A solution of trichloroethyl 6β-phenylacetamido-penicillanate 1β-oxide (28.86 g) and 5-mercapto-1-methyl-tetrazole (6.96 g) in toluene (800 cc.) is heated under reflux for 3 hours, and the water formed during the reaction is removed as it is formed by passing the condensate over calcium chloride before reintroducing it into the reaction mixture. The solvent is evaporated under reduced pressure (12 mm Hg) at 40°C; the residue obtained is dissolved in chloroform (50 cc.) and the solution obtained is chromatographed on a column of silica (400 g) (0.06-0.20 mm, pH neutral; diameter of the column: 4.8 cm., height: 55 cm.). Elution is carried out using chloroform, collecting 250 cc. fractions. Fractions 10 to 17 are combined and concentrated to dryness under reduced pressure (12 mm Hg) at 40°C. 4-(1-Methyl-5-tetrazolyl-dithio)-3-phenylacetamido-1-(1-trichloroethoxycarbonyl-2-methyl-prop-2-en-1-yl)-2-azetidinone (17.4 g.) is obtained in the form of a colourless pastelike product, the characteristics of which are as follows: Rf = 0.50 [silica gel; chloroform/ethyl acetate (50/50 by volume)].
