The compound 52 (1.0 g, 2.5 mmol) obtained in the same manner as in Example 21 (b) was dissolved in dry benzene (80 ml), treated with KOC(CH3)3 (684 mg, 6.1 mmol), and heated under reflux for 1 hour. After completion of the reaction, the solvent was evaporated and the residue was dried and suspended by adding a small amount of water under ice cooling. The aqueous layer was made neutral with 0.5N.HCl, and the residue was dissolved by adding chloroform. The mixed layer was removed to a separating funnel. After the solid was completely dissolved, water was added to wash the solution so as to make the chloroform layer neutral or weak acidic. The chloroform layer was separated, washed with saturated aquecus NaCl solution and dried over magnesium sulfate. The solvent was evaporated to give a white solid. The product was dissolved in chloroform and purified by silica gel column chromatography (developing solvent: benzene:ethyl acetate=4:1) to give the desired compound as white solid (801 mg, crude yield 58%). Recrystallization from the mixed solvent of methanol and water gave white crystals (654 mg, yield, 47%). m.p., 117°-118° C.
