[N:1]1[CH:6]=[CH:5][CH:4]=[CH:3][C:2]=1[C:7](O)=O.[NH2:10][C:11]1[CH:17]=[C:16]([N+:18]([O-])=O)[CH:15]=[CH:14][C:12]=1[S-:13].[Na+].[OH-].[Na+].[Sn](Cl)Cl>Cl>[N:1]1[CH:6]=[CH:5][CH:4]=[CH:3][C:2]=1[C:7]1[S:13][C:12]2[CH:14]=[CH:15][C:16]([NH2:18])=[CH:17][C:11]=2[N:10]=1PPA (500 g) is added to picolinic acid (24.6 g, 0.2 mol) and sodium 2-amino-4-nitrothiophenoxide (47 g, 0.2 mol). The mixture is heated at 120-130° C. for 6 hours. The resulting mixture is cooled and NaOH is added dropwise (pH=10). The product is filtered off and washed with water. The 2-(2-pyridyl)-5-nitrobenzothiazole is recrystallized from 9/1 isopropyl ether/methanol and is added to a solution of tin dichloride (61.6 g, 0.273 mol) in conc. HCl (50 ml) at 0° C. This mixture is heated at 95° C. for 120 minutes. It is cooled and NaOH is added dropwise (pH=10). The resulting mixture is extracted with chloroform and the combined organic phases are extracted with 0.5M HCl. The acidic aqueous phases are combined, basified with NaOH (pH=10) and extracted with ethyl acetate. The extracts are washed with water and evaporated. The solid obtained is recrystallized from isopropyl ether. Yield: 3.6 g; Rf (9/1 chloroform/methanol): 0.45; IR (KBr): 3400, 3325, 3215, 1590, 1430, 1320, 780 cm−1.
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