Circular Dichroism. LXXIV.CD of Benzene Derivatives with Several Identical Chiral Substituents

A comparison of the CD- and UV-spectra of benzene derivatives, which are mono-, 1,4-di- or 1,3,5-trisubstituted by (identical) chiral ligands, revealed that the rotational strength within the α-band of mono- and 1,3,5-trisubstituted benzenes is mainly derived from the magnetic transition moment of the benzene chromophore perpendicular to the ring, and from a corresponding electric transition moment stolen from a ligand transition. For the 1,4-disubstituted homologues, the main contribution to the rotational strength is associated with the electric transition moment in the plane of the benzene chromophore and with a corresponding magnetic transition moment, stolen from the A_1g → E_1g (D_6b-notation) transition within the same ring. For the 1,3,5-trisubstituted compounds, the 0–0-band is scarcely, or not at all, detectable in the UV-spectra, but distinctly recognizable in the CD-spectra. In a few cases Cotton-effects between the positions of the α- and the p-bands could be detected, which are ascribed to the magnetically allowed A_1g → E_1g (D_6b-notation) transition.     

Chiroptical Properties of Phorbol-12, 13, 20-triacetate and Some Other Phorbol Derivatives

The CD spectra of phorbol and of some of its derivatives, such as its 12, 13, 20-triacetate 1, show at least four bands. Three of them (called I, III, and IV) correspond to the R-, K-, and (perhaps) second n → π*-transition, respectively, of the enone system; the fourth band around 260 nm (band II) is of π → π*-origin and could be ascribed to an interaction of the lone pair of electrons on the oxygen atom at C(4) with the C=C-C=O system. The signs of the R-band as well as the K-band Cotton effects are very sensitive to chemical modifications around the chromophoric system in the cyclopentenones investigated. The absolute configuration of phorbol may be derived from the CD spectra of some of its derivatives, and, in particular, from the application of the “dibenzoate rule” to 12, 13-di-O-benzoyl phorbol-20-acetate (4).     

Circular Dichroism of Cardenolides

The relationship between structure and circular dichroism was studied for a series of cardenolides including some compounds with saturated lactone ring.