A highly efficient and convergent synthesis of a hexasaccharide, a dimer of the repeating unit of the antigen O2 polysaccharide of Stenotrophomonas maltophilia

A highly efficient and convergent synthesis of a hexasaccharide, which is a dimer of the repeating unit of the antigen O2 polysaccharide of Stenotrophomonas maltophilia, was achieved via coupling of 2,3,4-tri-O-acetyl-α-l-xylopyranosyl bromide with the tetrasaccharide, allyl 4-O-{3-O-[4-O-(3,4-di-O-benzoyl-α-l-rhamnopyranosyl)-2,3,6-tri-O-benzoyl-α-d-mannopyranosyl]-4-benzoyl-α-l-rhamnopyranosyl}-2,3,6-tri-O-benzoyl-α-d-mannopyranoside (18) by the Koenigs–Knorr method followed by deacylation. Compound 18 was readily prepared from the coupling of the disaccharide trichloroacetimidate, 4-O-(2-O-acetyl-3,4-di-O-benzoyl-α-l-rhamnopyranosyl)-2,3,6-tri-O-benzoyl-α-d-mannopyranosyl trichloroacetimidate (8) with the disaccharide acceptor, allyl 4-O-(2-O-acetyl-4-O-benzoyl-α-l-rhamnopyranosyl)-2,3,6-tri-O-benzoyl-α-d-mannopyranoside (16), and both 8 and 16 were prepared via the trichloroacetimidate method from simple starting materials. The sole use of acyl protecting groups substantially simplified protection and deprotection, and the allyl group at the reducing end of allyl 4-O-{2-O-[2,3,4-tri-O-acetyl-β-l-xylopyranosyl]-3-O-[4-O-(2-O-(2,3,4-tri-O-acetyl-β-l-xylopyranosyl)-3,4-di-O-benzoyl-α-l-rhamnopyranosyl)-2,3,6-tri-O-benzoyl-α-d-mannopyranosyl]-4-O-benzoyl-α-l-rhamnopyranosyl}-2,3,6-tri-O-benzoyl-α-d-mannopyranoside 19 allowed further chemical transformation.