N[C:2]1[CH:3]=[CH:4][CH:5]=[C:6]2[C:11]=1[CH:10]=[C:9]([S:12]([OH:15])(=[O:14])=[O:13])[CH:8]=[CH:7]2.[OH:16]S([O-])=O.[Na+].[OH-].[Na+].Cl>O>[OH:16][C:2]1[CH:3]=[CH:4][CH:5]=[C:6]2[C:11]=1[CH:10]=[C:9]([S:12]([OH:15])(=[O:14])=[O:13])[CH:8]=[CH:7]2A solution of 8-amino-2-naphthalenesulfonic acid (Aldrich, 2.40 g, 10.8 mmol) and 3.9M aq. NaHSO3 solution (20.0 mL, 77.0 mmol) in water (10.0 mL) was refluxed for 15 h. The reaction mixture was then basified with 30% aq. NaOH solution (430 mg, 10.8 mmol) and refluxed for 4 h. The reaction mixture was neutralized with conc. HCl and then concentrated in vacuo. The remaining solid was azeotroped with 1:1 MeOH/PhMe (3×) to give the title compound (2.41 g, 95%). 1H NMR (DMSO-d6) δ 9.43 (br s, 1H), 8.36 (s, 1H), 7.96 d, J=8.4 Hz, 1H), 7.88-7.78 (m, 2H), 7.52 (t, J=7.8 Hz, 1H), 7.43 (d, J=7.3 Hz, 1H); MS(ESI−) m/z 222.3 (M−H)−.
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