Potassium tertiary-butoxide (62 mg) was added to DMF (2 ml) and cooled to 0° C. Then a solution of the compound (100 mg) obtained in Example 51 in DMF (200 μl) was dropped therein to. After stirring for 15 minutes, diethyl ethoxymethylenemalonate was dropped therein to and the resultant mixture was stirred for 8 hours at room temperature. After treating in a conventional manner, it was subjected to silica gel column chromatography to thereby give 137 mg (75%) of the title compound as a colorless solid. The 1H-NMR and IR spectral data of this product was identical with those of the compound obtained in Example 63.
