A New Cycloaddition Process Involving Nitro Group Participation in Polynitroaromatic Chemistry

The reaction of ethyl vinyl ether (2 equiv) with 4,6-dinitrobenzofuroxan (DNBF, 1 equiv) in dichloromethane affords a mixture of two diastereomeric dihydrooxazine N-oxide adducts, 5a and 5b, in a 4:1 ratio. Further addition of the enol reagent to this mixture results in a second cycloaddition process with formation of a bis(dihydrooxazine N-oxide) product 6, which can also be obtained directly upon treatment of DNBF with excess ethyl vinyl ether. The observed condensations are accounted for in terms of inverse electron demand Diels−Alder cycloaddition processes between the enol ether dienophile and the heterodienyl moieties of DNBF, constituted first, by the 6-NO2 group conjugated to the 6,7-double bond and then by the 4-NO2 group and the 4,5-double bond of the carbocyclic ring. The configurations of the cycloadducts 5a and 5b have been determined on the basis of collected 1H NMR parameters, as well as NOE experiments. It thus appears that the configuration of the major diastereomer corresponds to the endo product while that of the minor one corresponds to the exo product of the first cycloaddition process. The results obtained emphasize a low aromatic character for the DNBF molecule.