General method: 6a-d (9.2 mmol) was dissolved in toluene (150 mL) under nitrogen protection, rhodium(II) acetate dimer (203 mg, 0.46 mmol) was added, and the reaction was heated to reflux for 1-2 hours. After completion of the reaction, the solvent was removed by distillation under reduced pressure, and the resulting residue was purified by silica gel column chromatography (eluent: hexane-ethyl acetate, 25:1 to 18:1, v/v) to afford the target product ethyl 6-chloroindole-2-carboxylate (7a-d) in 51.7%-58.6% overall yields.
