A suspension of methyltriphenylphosphonium bromide (3.59 g) in THF (30 mL) was cooled under nitrogen purging to −78° C., and 1.6 M n-butyllithium/hexane solution (5.79 ml) was added. The mixture was warmed to 0° C., and stirred for 20 min, and a solution of tert-butyl (6R,7R)-6-(3,4-dichlorophenyl)-7-formyl-1,4-oxazepane-4-carboxylate (2.89 g) in THF (50 ml) was added. After stirring at 0° C. for 20 min, and at room temperature for 2 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%-30%)) to give the title compound (1.10 g) as a colorless oil.
