Synthesis of four structural elements of xylose-containing carbohydrate chains from N-glycoproteins

The synthesis of the oligosaccharides β- d-Xyl p-(1→2)-β- d-Man p-OMe ( 12), β- d-Xyl p-(1→2)-[α- d-Man p-(1→6)]-β- d-Man p-OMe ( 17), β- d-Xyl p-(1→2)-[α- d-Man p-(1→3)]- β- d-Man p-OMe ( 21), and β- d-Xyl p-(1→2)-[α- d-Man p-(1→3)][α- d-Man p-(1→6)]-β- d-Man p-OMe ( 25) is described. Methyl 3- O-benzyl-4,6- O-isopropylidene-β- d-mannopyranoside ( 6) was prepared from the corresponding gluco-epimer ( 4) by oxidation, followed by stereoselective reduction. Condensation of 6 with 2,3,4-tri- O-acetyl-α- d-xylopyranosyl bromide in the presence of mercuric cyanide gave a 1:9 mixture of methyl 3- O-benzyl-4,6- O-isopropylidene-2- O-(2,3,4-tri- O-acetyl-α- ( 7a) and -β- d-xylopyranosyl)-β- d-mannopyranoside ( 7), and then 7 was converted into the acetylated disaccharide-glycoside 11. Regioselective mannosylation, with 2,3,4,6-tetra- O-acetyl-α- d-mannopyranosyl bromide, at position 6 of deisopropylidenated 7 ( 8), using mercuric bromide as a promoter, afforded the trisaccharide-glycoside derivative 13, which was transformed into the acetylated trisaccharide-glycoside 16. The disaccharide derivative 10, obtained from 8, and the trisaccharide derivative 15, obtained from 13, were glycosylated at position 3 with O-(2,3,4,6-tetra- O-acetyl-α- d-mannopyranosyl)trichloroacetimidate ( 19), using trimethylsilyl triflate as a promoter, giving rise to acetylated tri- ( 20) and tetra-saccharide ( 24) derivatives, respectively. O-Deacetylation of 11, 16, 20, and 24 gave 12, 17, 21, and 25, respectively.