Circular dichroism and stereochemistry of chiral conjugated cyclohexenones

An empirical analysis of CD spectra of variety of cyclohexenones, including variable temperature and solvent studies, reveals the presence of up to three Cotton effects in the 260-185 nm region, in addition to the well- recognized n-π^* Cotton effect of 300–350 nm. The long-wavelength band I and band II Cotton effects are ascribed respectively to the first (allowed) and second (formally forbidden) π-π^* transitions. The third (band III) Cotton effect at below 200 nm is of different nature and is attributed to the n-σ^* transition in the CO group.It is shown that CD spectra of cyclohexenones not only are influenced by the conformation of the chromophore but also reflect the presence of allylic axial substituents (n-π^* and band I), the absolute configuration of the cyclohexenone ring (band II), and the presence of axial alkyl groups in α′ or β′ positions (band III).