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Synlett 1992; 1992(4): 285-290
DOI: 10.1055/s-1992-21341
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C(5)-Epimerization in Glycopyranosides During the Cyclization of Anomeric Radicals: A Comparison with Glycofuranosides

Alain De Mesmaeker* , Adrian Waldner, Pascale Hoffmann, Paul Hug, Tammo Winkler
  • *Central Research Laboratories, Ciba-Geigy Ltd., CH-4002 Basel, Switzerland
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Publication Date:
08 March 2002 (online)

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The 5-exo cyclization of anomeric radicals in glycopyranosides is accompanied by the epimerization of the C(5)-chiral center. This results from an intramolecular hydrogen atom transfer from the axial C(5)-H bond to the α-cyclized radical. The extent of this intramolecular hydrogen atom transfer, and, consequently, of the C(5)-epimerization, depends on the concentration of tributyltin hydride, the temperature, and the nature and stereochemistry of the substituents at C(4) and C(6). In contrast, no epimerization could be detected in the case of an analogous glycofuranoside.