CONFORMATIONS OF ALKYLPIPERIDINE AMIDES

The nuclear magnetic resonance signals of the C-2 and C-6 protons of a series of alkylpiperidine benzamides coalesce at a temperature lower than that for the same protons in similar alkylpiperidine acetamides. This leads to the conclusion that the energy barrier to rotation about the C-N amide bonds is lower in the benzamides than in the acetamides as a result of increased steric interactions between the phenyl ring and the C-2 and C-6 substituents in the planar benzamide conformation. Such steric interactions between the amide and C-2 and C-6 alkyl substituents in both acetamides and benzamides are sufficient to cause conformational bias in the piperidine ring, resulting in the preference for axial configurations for the alkyl groups. These examples are a special case of the general concept of A(1,3>strain. The piperidine-containing molecule 3-benzoyl-3-azabicyclo [3.3.1] nonane has been found to have a chair-chair conformation. The nmr spectrum of 1-benzoyl-frcms-decahydroquinoline shows no variation with temperature change, suggesting that the amide group in this molecule has no preferred conformation. © 1968, American Chemical Society. All rights reserved.