Accessing optically pure 5,13-Dimethylaza[7]helicene via selective removal of α-methylbenzylamine chiral auxiliary by catalytic hydrogenation reaction

The synthesis of 5,13-dimethyl-9-aza[7]helicene is reported. A new method to access optically pure helicene with methyl substituent is established. The diastereomeric helical diamine is prepared from 5,13-diformyl-9-aza[7]helicene by attaching a chiral auxiliary and its isomers were physically separated by column chromatography. This diastereomeric diamine with two different benzyl bonds were subjected to Pd–C catalyzed hydrogenation at ambient condition, resulting in the hydrogenolysis at the less substituted site. The enantiomers of 5,13-dimethyl-9-aza[7]helicene were accessed, and their chiroptical properties were investigated.