To a suspension of sodium hydride (6.1 mg) in tetrahydrofuran (1.5 ml) was added a solution of the compound prepared in example 15 (70 mg) in tetrahydrofuran (3 ml) under an atmosphere of argon. The mixture was stirred for 30 min at room temperature. To the mixture were successively added methyl iodide (261 mg) and hexamethylphosphoramide (1 ml), After stirred for 2 h at room temperature, the mixture was diluted with ethyl acetate and water. The mixture was extracted with ethyl acetate. The extract was washed with water and a saturated aqueous solution of sodium chloride, successively, dried over anhydrous magnesium sulfate and evaporated. The residue was purified by silica gel column chromatography (ethyl acetate:hexane =2:3) to give the title compound (27 mg) having the following physical data.
