Hydrogen Atom Transfer Experiments Provide Chemical Evidence for the Conformational Differences between C- and O-Disaccharides

Abstract

The glycopyran-6-O-yl radical promoted hydrogen atom transfer reaction (HAT) between the two pyranose units of ¿-D-Manp-(1-4)-¿-D-Glcp and ¿-D-Manp-(1-4a)-4a-carba-¿-D-Glcp disaccharides provides supporting chemical evidence for the conformational differences between O- and C-glycosyl compounds. In the O-disaccharide the 6-alkoxyl radical, generated under oxidative or reductive conditions, abstracts exclusively the hydrogen at C-5' via a completely regioselective 1,8-HAT reaction. This may be attributable to the conformational restriction of the glycosidic and aglyconic bonds due principally to steric and stereoelectronic effects. On the contrary, very little regioselectivity is observed in the homologous C-disaccharide and a mixture of compounds generated by 1,5-, 1,6-, and 1,8-HAT processes where the abstraction occurs at hydrogen atoms positioned at C-4a, C-1', and C-5', respectively, has been obtained. This study has been extended to simpler O- and C-glycosides, where the aglycon was a straight n-alkyl alcohol tether of five atoms; in general, all of the results obtained are shown to be consistent with a major conformational flexibility of the C-glycosidic bond.