Two-dimensional NMR spectral studies of some 2,6-diarylpiperidin-4-ones

NOESY spectra have been recorded for t(3)-methyl-r(2), c (6)-diphenylpiperidin-4-one 1, its oxime 2, t(3)-isopropyl-r(2), c(6)-diphenylpiperidin-4-one 3, r(2), c(6)-bis(p-methoxyphenyl)piperidin-4-one 4 and r(2),c(6)-bis(o-methoxyphenyl)piperidin-4-one 5. For 5 C-13-H-1 HETCOR spectrum also has been recorded. In addition, proton-proton coupling constants and proton chemical shifts have been determined more precisely for 5 by recording its 1H NMR spectrum at 400 MHz. The C-13 chemical shifts of 5 are also reported. The NOESY spectra suggest that there could be significant nOe between 1,3-diaxial protons in the chair conformation. It is confirmed that in these compounds the aromatic protons ortho to the piperidine ring absorb at a lower field relative to the other aromatic protons. Furthermore, these ortho protons have significant nOes with the adjacent axial protons, but only weak nOes with the adjacent equatorial protons. For 5, in addition to conformation B with the o-methoxyl group syn to the benzylic hydrogen, conformation C with the o-methoxyl group anti to the benzylic hydrogen also should contribute significantly. The methoxyl group shields the ortho protons by magnetic anisotropic effect.