The so-called “cage dimer” pentacyclo[6,4,0,0^2,7,0^4,11,0^5·10]dodeca-3,6,9,12-tetraone obtained by irradiation of p-benzoquinone has been shown to have an “open” structure, and to be tricyclo[6,4,0,0^2,7]dodeca-4,10-diene-3,6,9,12-tetraone. This compound has been converted into biphenylene derivatives and has been reduced to the corresponding tetraol, whose tetramesylate with sodium iodide in acetone gives benzene, biphenyl, and an elusive intermediate which could be captured through the formation of Diels–Alder adducts with maleic anhydride.