Cope Rearrangement Versus a Novel Tandem Retro-Diels-Alder-Diels-Alder Reaction with Role Reversal

[reaction: see text] A reinvestigation of the thermolysis of 4,4-dibromotetracyclo[6.2.1.0(2,7).0(3,5)]undec-9-ene (2) affords diene 8 with a completely rearranged hydrocarbon skeleton via the isolable intermediate 4, along with cyclopentadiene and bromobenzene. DFT calculations show that the novel tandem retro-Diels-Alder-Diels-Alder reaction with role reversal is slightly less favored than the overall single-step Cope rearrangement.