A mixture of 5-(benzoyl-methyl-amino)-2,3-dihydro-indole-1-carboxylic acid tert-butyl ester (500 mg) and MnO2 (5 g) in toluene (20 mL) was heated at 80° C. for 1 h. Additional MnO2 (5 g) was introduced and the resulting mixture was stirred at 80° C. for 1 h. After cooling to room temperature, the mixture was filtered through celite and the filtrate was concentrated. The product was purified by chromatography (30% EtOAc-hexanes) to give 5-(benzoyl-methyl-amino)-indole-1-carboxylic acid tert-butyl ester (372 mg, 75%).
