73.5 g (290 mmol) of 2-[4-(p-chlorophenyl)-2-oxopyrrolidin-1-yl]-acetic acid are suspended in 900 ml of tetrahydrofuran; 92.5 g (350 mmol) of pentachlorophenol are added and the whole is stirred at room temperature until a clear solution is obtained. The solution is cooled in an ice bath, 65.8 g (319 mmol) of dicyclohexylcarbodiimide dissolved in 180 ml of tetrahydrofuran are added dropwise thereto within a period of 30 minutes and the whole is stirred for a further 1 hour in the ice bath and for a further 16 hours at room temperature; the resulting dicyclohexylurea is filtered off and the filtrate is concentrated to dryness by evaporation under reduced pressure at 60° and recrystallised from 400 ml of ethyl acetate. 2-[4-(p-chlorophenyl)-2-oxopyrrolidin-1-yl]-acetic acid pentachlorophenyl ester having a melting point of 135°-136° is obtained. Additional product can be obtained by concentrating the mother liquor.
