Trifluoroacetic acid → [(1S,6R)-7-Oxo-8-[(1R)-1-phenylethyl]-8-azabicyclo[4.3.0]non-3-en-1-yl]carboxylic acid

Trifluoroacetic acid (18.0 mL) was added to a solution of [(1S,6R)-7-oxo-8-[(1R)-1-phenylethyl]-8-azabicyclo[4.3.0]non-3-en-1-yl]carboxylic acid tert-butyl ester (2.04 g, 5.99 mmol) in dichloromethane (18.0 mL) with stirring under ice-cooling, and the mixture was stirred at room temperature for 1.5 hours. The reaction solution was concentrated under reduced pressure, and trifluoroacetic acid was azeotropically distilled with toluene (×3). A 1 mol/L sodium hydroxide solution (15.0 mL) was added to the resulting residue under ice-cooling, and the mixture was washed with diethyl ether (40 mL×2). The aqueous layer was made acidic with 1 mol/L hydrochloric acid (20 mL) under ice-cooling. Then, the precipitated crystals were collected by filtration and washed with 0.5 mol/L hydrochloric acid, ethyl acetate, and diethyl ether. The resulting crystals were dried under reduced pressure at 50° C. overnight to give 1.40 g of the title compound as white crystals. The aqueous layer of the filtrate was extracted with ethyl acetate (50 mL). The organic layers were combined, washed with water (150 mL) and brine (150 mL), and dried over anhydrous sodium sulfate. After filtration, the filtrate was concentrated under reduced pressure to give 250 mg of the title compound as white crystals.