PHOTOOXYGENATION OF TETRAMETHOXYBENZOBARRELENE - SYNTHESIS AND THERMAL-DECOMPOSITION OF BISDIOXETANE AND ENDOPEROXIDE

Photooxygenation of tetramethoxybenzobarrelene (2) afforded endoperoxide 4, and the diastereomeric bisdioxetanes syn,syn- and syn,anti-5 in good yields. Thermolysis of the bisdioxetane syn,syn-5 and separately of syn, anti-5 resulted in the monodioxetane trans-6 as major product, whose structure was rigorously established by X-ray analysis. On heating, the monodioxetanes trans- and cis-6 gave tetraester 7 nearly quatitatively. The homo Diels-Alder adduct 4 decomposed at room temperature to the keto ether 9 as the only isolable product. Partial photoxygenation of benzobarrelene 2 gave only the monodioxetane syn-3, no anti-3 was observed. Monodioxetane syn-3 decomposed at room temperature to the dihydroaphthalene 8, the naphthalene derivative 10 and dimethyl oxalate. Photooxygenation of dihydronaphthalene 8 gave quantitatively the dioxetane trans-6. The activation parameters (Delta H-double dagger , Delta S-double dagger and Delta G(double dagger) ) and excitation yields (Phi(S) and Phi(T)) for the bisdioxetane 5 were determined by standard chemiluminescence techniques. It is concluded that the two dioxetane rings in the bisdioxetane 5 cleave thermally successively with no evidence for thermally induced intramolecular sensitization to produce upper excited state products. The mechanism of the thermal decomposition of the bisdioxetane 5 and endoperoxide 4 is discussed.