4- Unsubstituted 2,4-dihydropyrazol-3-ones

Diazopyrazol-3-one salts have been prepared in situ and kept for a short period at low temperature by either diazotization of 4-aminopyrazol-3-ones or by diazo group transfer onto 4-unsubstituted 2,4-dihydropyrazol-3-ones from 4-toluenesulfo-nylazide or aziridinium salts. Coupling these diazonium salts with carbon nucleophiles is the most common reaction (04AHC(87)141). 

Under appropriate conditions 1,2-unsubstituted 1,2-dihydro- and 2,4-unsubstituted 2,4-dihydropyrazol-3-ones take part in nucleophilic substitutions at N2. In 2,4-dihydropyrazol-3-ones substituted at N1 nucleophilic substitution occurs at C4 as... 

From unsubstituted or 4,4-dimethylpyrazolidin-3-ones. Pyrazoli-din-3-ones 335a,b were oxidized to 4,5-dihydropyrazol-3-ones 336a,b with lead(IV) acetate at low temperature 336a isomerizes readily to... 

Bromination of 4-unsubstituted l,2-dihydropyrazol-3-ones is more rare. The reaction takes place with bromine or NBS and ALBN in chloroform. Thus pyrazol-3-ones 158a,b were treated with bromine to afford 4-bromopyrazol-3-ones 159a,b in 76 and 77% yield, respectively (89AP351). On the other hand treatment of pyrazol-3-one 158c with NBS and AIBN (a,a-azoisobutyronitrile) gave 4-bromopyrazol-3-one 159c in 73% yield (94SC2133) (Scheme 51). 

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