Chiroptical properties and optical and molecular orbital calculations for dictyopterenes A and B

Abstract

Circular dichroism spectra of the optically active cyclopropane hydrocarbons (1a) and (2), measured over a range of temperatures, showed these compounds to be configurationally identical. The c.d. spectrum was found to consist of two overlapping exciton pairs, corresponding to an equilibrium mixture of two solution conformers, for which thermodynamic parameters were calculated.

Optical calculations using the excition model were used to obtain the sign of the rotational strength for the long wavelength part of the exciton pair for different conformers. The result confirmed the absolute (R,R) configuration for (1a) obtained from oxidative degradation, and the conformational assignment made from spectral analysis showing that the trans,trans-conformer is responsible for the pair of excition bands at shorter wavelength region. Both CNDO/2 and ab initio calculations confirmed that this conformer was of the lowest energy.