To a solution of oxalyl chloride (1.52 g) in THF (20 mL) was added dimethyl sulfoxide (1.87 g) at −78° C., and the mixture was stirred for 20 min. A solution of tert-butyl (6R,7R)-6-(3,4-dichlorophenyl)-7-(hydroxymethyl)-1,4-oxazepane-4-carboxylate (3.0 g) in THF (10 mL) and triethylamine (7.78 mL) were added at −78° C., and the mixture was warmed to room temperature and stirred for 1 hr. 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. The residue was purified by silica gel column chromatography (eluent; hexane:ethyl acetate (0%-50%)) to give the title compound (2.89 g) as a colorless solid.
