Ampicillin → Bacampicillin

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A mixture of ampicillin Dane potassium salt (45.6 g), 1-bromoethyl ethyl carbonate (36.0 g), sodium bicarbonate (38.4 g) and acetonitrile (260 ml) is stirred for 3 hours at 40° C. The mixture is filtered and the cake is washed with acetonitrile (2×20 ml). Water (20 ml) is added and the mixture is stirred for 1 hour. Most of the solvent is removed by evaporation at 35°-40° C., under reduced pressure. Acetone (70 ml) and water (4.5 ml) are added. A mixture of concentrated hydrochloric acid (6.85 ml) and water (2.3 ml) is added slowly in order to keep the pH at 2-2.5. If, at the end, the pH is not stable, a small additional amount of 1:1 hydrochloric acid is added until a stable pH in the range 2-2.5 is obtained. The solution is dried and filtered. Butyl acetate (180 ml) and the solution is distilled under reduced pressure to a volume of ca. 40 ml. More butyl acetate (270 ml) is added and the product is allowed to crystallize at ca. 5° C. The mixture is filtered to give 32.5 g bacampicillin.

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