Abstract
The trimethylsilyl trifluoromethanesulfonate (TMSOTf)-catalyzed reaction of methyl 6-hydroxyhexanoate with 3-O-benzyl-4-(2,4-di-O-acetyl-3-deoxy-l-glycero-tetronamido)-4,6-dideoxy-2-O-levulinoyl-α-d-mannopyranosyl trichloroacetimidate followed by a two-step deprotection (hydrogenolysis over Pd/C catalyst and Zemplén deacylation, to simultaneously remove the acetyl and levulinoyl groups) gave 5-(methoxycarbonyl)pentyl 4-(3-deoxy-l-glycero-tetronamido)-4,6-dideoxy-α-d-mannopyranoside. The structure of the latter, for which crystals were obtained in the analytically pure state for the first time, followed from its NMR and high-resolution mass spectra and was confirmed by X-ray crystallography. The molecule has two approximately linear components; a line through the aglycon intersects a line through the mannosyl and tetronylamido groups at 120°. The crystal packing separates the aglycon groups from the tetronylamido and mannosyl groups, with only C-H…O hydrogen bonding among the aglycon groups and N-H…O, O-H…O and C-H…O links among the tetronylamido and mannosyl groups. A carbonyl oxygen atom accepts the strongest O-H…O hydrogen bond and two strong C-H…O hydrogen bonds. The geometric properties were compared with those of related molecules.
Highlights
O-specific polysaccharides (O-SP, O-antigens) are essential virulence factors and protective antigens of many pathogenic bacteria [1]
The crystal structure of 3, whose synthesis (Scheme 1) and isolation in the crystalline state and full characterization is described here for the first time, will aid in the interpretation of data resulting from a similar study in the Inaba series
The product of debenzylation, obtained in a virtually theoretical yield, was subjected to Zemplén transesterification. It simultaneously effected the removal of the acetyl and 2-O-levulinoyl groups, to give Compound 3 in a
Summary
O-specific polysaccharides (O-SP, O-antigens) are essential virulence factors and protective antigens of many pathogenic bacteria [1]. Following the pioneering work by Kenne et al [4] on the synthesis of the methyl α-glycoside of the terminal, monosaccharide determinant of the O-SP of Vibrio cholerae O1, serotype. The presence of the methoxycarbonyl group in the title, spacer-equipped Compound 3 described here makes it amenable to conversions to an array of derivatives suitable for conjugation to proteins through different chemical processes. It will be useful, within Vibrio cholerae O1 strains, for making tools for immunological/immunogenicity studies towards elucidating the molecular basis for serotype specificity, which often require glycoconjugates.
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