To a mixed solution of tert-butyl (6R,7S)-6-(3,4-dichlorophenyl)-7-ethenyl-1,4-oxazepane-4-carboxylate (1.08 g) and 4-methylmorpholine N-oxide (680 mg) in acetonitrile (6 mL)/acetone (6 mL)/water (6 ml) was added osmium(VIII) oxide immobilized catalyst I (Wako Pure Chemical Industries, Ltd. cat. 153-02581) (700 mg), and the mixture was stirred under nitrogen purging at room temperature overnight. The catalyst was filtered off through celite, and the filtrate was concentrated. Water was added, and the mixture was extracted with ethyl acetate. The organic layer was washed with brine, and dried over anhydrous magnesium sulfate, and the solvent was evaporated (product about 1.07 g). 900 mg therefrom was purified by HPLC (column: Sunrise C18-SAC 20×150 mm, mobile phase: water/acetonitrile (containing 5 mM ammonium acetate)) to give the title compound (diastereomer having a shorter retention time) (610 mg) as a colorless oil.
