O.[C:2]([OH:6])(=[O:5])[CH:3]=[O:4].C(N(CC)CC)C.C([O:16][C:17](=[O:31])[C:18](=O)[CH2:19][S:20][C:21]1[S:22][C:23]2[CH:29]=[CH:28][CH:27]=[CH:26][C:24]=2[N:25]=1)C>C(Cl)Cl.CO>[S:22]1[C:23]2[CH:29]=[CH:28][CH:27]=[CH:26][C:24]=2[N:25]=[C:21]1[S:20][C:19]1[CH:18]([C:17]([OH:31])=[O:16])[O:5][C:2](=[O:6])[C:3]=1[OH:4]To a solution of glyoxylic acid monohydrate (98 mg, 1.03 mmol) in 6 mL methylenechloride/methanol (5:1) was added triethylamine (418 μL, 3.0 mmol) and stirred viogorously. 3-(benzothiazol-2-ylsulfanyl)-2-oxo-propionic acid ethyl ester (281 mg, 1.06 mmol) in 6 mL methylenechloride/methanol (5:1) was added dropwise over a period of 3 hours and the reaction was stirred overnight (15 hrs) at room temperature. The reaction mixture was then neutralized with NaH2PO4 and washed with ethylacetate. The resulting very polar product remained in the aqueous layer and was purified by reverse-phase HPLC. The pure fractions were pooled and concentrated under reduced pressure. Upon evaporation of solvent, 3-(benzothiazol-2-ylsulfanyl)-4-hydroxy-5-oxo-2,5-dihydro-furan-2-carboxylic acid precipitated out (70 mg) as a white solid which was filtered and washed with water. The filtrate was collected and lyophilized to afford a pale yellow solid which was determined to be the triethylamine salt of the desired product. 1H-NMR (MeOD, 300 MHz) δ (ppm): 7.92-7.87 (m, 2H), 7.51-7.38 (m, 2H), 5.68 (s, 1H). MS (ESI) m/z: 310 (M+H+).
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