3- Pyrazolin-5-ones sulfonation

TABLE XXXV. 2-Pyrazolin-5-ones. Sulfonic Acids ... 

Dimethyl-2-phenyl-3,4-pyrazolidinedione and its 1,5-diphenyI-2-methyl analog are the only compounds of their class known. The former has been synthesized by treatment of l-(2,3-dioxobutyryl)-l-phenyl-2-methyl-2-nitrosohydrazine hydrate with sodium bisulfite" and by methylation of 4-hydroxy-3-methyl-l-phenyl-2-pyrazolin-5-one.820 The latter compound has been prepared in the same way.1243 The dimethyl phenyl compound has also been isolated from urine as a metabolic product of 4-dimethylamino-2,3-dimethyl-l-phenyl-3-pyra-zolin-5-one617 and it is formed by treatment of the same compound with sulfuric acid.618 The 4-thiono analog is also known. Reduction of 2-3-dimethyl-l-phenyl-5-oxo-3-pyrazolin-4-sulfonic acid using zinc in acid gives l,5-dimethyl-2-phenyl-4-thiono-3-pyrazolidinone.1249... 

Sulfonation of 2- and 3-pyrazolin-5-ones occurs at position 4. Thus 3-methyl-1-phenyl-pyrazolin-5-one with 20% oleum at 10-15 °C yields the corresponding 4-sulfonic acid (304) (64HC(20)l). Higher temperatures cause sulfonation not only of the heterocycle but also of the phenyl group. Antipyrine treated with sulfuric acid and acetic anhydride yields (305). 

The equilibrium in pyrazolin-5-ones, containing one or more carboxylate or sulfonate group, is extremely sensitive to the pH of the medium. For example, pyrazolinone 105c, which exists in D2O solution at pH 2 as an enol... 

The vast majority of reactions undergone by 3-pyrazolin-5-ones occur at the 4-position. These pyrazolinones react by direct substitution, very much as do activated benzene rings. Such reactions as halogenation, nitrosation, sulfonation, acylation, the Mannich reaction and many others give 4-substituted-3-pyrazolin-5-ones. 

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