ROTATIONAL-ISOMERISM ABOUT THE C-ALPHA-CO BOND IN PROLINE DERIVATIVES - H-1 AND C-13 NMR-STUDIES OF "BENZYLOXYCARBONYL-PRO-N-METHYLAMIDE AND PIVALOYL-PRO-N-METHYLAMIDE

Abstract

The 270 MHz 1H NMR spectrum of benzyloxycarbonyl-Pro-N-methylamide in CDCl3 is exchange broadened at 293.degree. K. Spectral lines due to 2 species are frozen out at 253.degree. K and a dynamically averaged spectrum is obtained at 323.degree. K. A selective broadening of the C.beta. and C.gamma. resonances in the 13C NMR spectrum is observed at 253.degree. 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 detected in pivaloyl-Pro-N-methylamide where cis-trans interconversion about the imide bond is precluded by the bulky tert-butyl group. The rate process is attributed to rotation about the C.alpha.-CO bond (.psi.) and a barrier (.DELTA.G#) of 14 kcal mol-1 is estimated. 13C NMR 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.