Synthesis and structures of helical polycyclic aromatic hydrocarbons bearing aryl substituents at the most sterically hindered positions

A three-step synthetic sequence starting from condensation between a benzannulated enediyne and an aryl tert-butyl ketone was established to provide easy access to angularly fused polycyclic aromatic hydrocarbons bearing one or two aryl substituents at the most sterically hindered positions to cause helical twists. The dynamic behaviors involving the helix inversion and the restricted rotation of the aryl substituents were investigated by temperature-dependent NMR studies. The X-ray structure of an indeno-fused 1-phenylpentahelicene derivative showed severe distortion of the [5]helicene system from planarity.