Conformational analysis and ¹³C NMR spectra of some 2,6‐diarylpiperidin‐4‐ones

The ¹³C NMR spectra of several methyl substituted 2,6‐diaryl‐4‐piperidones (aryl = phenyl, o‐chlorophenyl) have been measured. Substituent parameters for methyl and chloro groups have also been derived and are discussed in terms of their steric and electronic interactions. Both the methyl substitution at C‐3 and chlorine substitution in the ortho position of the phenyl ring cause significant changes in the ¹³C chemical shifts, which can be satisfactorily explained on the basis of steric interactions taking place in the preferred conformations of these molecules. In compounds with o‐chloro groups, the aryl ring is oriented such that the chlorine is syn to the axial hydrogen on the aryl‐bearing carbon. The shielding effect of the carbonyl group is found to be more pronounced on an adjacent equatorial methyl than on an adjacent axial methyl group.