Ethyl bromopyruvate (0.070 mL, 0.50 mmol) was added to a solution of tert-butyl (6R,7R)-6-(2-amino-2-thioxoethyl)-7-(3,4-dichlorophenyl)-1,4-oxazepane-4-carboxylate (140 mg, 0.33 mmol) in EtOH (5 mL). The reaction mixture was stirred at 80° C. for 2 hr, triethylamine (0.140 mL, 1.00 mmol) and di-tert-butyl dicarbonate (0.115 mL, 0.50 mmol) were added at room temperature, and the mixture was stirred at room temperature for 5 hr. To the reaction mixture was added aqueous sodium hydrogen carbonate solution, and the mixture was extracted with ethyl acetate. The extract was washed with brine, and dried over anhydrous sodium sulfate. The solvent was evaporated under reduced pressure. The residue was purified by silica gel column chromatography to give the title compound (173 mg).
