Synthesis of oxa[9]helicene derivatives by cyclodehydration of 1,1′-bibenzo[c]phenanthrene-2,2′-diol using phosphorus pentoxide

A new one-step cyclization reaction has been established for the synthesis of oxa[9]helicene derivatives, containing eight ortho-condensed benzene rings and one furan ring. 1,1′-Bibenzo[c]phenanthrene-2,2′-diols (HEBPOLs) afforded the corresponding oxa[9]helicenes by refluxing with phosphorous pentoxide (P₂O₅) in o-dichlorobenzene. Enantiomerically pure oxa[9]helicenes were also synthesized with retention of the configuration and enantiopurity (>98% ee) from the corresponding enantiomerically pure HEBPOLs.