[CH2:1]([S:3][CH:4]1[C:13]2[C:8](=[C:9]([Cl:20])[CH:10]=[C:11]([C:15]([O:17]CC)=[O:16])[C:12]=2[CH3:14])[S:7][CH2:6][CH2:5]1)[CH3:2].[OH-].[K+].C(O)C>O>[CH2:1]([S:3][CH:4]1[C:13]2[C:8](=[C:9]([Cl:20])[CH:10]=[C:11]([C:15]([OH:17])=[O:16])[C:12]=2[CH3:14])[S:7][CH2:6][CH2:5]1)[CH3:2]0.75 Gram (2.3 mmol) of 4-ethylthio-5-methyl-6-ethoxycarbonyl-8-chlorothiochroman, 0.47 g (7.1 mmol) of potassium hydroxide, 5 ml of ethanol and 2 ml of water were mixed, and the mixture was stirred under heat at 60° C. for 2 hours. After the completion of the reaction, the solvent was distilled off, 5 ml of water was added, and then 2N hydrochloric acid was added to adjust a pH of 1. Then, the mixture was extracted with ethyl acetate twice. An organic layer was washed with a saturated sodium chloride aqueous solution and dried over anhydrous sodium sulfate, and the solvent was distilled off to give 1.0 g (yield 100%) of 4-ethylthio-5-methyl-6-carboxy-8-chlorothiochroman.
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