Dissolved in 40 ml of dimethylformamide were 8.54 g of L-phenylalanine benzyl ester p-toluenesulfonate and 3.40 g of 2-bromo-γ-butyrolactone. After a dropwise addition of 4.04 g of triethylamine, the resultant mixture was heated and stirred at 70° C. for 18 hours. The reaction mixture was poured into chilled water, followed by extraction with ethyl acetate. The extract was washed successively with water, a 2% aqueous solution of sodium hydrogencarbonate, and water. After drying the extract over anhydrous sodium sulfate, it was distilled under reduced pressure. The residue was isolated and purified by chromatography on a silica gel column (toluene:ethyl acetate=20:1-5:1). From initial fractions of the column effluent, 1.75 g of N-(2-oxo-3-tetrahydrofuranyl)-L-phenylalanine benzyl ester (Isomer A) was obtained as a colorless oily substance.
