To a mixed solution of (6R,7R)-4-(tert-butoxycarbonyl)-7-(3,4-dichlorophenyl)-1,4-oxazepane-6-carboxylic acid (0.82 g) in toluene (10 ml) and methanol (2 mL) was added trimethylsilyldiazomethane (1.576 mL, 2 mol/l hexane solution) at 0° C. and the mixture was stirred for 30 min. Acetic acid was added until the solution became colorless. The solvent was evaporated under reduced pressure, and the residue was extracted with ethyl acetate and water. The organic layer was washed with brine, and dried over anhydrous sodium sulfate. The solvent was evaporated under reduced pressure to give the title compound (0.51 g).
