Rotational equilibria in boat conformations of N-acyl-r-2,c-6-di(2-hetero- aryl)piperidin-4-ones using NMR spectra and semiempirical MO calculations

Four N-acyl-r-2,c-6-di(2-heteroaryl)piperidin-4-ones 3-6 have been prepared and their conformational preferences examined using 1 H and 13 C NMR spectral studies and semiempirical MO calculations. Solution state conformational studies on the compounds 3-6 by 1 H and 13 C NMR spectral methods indicated the preference for the boat conformation with coplanar orientation of N-acyl groups. The energy barrier for N-C rotation in 3 has been found to be 58.4 kJ mol -1 using variable temperature 1 H NMR spectral studies. The semiempirical MO calculations, using AMI method, also suggested the preference of boat conformation for the N-acyl derivatives 3-6.