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© Research
Publication : Journal of Carbohydrate Chemistry

Synthesis of the methyl glycosides of a di- and two trisaccharide fragments specific for the Shigella flexneri serotype 2a O-antigen

Scientific Fields
Diseases
Organisms
Applications
Technique

Published in Journal of Carbohydrate Chemistry - 01 Jan 2000

Mulard LA, Costachel C, Sansonetti PJ

J. Carbohydr. Chem. 2000; 19: 849-77

The stereocontrolled synthesis of methyl a-D-glucopyranosyl-(l—>4)-a-Lrhamnopyranoside (EC, 1), methyl a-L-rhamnopyranosyl-(l—»3)-[a-D-glucopyranosyl-(l—>4)]-a-L-rhamnopyranoside (B(E)C, 3) and methyl a-D-glucopyranosyl-(l—>4)-a-L-rhamnopyranosyl-(l—>3)-2-acetamido-2-deoxy-b-D-glucopyranoside (ECD, 4) is described; these constitute the methyl glycosides of branched and linear fragments of the O-specific polysaccharide of Shigella flexneri serotype 2a. Emphasis was put on the construction of the 1,2-cis EC glycosidic linkage resulting in the selection of 2,3,4,6-tetra-O-benzyl-a-D-glucopyranosyl fluoride (8) as the donor. Condensation of methyl 2,3-0-isopropylidene-4-0-trimethylsilyl-a-L-rhamnopyranoside (11) and 8 afforded the fully protected aE-disaccharide 20, as a common intermediate in the synthesis of 1 and 3, together with the corresponding bE-anomer 21. Deacetalation and regioselective benzoylation of 20, followed by glycosylation with 2,3,4-tri-O-benzoyl-a-L-rhamnopyranosyl trichloroacetimidate (15) afforded the branched trisaccharide 25. Full deprotection of 20 and 25 afforded the targets 1 and 3, respectively. The corresponding bE-disaccharide, namely, methyl b-D-glucopyranosyl-(l—»4)-a-L-rhamnopyranoside (bEC, 2) was prepared analogously from 21. Two routes to trisaccharide 4 were considered. Route 1 involved the coupling of a precursor to residue E and a disaccharide CD. Route 2 was based on the condensation of an appropriate EC donor and a precursor to residue D. The former route afforded a 1:2 mixture of the aE and bE condensation products which could not be separated, neither at this stage, nor after deacetalation. In route 2, the required aE-anomer was isolated at the disaccharide stage and transformed into 2,3,4,6-tetra-O-benzyl-a-D-glucopyranosyl-(l—»4)-2,3-di-0-benzoyl-a-L-rhamnopyranosyl trichloroacetimidate (48) as the EC donor. Methyl 2-acetamido-2-deoxy-4,6-O isopropylidene-b-D-glucopyranoside (19) was preferred to its benzylidene analogue as the precursor to residue D. Condensation of 19 and 48 and stepwise deprotection of the glycosylation product afforded the target 4.