Abstract Reactions of arenediazonium chlorides with ethyl 2-methyl-and 2-chloro-4- oxobutanoates gave, respectively, ethyl 2-(arylhydrazono) propanoates and chloro (arylhydrazono) acetates. Ethyl 2-(arylhydrazono)-propanoates reacted with the Vilsmeier- Haak reagent to give ethyl 1-aryl-4-formyl-1 H-pyrazole-3-carboxylates. Ethyl 1-aryl-4-acetyl- 5-methyl-1 H-pyrazole-3-carboxylates were obtained by reaction of chloro (arylhydrazono) ...