To a stirred solution of 2,3-di-n-octyloxypropyl iodide (529 mg) in dimethylsulfoxide (14 ml) was added 7-[2-(2-aminothiazol-4-yl)-2-methoxyiminoacetamido]-3-cephem-4-carboxylic acid (syn isomer, 1427 mg) and triethylamine (0.52 ml) at room temperature, and stirred at 40° C. for 4 hours and at 50° C. for 4 hours. The resultant mixture was poured into ice water (200 ml), adjusted to pH 8 with an aqueous solution of sodium bicarbonate and extracted with ethyl acetate (140 ml). The extract was washed with a saturated aqueous solution of sodium chloride (50 ml) and dried over anhydrous magnesium sulfate. The solvent was evaporated under reduced pressure. After washing the residue with n-hexane (8 ml), the residue was tritulated with petroleum ether and then the precipitates were collected by filtration to give 2,3-di-n-octyloxypropyl 7-[2-(2-aminothiazol-4-yl)-2-methoxyiminoacetamido]-3-cephem-4-carboxylate (syn isomer, 160 mg), mp. 75° to 78° C.
