Pyrolysis of 2-methylene-3,4,5,6-dibenzo-3′,4′-(9,10-phenanthro)spirobicyclohexane to yield tetrabenzanthracene and ethylene

The pyrolysis of 2-methylene-3,4,5,6-dibenzo-3′,4′-(9,10-phenanthro)spirobicyclohexane (3) does not lead to an isomer, as suggested in the literature, but yields tetrabenz(a,c,h,j) anthracene (4). Concerted and two-step reactions, the latter involving an ene rearrangement, are considered for the loss of the two-carbon fragment from 3.