To a solution of (3S,4R)-3-[(1R)-1-(t-butyldimethylsilyloxy)ethyl]-4-[(1RS)-1-methyl-2-oxo-2-(2-thioxothiazolidin-3-yl)ethyl]-2-azetidinone obtained in Example 2 (797 mg) in methanol (16 ml) was added potassium carbonate (410 mg) at 0° C. and the mixture was stirred at room temperature for 10 minutes. The reaction mixture was poured into a mixture of ethyl acetate (40 ml), saturated aqueous sodium chloride (25 ml) and 1N hydrochloric acid (3 ml). The organic layer was separated, washed with saturated aqueous sodium chloride solution (30 ml ×2), dried over magnesium sulfate and evaporated. The residue was chromatographed on silica gel (21 g) eluting with a mixture of hexane and ethyl acetate (3:2, V/V) to give methyl (2R)-2-[(2S,3S)-3-{(1R)-1-(t-butyldimethylsilyloxy)ethyl}-4-oxoazetidin-2-yl]propionate (597 mg) containing a very small amount of methyl (2S)-2-[(2S,3S)-3-{(1R)-1-(t-butyldimethylsilyloxy)ethyl}-4-oxoazetidin-2-yl]propionate.
