Rotational barriers around the C2-Ar bond in hydrochlorides of 1,5,5-trimethyl-2-(3,4,5-trimethoxyphenyl)-and 1,2,2-trimethyl-6-(3,4,5-trimethoxyphenyl)-4-piperidones (4 and 5) were determined to be 46.0 kJ mol1 at −30°C and 59.4 kJ mol-1 at 40°C, respectively. Molecular mechanics (MM) calculations on phenyl analogues agree with these values. The barriers for the free bases 4 and 5 (27.7 kJ mol1 and 36.4 kJ mol-1, respectively) were obtained by MM calculations on the analogous 2-phenyl-4-piperidones. The 6-gem-dimethyl substituents in 5-HC1 promote a strong long-range effect and thus raise the C2-Ar rotational barrier, whereas 5-gem-dimethyl substitution (4-HCl) does not cause this effect. The less constrained phenyl rotation in these 2-arylpiperidones takes place on a twist-boat conformation as suggested by MM calculations. © 1990, American Chemical Society. All rights reserved.