Phosphorus oxychloride (0.5 ml.) was added to a solution of N,N-dimethylformamide (0.42 ml) in dry ethyl acetate (1 ml.) under cooling, and 2-methoxyimino-3,3-ethylenedioxy-4-bromobutyric acid (syn isomer, 1.2 g.) was added thereto under cooling and stirred at the same temperature. The solution was added to a stirred solution of 7-amino-3-hydroxycepham-4-carboxylic acid (1.0 g.) and trimethylsilylacetamide (6.0 g.) in dry ethyl acetate (50 ml.), at -20° C. and stirred at the same temperature for an hour. Ethyl acetate (20 ml.) and water (10 ml) were added to the resultant solution and the ethyl acetate layer was separated. The aqueous layer was extracted with ethyl acetate (20 ml.). The ethyl acetate layer and the extract were combined, and extracted with a saturated aqueous solution of sodium bicarbonate (20 ml.), and water (20 ml.) successively. After ethyl acetate (50 ml) was added thereto, the aqueous solution was adjusted to pH 2.0 with 6N hydrochloric acid, and the ethyl acetate layer was separated. The aqueous layer was extracted with ethyl acetate (25 ml×2). The ethyl acetate layer and the extract were combined, washed with a saturated sodium chloride aqueous solution and dried over magnesium sulfate. After evaporating the solvent is vacuo, the residue was pulverized with diisopropyl ether. The precipitates were collected by filtration to give 7-(2-methoxyimino-3,3-ethylenedioxy-4-bromobutyramido)-3-hydroxycepham-4-carboxylic acid (syn isomer, 1.8 g.).
