Electrochemical Acetoxylation of N-Acetylindolines and iV-Acetylindoles. A New Synthesis of Indigos

Electrochemical acetoxylation of N-acetylindolines 3 in AcOH-EtsN at potentials 1.1-1.7 V vs. SCE, 4 faradays/ mol of electricity, using platinum electrodes afforded the corresponding 2,3-diacetoxyindolines 5 in 70-77% yields. Likewise, N-acetylindoles 4 gave 5 in 76-82% yields. The acetate 5 could also be prepared from indoline (2) without isolating the intermediates 3 and 4. Thermal decomposition of 5 at 140-145 °C gave N-acetylindoxyl acetates 7 in 81-87% yields and subsequent hydrolysis with 1 M aqueous sodium hydroxide provided indigos in 86-96% yields. Electrochemical bromination of 3a (X = H) using various alkali bromides led to the corresponding bromide 3b (X = Br) in 95-99% yields, which can be used as a precursor of bromoindigo synthesis.