| Abstract
| - Treatment of 2‘-carbonyl-α-C-glycopyranosides of gluco, galacto, manno, 2-deoxy, and 2-azido sugarswith 4% NaOMe resulted in anomeric epimerization to give their respective β-anomers in good toexcellent yields. The epimerization of the 2‘-aldehyde of α-C-galactopyranoside (10) in deuteriummethanol, which afforded the β-anomer with exclusive deuterium replacements at the 1‘-position,excluded the possibility of the exo-glycal as being involved as an intermediate. When 2‘-aldehyde(36) and 2‘-ketone (41) of 2,3-di-O-benzyl-α/β-l-C-arabinofuranoside were used as substrates wewere able to obtain the respective equatorial α-C-arabinopyranosides (37 and 42). These observationsconfirmed that the epimerization involves an acyclic α,β-unsaturated aldehyde or ketone, which isformed by the enolation of 2‘-carbonyl-α-C-glycoside with subsequent β-elimination. Thereafter anintramolecular hetero-Michael cycloaddition occurs, leading to the formation of thermodynamicallycontrolled stable products, which were exclusively the equatorial C-glycopyranosides, except inthe case of 2‘-carbonyl-C-furanosides, where a mixture of two anomers was obtained.
|
