To DMF (2 ml) was added potassium tertiary-butoxide (226 mg) under ice-cooling. After dropping a solution of the compound (100 mg) obtained in Example 51 and diethyl ethoxymethylenemalonate (293 mg) in DMF (0.5 ml), the resultant mixture was stirred at room temperature for 18 hours. After treating in a conventional manner, the obtained residue was subjected to silica gel column chromatography to thereby give 113 mg (65%) of the title compound. The physical constants of the obtained compounds was identical with those described in Japanese Patent No. 2,769,174.
