PURPOSE: Title compoud used as an intermediate for indigo dyes is produced from readily available indolines by mild electrolytic oxidation without application of the alkali fusion process requiring the treatment at elevated temperatures.

CONSTITUTION: The electrolytic oxidation of 1-acylindole such as 1-acetyl-indole, 1-acetyl-4-methylindole or 1-acylindoline such as 1-acetylindoline or 1-acetyl-4- methylindoline is conducted using a solvent system mainly containing a lower aliphatic carboxylic acid such as acetic acid in the presence of a base such as triethylamine, pyridine or diazabicycloundecane in the temperature range from 10 to 30°C to produce the objective compound. The electrodes employed in the above oxidation is preferably made of platinum.