A New Redox-Denitration Reaction of Aromatic Nitro Compounds.

When 4,4-dimethyl-1-(2-nitrophenyl)pyrazolidin-3-one 9 is heated in pyridine containing pyridine hydrochloride it is transformed into 4,4-dimethyl-1-phenylpyrazolidin-3,5-dione 10 in which the methylene group has been oxidised and the nitro group has disappeared; two further examples of the same reaction, of pyrazolidinones 4 and 7, are reported together with a mechanistic rationalisation of this curious reaction.