Synthesis and NMR spectral studies of some 2,6-diarylpiperidin-4-one O-benzyloximes

Variously substituted 2,6-diarylpiperidin-4-one O-benzyloximes were synthesized by the direct condensation of the corresponding 2,6-diarylpiperidin-4-ones with O-benzylhydroxylamine hydrochloride. All the synthesized compounds are characterized by IR, Mass and NMR spectral studies. NMR spectral assignments are made unambiguously by their one-dimensional (1H NMR and 13C NMR) and two-dimensional (1H–1H COSY, NOESY, HSQC and HMBC) NMR spectra. All the synthesized compounds are resulted as single isomer, i.e., exclusively E isomer (9–14). The conformational preference of 2,6-diarylpiperidin-4-one oxime ethers with and without alkyl substituents at C-3 and C-5 has also been discussed using the spectral studies. The observed chemical shifts and coupling constants suggest that compounds 8–13 adopt normal chair conformation with equatorial orientation of all the substituents while compound 14 contributes significant boat conformation along with the predominant chair conformation in solution. The effect of oximination on ring carbons, their associated protons, alkyl substituents and ipso carbons are studied. Every proton in the piperidone ring of the oxime ether is observed as distinct signal due to oximination. The order of chemical shift magnitude in compound 8 is H-2a > H-6a > H-5e > H-3e > H-3a > H-5a. For 9–12, the order is H-6a > H-5e > H-2a > H-3a > H-5a, for 13, H-6a > H-2a > H-5e > H-3a > H-5a and for 14, the order is H-2a > H-6a > H-5e > H-3a > H-5a while the 13C chemical shift magnitude for 8–14 due to oximination is C-2 > C-6 > C-3 > C-5.