The photochemistry of the trans-1,3,5-hexatriene-p-benzoquinone Diels–Alder adduct has been investigated. Irradiation of this material in its long wavelength n → π* absorption band leads to a single photoproduct in high yield. A single crystal X-ray structure determination shows this product to be tetracyclo[6.4.0.02,11.04,9]dodec-6-ene-3,10-dione, the result of a novel regiospecific intramolecular 2 + 2 photoaddition between the side chain vinyl group and the enedione double bond. Crystals of tetracyclo[6.4.0.02,11.04,9]dodec-6-ene-3,10-dione are monoclinic, a = 7.5290(7), b = 19.072(1), c = 6.7237(4) Å, β = 107.918(6) °, Z = 4, space group P21/n. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to a final R of 0.041 for 1603 reflections with I ≥ 3σ(I). The structure consists of well-separated molecular units. Bond distances are: C=O, 1.216 and 1.219(2), C=C, 1.318(2), mean C(sp3)—C(sp2), 1.508, mean C(sp3)—C(sp3), 1.560 in the four-membered ring and 1.544 elsewhere, mean C(sp3)—H, 0.99, and mean C(sp2)—H, 0.95 Å. Bond distances between nonhy drogen atoms have been corrected for thermal motion. A possible rationalization for the observed photoprocess is discussed in terms of conformational and radical stability effects.
