Chiroptical Properties of the Cisoid Enone Chromophore

Abstract

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.

Chiroptical Properties of cisoid Enones from Circular Dichroism (CD) and Anisotropic Circular Dichroism (ACD) Spectroscopy

“…The resulting sign change of the values of De without a corresponding change of helicity fits in the model given earlier. [10,12] With the same assumption for the order parameter determination as for the n ± p* transition, the analysis of De A for the p ± p* transitions has been performed for De* 33 and (De* 11 De* 22 (Figures 5 and 6 and Table 3). For the asubstituted compounds 1 and 3 the values of De* 33 as well as Table 3).…”

“…[10] The ACD measurements for ketosteroids with an additional ring system confirm this conclusion. [12] As a basis for this conclusion it has been demonstrated that the ACD data allow for the experimental determination of the contributions of different vibrational progressions to the CD band. [14±17] The discussed above observations encouraged us to apply ACD spectroscopy systematically to solve stereochemical problems arising in attempts to correlate signs of CD bands and the absolute configuration of cisoid enone moieties.…”

“…Recently we have reported a study on the chiroptical properties of cisoid enones, where, in general, the positive (negative) sign of the CE associated with the n ± p* transition reflects the positive (negative) enone helicity. [12] However, one of the examples discussed there indicated that substituents located in the vicinity of the chromophoric system may change the sign of the CD band without a corresponding change of the local helicity of the chromophore. There are additional literature reports indicating that the introduction of an a'-axial bromo substituent leads to a sign change of the n ± p* CD band, for example, to a behavior contrary to the prediction based on the helicity rule.…”

Enantiospecific Synthesis and Chiroptical Properties of Bicyclic Enones

“…Recently a study on the chiroptical properties of cisoid enones showed that, in general, the positive (negative) sign of the CE associated with the nǞπ* transition reflects the positive (negative) enone helicity. [20] The sign of the torsional angles in enones 4 and 8 correlates with the long-wavelength sign of the CE in the bisignate CD curve (Figure 1). [21] The sign of the shorter-wavelength band is opposite to that of the longer-wavelength band.…”

Synthesis, Molecular Structure, and Chiroptical Properties of Dibenzylidene Derivatives of Bicyclo[3.3.1]nonane and Brexane

“…The α,β-unsaturated ketone chromophore has been extensively investigated in recent years and a number of sector and/or helicity rules have been proposed to correlate the absolute configuration of enones with the sign of the observed CEs in the range 220 to 350 nm, ascribed to π→π* and n→π* transitions. 21,22 Studies on the chiroptical properties of cisoid enones showed that, in general, the positive sign of n→π* CE reflects the positive enone helicity (positive C=CÀC=O torsion angle), whereas the second π→π* CE is of the opposite sign. By contrast, the styrene chromophore was studied in less detail.…”