Synthesis of Enantiomerically Pure Oxa[9]helicene Derivatives by a Nucleophilic Cyclodehydration Reaction of Helical 1,1′-Bibenzo[c]phenanthrenylidene-2,2′-dione

Enantiomerically pure 9-substituted 11-oxa[9]helicene derivatives have been synthesized through furan-ring formation by a nucleophilic cyclodehydration reaction of enantiomerically pure helical polycondensed 2,2'-diphenoquinone derivatives (1,1'-bibenzo[c]phenanthrenylidene-2,2'-diones). (P)-2,2'-diphenoquinone derivatives afforded (P)-oxa[9]helicenes, whereas (M)-2,2'-diphenoquinone derivatives afforded the corresponding (M)-oxa[9]helicenes. Therefore, the ring-closing reaction afforded the corresponding enantiomerically pure products without decreasing the enantiomeric excess, and it proceeded stereospecifically with retention of the configuration. The thermal stability of an oxa[9]helicene was studied by determining the decrease in its enantiomeric excess at various temperatures, and its racemization barrier was found to be 165.6 kJ/mol.