To a mixture of 1-[(1R,2R)-2-(2,4-difluorophenyl)-2-hydroxy-1-methyl-3-(1H-1,2,4-triazol-1-yl)propyl]-3-[4-(1H-tetrazol-1-yl)phenyl]-2-imidazolidinone(1.31 g) and ethyl iodomethyl carbonate (1.25 g) was added acetonitrile (20 ml) and the mixture was stirred for 14 hours at 60° C. under an argon atmosphere. The reaction mixture was concentrated under reduced pressure and the residue was submitted to silica gel flush chromatography (eluent: ethyl acetate→acetone→acetone/ethanol=4/1). The fraction containing the desired compound was concentrated under reduced pressure. The residue was subjected to ODS column chromatography (eluent: methanol/water=3/2) to give 1-[(2R,3R)-2-(2,4-difluorophenyl)-2-hydroxy-3-[2-oxo-3-[4-(1H-tetrazol-1-yl)phenyl]-1-imidazolidinyl]butyl]-4-ethoxycarbonyloxymethyl-1H-1,2,4-triazolium iodide (Compound 17, 1.1 g) as a pale yellow powder.
