[O:1]1[CH:5]=[CH:4][N:3]=[C:2]1[CH:6]([C:12]1[CH:17]=[CH:16][C:15]([O:18]C2CCCCO2)=[CH:14][CH:13]=1)[CH2:7][C:8]([O:10][CH3:11])=[O:9].O.C1(C)C=CC(S(O)(=O)=O)=CC=1>CO>[OH:18][C:15]1[CH:16]=[CH:17][C:12]([CH:6]([C:2]2[O:1][CH:5]=[CH:4][N:3]=2)[CH2:7][C:8]([O:10][CH3:11])=[O:9])=[CH:13][CH:14]=1A mixture of M11.4 (2.1 mmol), p-toluenesulfonic acid monohydrate (0.57 mmol) in MeOH (15 mL) was stirred at room temperature for 1.5 hours. After it was quenched with NaHCO3 (aqueous) solution, MeOH was removed by rotary evaporator. The residue was extracted with EtOAc, and the combined organic phase was washed with brine, dried over anhydrous sodium sulfate, and filtered through short plug of silica gel. After removing solvent, compound M11 was obtained. MS ESI (pos.) m/e 248.1 (M+H). 1H NMR (500 MHz) (DMSO-d6) δ 9.04 (s, 1H); 7.99 (s, 1H); 7.14 (s, 1H); 7.05 (m, 2H); 6.72 (m, 2H); 4.49-4.52 (m, 1H); 3.57 (s, 1H); 3.22-3.27 (m, 1H); 2.89-2.94 (m, 1H).
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