Acetic acid (10 cc) was added to 6-(2-phenoxyacetamido)-2,2-dimethylpenam-3-carboxylic acid (3.50 g.), and sodium tungstate dihydrate (100 mg.) was further added thereto. To the mixture was added 30% hydrogen peroxide (1.2 cc) while cooling in an ice bath, and the mixture was stirred for 1 hour. Water was added to the mixture and the precipitate was collected by filtration, washed with water and dried to yield crystals (3.24 g.) of 6-(2-phenoxyacetamido)-2,2-dimethylpenam-3-carboxylic acid-1-oxide, m.p. 166°-167° C.
