CONFORMATIONAL-ANALYSIS OF SOME HETEROCYCLIC-SYSTEMS .1. CONFORMATIONAL-ANALYSIS OF SUBSTITUTED N-PHENYL-CARBAMOYPIPERIDIN-4-ONES

The conformations of variously substituted N-phenylcarbamoylpiperidin- 4-ones have been analysed using IR, H-1 and C-13 NMR data. The spectra l data suggest the coplanar orientation of the phenylcarbamoyl moiety in all the compounds with respect to dynamically averaged plane of the piperidone ring system. The non-observation of anisochronous signals for the protons/carbons adjacent to the ring nitrogen, despite the cop lanar orientation of the phenylcarbamoyl moiety is related to the fast er rotation around the N-CO bond (i.e., lower activation energy barrie r, Delta G#) compared to the NMR time scale. While the symmetrically s ubstituted compounds viz., 6 and 10 are assumed to have a sofa conform ation, a twist boat conformation is preferred by the other unsymmetric ally substituted piperidones (7,8 and 9). In case of 8, the proposed c onformation in solution is further cooroborated by the solid state con formation derived by X-ray diffraction study. The importance of A((1,3 )) strain indicating the preferred conformations has also been describ ed.