An X-ray and conformational study of 5,7-dihydro-1,11-dimethyl-6H-dibenzo[a,c]cyclohepten-6-one and its methyl derivatives

The crystal structures for the title compound 1 and its α-methyl derivatives 2–4 have been determined. Molecular mechanics calculations of the conformation of minimum energy for 1, 3, and 4 correspond closely to the symmetric structures observed in the crystal. For 2, rotation at the CO—CH2 bond by 24° produced an asymmetric conformation matching that seen in the solid state. The calculated changes in energy as a function of the torsional angle for 1 and 2 were compared with those for cyclohexanone and the axial conformer of the α-methyl derivative. The energy requirements for conformational change in the seven-membered ring were significantly greater than for the cyclohexane analogue. The calculated energy differences between diastereomeric methyl derivatives 2–4, 6, and 7 were in agreement with epimerization experiments. The distance of closest approach observed between a proton of the axial methyls of 2 and 3 and the distal benzene ring is smaller than expected on the basis of van der Waals' radii.