To a solution of ethyl 4-[3-(3-nitrobenzoyl)indol-1-yl]butyrate (0.5 g) in tetrahydrofuran (10 ml) were added sodium borohydride (99.4 mg) and boron trifluoride etherate (0.485 ml) at -25° C., and the mixture was stirred at the same temperature for 2 hours. The mixture was poured into a mixture of ethyl acetate and ice water. The organic layer was separated, washed with water and brine, and dried over magnesium sulfate. After evaporation of the solvent, the residue was chromatographed on silica gel (25 g) eluting with a mixture of ethyl acetate and hexane (1:4) to give ethyl 4-[3-(3-nitrobenzyl)indol-1-yl]butyrate (173 mg) as a yellow oil.
