1. To a stirred suspension of 4-aminobenzoic acid (50 g, 0.364 mol) in a concentrated hydrochloric acid/water (46 mL/283 mL) mixture was added cyanamide (35 g, 0.839 mol) at room temperature.
2. The reaction mixture was heated to reflux at 100°C for 6 hours.
3. After completion of the reaction, the mixture was allowed to stand at room temperature for 16 hours (without stirring).
4. The precipitated solid was collected by filtration and washed with cold water (100 mL).
5. Suspend the resulting solid in aqueous potassium carbonate (30 g K2CO3 dissolved in 400 mL of water) and stir for 30 minutes.
6. The solid was filtered again, washed with water (250 mL) and subsequently dried under vacuum to afford the intermediate Int-2A (35 g, 53% yield) as a white solid.
7. Acetyl chloride (35 mL, 490 mmol) was slowly added dropwise to a stirred methanol (350 mL) suspension of Int-2A (35 g, 195.5 mmol) at 0°C and protected by an inert atmosphere for 20 minutes.
8. After the dropwise addition, the reaction mixture was stirred at room temperature for 16 hours.
9. After confirming complete consumption of the raw material by thin layer chromatography (TLC), the reaction mixture was neutralized with solid sodium bicarbonate at 0 °C.
10. Excess sodium bicarbonate was removed by filtration and the filtrate was concentrated under vacuum to give the crude product.
11. The crude product was washed with ethyl acetate (100 mL) to afford the purified intermediate Int-2B (35 g, 92% yield) as a white solid.
