AN X-RAY AND CONFORMATIONAL STUDY OF DRO-1,11-DIMETHYL-6H-DIBENZO[A,C]CYCLOHEPTEN-6-ONE AND ITS METHYL-DERIVATIVES

The crystal structures for the title compound 1 and its alpha-methyl d erivatives 2-4 have been determined. Molecular mechanics calculations of the conformation of minimum energy for 1, 3, and 4 correspond close ly to the symmetric structures observed in the crystal. For 2, rotatio n at the CO-CH2 bond by 24-degrees produced an asymmetric conformation matching that seen in the solid state. The calculated changes in ener gy as a function of the torsional angle for 1 and 2 were compared with those for cyclohexanone and the axial conformer of the alpha-methyl d erivative. The energy requirements for conformational change in the se ven-membered ring were significantly greater than for the cyclohexane analogue. The calculated energy differences between diastereomeric met hyl derivatives 2-4, 6, and 7 were in agreement with epimerization exp eriments. The distance of closest approach observed between a proton o f the axial methyls of 2 and 3 and the distal benzene ring is smaller than expected on the basis of van der Waals' radii.