Synthesis and properties of heterohelicenes as molecular springs

Helicenes are a class of ortho-condensed polycyclic helical aromatic compounds, which have high optical rotational values and optical stability. In this report, we wish to describe the synthesis, of optically active heterohelicenes and their characteristic structures. Three approaches to the synthesis of optically active helicenes were studied: (1) diastereoselective photocyclization of 1,2-diarylethylenes, (2) kinetic resolution of racemic helicenediols 9 by lipase-catalyzed enantioselective transesterification, and (3) diastereoselective biaryl coupling and stereospecific transformation of axial chirality to helicity. The racemic and optically active helicenediols 9 crystallize with ethanol, 1, 2-dichloroethane and testosterone to form clathrates. Single crystal X-ray analyses of the clathrates show that 9 has flexibilty and the dihedral angles between terminal thiophene rings vary from 38.0degrees to 54.5degrees, indicating that helicenediol 9 acts as a molecular spring. The bridged heterohelicenes 20 and 21 were also synthesized in order to determine the relationship between helical structures and spectral properties.