To a mixed solution of tert-butyl (6R,7R)-6-(3,4-dichlorophenyl)-7-(hydroxymethyl)-1,4-oxazepane-4-carboxylate (3.0 g) and sodium periodate (3.41 g) in acetonitrile (16 mL)/ethyl acetate (16 mL)/water (24 mL) was added ruthenium (III) chloride (17 mg), and the mixture was stirred at room temperature for 10 min. The solid was filtered off through celite, and the filtrate was concentrated. The residue was crystallized from ethyl acetate to give the title compound (311 mg) as a colorless amorphous powder.
