and an n chlorinating agent in the presence of a poly 4 vinylpyridine divinylbenzene copolymer containing approximately 2 5 cross linking as described in co pending application ser no 960 347 filed this even date in a preferred embodiment p nitrobenzyl 6 phenoxyacetamido 2 2 dimethylpenam 3 carboxylate 1 oxide is reacted in anhydrous toluene with approximately 1 1 molar equivalents of n chlorophthalimide in the presence of between about 1 and about 3 grams per gram of penicillin sulfoxide of poly 4 vinylpyridine divinylbenzene cross linked to the extent of between about 2 and about 5 the reaction is carried out at the reflux temperature for about 100 minutes the reaction suspension is filtered to remove the insoluble copolymer and phthalimide and the filtrate containing p nitrobenzyl 3 methyl 2 2 chlorosulfinyl 4 oxo 3 phenoxyacetamido azetidinyl 3 butenoate is cooled to a temperature between about 0 u00b0 and 10 u00b0 c about 1 molar equivalent of diethyl ether is added to the cold filtrate followed by the addition of between about 2 and 2 5 molar equivalents of stannic chloride with the addition of the stannic chloride the sulfinyl chloride stannic chlorideoxo ligand complex precipitates from the cold filtrate and is stirred in the solvent for between about 3 and about 24 hours to ensure complete formation of the complex thereafter the complex is filtered washed on the filter with a.