Cyclophilic Reactions of Allene‐1,3‐dicarboxylic Esters. Part 4. Pyrazolone Formation and a Novel Indole Synthesis from the Nucleophilic Addition of 1,2‐Diphenylhydrazine to Dimethyl Allene‐1,3‐dicarboxylate

Experiments are described for the addition of 1,2‐diphenylhydrazine to dimethyl allene‐1,3‐dicarboxylate and the subsequent conversion of dimethyl 3‐(1,2‐diphenylhydrazino)pent‐2‐enedioate (3) to either the pyrazolone (4) or a mixture of the indole (5) and the pyrazolone (4).