Methanol → 2-[4-(Cyclopentylamino)-2-fluoro-phenyl]propionic acid

To a solution of 1.50 g (7.6 mmol) of compound (7) in a solvent mixture of 10 mL of methanol and 0.2 mL of acetic acid, 1.4 mL (15.2 mmol) of cyclopentanone and 0.96 g (15.2 mmol) of sodium cyanoborohydride (NaBH3CN) were added. The obtained solution was stirred at room temperature for 12 hours. Next, the reaction mixture was dried under a vacuum and extracted with dichloromethane, followed by drying over anhydrous sodium sulfate and subsequent filtration. The filtrate was dried under a vacuum, and the residue was purified by silica gel chromatograph (n-hexane/ethyl acetate 3:2) to give a methyl ester of compound (11a). This compound was hydrolyzed with sodium hydroxide to give 1.09 g (yield 54%) of compound (11a) of interest as brown powder.