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

The crystal structures for four alpha-chloro derivatives 2-5 of the ti tled ketone 1 have been determined, confirming their earlier assignmen ts of stereochemistry. Variations in the conformations of 2-5 suggeste d some flexibility in the central seven-membered ring. Molecular mecha nics calculations of the changes in energy as a function of the intern al torsional angles at the carbonyl of these chloro derivatives indica ted the ring flexibility to be comparable to that of cyclohexanone. In the three compounds containing an axial chlorine the distance between the chlorine atom and two of the carbon atoms of the more remote benz ene ring was 3.2 +/- 0.1 Angstrom, suggesting the value for the thickn ess of a benzene ring may be significantly smaller than the accepted v alue when it is interacting with a single atom.