Transformation of 4‐(1‐dimethylaminoethylidene)‐2‐phenyl‐5(4H)‐oxazolone into methyl 2‐benzoylamino‐3‐oxobutanoate. The synthesis of 1‐substituted 4‐benzoylamino‐3‐methyl‐5(2H)‐pyrazolones

Methyl 2‐benzoylamino‐3‐oxobutanoate (3) was prepared from hippuric acid (1) which was converted with N,N‐dimethylacetamide and phosphorus oxychloride into 4‐(1‐dimethylaminoethylidene)‐2‐phenyl‐5(4H)‐oxazolone (2) followed by hydrolysis with hydrochloric acid in methanol. Compound 3 was treated with hydrazines 4 to give 4‐benzoylamino‐3‐methyl‐1H‐pyrazol‐5(2H)‐one (6a) and its 1‐substituted derivatives 6b‐j. The corresponding hydrazones 5f, i, j were isolated as intermediates.