(untitled) - Walton, J C; Studer, A (Acc. Chem. Res., 2005)
As a referee has pointed out, the aromatization process leading to byproduct 13 is most likely the result of a radical-chain process whereby an initiating radical abstracts the doubly allylic hydrogen from cycloadduct 12, and the ensuing cyclohexadienyl radical then undergoes aromatization by elimination of a methyl radical. The latter, in turn, abstracts the doubly allylic hydrogen of another molecule of 12, thereby propagating the chain; see: