In an argon stream, a solution of dry trifluoroacetic acid (0.2 ml) in methylene chloride (3 ml) is added dropwise to a mixture of dimethyl sulfoxide (0.1 ml) and methylene chloride (10 ml) at -65° C. or lower, and the mixture is stirred at -70° C. for 0.5 hours. A solution of cis-1-(tert-butyldimethylsilyl)-3-[(R)-1-hydroxyethyl]-4-trimethylsilylethynyl-2-azetidinone (0.4 g) in methylene chloride (4 ml) is added dropwise at -65° C. or lower and the mixture is stirred at the same temperature for 0.5 hour. After addition of triethylamine (0.3 ml) at the same temperature, the temperature is gradually raised to room temperature. The reaction mixture is poured into a mixture of ethyl acetate (70 ml) and water (100 ml), and the organic layer is separated, washed with aqueous sodium chloride, dried over magnesium sulfate and concentrated under reduced pressure. The residue is subjected to silica gel column chromatography (eluent: 3% ethyl acetate-97% n-hexane) to give trans i.e. (3S,4S)-3-acetyl-1-(tert-butyldimethylsilyl)-4-trimethylsilylethynyl-2-azetidinone (0.13 g)
