General procedure for the synthesis of 2-amino-4,6-dichlorobenzoic acid from 4,6-dichlorodihydroindole-2,3-dione:
3) Preparation of intermediate I-3;
At room temperature, Intermediate I-2 (5.0 g, 23.1 mmol) prepared from Example I (2) of the above isolation was dissolved in 75 mL of 1N NaOH (aq), and hydrogen peroxide (28% v/v, 10 mL) was added in batches. The reaction mixture was stirred for 2 hours and then filtered to remove the insoluble dark brown solid. Subsequently, the filtrate was carefully acidified with hydrochloric acid to pH 2. The yellow precipitate formed was collected, washed with water and dried under vacuum. The target compound 2-amino-4,6-dichlorobenzoic acid (3.90 g, 82% yield) was obtained as an ivory colored solid by recrystallization from benzene.
TLC Rf = 0.1 (EtOAc:hexane = 1:1);
Melting point 188-189°C;
1H NMR (DMSO-d6) δ 6.76 (d, J=1.9Hz, 1H, ArH), 6.85 (d, J=1.9Hz, 1H, ArH);
MS (EI) m/e 206 [M+], 162 [M+-CO2].
