CDI (0.991 g, 6.11 mmol) was added to a solution of [(6R,7R)-4-(tert-butoxycarbonyl)-7-(3,4-dichlorophenyl)-1,4-oxazepan-6-yl]acetic acid (1.9 g, 4.70 mmol) in THF (20 ml), and the mixture was stirred at room temperature for 1 hr. To the reaction mixture were added potassium 3-ethoxy-3-oxopropanoate (0.880 g, 5.17 mmol) and magnesium chloride (0.492 g, 5.17 mmol). The reaction mixture was stirred at 60° C. for 5 hr. To the reaction mixture was added 1 N hydrochloric acid, 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 (1.959 g).
