Synthesis of a hexasaccharide fragment of the O-deacetylated GXM of C. neoformans serotype B.
beta-D-Xylp-(1-->4)-alpha-D-Manp-(1-->3)-[beta-D-Xylp-(1-->2)]-alpha-D-Manp-(1-->3)-[beta-D-Xylp-(1-->2)]-alpha-D-Manp, the fragment of the exopolysaccharide from Cryptococcus neoformans serovar B, was synthesized as its methyl glycoside. Thus, acetylation of allyl 3-O-benzoyl-4,6-O-benzylidene-alpha-D-mannopyranoside (1) followed by debenzylidenation and selective 6-O-benzoylation afforded allyl 2-O-acetyl-3,6-di-O-benzoyl-alpha-D-mannopyranoside (4). Glycosylation of 4 with 2,3,4-tri-O-benzoyl-D-xylopyranosyl trichloroacetimidate (5) furnished the beta-(1-->4)-linked disaccharide 6. Deallylation followed by trichloroacetimidate formation gave the disaccharide donor 8, and subsequent coupling with allyl 2,3,4-tri-O-benzoyl-beta-D-xylopyranosyl-(1-->2)-4,6-di-O-benzoyl-alpha-D-mannopyranoside (9), produced the tetrasaccharide 10. Reiteration of deallylation and trichloroacetimidate formation from 10 yielded the tetrasaccharide donor 12. The downstream disaccharide acceptor 18 was obtained by condensation of 5 with methyl 3-O-acetyl-4,6-O-benzylidene-alpha-D-mannopyranoside, followed by debenzylidenation, benzoylation, and selective 3-O-deacetylation. Coupling of 18 with 12 afforded the hexasaccharide 19, and subsequent deprotection gave the hexasaccharide glycoside 20. Selective 2"-O-deacetylation of 19 gave the hexasaccharide acceptor 21. Condensation of 21 with glucopyranosyluronate imidate 22 did not produce the expected heptasaccharide glycoside; instead, a transacetylation product 19 was obtained. Meanwhile, there was no reaction between 21 and the bromide donor 23.
Zhao
Wei
W
Research Center for Eco-Environmental Sciences, Academia Sinica, PO Box 2871, Beijing 100085, China.
Kong
Fanzuo
F
D000081:Acetamides; D000107:Acetylation; D001965:Bromides / Q000737:chemistry; D002240:Carbohydrate Sequence; D062845:Chloroacetates; D003455:Cryptococcus neoformans / Q000378:metabolism; D020723:Glucuronic Acid / Q000737:chemistry*; D006027:Glycosides / Q000737:chemistry; D008358:Mannose / Q000737:chemistry*; D008956:Models, Chemical; D008969:Molecular Sequence Data; D011134:Polysaccharides / Q000737:chemistry*; D011714:Pyrans / Q000737:chemistry; D013696:Temperature; D014238:Trichloroacetic Acid / Q000737:chemistry; D014994:Xylose / Q000737:chemistry*
D000081:Acetamides; D001965:Bromides; D062845:Chloroacetates; D006027:Glycosides; D011134:Polysaccharides; D011714:Pyrans; D014238:Trichloroacetic Acid; C027478:glucuronoxylomannan; D020723:Glucuronic Acid; D014994:Xylose; C009528:trichloroacetamide; D008358:Mannose