J Carbohydr Chem 2000; 19: 503-26
The stepwise synthesis of methyl a-D-glucopyranosyl-(l—»3)-a-L-rhamnopyranosyl-(l-»3)-a-L-rhamnopyranoside (EBC-OMe, 1), methyl oc-L-rhamnopyranosyl-(l—>2)-[a-D-glucopyranosyl-(l—>3)]-a-L-rhamnopyranosyl-(l-> 3)-a-L-rhamnopyranoside (A(E)BC-OMe, 2), and methyl 2-acetamido-2-deoxy-P-D-glucopyranosyl-(l->2)-a-Lrhamnopyranosyl-(l->2)-[a-D-glucopyranosyl-(l->3)]-a-L-rhamnopyranosyl-(l->3)-a-L-rhamnopyranoside (DA(E)BC-OMe, 3) is described. Compounds 1,2 and 3 constitute the methyl glycosides of fragments of the O-specific polysaccharide of Shigella flexneri
serotype 5a. Methyl 2,4-di-0-benzoyl-a-L-rharnnopyranosyl-(l-»3)-2,4-di-0-benzoyl-a-L-rhamnopyranoside was an appropriate BC precursor for the synthesis of 1. For the synthesis of the branched targets 2 and 3 , a benzyl group was best suited at position 2 of
rhamnose C. Thus, methyl 4-0-benzyl-a-L-rhamnopyranosyl-(l-»3)-2,4-di-O-benzyl-a-L-rhamnopyranoside was the key intermediate to the BC portion. In all cases, 2,3,4,6-tetra-O-benzyl-a-D-glucopyranosyl fluoride was a convenient E precursor, whenused in combination with titanium tetrafluoride. All along, attention was paid to steric hindrance as a factor of major impact on the condensation steps outcome. Therefore, based on previous experience, 2-0-acetyl-3,4-di-0-allyl-a-L-rhamnopyranosyl trichloroacetimidate and 3,4,6-tri-O-acetyl-2-deoxy-2-trichloroacetamido-a-D-glucopyranosyl trichloroacetimidate were used as donors. Both suited all requirements when used as key precursors for residues A and D in the synthesis of 3, respectively.