Condensations of 4: 4'-diethoxycarbonylhydrazobenzene with alkylmalonyl chlorides give 4- alkyl-3: 5-dioxo-1: 2-di-pethoxycarbonylphenylpyrazolidines which on hydrolysis yield the corresponding carboxylic acids. Oxidation of methyl 4-aminosalicylate by sodium perborate