[SH:1][CH2:2][C:3]([NH:5][C:6]1[CH:14]=[CH:13][C:9]([C:10]([OH:12])=[O:11])=[CH:8][CH:7]=1)=[O:4].C([O-])([O-])=O.[K+].[K+].[CH2:21](I)[CH3:22].[OH-].[Na+]>O1CCCC1.O>[CH2:21]([S:1][CH2:2][C:3]([NH:5][C:6]1[CH:14]=[CH:13][C:9]([C:10]([OH:12])=[O:11])=[CH:8][CH:7]=1)=[O:4])[CH3:22]15 g of 4-mercaptoacetylaminobenzoic acid are dissolved in 450 ml of tetrahydrofuran, and 39.2 g of potash and 17.24 ml of ethyl iodide in 50 ml of tetrahydrofuran are added thereto. The mixture is stirred overnight and 200 ml of water and a small amount of 2N NaOH are added thereto. The cloudy solution is filtered, and the filtrate is acidified with concentrated HCl. The yellow crystals are filtered off with suction and recrystallised from isopropanol, yielding 4-ethylmercaptoacetylaminobenzoic acid having a melting point of 209°-211°.
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