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GENERAL METHOD: Cesium carbonate (829 mg, 6 mmol) was added to a solution of indole-6-carboxaldehyde (435 mg, 3 mmol) in acetonitrile (30 mL) and the mixture was heated to reflux for 2 hours. Subsequently, ethyl iodide (3.3 mmol) was added to the reaction system and heating and refluxing was continued for 1 hour. Upon completion of the reaction, the solvent was removed by distillation under reduced pressure, water was added to the residue and extracted three times with ethyl acetate. The organic layers were combined and dried with anhydrous sodium sulfate and subsequently concentrated under vacuum. Finally, the resulting residue was purified by fast column chromatography to afford the target product 1-ethyl-1H-indole-6-carbaldehyde.

[1] Bioorganic and Medicinal Chemistry, 2014, vol. 22, # 7, p. 2060 - 2079
[2] Patent: WO2014/142363, 2014, A1. Location in patent: Paragraph 0405