0.326 g. of 7-(3-Hydroxy-5-oxo-1-cyclopenten-1-yl)-heptanoic acid were dissolved in 20 ml. of dry acetone. At -20° C., 0.23 ml. of triehtylamine were added and then 0.216 ml. isobutyl chloroformate. After 5 minutes, the temperature was permitted to increase to 25° C., and 0.4 g. of p-benzoylaminophenol dissolved in 10 ml. of dry pyridine was added dropwise. After 2 hours, the solvent was distilled off; the residue was extracted in ethyl acetate; the organic phase was washed with water and dried over sodium sulphate. The solvent was distilled off. After chromatographic purification of the residue (silica gel/ethyl acetate), 7-(3-hydroxy-5-oxo-1-cyclopenten-1-yl)-heptanoic acid p-benzoylaminophenyl ester (m.p. 168°-169° C.) was obtained.
