Transformations of phenylhydrazones of dialkyl 2‐oxo‐propane‐1,3‐dicarboxylate and of ethyl acetoacetate in concentrated sulfuric acid

The hydrazones 3a,b , prepared from phenylhydrazine ( 1 ) and dialkyl 2‐oxopropane‐1,3‐dicarboxylate ( 2a,b ) were converted in concentrated sulfuric acid at −5 °C into a mixture of alkyl (3‐carboxyindol‐2‐yl)acetates ( 5a,b ), and ethyl (5‐ethoxy‐1‐phenyl‐1 H ‐pyrazol‐3‐yl)acetate 6 . The hydrazone 8 , prepared from 1 and ethyl acetoacetate ( 7 ) was transformed under the same conditions into a mixture of five compounds: ethyl 2‐methylindol‐3‐carboxylate ( 9 ), 2‐methylindol‐3‐carboxylic acid ( 10 ), 2‐methylindol ( 11 ), 5‐ethoxy‐3‐methyl‐1‐phenyl‐1 H ‐pyrazole ( 12 ), and 3‐methyl‐1‐phenyl‐1 H ‐pyrazol‐5‐one ( 13 ).