Epoxidation of 1,4-dihydro-1,4-methanoanthraquinones. Photochemistry and crystal structure determination of the products

The 1,4-dihydro-1,4-methanoanthraquinones (5), (6), and (10) afforded respectively the pairs of epoxides (2) and (3), (7) and (8), and (11) and (12), on treatment with hydrogen peroxide-aqueous sodium carbonate. When irradiated with sunlight, only one isomer of each pair, viz. (3), (8), and (12) underwent rearrangement, giving the quinones (4), (9), and (13) respectively. The stereochemistry of the photolabile epoxides was obtained by a crystal structure determination for the bromo-derivative (11). This was carried out by single-crystal X-ray diffraction at 295 K and refined by full-matrix least squares to R 0.062 (669 reflections). Crystals are monoclinic, P2₁/n, a = 5.870(5), b = 18.13(1), c = 11.475(6) Å, β = 95.84(6)°, and Z = 4.