A synthetic strategy to xylose-containing thioglycoside tri- and tetrasaccharide building blocks corresponding to Cryptococcus neoformans capsular polysaccharide structures

As part of an ongoing project aimed at developing vaccine candidates against Cryptococcus neoformans the prepn. of tri- and tetrasaccharide thioglycoside building blocks, to be used in construction of structurally defined part structures of C. neoformans GXM capsular polysaccharide, was investigated. Using a naphthalenylmethyl (NAP) ether as a temporary protecting group and trichloroacetimidate donors in optimized glycosylations the target building blocks, Et 6-O-acetyl-2,4-di-O-benzyl-3-O-(2-naphthalenylmethyl)-α-D-mannopyranosyl-(1→3)-[2,3,4-tri-O-benzyl-β-D-xylopyranosyl-(1→2)]-4,6-di-O-benzyl-1-thio-α-D-mannopyranoside (16) and Et 2,3,4-tri-O-benzyl-β-D-xylopyranosyl-(1→2)-4,6-di-O-benzyl-3-O-(2-naphthalenylmethyl)-α-D-mannopyranosyl-(1→3)-[2,3,4-tri-O-benzyl-β-D-xylopyra-nosyl-(1→2)]-6-O-acetyl-4-O-benzyl-1-thio-α-D-mannopyranoside (21), were efficiently prepd. These synthesized thiosaccharide building blocks were then used as donors in high-yielding (∼90%) DMTST promoted glycosylations to a spacer-contg. acceptor to, after deprotection, afford GXM polysaccharide part structures ready for protein conjugation to give vaccine candidates. Also, the NAP groups in the building blocks were removed to obtain tri- and tetrasaccharide acceptors suitable for further elongation towards larger thiosaccharide building blocks.