Preparation of some (1 goes to 6)-linked disaccharides, and their derivatives suitable for protein modification

Synthetic methods for the preparation of per-O-acetylated, (1 goes to 6)-linked disaccharides containing either a D-galactose or a D-glucose residue at the reducing end are described. In these methods, 1,2,3,4-tetra-O-acetyl-6-O-trityl-beta-D-glucopyranose was first converted into 1,2,3,4-tetra-O-acetyl-beta-D-glucopyranose (1) by rapid treatment with 90% trifluoroacetic acid, followed by rapid isolation designed to minimize O-acyl migration. Disaccharides acid, followed by rapid isolation designed to minimize )-acyl migration. Disaccharides were formed by glycosylation of 1 or 1,2:3,4-di-O-isopropylidene-D-galactopyranose with per-O-acetylglycosyl halides. Isopropylidene groups in the resulting disaccharide, if present were removed, and the disaccharide was per-O-acetylated. Per-O-acetylated beta-Gal-(1 goes to 6)-Glc and beta-GlcNAc-(1 goes to 6)-Gal, and a mixture of per-O-acetylated alpha-Gal-(1 goes to 6)-Gal and beta-Gal-(1 goes to 6)-Gal (in the ratio of 3:7) were thus obtained. The per-O-acetylated Gal-(1 goes to 6)-Gal disaccharides were converted, by a reaction sequence previously reported, into (2,2-dimethoxyethyl)aminocarbonylmethyl 1-thio-beta-D-glycosides, which could then be coupled to proteins via reductive alkylation. For the anomeric mixture of per-O-acetylated Gal-(1 goes to 6(-Gal, conversion into the corresponding 1-thioglycoside permitted resolution of the isomers by chromatography on silica gel. When disaccharides, as borate complexes, were chromatographed on a column of a strong, anion-exchange resin, all of the (1 goes to 6)-linked disaccharides of neutral sugars tested (including melibiose) were eluted later than analogous disaccharides having other linkages, and also later than any neutral monosaccharides.