Ring reversal of cis-cyclohexane-1,2,3,4,5,6-hexacarboxylic acid and its hexamethyl ester

Abstract

cis-Cyclohexane-1,2,3,4,5,6-hexacarboxylic acid and its hexamethyl ester have been synthesized and shown, by n.m.r., to exist in solution at room temperature as an equilibrium of slowly exchanging chair conformations. From complete line shape analysis of the ¹³C n.m.r. spectra measured at different temperatures the activation parameters have been determined. A relatively high value (ca. 17 kcal mol^–1) of free energy of activation has been found for both molecules, by analogy with the hexamethylcyclohexane case. The energy barrier of the acid has been calculated with the method of molecular mechanics and the computer program MOLBD3: the value obtained (16 kcal mol^–1) is a slight overestimate by comparison with the observed value of 13–14.5 kcal mol^–1.