1. 290 kg of methylamine solution was added to the reactor, 59 kg of methyl 4-(2-chlorophenyl)-2-(((2-(1,3-dioxoisoindolin-2-yl)ethoxy)methyl)-6-methyl-1,4-dihydropyridine-3-carboxylic acid ethyl ester-5-carboxylate (o-benzoylcarbamodiphenyl) was added with stirring, and the reaction was carried out with stirring for 20 hrs. at 25 °C. After completion of the reaction, the reaction was cooled to 20°C and the reaction mixture was filtered. The filtrate was washed with potable water until the pH of the wash water was ≤ 10, followed by rinsing the filtrate with purified water. The filtrate residue was dried at 60°C until the moisture content was ≤1.5%, cooled at room temperature and discharged to obtain amlodipine.
2. 10 g of amlodipine and 100 ml of water were added to a 250 ml aubergine flask and stirred at room temperature. 3.9 g of maleic acid was dissolved in 40 ml of water and slowly added dropwise to the amlodipine solution at a constant temperature of 30°C. The solution was then stirred at room temperature. After completion of the dropwise addition, stirring was continued for 4 h. Subsequently, filtration was carried out. The filter cake was washed with water twice, and the filter cake was dried under vacuum at 65 °C for more than 8 h. A white amlodipine maleate crude product was obtained with 91% yield and 99.2% purity.
3. 10 g of crude amlodipine maleate was taken and added to a 250 ml aubergine bottle, 80 ml of methanol was added and heated to 60 °C with stirring until completely dissolved. Subsequently cooled to 50°C, 1 g of activated carbon was added, stirred well and then stirred at 65°C for 30 minutes. The activated carbon was removed by hot filtration and the aubergine bottle was washed with 20 ml of methanol. The filtrate was distilled under reduced pressure, concentrated to 40 ml and then 120 ml of ethyl acetate was added and azeotropic distillation was carried out to concentrate the volume to 50 ml. After cooling at room temperature, it was further cooled and stirred at 0°C for 2 hrs, filtered, dried under reduced pressure to give amlodipine maleate in 91.5% yield and 99.5% purity.
