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 (0.5 g), chloromethyl isopropyl carbonate (3.17 g) and acetonitrile (1 ml) was stirred for 6 hours at 100° C. After having been cooled, the mixture was diluted with diisopropyl ether (10 ml). The resulting powder was collected by filtration. The powder was subjected to silica gel chromatography (eluent: ethyl acetate→acetone→acetone/ethanol=10/1→acetone/ethanol=5/1). The solvent was distilled off, and the residue was crystallized from ethanol/acetone 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-(isopropoxycarbonyloxy)methyl-1H-1,2,4-triazolium chloride (Compound 18, 110 mg) as white powdery crystals.
