Synthesis, and carbon-13 n.m.r. study, of O-α-
-rhamnopyranosyl-(1→3)-O-α-
-rhamnopyranosyl-(1→2)-
-rhamnopyranose and O-α-
-rhamnopyranosyl-(1→3)-O-α-
-rhamnopyranosyl (1→3)-
-rhamnopyranose, constituents of bacterial, cell-wall polysaccharides |
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Authors: | Vince Pozsgay |
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Abstract: | O-α-
-Rhamnopyranosyl-(1→3)-
-rhamnopyranose (19) and O-α-
-rhamnopyranosyl-(1→2)-
-rhamnopyranose were obtained by reaction of benzyl 2,4- (7) and 3,4-di-O-benzyl-α-
-rhamnopyranoside (8) with 2,3,4-tri-O-acetyl-α-
-rhamnopyranosyl bromide, followed by deprotection. The per-O-acetyl α-bromide (18) of 19 yielded, by reaction with 8 and 7, the protected derivatives of the title trisaccharides (25 and 23, respectively), from which 25 and 23 were obtained by Zemplén deacetylation and catalytic hydrogenolysis, With benzyl 2,3,4-tri-O-benzyl-β-
-galactopyranoside, compound 18 gave an ≈3:2 mixture of benzyl 2,3,4-tri-O-benzyl-6-O-2,4-di-O-acetyl-3-O-(2,3,4-tri-O-acetyl-α-
-rhamnopyranosyl)-α-
-rhamnopyranosyl]-β-
-galactopyranoside and 4-O-acetyl-3-O-(2,3,4-tri-O-acetyl-α-
-rhamnopyranosyl)-β-
-rhamnopyranose 1,2-(1,2,3,4-tetra-O-benzyl-β-
-galactopyranose-6-yl (orthoacetate). The downfield shift at the α-carbon atom induced by α-
-rhamnopyranosylation at HO-2 or -3 of a free α-
-rhamnopyranose is 7.4-8.2 p.p.m., ≈1 p.p.m. higher than when the (reducing-end) rhamnose residue is benzyl-protected (6.6-6.9 p.p.m.). α-
-Rhamnopyranosylation of HO-6 of gb-
-galactopyranose deshields the C-6 atom by 5.7 p.p.m. The 1 2-orthoester ring structure O2,C(me)OR] gives characteristic resonances at 24.5 ±0.2 p.p.m. for the methyl, and at 124.0 ±0.5 p.p.m. for the quaternary, carbon atom. |
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Keywords: | |
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