To 300 ml of dried dimethylformamide, was added 14.56 g (0.12 mol.) of potassium tert-butoxide and 18 g (0.1 mol.) of 1-phenyl-5-mercaptotetrazole. The mixture was stirred for 20 minutes at room temperature. Thereafter, to the mixture cooled to 5° C. and kept there, was gradually added dropwise a solution prepared by adding 25 g (0.1 mol.) of 3-chloro-3-phenylphthalide to 75 ml of dried dimethylformamide. At the conclusion of the dropwise addition, the cooling bath was removed, and the mixture was stirred for 2 hours at room temperature. Thereto, 300 ml of a saturated aqueous solution of ammonium chloride was added. The reaction mixture was extracted with ethyl acetate repeatedly. The extract was washed with water, and the resulting organic layer was dried over anhydrous sodium sulfate. Sodium sulfate was filtered off with suction, and the filtrate was concentrated to yield 35 g of oily substance. This oily substance was separated and purified by flash column chromatography (silica gel: 700 g, eluate: hexane-ethyl acetate (3:1 by vol.) mixture). Thus, the intended compound, 3-phenyl-3-(1-phenyltetrazole-5-ylthio)phthalide (1), was obtained in the form of white crystals. Yield 21.5 g, Melting Point 137°-139° C.
