[N:1]1[CH:6]=[CH:5][CH:4]=[CH:3][C:2]=1[S:7][S:8][CH2:9][CH2:10][CH2:11][C:12]([OH:14])=[O:13].N1C=CC=CC=1SSC1C=CC=CN=1.[S:29](Cl)(=[O:32])(=[O:31])[OH:30].[OH-].[Na+]>ClCCCl.CCOC(C)=O.CCCCCC>[N:1]1[CH:6]=[CH:5][CH:4]=[CH:3][C:2]=1[S:7][S:8][CH2:9][CH2:10][CH:11]([S:29]([OH:32])(=[O:31])=[O:30])[C:12]([OH:14])=[O:13]To a solution of 4-(pyridin-2-yldisulfanyl)butanoic acid (725 mg, 3.16 mmol) and 1,2-di(pyridin-2-yl)disulfane (700 mg, 3.18 mmol) in 15 ml of DCE (1,2-dichloroethane) at 75° C. was added chlorosulfuric acid (300 μL, 4.51 mmol). After stirring for 20 min at 75° C., another portion of chlorosulfuric acid (300 μL, 4.51 mmol) was added. The mixture was allowed to stir at 75° C. for 20 min, then another portion of chlorosulfuric acid (200 μL, 3.0 mmol) was added. Again 25 min later, the final portion of chlorosulfuric acid (200 μL, 3.0 mmol) was added and the mixture was stirred for further 25 min. The mixture was immediately cooled on ice bath, neutralized with 1M NaOH to pH ˜7, diluted with EtOAc/Hexane (1:1), separated, and the organic layer was washed with pure water (3×25 ml) while the generated each of aqueous layer was washed with EtOAc/Hexane (1:1, 35 ml). The aqueous layers were combined, acidified with HCl/HOAc to pH 3˜4, concentrated to ˜10 ml, diluted with MeCN (100 ml), sonicated (or quickly stirred) for 1 h, filtered, and the resulting pellet washed with water/MeCN (1:10). The solution was then concentrated and purified on a SiO2 cartridge (40 g) eluting with water/MeCN/HOAc (1:10:0.01). To the pooled fractions containing the product, DMF (˜5 ml) was added and evaporated to dryness to afford 4-(pyridin-2-yldisulfanyl)-2-sulfobutanoic acid (295 mg, 0.954 mmol, 30.2% yield).
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