| Abstract
| - In investigating the reactivities of aza analogues of super-electrophile 4,6-dinitrobenzofuroxan(DNBF, 1), we have found that the nitro-substituted pyridofuroxan 2 gives a remarkably stablehydrate 3 in aqueous solution (as evidenced by the requirement of ca. 50% H2SO4 (H0 ∼ −3) forcomplete recovery of 2). The equilibrium constant KH2O for hydration of 2 is estimated to be ≥ 100,being comparable only with the KH2O values reported for hydration of highly activated neutralpolyazaaromatics such as 2- and 6-hydroxypteridines or 7-azapteridine. Interestingly, the NH groupof 3 undergoes ionization at rather low pH (p= 5.79), affording an anionic hydroxy σ-adduct4 which is thermodynamically 108 times more stable than the related σ-adduct of pteridine. Theexperimental evidence is that 4 is slightly more stable than the hydroxy σ-adduct of DNBF,indicating not only that 2 ranks among the most electrophilic heteroaromatics known to date butalso than an aza group may in fact be as efficient as a nitro group in promoting σ-complex formation.2 is also found to be a versatile Diels−Alder reagent, as a result of the low aromatic character ofits six-membered ring. Upon treatment of 2 with cyclopentadiene and 2,3-dimethylbutadiene, variousreactivity patterns have been observed. These led to different cycloadducts arising from normal aswell as inverse electron-demand condensations involving the pyridine ring as the dienophile or theheterodiene contributor. Altogether, the results reveal major differences between the reactivity of2 and that of DNBF, with in particular a remarkable tendency of the pyridofuroxan adducts toundergo covalent hydration, resulting in the formation of stable carbinolamines. Also noteworthyis the characterization of a diadduct which results from a Diels−Alder trapping of the o-dinitrosointermediate involved in the exchange of the 1-oxide and 3-oxide tautomers of 2.
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