Electrophilic transannular cyclization of octadehydrodibenzo[12]annulene reexamined: indication of the formation of both anti- and syn-indenofluorenes.

The reaction of tetrabutoxyoctadehydrodibenzo[12]annulene 2f with iodine under aerobic conditions was reexamined. Contrary to previous reports, the present results revealed the formation of both anti-diiodoindenofluorenedione and its syn isomer through the oxidation of the respective tetraiodoindenofluorenes, indicating the occurrence of two modes of iodine-induced transannular cyclization. This was supported by the reaction of 2f with bromine, which gave anti- and syn-hexabromodihydroindenofluorenes through interception of indenofluorene intermediates by bromine. The hexabromides were transformed into the corresponding dibromodiones by hydrolysis.