Synthesis of pyridazino[4,5‐ b ]indole derivatives from 2‐(3‐carboxy‐1‐methylindole)acetic acid anhydride

2‐(3‐Carboxy‐1‐methylindole)acetic acid anhydride ( 1 ) reacts with aryldiazonium salts to give 85–95% of the corresponding α‐hydrazono anhydrides 2 . Treating 2 with boiling hydrazine hydrate in xylene, the respective 2‐aryl‐4‐carbohydrazido‐5‐methyl‐1‐oxo‐1,2‐dihydro‐5 H /‐pyridazino[4,5‐ b ]indoles 3 were obtained (47–67%), and these compounds characterized as the respective benzylidene derivatives 4 . Compounds 2 react with amines (aniline, morpholine, piperidine) to give the respective 2‐(3‐carboxy‐1‐methylindole)aceta‐mide 5 or the respective 2‐aryl‐4‐carboxamido‐5‐methyl‐5 H ‐pyridazino[4,5‐ b ]indole 6 , the product obtained depending on the structure of the aryl substituent. Boiling 2b (aryl = 4‐chlorophenyl) with 5% sodium hydroxide gave (80%) 2‐(3‐carboxy‐1‐methylindole)acetic acid ( 7 ). Hydrolysis of 2b gave a mixture of 7 and 2‐(3‐carboxy‐1‐methylindolyl)‐2‐(4‐chlorophenylhydrazono)acetic acid ( 8 ).