Conformational Preference and Configurational Equilibria of the Methyl 3-Deoxy-3-nitro-pentopyranosides and their Nitronates

Abstract

The conformations of four methyl 3-deoxy-3-nitropentopyranosides and their sodium nitronates in aqueous solution were examined by n.m.r. spectroscopy, and the results are explained by calculation of free energies of non-bonded interactions. It is shown that protonation of glycoside nitronates is a kinetically controlled process. The β-epimerization of nitronates was investigated, and relative thermodynamic stabilities of epimers were determined and correlated with conformations. The non-bonded interaction between the nitronate grouping and an adjacent, equatorial hydroxyl group (A(1,3) effect) must be associated with a free energy in excess of 2 kcal/mol. The influence of this effect upon conformational and configurational equilibria in glycoside nitronates and upon nitromethylene acidity is emphasized. The compounds were found to differ in their behavior during early stages of the β-epimerization reaction, which was linked to differential nitromethylene acidities. Mechanisms for the reaction are discussed.