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

A heptasaccharide, beta-D-Xylp-(1-->2)-alpha-D-Manp-(1-->3)-[beta-D-Xylp-(1-->2)]-alpha-D-Manp-(1-->3)-[beta-D-GlcpA-(1-->2)][beta-D-Xylp-(1-->4)]-alpha-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-beta-D-xylopyranosyl-(1-->2)-3,4,6-tri-O-benzoyl-alpha-d-mannopyranosyl trichloroacetimidate (7) and allyl 2,3,4-tri-O-benzoyl-beta-D-xylopyranosyl-(1-->2)-4,6-di-O-benzoyl-alpha-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-beta-d-xylopyranosyl-(1-->4)-2-O-acetyl-6-O-benzoyl-alpha-D-mannopyranoside (18) with 11 followed by selective deacetylation yielded hexasaccharide acceptor 20. Coupling of 20 with methyl 2,3,4-tri-O-acetyl-alpha-D-glucopyranosyluronate bromide (21) and subsequent deprotection furnished the target heptaoside. A hexasaccharide fragment, alpha-D-Manp-(1-->3)-[beta-D-Xylp-(1-->2)]-alpha-D-Manp-(1-->3)-[beta-D-GlcpA-(1-->2)][beta-D-Xylp-(1-->4)]-alpha-D-Manp, was also similarly synthesized as its methyl glycoside.