A major obstacle to advances in glycobiology and glyco-medicine is the lack of pure and structurally well-defined carbohydrates and glycoconjugates. In many cases, well-defined oligosaccharides can only be obtained by chemical synthesis. Unfortunately, no general method is available for the preparation of complex carbohydrates of biological importance and one of the main stumbling blocks is the lack of a reliable methodology for the stereoselective formation of 1,2-cis-glycosides. Our preliminary studies have shown that a glycosyl donor substituted with a chiral auxiliary, such as a (S)-(phenylthiomethyl)benzyl ether, can be employed for the stereoselective introduction of 1,2-cis glycosides such as a-glucosides and a-galactosides. Neighboring group participation by the chiral auxiliary leads to a quasi-stable anomeric sulfonium ion, which due to steric and electronic factors is formed as a trans-decalin ring system. Displacement of the ?-sulfonium ion by a hydroxyl leads to the stereoselective introduction of a-glycosides.
The aim of this application is to further develop the synthetic methodology and apply it to the preparation of oligosaccharides that have the potential to be developed as vaccines. The compatibility of the auxiliaries with commonly used glycosylation protocols and protecting group manipulations will be established. The auxiliary based methodology will be extended to the introduction of a wide variety of 1,2-cis-glycosides. A new generation of chiral auxiliary will be developed with improved properties such better acid stability, selective removable in the presence of sensitive protecting groups, and compatible with highly reactive glycosyl donors. Finally, the new methodology will be applied to the synthesis of oligosaccharides derived from Francisella tularensis and Bacillus anthracis, which are category-A bio-terrorism agents. Immunological experiments will be performed to establish whether the synthetic compounds can be employed as vaccine candidates.
A major obstacle to advances in glycobiology and glyco-medicine is the lack of pure and structurally well-defined carbohydrates. It is proposed to develop new synthetic methodologies for the preparation for well-defined complex carbohydrates. The new methodology will be applied to the preparation of oligosaccharides derived from pathogens, which are classified as category-A bio- terrorism agents. Immunological experiments will be performed to establish whether the synthetic compounds can be employed as vaccine candidates. ? ? ?
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