Conformational Analysis and Absolute Configuration of Axially Chiral 1-Aryl and 1,3-Bisaryl-xanthines.

The xanthine scaffold is known to be the forefather of a class of biological active molecules. Xanthine is a planar framework in which an aryl substituent linked in the 1 or 3 position is driven out of the xanthine plane because of the steric hindrance exerted by the two carbonyls. This work analyses the stereodynamics of some 1-aryl and 1,3-bisaryl-xanthines and describes the steric requirements needed to produce stable heteroaromatic atropisomers or diastereoisomers, with one or two N-Csp2 stereogenic axes. The N-C racemization barrier was found to be bigger than 25 kcal/mol. The absolute configurations of the novel atropisomers has been assigned using TD-DFT simulation of ECD spectra.