1,8-Diazabicyclo[5.4.0]-7-undecene (2.17 ml, 14.5 mmol) and Eschenmoser salt (3.66 g, 19.8 mmol) were added to a solution of (3S)-3-(3-oxo-1-propyl)-5-oxo-1-[(1R)-1-phenylethyl]pyrrolidine-3-carboxylic acid tert-butyl ester in dichloromethane (88.0 ml) in a nitrogen atmosphere, and the mixture was stirred at room temperature for 16 hours. Thereafter, Eschenmoser salt (1.22 g, 6.59 mmol) was added, and the mixture was stirred at room temperature for three hours. A saturated ammonium chloride solution (150 ml) was added to the reaction solution, followed by extraction with chloroform (100 ml×1, 150 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→60:40→50:50→34:66) to give 3.14 g of the title compound as white crystals.
