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C. H. V. KUMAR ET AL.
Synthesis of Anthranilic Acids
To a stirred solution of indole=indigo (10 mmol) and bromamine-T (30 mmol)
in alkaline (pH 12) acetonitrile=water (1:1) mixture (20 mL), PdCl2 (1 mol%) was
added, and the mixture was heated at 60 ꢀC for 2–4 h. The reaction progress was mon-
itored by thin-layer chromatography (TLC). After completion of the reaction, the
reduction product of bromamine-B, PhSO2NH2, was extracted with ethyl acetate,
identified by TLC, and confirmed by MS analysis. The aqueous part of the reaction
mixture was neutralized with acid, and the solvent was evaporated under reduced
pressure. The residue was dissolved in dichloromethane. The dichloromethane layer
was washed twice with water and dried over sodium sulfate. The solvent was evapo-
rated under reduced pressure, and the residue thus obtained was purified by passing
through a short silica-gel column using dichloromethane as eluent. Evaporation of
the solvent yielded anthranilic acids in around 95% yield. Alternatively, after extract-
ing out p-toluenesulfonamide, the aqueous layer was neutralized with acid and the
anthranilic acids were estimated as their zinc anthranilates. The procedure for the esti-
mation is as follows: anthranilate was precipitated as its zinc salt.[22] To the reaction
mixture, a calculated volume of 1 mol dmꢁ3 HCl was added, followed by 10 mL of pH
5.0 buffer and 10 mL of 1% zinc sulfate. The precipitate formed was filtered, dried at
105–110 ꢀC, and weighed, and the recovery of (C7H6O2N)2Zn was around 95%. The
reaction times and yields are given in Table 1. The products were identified by TLC
and mp by comparing with authentic samples. Further, they were subjected to MS
analysis, which showed a molecular ion peak of 137 amu, confirming anthranilic acid.
Other peaks observed in the mass spectrum can be interpreted in accordance with the
observed structure.
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