Evaluation of the equilibrium and activation parameters for the interconversion of the conformational isomers of some N-acylprolines by nuclear magnetic resonance spectroscopy

Abstract

Study of the ¹H n.m.r. spectra of six N-acyl-L-prolines (I)–(VI) over a range of temperatures has enabled the equilibrium and activation parameters for the interconversion of the cis- and trans-conformers to be evaluated. The energy barrier for the cis–trans-interconversion in (I), involving a urethane bond, is much lower than that for the other compounds, involving peptide bonds; the significance of this difference for the well known resistance of urethanes to racemisation in peptide synthesis is discussed.