Ethyl 4-[3-(4-amino-3-hydroxybenzoyl)indol-1-yl]butanoate (5.0 g) was suspended in a mixed solvent containing methanol (50 ml) and water (5 ml), and 4-ethylbenzaldehyde (3.66 g) and sodium cyanoborohydride (1.80 g) were added to the suspension. Acetic acid (2.5 ml) was further added dropwise to the mixture over five minutes, and the resultant mixture was stirred at room temperature for one hour. Water (50 ml) was added to the reaction mixture, and crystals so precipitated were collected by filtration. The produced crystals were recrystallized from ethanol, to thereby obtain 5.39 g of ethyl 4-{3-[4-(4-ethylphenyl)methylamino-3-hydroxybenzoyl)indol-1-yl}butanoate as yellow crystals.
