The general procedure for the synthesis of 2-amino-4,6-dimethoxybenzoic acid from 4,6-dimethoxy-2,3-dioxoindoline is as follows:
1. 3,5-dimethoxyaniline (199 g, 1.30 mol) was dissolved in ether (5.0 L) in a 5 L triple-necked flask and cooled to 0°C.
2. After 45 min, HCl gas (227 g) was passed into the solution, keeping the reaction temperature at 10°C for 45 min.
3. The reaction mixture was filtered, washed with 4 L of ether, followed by isopropyl acetate under high vacuum and dried at 45 °C overnight to give a white solid hydrochloride (242.3 g, 98% yield).
4. The above hydrochloride (20 g, 0.05 mol) was mixed with oxalyl chloride (33 mL) in a three-necked flask equipped with a reflux condenser and heated to an external temperature of 170 °C for 2 h. During this time, oxalyl chloride was removed by distillation.
5. The flask was cooled to 0 °C, methanol (40 mL) was added, heated to reflux for 45 min, filtered while hot, and washed with methanol (80 mL) to obtain a yellow-green solid 4,6-dimethoxyisatin (17.2 g, yield 79%).
6. Dissolve indigo red (162g, 0.78 mol) in 40% aqueous NaOH solution (1.5L), heat to an external temperature of 70°C, and slowly add 35% H2O2 (405mL). After each batch of H2O2 was added, the internal temperature of the reaction was increased from 64°C to 80°C.
7. After addition, the reaction was continued at 70 °C until the bubbles disappeared and stirred for an additional 2 hours, followed by cooling to room temperature and stirring overnight.
8. the mixture was reheated to 70 °C, additional H2O2 (75 mL) was added and stirring was continued for 2 hours until the reaction was complete.
9. Cooled to 10 °C, saturated Na2S2O3 solution (150 mL) was added and the pH was adjusted to 8 with 37% HCl (1.6 L) and to pH 6 with glacial acetic acid (75 mL), keeping the reaction temperature at no more than 40 °C during this time.
10. The reaction mixture was filtered and washed with water (4L) to give a brown solid amino acid (83.7g, 55% yield).
11. Dissolve the amino acid (82.7 g, 0.42 mol) in anhydrous THF (4.2 L), add EDCl (89.2 g, 0.48 mol), HOBT (65 g, 0.48 mol), and NMM (51.3 mL), and stir at room temperature for 3 hours.
12. add 50% aqueous NH3 (83 mL) and stir at room temperature for 16 hours.
13. added water (1.25L), extracted with DCM (2 x 250mL), combined the organic phases, washed with water (2 x 500mL), concentrated and slurried with ether (550mL), filtered, and dried under high vacuum to give 4,6-dimethoxy-2-aminobenzamide as a brown solid (46.7g, 57% yield).
14. 2-Amino-4,6-dimethoxybenzamide (1.06 g, 5.4 mmol), 3,5-dimethyl-4-hydroxybenzaldehyde (0.810 g, 5.4 mmol), K2CO3 (0.747 g, 5.4 mmol), and I2 (1.645 g, 6.5 mmol) were dissolved in DMF (20 mL) and heated at 80 °C for 12 hour.
15. Cooled to room temperature, poured into crushed ice, collected the solid and purified by column chromatography to afford 2-(4-hydroxy-3,5-dimethylphenyl)-5,7-dimethoxyquinazolin-4(3H)-one as a white solid (0.9 g, 51% yield).
Melting point: 291-293°C.
