Conformation and Circular Dichroism of Several N-Acyl-l-prolines

Abstract NMR and CD studies were carried out for acetyl-l-proline, isovaleroyl-l-proline, isobutyroyl-l-proline, pivaloyl-l-proline and benzoyl-l-proline. It was found that N-acetyl-l-proline has two rotational isomers about the amide bond, S-trans and S-cis conformers even at room temperature, and that the population of the former is greater than that of the latter except for pivaloyl-l-proline, which consists only of the S-trans conformer. The two isomeric N-acyl-l-prolines showed different CD peaks of opposite sign associated with the n→π* transition of the amide group.