To a solution of 2,2,6,6-tetramethylpiperidine (322 mg) in tetrahydrofuran (5.0 ml) was added dropwise a solution of n-butyllithium (1.6M n-hexane solution 1.3 ml) at −70˜−60° C. and the solution was stirred at 0° C. for 30 minutes. A solution of benzyl 1-tert-butoxycarbonylindole-4-carboxylate (500 mg) in tetrahydrofuran (2.5 ml) was added dropwise to the above solution at −70˜−60° C. and the mixture was stirred at −70° C. for 30 minutes. To the mixture was added a solution of ethyl chloroformate (185 mg) in tetrahydrofuran (2.5 ml) at such a rate as to maintain the temperature below −60° C. The solution was stirred at −70° C. for 2 hours and the reaction was quenched with aqueous saturated ammonium chloride solution at −20° C. The aqueous solution was extracted with ethyl acetate. Drying, filtering and removal of solvents afforded a crude product. The crude product was chromatographed on silica gel (eluent; n-hexane:ethyl acetate=15:1) to give benzyl 1-tert-butoxycarbonyl-2-ethoxycarbonylindole-4-carboxylate (100 mg) as a colorless oil.
