To a solution of 16.4 g (21.9 mM) of benzhydryl 7β-phenoxyacetamido-3-diphenylphosphoryloxy-3-cephem-4-carboxylate in 128 ml of N,N-dimethylformamide were added at -10° C. a solution of 1.93 g (24.1 mM) of 70% sodium hydrosulfide in 86 ml of N,N-dimethylformamide and 4.25 g (32.9 mM) of diisopropylethylamine, and the mixture was stirred for 45 minutes. After the reaction, 200 ml of water was added and the mixture was extracted with 200 ml of diethyl ether. The aqueous layer was adjusted to pH 2 to 3 with 0.5% hydrochloric acid and extracted with ethyl acetate (300 ml), and the extract was washed with a saturated aqueous sodium chloride solution (200 ml×2) and dried over anhydrous magnesium sulfate. The solvent was evaporated and the residue was chromatographed over silica gel column (eluent; ethyl acetate:benzene=1:2 to 2:1) to give 10.4 g of benzhydryl 7β -phenoxyacetamido-3-mercapto-3-cephem-4-carboxylate.
