Facile Synthesis of the Heptasaccharide Repeating Unit of O‐Deacetylated GXM of C. neoformans Serotype B.

Abstract

Abstract A heptasaccharide, β- d -Xyl p -(1→2)-α- d -Man p -(1→3)-[β- d -Xyl p -(1→2)]-α- d -Man p -(1→3)-[β- d -Glc p A-(1→2)][β- d -Xyl p -(1→4)]-α- d -Man p , 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 -Man p -(1→3)-[β- d -Xyl p -(1→2)]-α- d -Man p -(1→3)-[β- d -Glc p A-(1→2)][β- d -Xyl p -(1→4)]-α- d -Man p , was also similarly synthesized as its methyl glycoside.