THE PHOTOCHEMISTRY OF 8-METHYL-2,3-BENZOBICYCLO[4.2.0]OCTA-2,4,7-TRIENE

8-Methyl-2,3-benzobicyclo[4.2.0]octa-2,4,7-triene (20) was prepared by Shapiro reaction from -methyl-2,3-benzobicyclo[4.2.0]octa-2,7-dien-4- one (19). Triene 20 was found to be thermally and photochemically reac tive. 6-Methylbenzocyclooctatetraene (i.e., 6-methyl COT 21) was the u nique product formed when solutions of 20 were heated at 150 degrees C for 10 h. The direct irradiation of triene 20 gave 6-methylbenzosemib ullvalene (i.e., SB 22, 46%, Phi = 0.073), l-7,8-benzotetra-cyclo[3.3. 0(2,4).0(3,6)]oct-7-ene (23, 35%, Phi = 0.062), 6-methyl COT 21 (4%, P hi = 0.008), 7-methyl COT 24 (1%, Phi = 0.002), and naphthalene (3%, P hi = 0.006). Sensitized irradiations of 20 gave SB 22 (45%, Phi = 0.02 2), 6-methyl COT 21 (2%, Phi = 0.001), and naphthalene (4%, Phi = 0.00 2). Studies with deuterium-labelled triene (20a) indicated that the se mibullvalene 22 was produced from both S-1 and T-1 excited states of 2 0 by Zimmerman di-pi-methane rearrangement.