Chiroptical studies. Part LXXXIX. An empirical analysis of the circular dichroism of hexahydroindanones, bicyclo[4.2.0]octan-7-ones, and their polycyclic analogues

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Abstract
An empirical analysis of c.d. data for five-membered ring ketones of the hexahydroindanone type, including androstan-15-ones, -16-ones, and -17-ones and similar compounds, has established the main features of the relationships between Cotton effects and structural and conformational types in this series. The analysis has been extended to a few compounds of steroid-like structure which include a cyclobutanone ring (bicyclo[4.2.0]octan-7-ones). Five stereochemical types of hexahydroindanone are recognised, three for hexahydroindan-1-ones and two for hexahydroindan-2-ones.In the first stage of the analysis, estimates of c.d. contributions (δΔε) are obtained from experimental data for the following units: (a) the bicyclic systems, (b) angular methyl groups, and (c)‘third rings’. Estimates of the contribution of the ‘second’(cyclohexane) ring to the total value of Δε for each of the bicyclic systems are then obtained by regarding the effects of individual bonds of the ‘second’ ring as equivalent to those of C–Me bonds at ‘α’ or ‘β’ positions of the cyclopentanone ring. The δΔε values obtained in this way for ‘second’ rings are used to derive δΔε values for the chiral cyclopentanone rings themselves. The values confirm earlier suggestions that the sign and magnitude of the Cotton effect is controlled largely by the chirality of the cyclopentanone ring itself, although other major contributions may come from any ‘α’-quasi-axial substituents present at the ring junction. Effects of alkyl substituents elsewhere on the cyclopentanone ring are discussed briefly.

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