To a suspension of 3-hydroxy-7beta-phenylacetylaminocepham-4-carboxylic acid diphenylmethyl ester (2) in dichloromethane (13 parts) at 0° C. are added pyridine (6 equivalents) and acetic anhydride (6 equivalents). After 14 hours at 0° to 5° C., the mixture is diluted with ice-water. The organic layer is taken, washed with hydrochloric acid, aqueous sodium hydrogen carbonate, and water, dried, and concentrated. The reaction is crystallized from a mixture of dichloromethane and hexane to give 3-acetoxy-7beta-phenylacetylaminocepham-4-carboxylic acid diphenylmethyl ester (3a). Yield: 98.1%. mp. 187°-188° C. (decomposition).
