Facile synthesis of the heptasaccharide repeating unit of O-deacetylated GXM of C. neoformans serotype B Original Research Article

A heptasaccharide, β-d-Xylp-(1→2)-α-d-Manp-(1→3)-[β-d-Xylp-(1→2)]-α-d-Manp-(1→3)-[β-d-GlcpA-(1→2)][β-d-Xylp-(1→4)]-α-d-Manp, the repeating unit of the exopolysaccharide from Cryptococcus neoformans serovar B, was synthesized as its methyl glycoside. Thus 2,3,4-tri-O-benzoyl-β-d-xylopyranosyl-(1→2)-3,4,6-tri-O-benzoyl-α-d-mannopyranosyl trichloroacetimidate (7) and allyl 2,3,4-tri-O-benzoyl-β-d-xylopyranosyl-(1→2)-4,6-di-O-benzoyl-α-d-mannopyranoside (8), readily obtained from the corresponding monosaccharide derivatives via simple transformation, were coupled to give a (1→3)-linked tetrasaccharide 9. Deallylation of 9 followed by trichloroacetimidate formation produced the tetrasaccharide donor 11. Condensation of methyl 2,3,4-tri-O-benzoyl-β-d-xylopyranosyl-(1→4)-2-O-acetyl-6-O-benzoyl-α-d-mannopyranoside (18) with 11 followed by selective deacetylation yielded hexasaccharide acceptor 20. Coupling of 20 with methyl 2,3,4-tri-O-acetyl-α-d-glucopyranosyluronate bromide (21) and subsequent deprotection furnished the target heptaoside. A hexasaccharide fragment, α-d-Manp-(1→3)-[β-d-Xylp-(1→2)]-α-d-Manp-(1→3)-[β-d-GlcpA-(1→2)][β-d-Xylp-(1→4)]-α-d-Manp, was also similarly synthesized as its methyl glycoside.