J. Carbohydr. Chem. 2000; 19: 1131-50
Starting from the known methyl 2,3,4,6-tetra-O-benzyl-a-D-gIucopyranosyl-(1—»4)-2-0-benzoyl-a-L-rhamnopyranoside, the stepwise linear syntheses of methyl a-Lrhamnopyranosyl-(l-»2)-a-L-rhamnopyranosyl-(l—> 3)-[oc-D-gIucopyranosyI-(l—»4)]-a-L-rhamnopyranoside (AB(E)C, 4), and methyl 2-acetamido-2-deoxy-P-D-glucopyranosyl-(l—>2)-a-L-rhamnopyranosyl-(l-» 2)-a-L-rhamnopyranosyl-(l—» 3)-[a-D-glucopyranosyl-(l—»4)]-a-L-rhamnopyranoside (DAB(E)C, 5) are described; these constitute themethyl glycosides of a branched tetra- and pentasaccharide fragments of the 0-specificpolysaccharide of Shigella flexneri serotype 2a, respectively. The chemoselective Odeacetylation at position 2Q and/or 2A of key tri- and tetrasaccharide intermediates
bearing a protecting group at position 2c was a limiting factor. As such a step occurred once in the synthesis of 4 and twice in the synthesis of 5, the regioselective introduction of residue A on a B(E)C diol precursor (12) and that of residue D on an AB(E)C diol precursor (19) was also attempted. In all cases, a trichloroacetimidate donor was involved. The latter pathway was found satisfactory for the construction of the target 4 using the appropriate tri-0-benzoyl rhamnosyl donor. However, attempted chain elongation of 12 using 2-0-acetyl-3,4-di-O-benzyI-a-L-rhamnopyranosyl trichloroacetimidate (8) resulted in an inseparable mixture which needed to be benzoylated to allow the isolation of the target tetrasaccharide. Besides, condensation of the corresponding tetrasaccharide acceptor and the A^-acetylglucosaminyl donor was sluggish. As the target pentasaccharide was isolated in a poor yield, this route was abandoned.