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• Synthesis

  

  Synthesis of Methyl p-aminobenzoate hydrochloride (3)

  In a 1L flask, 500mL of anhydrous methanol was added and cooled in an ice-water bath. 65mL of thionyl chloride was slowly added dropwise, with the internal temperature controlled not to exceed 15℃. After the addition was complete, the mixture was stirred for 15min, and then 100g of p-aminobenzoic acid was added at once. The mixture was slowly heated under reflux for 3.5h, cooled, and a white solid precipitated. The solid was filtered, washed with cold methanol, and dried to obtain 136.1g of intermediate 3, with an almost quantitative yield.

  Synthesis of methyl hydrazine benzoate hydrochloride (5) In a 1L three-necked flask, add 300mL of water, slowly add 55mL (0.6mol) of concentrated hydrochloric acid, then add 50g (0.27mol) of intermediate 3, cool to below -10℃, and dropwise add 50mL of aqueous solution containing 20g (0.29mol) of NaNO2, controlling the temperature of the reaction solution to not exceed 0℃. When the addition is almost complete, frequently check with starch-KI test paper. If the test paper turns blue quickly a few minutes after adding the NaNO2 solution, stop the addition and continue stirring at <0℃ for later use. In another 1L three-necked flask, add 280mL of water, and add 87g (0.69mol) of Na2SO3 while stirring, so that most of the solid dissolves. Then adjust the pH of the Na2SO3 solution to 6.5~6.8 with concentrated hydrochloric acid. The above diazotization solution is slowly added to the Na2SO3 solution in a thin stream, while controlling the temperature of the reaction solution to not exceed 20℃. After the addition, stir at room temperature for 0.5 h, then add 20 ml of concentrated hydrochloric acid, slowly heat to 90 °C and reflux for 3 h, let it cool naturally to room temperature, stir at 10 °C for 0.5 h, filter, wash the filter cake with 20 mL of 10% (mass fraction) hydrochloric acid, then wash with ice water, and dry under vacuum under reduced pressure to obtain 48.6 g of yellow solid, yield 90%, mp 233~235 °C. Synthesis of 4-Hydrazinylbenzoic acid (1) In a 500 mL flask, add 30 g (0.148 mol) of intermediate 5 and 200 mL of 1:1 (V/V) ethanol-water, stir, and slowly add 40 ml of 50% (mass fraction) NaOH solution. Under nitrogen protection, heat to reflux until there is no raw material, which takes about 3 h, and recover all the ethanol. The mixture was cooled to room temperature, and the pH of the reaction solution was adjusted to 4-4.1 with concentrated hydrochloric acid. Stirring was continued at 10°C for 1 hour. The solution was filtered, washed with a small amount of ice water, and dried under vacuum to obtain 25.5 g of a yellow solid, with a yield of 91%.;

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