Potassium hydroxide (40 mg) was dissolved in ethanol (1.25 ml), and the solution was diluted with DMSO (25 mL). To this solution were added tert-butyl 7-(3,4-dichlorophenyl)-6-oxo-1,4-oxazepane-4-carboxylate (2.60 g) and paraformaldehyde (267 mg), and the mixture was stirred at room temperature for 30 min. The reaction solution was neutralized with 1 N hydrochloric acid, brine was added, and the mixture was extracted with ethyl acetate. The organic layer was washed twice with brine, and dried over anhydrous magnesium sulfate, and the solvent was evaporated under reduced pressure. The residue was purified by silica gel chromatography (hexane/ethyl acetate) to give the title compound (1.27 g).
