To a mixed solution of the compound (0.15 g) obtained in Example 1 and potassium carbonate (0.047 g) in DMF (2 ml) and water (0.02 ml), 5-(chloromethyl)-2,4-dihydro-3H-1,2,4-triazol-3-one (0.046 g) synthesized by the method described in the document (e.g., Tetrahedron Letters, Vol. 41, pages 8661-8664) was added at 0° C., and the reaction mixture was stirred at 0° C. for 3 hours. The reaction mixture was poured into water, and then the product was extracted with ethyl acetate. The organic layer was washed with 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 crystals (0.050 g, 29%).
