The circular dichroism and absorption spectra of alkaloids containing the aniline chromophore. The absolute configuration of calycanthine

Abstract

The circular dichroism and absorption spectra of calycanthine, carcurine-II, and of chimonanthine and related alkaloids containing two non-coplanar aniline chromophores are reported. The results are analysed by means of a model in which the excitation dipoles of the two chromophores couple to give a resultant helical charge-displacement responsible for the optical activity of the corresponding absorption band. The relative frequencies and signs of the rotational strengths calculated for one of the optical isomers of calycanthine correspond to the positions and signs of the observed circular dichroism bands, and thus establish the absolute configuration of the alkaloid. The factors limiting the application of the method to the circular dichroism spectra of caracurine-II and of the chimonanthine group are discussed.