The circular dichroism and absolute configuration of Tröger's base

Abstract

The circular dichroism and absorption spectra of the (+)- and (–)-isomers of Tröger's base (1,2′-methylene-3-p-tolyl-6-methyl-1,2,3,4-tetrahydroquinazoline) are reported. The spectra are analysed by means of the coupled dipole model in which the excitation moments of the two aniline chromophores interact to give the helical charge displacements responsible for the optical rotatory power of the corresponding absorption bands. The relative frequencies and signs of the rotational strengths calculated for the (1R,3R) configuration of the more stable conformation of Tröger's base correspond to the positions and the signs of the circular dichroism bands observed in the spectrum of the (+)-isomer, establishing the stereochemical configuration of the enantiomers. The empirical comparison of Cotton-effect curves is of limited validity for relating stereochemical configurations.