiolabelled 3,5-di-O-[14C]acetyl-4-O-(2,3,4,6-tetra-O-methyl-β -d-galactopyranosyl)-2,6-di-O-methyl-d-glucose diethyl dithioacetal (7) and 5-O-[14C]acetyl-2,3,4-tri-O-methyl-l-fucose diethyl dithioacetal (8). The former was further degraded via the bis(sulfone), and thereby 2,3,4,6-tetra-O-methyl-d-galactose (13) was expelled. The monosaccharide branches, fucose and galactose, were identified as derivatives 8 and 13, respectively, by comparison with authentic samples. Isolation of microquantities of products was carried out by preparative TLC.