Chiroptical Properties of the Cisoid Enone Chromophore

The circular dichroism (CD) and circular dichroism of anisotropic samples (ACD) of cisoid enones
3-methyl-3α,5α-3,3‘,4‘,5‘-tetrahydrobenzo[2,3]cholest-2-en-1-one (4) and its 3β-diastereoisomer 7 have been
investigated. The relation between structures, obtained from energy minimization and from X-ray analysis,
and the signs of the Cotton effects (CEs) is discussed in terms of previously published rules., A modification
of recently published rules connecting signs of the nπ* and band II CEs with enone chirality is postulated.
The ACD results allow an interpretation of the breakdown of the helicity rule for the nπ* band of 7 as a
consequence of a different variation of the different coordinates Δ

in Δε = (Δ

+ Δ

+ Δ

)/3 by
vibronic coupling in comparison to the coordinates Δ

of 4. This change of the size of the Δ

of 7
originates from a different perturbation of the enone chromophore by the cyclohexene A‘ ring which causes
a non-symmetrically deformed A ring system in 7 (non-symmetrical boat conformation) compared with the
slightly distorted symmetrical chair conformation in 4.