Absolute configuration of cholesteryl chloride and cholesteryl bromide

Abstract

We report a direct X-ray determination of the absolute configuration of two cholesterogens, cholesteryl chloride and the isomorphous cholesteryl bromide. The crystal and molecular structures of these two compounds (without regard to the absolute configuration) have already been analyzed and described elsewhere. In the present study the anomalous scattering of CrK, radiation by the halogen atoms was used and the intensities of the Bijvoet pairs of reflectiqns were estimated by the photographic method. By comparison with the calculated values of the Bijvoet differences, the absolute configurations of the seven asymmetric carbon atoms in the molecules were found to be 3S, 8S, 9S, 10R, 13R, 14s and 17R, employing the standard notation of Cahn, Ingold and Prelog. In addition, circular dichroism curves for both the compounds (in solution) were obtained and the observed negative Cotton effect was found to be in conformity with $he results of the X-ray analysis. Optical rotation measurements in the liquid crystalline phase were also carried out and it was established that both compounds form a right-handed helix. The study provides, probably for the first time, an unambiguous relationship between the absolute configuration of the molecule and the sign of the helix in the cholesteric phase.