Sodium carbonate (212 mg.) was dissolved in a suspension of 2-methyl-2,3-methylene-6-aminopenam-3-carboxylic acid (428 mg.) in water (4 ml.). To a solution was added N-ethoxycarbonylphthalimide (440 mg.) all at once, and then stirred at room temprature for 2 hours. The resultant mixture was washed with methylene chloride. Methylene chloride (20 ml.) was added to the mixture, and then the solution was acidified with 1/4N-hydrochloric acid (16 ml.) and extracted. The extract was washed with water, dried over magnesium sulfate and then the solvent removed under reduced pressure to give amorphous product. The product was dissolved in methanol (8 ml.). To the solution was added an aqueous solution of N,N-dibenzylethylenediamine acetate (220 mg.) in water (4 ml.) to give colorless crystals of N,N-dibenzylethylenediamine salt (280 mg.) of 2-methyl-2,3-methylene-6-phthalimidopenam-3-carboxylic acid, mp 181° to 182°C (dec.).
