Structure, Stereodynamics and Absolute Configuration of the Atropisomers of Hindered Arylanthraquinones

Anthraquinone substituted by 2-methyl-1-naphthyl groups in positions 1,8 yields syn (meso) and anti (racemic) isomers (red and yellow colored, respectively) that interconvert with a barrier of 35.4 kcal mol−1 in solution. Their structures were identified by NOE experiments in solution and X-ray diffraction in the solids. The racemic anti form (C2 point group) entails two atropisomers that were separated by enantioselective HPLC: the absolute configuration was assigned by TD-DFT simulation of the ECD spectrum. Two atropisomers were also separated and assigned in the case of anthraquinone bearing a single 2-methyl-1-naphthyl substituent in position 1.