CHIROPTICAL PROPERTIES OF THE CISOID ENONE CHROMOPHORE

The circular dichroism (CD) and circular dichroism of anisotropic samp les (ACD) of cisoid enones 3-methyl-3 alpha,5 -3,3',4',5'-tetrahydrobe nzo[2,3]cholest-2-en-1-one (4) and its 3 beta-diastereoisomer 7 have b een investigated. The relation between structures, obtained from energ y minimization and from X-ray analysis, and the signs of the Cotton ef fects (CEs) is discussed in terms of previously published rules.(1,2) A modification of recently published rules connecting signs of the n p i and band II CEs with enone chirality is postulated. The ACD results allow an interpretation of the breakdown of the helicity rule for the n pi band of 7 as a consequence of a different variation of the diff erent coordinates Delta epsilon(ii) in Delta epsilon = (Delta epsilon (11) + Delta epsilon(22)* + Delta epsilon(33)*)/3 by vibronic couplin g in comparison to the coordinates Delta epsilon(ii) of 4. This chang e of the size of the Delta epsilon(ii) of 7 originates from a differe nt perturbation of the enone chromophore by the cyclohexene A' ring wh ich causes a non-symmetrically deformed A ring system in 7 (non-symmet rical boat conformation) compared with the slightly distorted symmetri cal chair conformation in 4.