New reactivity patterns in the Diels-Alder reactivity of nitrobenzofuroxans

The reaction of cyclopentadiene with 4-nitro-6-trifluoromethylsulfonylbenzofuroxan 5 in dichloromethane or chloroform proceeds stereoselectively at 0 degrees C to afford a single compound 6, which is shown to result from an inverse electron-demand Diels-Alder condensation involving the carbocyclic ring of 5 as the diene contributor. However, the adduct 6, whose X-ray structure could be obtained, is not the thermodynamically stable product of the interaction. Keeping a solution of 6 at room temperature results after a few days in the isolation of a new adduct 7a which arises from a regioselective and stereoselective normal electron-demand Diels-Alder condensation involving the C(4)C(5) double bond of 5 as the dienophile contributor. The carbodienic behaviour of 5 as well as the preferred dienophilic reactivity of the C(4)C(5) rather than of the C(6)C(7) double bond, represent two new reactivity patterns in the chemistry of the nitrobenzofuroxans.