Both 1H-DNMR and 13C-DNMR has been applied to determine the rotational C(sp2)-C(sp3) barrier in hydrochloride of trans-1, 3-dimethyl-2-(3,4,5-trimethoxyphenyl)-4-piperidone (3) synthesized via acid catalyzed cyclization of the precursor iminoketal 4. 2D-NMR experiments allowed the assignment of practically all the 1H- and 13C-NMR signals. Rotational barrier around the C2-Ar bond was shown to be 55.9 KJ/mol-1 at 20 °C by analysis of the variable temperature 1H-NMR spectra and a value of 55.3 KJ mol-1 at 13 °C was found using 13C-NMR. This last procedure was also applied to determine the rotational barrier of 1,3,3-trimethyl-2-(3,4,5-trimethoxyphenyl) -4-piperidone (1) hydrochloride. All the experimental barriers are discussed in terms of the influence of the piperidine ring substituents with the aid of molecular mechanical calculations. © 1986.