Stereodynamics of diheteroarylpiperidines and their derivatives

The stereodynamics of a number of N-nitroso-cis-r-2,c-6-di(2-heteroaryl)- a nd N-nitroso-cis-r-2,c-6-diarylpiperidines (12-17) have been studied. The c onformational preferences have been correlated with those of N-nitroso-ris- r-2,c-6-dimethylpiperidine (1) and N-nitroso-cis-r-2,c-6-diphenylpiperidine (2a-f, 3a-c). Competition between A(1,3)-strain, torsional strain and 1,3- diaxial interaction appears to show varied influence on the ring geometry o f all these N-nitrosopiperidines. Solution state conformational studies by NMR methods on N-nitroso compounds 12-17 reveal the preference of twist-boa t conformation with a flattening at nitrogen end of the ring due to the cop lanar nitroso group. The aryl substituents at C2 and CG carbons are found t o prefer pseudo-equatorial and pseudo-axial orientations, respectively. The semiempirical molecular orbital calculations, by using AMI method, suggest the preference for twist-boat forms for 12-17. The X-ray diffraction studi es on these N-nitrosopiperidin-4-ones also confirm the preference for the n on-chair conformations.