Synthesis of the methyl glycosides of a di- and two trisaccharide fragments specific for the Shigella flexneri serotype 2a O-antigen -: Part 4

The stereocontrolled synthesis of methyl alpha-D-glucopyranosyl-(1-->4)-alpha-L-rhamnopyranoside (EC, 1), methyl alpha-L-rhamnopyranosyl-(1-->3)-[alpha-D-glucopyra- osyl-(1-->4)]-alpha-L-rhamnopyranoside (B(E)C, 3) and methyl alpha-D-glucopyranosyl-(1-->4)-alpha-L-rhamnopyranosyl-(1-->3)-2-acetamido-2-deoxy-beta-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-alpha-D-glucopyranosyl fluoride (8) as the donor. Condensation of methyl 2,3-O-isopropylidene-4-O-trimethylsilyloside-alpha-L-rhamnopyranoside (11) and 8 afforded the fully protected alpha E-disaccharide 20, as a common intermediate in the synthesis of 1 and 3, together with the corresponding beta E-anomer 21. Deacetalation and regioselective benzoylation of 20, followed by glycosylation with 2,3,4-tri-O-benzoyl-alpha-L-rhamnopyranosyl trichloroacetimidate (15) afforded the branched trisaccharide 25. Full deprotection of 20 and 25 afforded the targets 1 and 3, respectively. The corresponding beta E-disaccharide, namely, methyl beta-D-glucopyranosyl-(1-->4)-a-L-rhamnopyranoside (PEC, 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 alpha E and PE condensation products which could not be separated, neither at this stage, nor after deacetalation. In route 2, the required alpha E-anomer was isolated at the disaccharide stage and transformed into 2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl-(1-->4)-2,3-di-O-benzoyl-alpha-L-rhamnopyranosyl trichloroacetimidate (48) as the EC donor. Methyl 2-acetamido-2-deoxy-4,6-O-isopropylidene-beta-D-glucopyran-oside (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.