Rotational isomerism about the C alpha-CO bond in proline derivatives. 1H and 13C N.m.r. studies of benzyloxycarbonyl-Pro-N-methylamide and pivaloyl-pro-N-methylamide

Abstract

The 270 MHz 1H n.m.r. spectrum of benzyloxycarbonyl-Pro-N-methylamide in CDCl3 is exchange broadened at 293 degrees K. Spectral lines due to two species are frozen out at 253 degrees K and a dynamically averaged spectrum is obtained at 323 degrees K. A selective broadening of the C beta and C gamma resonances in the 13C n.m.r. spectrum is observed at 253 degrees K, with a splitting of the C beta and C gamma resonances into a pair of lines of unequal intensity. A similar broadening of C beta and C gamma peaks is also detected in pivaloyl-Pro-N-methylamide where cis-trans interconversion about the imide bond is precluded by the bulky t-butyl group. The rate process is thus attributed to rotation about the C alpha-CO bond (psi) and a barrier (delta G#) of 14 kcal mol-1 is estimated. 13C n.m.r. data for pivaloyl-Pro-N-methylamide in a number of solvents is presented and the differences in the C beta and C gamma chemical shifts are interpreted in terms of rotational isomerism about the C alpha-CO bond.