To a solution of sodium hydrogen carbonate (84 mg) in water (1 ml) is added dimedone (84 mg), and the mixture is dissolved by treating with ultrasonics. Thereto is added ethanol (8 ml), and further treated with ultrasonics for five minutes. To the mixture are added palladium diacetate (11 mg), triethyl phosphite (0.06 ml) and (1R,5S,6S)-2-[(4R)-pyrrolidine-2-thion-4-ylthio]-6-[(1R)-1-hydroxyethyl]-1-methylcarbapen-2-em-3-carboxylic acid allyl ester (383 mg) under nitrogen atmosphere, and the mixture is stirred at 35°-38° C. for one hour, and stirred at 20° C. for 10 minutes, and then stirred at 5° C. for 20 minutes. The precipitated crystals are collected by filtration to give (1R,5S,6S)-2-[(4R)-pyrrolidine-2-thion-4-ylthio]-6-[(1R)-1-hydroxyethyl]-1-methylcarbapen-2-em-3-carboxylic acid sodium salt (157 mg). The mother liquor is concentrated under reduced pressure to remove the ethanol, and thereto is added water (20 ml). The mixture is washed five times with chloroform (20 ml), and the aqueous layer is subjected to lyophilization to give the above sodium salt (168 mg).
