Synthesis of enantiomerically pure oxa[9]helicene derivatives via 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 via a nucleophilic cyclodehydration reaction of enantiomerically pure helical polycondensed-2,2'-diphenoquinone derivatives (1,1'-bibenzo[c]phenanthrenylidene-2,2'-dione). (P)-2,2'-diphenoquinone derivatives afforded (P)-oxa[9]helicenes, while (M)-2,2'-diphenoquinone derivatives afforded their corresponding (M)-oxa[9]helicenes. Thus, the ring-closing reaction afforded the corresponding enantiomerically pure products without decreasing the enantiomeric excess and proceeded stereospecifically while retention the configuration. The thermal stability of oxa[9]helicene was studied by determining the decrease in the enantiomeric excess at various temperatures, and its racemization barrier was found to be 165.6 kJmol-1.