Nitrosation at C‐4 of 1‐n‐alkyl‐3‐methyl‐5‐pyrazolones is achieved in about 50% yield with sodium nitrite in hydrochloric acid medium. A tautomeric equilibrium in solution with a proton moving from the OH at C‐5 to N‐2 in the nitrosated pyrazolones is proposed.

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The reaction of 1‐substituted‐3‐methylpyrazol‐5‐ones 1 with alkyl chloroformates and calcium hydroxide in dioxane have been studied. With 1‐phenyl‐3‐methylpyrazol‐5‐one, the isolated product was alkyl 3‐methyl‐5‐oxo‐1‐phenylpyrazole‐4‐carboxylate 2 but with 1‐alkyl‐3‐methylpyrazol‐5‐one formation of 1‐alkyl‐5‐alkoxycarbonyloxy‐3‐methylpyrazole 3 was observed. Replacement of alkyl chloroformate by bis(alkoxythiocarbonyl) sulfide results in the formation of 4‐alkoxythiocarbonyl derivatives 4 in low yield with both 1‐substituted‐3‐methylpyrazol‐5‐ones.