To a solution of 1-benzyloxycarbonyl-2-indoline carboxylic acid (320 mg, 1.05 mmol) and triethylamine (106 mg, 1.05 mmol) in chloroform (20 ml), ethyl chloroformate (109 mg, 1 mmol) was added at -15° C. The reaction mixture was stirred at a bath temperature of -10 to 5° C. for 1 hr. Then a precooled solution of (3S,4S)-3-amino-4-acetoxy-azetidin-2-one in ethyl acetate was added at -15° C. and stirring was continued at a bath temperature of -15 to 5° C. for 1 hr. After removal of solvent, the residue was dissolved in ethyl acetate, washed with water, brine and dried over sodium sulfate. After removal of solvent, the residue was purified by silica gel column chromatography using hexane-ethyl acetate (1:3) as eluent and the title compound was obtained.
