To a stirred solution of 3.58 g of (3S)-2-oxo-3-[[(phenylmethoxy)carbonyl]amino]-1-azetidinesulfonic acid, tetrabutylammonium salt and 960 mg of 8-hydroxyquinoline in 10 ml of dichloroethane was added 1.53 g of camphorsulfonic acid. An additional 5 ml of dichloroethane was added and the mixture was stirred to effect solution and a seed crystal of the desired product was added to initiate precipitation. After ca. 5 minutes, 15 ml of ethyl acetate was added slowly. After an additional 30 minutes, the solid was isolated by filtration, washed with 1:1 dichloromethane/ethyl acetate and dried in vacuo to afford 1.98 g of the title compound as a fine, light yellow powder.
