1.0 Gram (3.3 mmol) of 4-ethylthio-5-methyl-6-carboxy-8-chlorothiochroman was dissolved in 5 ml of t-amyl alcohol, and 0.45 g (4.0 mmol, 1.2 eq.) of 1-ethyl-5-hydroxypyrazole and 0.83 g (4.0 mmol, 1.2 eq.) of N,N'-dicyclohexylcarbodiimide were added. The mixture was stirred at room temperature for 2.5 hours. After the completion of the reaction, ethyl acetate and hexane were added, and liberated N,N'-dicyclohexylurea was filtered off. The filtrate was concentrated and purified by column chromatography (silica gel/hexane+ethyl acetate=5:1) to give 1.4 g (yield 100%) of 4-ethylthio-5-methyl-6-(1-ethylpyrazol-5-yl)oxycarbonyl-8-chlorothiochroman.
