SYNTHESIS OF A SELECTIVELY PROTECTED TRISACCHARIDE BUILDING BLOCK THAT IS PART OF XYLOSE-CONTAINING CARBOHYDRATE CHAINS FROM N-GLYCOPROTEINS

The synthesis is reported of ethyl 4-O-[3-O-allyl-4,6-O-isopropylidene-2-O-(2,3,4-tri-O-acetyl-beta-D-xylopyranosyl)-beta-D-mannopyranosyl]-3,6-di-O-benzyl-2-deoxy-2-phthalimido-1-thio-beta-D-glucopyranoside (16), a key intermediate in the synthesis of xylose-containing carbohydrate chains from N-glycoproteins. Condensation of ethyl 3,6-di-O-benzyl-2-deoxy-2-phthalimido-1-thio-beta-D-glucopyranoside (5) with 2,4,6-tri-O-acetyl-3-O-allyl-alpha-D-glucopyranosyl bromide, using silver triflate as a promoter, gave the beta-linked disaccharide derivative 8 (84%). O-Deacetylation of 8 and then isopropylidenation afforded 10, which was converted via oxidation-reduction into ethyl 4-O-(3-O-allyl-4,6-O-isopropylidene-beta-D-mannopyranosyl)-3, 6-di-O-benzyl-2-deoxy-2-phthalimido-1-thio-beta-D-glucopyranoside (12). Silver triflate-promoted condensation of 12 with 2,3,4-tri-O-acetyl-alpha-D-xylopyranosyl bromide gave 16 (71%). The Xyl p unit in 16 and in de-isopropylidenated 16 (17) existed in the C-1(4)(D) conformation, but that in 0-deacetylated 17 (18) existed in the C-4(1)(D) conformation.