2,2,2-trichloroethyl 6-(2-phenylacetamido)-2,2-dimethylpenam-3-carboxylate (466 mg.) was added to acetic acid (2 cc), and sodium percarbonate (377 mg.) was added thereto while cooling in an ice bath. The mixture was stirred for 30 minutes, and sodium tungstate dihydrate (40 mg.), acetic acid (1 cc) and water (1 cc) were added thereto. The mixture was stirred for 3 hours, and then water (50 cc) was added thereto. The precipitate was collected by filtration, washed with water and dried to yield crude crystals (400 mg.) of 2,2,2-trichloroethyl 6-(2-phenylacetamido)-2,2-dimethylpenam-3-carboxylate-1-oxide, m.p. 165° - 167° C.
