Temperature-dependent, competitive 1,3-acyl shift versus decarbonylation of a cyclopropanone intermediate.

Photooxygenation of 1,1,3-trimethyl-1,2-dihydropentalene gives an unstable endoperoxide which upon decomposition delivers a bicyclic cyclopropanone intermediate; this species either extrudes CO to give a cycloheptadienone or undergoes a 1,3-acyl shift, both processes occurring most likely in a stepwise manner via diradical intermediates. Alternatively, C3a-C4 cleavage in the dioxygen diradical derived from the endoperoxide yields a 2-cyclopropyl substituted cyclopentadienone epoxide.