Conformational study of some N-acyl-2r,6c-diphenylpiperidin-4-one oximes using NMR spectra

Stereochemistry of N-benzoyl-2r,6c-diphenylpiperidin-4-one oxime (1), N-benzoyl-3t-methyl-2r,6c-diphenylpiperidin-4-one oxime (2), N-benzoyl-3t-ethyl-2r,6c-diphenylpiperidin-4-one oxime (3), N-acetyl-2r,6c-diphenylpiperidin-4-one oxime (4) and N-acetyl-3t-methyl-2r,6c-diphenylpiperidin-4-one oxime (5) has been studied using 1H, 13C and two-dimensional NMR spectra. Analysis of the spectral data shows that in 2, 3 and 5 the NOH group is anti to the alkyl group at C-3. All the five compounds seem to exist largely in boat conformations with one phenyl group in the flagpole position. Analysis on the reported spectral data on N-phenylcarbamoyl- and N-benzoyl-2r,6c-diphenylpiperidin-4-ones suggests that these compounds also largely exist in boat conformations with one phenyl group in the flagpole position. Chair conformation with axial phenyl groups may contribute to some extent. The relative populations of the boat and chair conformations have been estimated for all compounds except 5. The results on 1 and 2 are supported by the observed NOEs in their NOESY spectra. The geometry of the boat form in solution is probably in between that of a classical boat and twist-boat.