The Diels–Alder reactivity of nitrobenzofuroxans: mono- and di-adducts of isoprene and 2,3-dimethylbutadiene. New convenient precursors to naphtho- and phenanthreno-furoxanic and -furazanic structures

The reactions of a series of differently substituted nitrobenzofuroxans with isoprene and 2,3-dimethylbutadiene have been investigated. A variety of mono- and di-adducts resulting from normal electron demand Diels–Alder condensations involving the activated C6C7 and/or C4C5 double bonds of the carbocyclic ring as the dienophile contributors have been identified and structurally characterized. The regioselectivity of the reactions is found to be strongly dependent on the substitution pattern of this ring. In the 4-nitro-6-X-series, the diene molecule first adds to the C6C7 double bond if X is a strong electron-withdrawing substituent (X=NO2, SO2CF3) but to the nitroactivated C4C5 double bond if X is a moderately activating substituent (X=CN, CF3). Subsequent addition of a second molecule of diene occurs to give highly stereoselective diadducts in the 6-cyano, 6-trifluoromethyl and 6-nitro systems. Contrasting with this behavior, only monoadducts corresponding to the addition of diene to the nitroactivated C6C7 double bond were obtained in the 4-X-6-nitro-series (X=CN, CF3). 4,6-Dinitrotetrazolo[1,5-a]pyridine reacts similarly to 4,6-dinitrobenzofuroxan, i.e. highly stereoselective diadducts are formed on the reaction with isoprene and 2,3-dimethylbutadiene. A most significant finding is that the treatment of some of the isolated mono- and di-adducts by a strong base like t-BuOK results in a facile β-elimination of nitrous acid. Concomitant oxidative rearomatization of the resulting cyclohexadiene moieties then occurs spontaneously to afford otherwise difficultly available naphtho- or phenanthreno furoxanic or furazanic structures as well as azaphenanthrenotetrazoles.