Synthesis and Structures of Diindeno-Fused 1,12-Diphenylbenzo[c]phenanthrene and 1,14-Diphenyl[5]helicene Bearing Severe Helical Twists

Diindeno-fused 1,12-diphenylbenzo[c]phenanthrene 15 and 1,14-diphenyl[5]helicene 20 were prepared in a four-step synthetic sequence from 2,7-naphthalenedicarboxylic acid and 3,6-phenanthrenedicarboxylic acid, respectively. The X-ray structures of 15 and 20 show that the fused ring systems possess severe helical twists. The rotational barriers of the phenyl substituents were determined by the analysis of temperature-dependent ¹H NMR spectra.