To a solution of the compound (0.20 g) obtained in Example 1 and Et3N (0.19 ml) in acetonitrile (5 ml), methyl isocyanate (0.050 ml) was added, and the reaction mixture was stirred at room temperature for 14 hours. The reaction mixture was poured into water, and then the product was. extracted with ethyl acetate. The organic layer was washed with an aqueous 10% citric acid solution and saturated brine, dried, and then the solvent was evaporated under reduced pressure. The obtained residue was isolated and purified by silica gel column chromatography (ethyl acetate:methanol=20:1) to obtain the title compound as colorless oil (0.20 g, 95%).
