A solution of 93.4 g of 5-oxo-3-[4-(trifluoromethyl)phenyl]-2-pyrrolidinepropanoic acid methyl ester and 5-oxo-β-[4-(trifluoromethyl)phenyl]-2-pyrrolidinepropanoic acid methyl ester in 370 ml of 1 N aqueous sodium hydroxide solution is heated to 60° C. for four hours. Excess aqueous hydrochloric acid (1 N, 380 ml) is added and the solution concentrated under reduced pressure to give 5-oxo-β-[4-(trifluoromethyl)phenyl]-2-pyrrolidinepropanoic acid and 5-oxo-3-[4-(trifluoromethyl)phenyl]-2-pyrrolidinepropanoic acid. The acids are dissolved in 300 ml of acetic anhydride, and the solution is heated to 90° C. The solution is filtered hot and the filtrate concentrated in vacuo. The oil is chromatographed on silica gel (200-400 mesh) (elution with chloroform) to give cis-and trans-dihydro-1-(4-trifluoro-methylphenyl)-1H-pyrrolizine-3,5-(2H,6H)-diones as a white solid with a mp 173°-181° C.
