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-β- d- xylopyranosyl)-β- d-mannopyranosyl]-3,6-di- O-benzyl-2-deoxy-2-phthalimido-1-thio-β- 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-β- d-glucopyranoside ( 5) with 2,4,6-tri- O-acetyl-3- O-allyl-α- d-glucopyranosyl bromide, using silver triflate as a promoter, gave the β-linked disaccharide derivative 8 (84%). O-Deacetylation of 8 and then isopropylidenation afforded 10, which was converted via oxidation-reduction into ethyl 4- O-allyl-4,6- O-isopropylidene-β- d-mannopyranosyl)-3, 6-di- O-benzyl-2-deoxy-2-phthalimido-1-thio-β- d-glucopyranoside ( 12). Silver triflate-promoted condensation of 12 with 2,3,4-tri- O-acetyl-α- d-xylopyranosyl bromide gave 16 (71%). The Xyl p unit in 16 and in de-isopropylidenated 16 ( 17) existed in the 1 C 4( d) conformation, but that in O-deacetylated 17 ( 18) existed in the 4 C 1( d) conformation.