To the compound (6) was added concentrated aqueous hydrochloric acid (50 ml). The mixture was heated at 90° C. for 2 hours and then cooled down. To the mixture were added water (50 ml) and toluene (20 ml). The aqueous layer was separated and neutralized by 30% aqueous sodium hydroxide. The compound (12) was extracted with ethyl acetate (50 ml), and the ethyl acetate layer was washed with water (30 ml). Each aqueous layer was extracted with ethyl acetate (20 ml). The ethyl acetate layer was collected and concentrated under reduced pressure to yield oily residue. To the oily residue was slowly added diisopropyl ether (50 ml) for crystallization. The obtained slurry was stirred at room temperature for 30 minutes, filtered, washed with diisopropyl ether (30 ml) and dried to yield the compound (12) (10.4 g) as a white crystal. Yield from the compound (22):82%
