Carboxyl and aromatic chromophores: optical rotatory dispersion and circular dichroism studies

Abstract

Until recently the carbonyl chromophore has been of prime importance in the appli­cation of optical rotatory dispersion (o. r. d.) and circular dichroism (c. d.) to organic structural problems (Djerassi 1960; Crabbé 1965). The reasons were, first, the accessibility of the n → π * band of the carbonyl group at 290 nm to the first com­mercial spectropolarimeters; secondly, the availability of many carbonyl com­pounds of known stereochemistry, on which Djerassi and subsequently others worked so intensively; and, thirdly, a few years later the development of the octant rule as a theoretical background to the extensive collection of experimental data which had then been made by Djerassi (Moffitt et al . 1961). In this treatment we might say that one looks at the asymmetry of the molecule through the ‘eyes’ of the relevant chromophore; in less anthropomorphic terms, one considers the symmetry planes of the orbitals involved in the 290 nm transition as a frame of reference. It is appropriate to consider the logical order in which the octant rule was applied to carbonyl compounds of increasing flexibility. Djerassi had very wisely started with ketones of rigid conformation, trans -decalones (e. g. I) and their polycyclic ana­logues; the work then passed to more flexible compounds such as the cis -decalones (II), the monocyclic ketones (III) and then finally to open-chain ketones, including steroid side-chain ketones (e. g. IV); with these latter flexible compounds, the o. r. d. method is a valuable probe for conformational studies (Crabbé 1965, pp. 134-43). The c. d. treatment was applied initially by the Roussel-Uclaf group in Paris to similar series of ketones (Velluz, Legrand & Grosjean 1965).