7-Amino-2-carboxy-3-methyl-8-oxo-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene (371 g) is dissolved in a solution of sodium bicarbonate (307 g) in a mixture of distilled water (2 liters) and dioxane (2 liters). A solution of di-tert.-butyl carbonate (421 g) in dioxane (2 liters) is added in the course of 10 minutes. The reaction mixture is stirred for 48 hours at 25° C. The suspension obtained is concentrated under reduced pressure (20 mm Hg) at 50° C. to a residual volume of about 2 liters, and is then diluted with ethyl acetate (1 liter) and distilled water (2 liters). The aqueous phase is decanted, washed with ethyl acetate (500 cc) and acidified to pH 2 with 6 N hydrochloric acid in the presence of ethyl acetate (1500 cc). The aqueous phase is extracted with ethyl acetate (2×1 liter). The combined organic phases are washed with a saturated sodium chloride solution (2×250 cc) and dried over sodium sulphate. After filtration, the solvent is evaporated under reduced pressure (20 mm Hg) at 50° C. 7-tert.-Butoxycarbonylamino-2-carboxy-3-methyl-8-oxo-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene (486 g) is obtained in the form of yellow crystals (m.p.=190° C., with decomposition).
