CRYSTAL-STRUCTURE AND PHOTOCHEMISTRY OF DIMETHYL ETRAMETHYL-1,4-ETHENONAPHTHALENE-2,3-DICARBOXYLATE

The photochemical rearrangement of a bridgehead-methylated 1,4-ethenon aphthalene has been studied by crystal structure analyses of (1b) and two of its photoproducts (2b and 3b). Crystal data: T = 295 K, Cu K al pha, lambda = 1.54178 Angstrom, C20H22O4, M(r) = 326.39. Dimethyl etra methyl-1,4-ethenonaphthalene-2,3-dicarboxylate (1b), P2(1)c, a = 12.64 3 (1), b = 9.288 (1), c = 15.153 (1) Angstrom, beta = 105.70 (1)degree s, Z = 4, R = 0.037 for 1974 reflections with I > 3 sigma(I). Dimethyl nzo[a]cyclopropa[c,d]pentalene-6b,6c-dicarboxylate (2b), Pca2(1), a = 7.325 (1), b = 14.781 (1), c = 15.535 (1) Angstrom, Z = 4, R = 0.045 for 1419 reflections. Dimethyl ramethylbenzo-cyclooctatetraene-5,10-di carboxylate (3b), Pbca, a = 17.716 (2), b = 15.298 (2), c = 13.210 (3) Angstrom, Z = 8, R = 0.046 for 2265 reflections. Molecule (1b) exhibi ts steric strain, as a result of short intramolecular Me...Me contacts [2.963 (5) and 3.013 (5) Angstrom]. The formation of photoproducts (2 b) and (3b) can be rationalized in terms of mechanisms involving benzo -vinyl bonding, which relieves the steric crowding between methyl grou ps.