A mixture of sodium hydrogen carbonate (0.84 g) in water (10 ml), dimedone (0.84 g) and tetrahydrofuran (80 ml) is treated with ultrasonics, and thereto is added palladium diacetate (0.11 g) and triethyl phosphite (0.58 g) under nitrogen atmosphere, and the mixture is stirred for three minutes. To the mixture is added (1R,5S,6S)-2-[(4R)-pyrrolidine-2-thion-4-ylthio]-6-[(1R)-1-hydroxyethyl]-1-methylcarbapen-2-em-3-carboxylic acid allyl ester (3.83 g), and the mixture is stirred at 35°-37° C. for 45 minutes. The mixture is stirred at 5° C. for 30 minutes, and the precipitated crystals are collected by filtration, washed with tetrahydrofuran, and dried under reduced pressure to give (1R,5S,6S)-2-[(4R)-pyrrolidine-2-thion-4-ylthio]-6-[(1R)-1-hydroxyethyl]-1-methyl-carbapen- 2-em-3-carboxylic acid sodium salt (3.28 g).
