Sodium tungstate dihydrate (500 mg.) was added to a solution of 2,2,2-trichloroethyl 6-(2-phenylacetamido)-2,2-dimethylpenam-3-carboxylate (31.5 g.) in acetic acid (150 cc.), and to the mixture was added, dropwise, 30% hydrogen peroxide (9.1 cc) while cooling in an ice bath. The mixture was stirred for 1.5 hours, and then water was added thereto. The precipitate was collected by filtration, washed with water and dried to yield crystals (33.2 g.) of 2,2,2-trichloroethyl 6-(2-phenylacetamido)2,2-dimethylpenam-3-carboxylate-1-oxide, m.p. 165.5° - 168.5° C.
