5-Methyl-4-unsubstituted pyrazol-3-ones

Many reagents are able to chlorinate aromatic pyrazole derivatives chlorine-water, chlorine in carbon tetrachloride, hypochlorous acid, chlorine in acetic acid (one of the best experimental procedures), hydrochloric acid and hydrogen peroxide in acetic acid, sulfuryl chloride (another useful procedure), etc. iV-Unsubstituted pyrazoles are often used as silver salts. When methyl groups are present they are sometimes chlorinated yielding CCI3 groups. Formation of dimers and trimers (308 R = C1) has also been observed. 

Unsubstituted hydrazine reacts with 4-dialkylaminobut-3-yn-2-ones in dry THF and methylene chloride (50°C, 3 h) to form 3(5)-dialkylamino-5(3)-methyl-pyrazoles (364) in 56-63% yield (87ZOR1635). 

Alkylation of pyrazol-3-ones usually occurs not only on side-chain substituents such as primary amino groups but also on the nitrogen atom of the unsubstituted lactam group. Alkylation can also occur on a stabilized carbanion generated from a methyl group by a strong base. 

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