Ethanol (43.9 ml) was added to sodium borohydride (644 mg, 17.0 mmol) in a nitrogen atmosphere, and the mixture was cooled with ice-acetone. Cerium chloride heptahydrate (6.55 g, 17.6 mmol) and (3S)-3-(3-oxo-2-methylene-1-propyl)-5-oxo-1-[(1R)-1-phenylethyl]pyrrolidine-3-carboxylic acid tert-butyl ester (3.14 g, 6.55 mmol) were added under cooling. The mixture was stirred under cooling for one hour, and then cerium chloride heptahydrate (1.64 g, 4.40 mmol) and sodium borohydride (166 mg, 4.39 mmol) were added. The mixture was stirred under cooling for 30 minutes and then ice-cooled. A saturated ammonium chloride solution (150 ml) was added to the reaction solution. The mixture was concentrated under reduced pressure and ethanol was removed. The mixture was extracted with ethyl acetate (150 ml×1, 100 ml×1). The organic layer was washed with brine (200 ml) and then dried over anhydrous sodium sulfate. After filtration, the filtrate was concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography (hexane:ethyl acetate=100:0→90:10→66:34→50:50→34:66→25:75→17:83→5:95) to give 2.83 g of the title compound.
