Journal
NATURE CHEMISTRY
Volume 4, Issue 8, Pages 663-667Publisher
NATURE PUBLISHING GROUP
DOI: 10.1038/NCHEM.1404
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Funding
- Natural Sciences and Engineering Research Council (NSERC) of Canada
- National Institutes of Health (NIH) [GM62160]
- Ministere de l'Education Nationale de la Recherche et de la Technologie
- NSERC
- Canada Research Chairs programme
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Although arguably the most important reaction in glycoscience, chemical glycosylations are among the least well understood of organic chemical reactions, resulting in an unnecessarily high degree of empiricism and a brake on rational development in this critical area. To address this problem, primary C-13 kinetic isotope effects have now been determined for the formation of beta- and alpha-manno- and glucopyranosides using a natural abundance NMR method. In contrast to the common current assumption, for three of the four cases studied the experimental and computed values are indicative of associative displacement of the intermediate covalent glycosyl trifluoromethanesulfonates. For the formation of the alpha-mannopyranosides, the experimentally determined KIE differs significantly from that computed for an associative displacement, which is strongly suggestive of a dissociative mechanism that approaches the intermediacy of a glycosyl oxocarbenium ion. The application of analogous experiments to other glycosylation systems should shed further light on their mechanisms and thus assist in the design of better reactions conditions with improved stereoselectivity.
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