Cope Rearrangement versus a Novel Tandem Retro-Diels−Alder−Diels−Alder Reaction with Role Reversal^†

A reinvestigation of the thermolysis of 4,4-dibromotetracyclo[6.2.1.0^2,7.0^3,5]undec-9-ene (<b>2</b>) affords diene <b>8</b> with a completely rearranged hydrocarbon skeleton via the isolable intermediate <b>4</b>, along with cyclopentadiene and bromobenzene. DFT calculations show that the novel tandem retro-Diels−Alder−Diels−Alder reaction <i>with role reversal</i> is slightly less favored than the overall single-step Cope rearrangement.