Tautomerism in the 5-pyrazolone series 1(H)-5-pyrazolones and indazolones

Structure (I) is shown to be the correct formulation for the compound hitherto considered to be the “azine” of ethyl acetoacetate; I rearranges to IV. Both IV, 1(H)-3-methyl, and 1(H)-3-phenyl, 5-pyrazolones and related compounds are shown to exist in the enolic form (VII) in the solid state and predominantly so in the solvents in which they are soluble. Similarly the indazolones are found to be represented exclusively by the enolized structure XII. The acetates of both series are reformulated where necessary on the basis of spectroscopic data, and the high frequencies found for the carbonyl groups in the IR are shown to reflect a lack of conjugation with the π-electrons of the heterocyclic ring.