4-Nitrobenzofuroxans, tautomerism

The principal methods for forming the heterocyclic ring of benzofuroxans involve oxidation of o-quinone dioximes, thermolysis of o-nitroaryl azides, and oxidation of o-nitroanilines (Scheme 25). Ring chain tautomerism (Section 4.05.5.2.1) for the A -oxides of asymmetrically substituted benzofuroxans is more facile than for monocyclic analogues and mixtures of isomers may result. Benzofuroxans are also formed by Boulton-Katritzky rearrangement of 7-nitro-2,l-benzisoxazoles and 4(7)-nitrobenzofuroxans (Section 4.05.5.2.5). 

The tautomerism of nitrated 5(6)-fruorobenzofuroxans has been studied by Charushin and colleagues using dynamic H, 13C, 19F NMR spectroscopy [758], It has been discovered that 4-nitro-5-hydroxy-6-fluorobenzofuroxan, on dissolving in DMSO, transforms to 4-hydroxy-5-fluoro-7-nitrobenzofuroxan in a result of the Boulton-Katritzky rearrangement [758],... 

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