To a solution of 345 mg of 2-methylthio-5-(6-methoxycarbonylhexylidene)-4-(3-hydroxy-1-octenyl)-2-cyclopentenone obtained in Example 34 dissolved in 20 ml of acetone was added 220 ml of 0.1M phosphate buffer of pH 8. While the mixture was stirred, 24 mg of pig liver esterase was added thereto, and the mixture was stirred at 30-35° C. for 150 hours. After the pH was adjusted to 4 with 0.1N hydrochloric acid, ammonium sulfate was added to saturation and ethyl acetate was added, followed by filtration. The filtrate was extracted with ethyl acetate, and the organic layers were combined and washed with saturated aqueous sodium chloride. The product was dried over anhydrous magnesium sulfate, filtered and concentrated, followed by silica gel chromatography to give 193 mg (yield 58%) of 2-methylthio-5-(6-carboxyhexylidene)-4-(3-hydroxy-1-octenyl)-2-cyclopentenone.
