To a mixture of methyl 4-{[cyclopentyl(3-methyl-1-benzothiophen-2-yl)methyl]amino}benzoate (1.56 g) synthesized above, tetrahydrofuran (20 mL) and ethanol (20 mL) was added 1N aqueous sodium hydroxide solution (20.0 mL), and the mixture was stirred with heating under reflux for 4 hr, and concentrated under reduced pressure. The residue was dissolved in water (40 mL), and 1N hydrochloric acid (20.0 mL) was added at 0° C. The resulting precipitate was collected by filtration, and the obtained pale-brown solid was dissolved in ethyl acetate. The solution was washed with saturated brine, dried over magnesium sulfate, and concentrated under reduced pressure to give the title object compound (1.42 g, 95%) as a pale-brown solid.
