Na(OAC)3BH (495 mg, 2.33 mmol) was added to a solution of the (2R,3S)-2-(4-aminophenyl)-1-(2-methylbenzoyl)piperidine-3-carboxylic acid ethyl ester (612 mg, 1.67 mmol), cyclopentanone (140 mg, 1.67 mmol) and acetic acid (100 mg, 1.67 mmol) in dry dichloroethane at r.t and the reaction mixture was heated to 50° C. for 4 h, cooled to r.t and stirred for 48 h. It was then diluted with CH2Cl2 (30 mL), washed with saturated aqueous NaHCO3 solution, dried and concentrated in vacuo. The residue was purified by ISCO flash column using ethyl acete and hexanes as mobile phase (40 g column, 0-40% gradient) to afford (2R,3S)-2-(4-Cyclopentylaminophenyl)-1-(2-methylbenzoyl)piperidine-3-carboxylic acid ethyl ester (450 mg).
