Example 72: Synthesis of 3-oxo-2,3-dihydro-[1,2,4]triazolo[4,3-a]pyridine-6-carbonyl chloride:
1. 6-Chloronicotinic acid (20 g, 127 mmol) was used as raw material, which was dissolved in 200 mL of ethanol and hydrazine hydrate (14.8 mL, 317 mmol) was added. The reaction mixture was refluxed overnight, cooled to room temperature and precipitated as a solid, which was collected by filtration and washed with petroleum ether/ethyl acetate (2:1) to give 18 g of 6-hydrazinonicotinic acid as a yellow solid (yield: 91%).1H NMR (DMSO, 400 MHz) δ: 8.52 (s, 1H), 7.88 (dd, J = 2.0 Hz, 8.8 Hz, 1H), 6.63 (d, J = 8.8 Hz, 1H).
2. 6-Hydrazinonicotinic acid (18 g, 117 mmol) was suspended in 100 mL of anhydrous tetrahydrofuran, anhydrous pyridine (23 g, 293 mmol) was added, and after stirring at room temperature for 15 min, bis(trichloromethyl) carbonate (80 g, 270 mmol) was slowly added. The reaction mixture was stirred at room temperature overnight, methanol was added and stirring was continued for 15 h. A solid was precipitated, which was collected by filtration and washed with petroleum ether/ethyl acetate (1:1) to afford 20 g of dimethyl 3-oxo-[1,2,4]triazolo[4,3-a]pyridine-2,6(3H)-dicarboxylate.1H NMR (CDCl3, 400 MHz) δ: 3.95 (s, 3H) , 4.13 (s, 1H), 7.13 (dd, J = 1.6 Hz, 10 Hz, 1H), 7.71 (dd, J = 1.6 Hz, 10 Hz, 1H), 8.55 (t, J = 2 Hz, 1H).
3. 3-Oxo-[1,2,4]triazolo[4,3-a]pyridine-2,6(3H)-dicarboxylic acid dimethyl ester (10 g, 40 mmol) was dissolved in 60 mL of tetrahydrofuran, 10 mL of water and LiOH-H2O (4.8 g, 114 mmol) were added, and after stirring at room temperature for 6 h, the tetrahydrofuran was removed by vacuum concentration, and the tetrahydrofuran was adjusted to pH 2 by adding 1 mL of HCl solution. The pH was adjusted to 2. A solid was precipitated and collected to give 11 g of 3-oxo-2,3-dihydro-[1,2,4]triazolo[4,3-a]pyridine-6-carboxylic acid, which was used directly in the next step.1H NMR (DMSO, 400 MHz) δ: 7.23 (d, J = 10 Hz, 1H), 7.48 (dd, J = 1.6 Hz, 10 Hz, 1H), 8.25 (s, 1H), 8.25 (s, 1H), 8.25 (s, 1H) and 8.25 (s, 1H). 8.25 (s, 1H), 9.13 (s, 1H), 12.89 (s, 1H).
4. 3-Oxo-2,3-dihydro-[1,2,4]triazolo[4,3-a]pyridine-6-carboxylic acid (3 g, 16.8 mmol) was dissolved in 60 mL of anhydrous dichloromethane, oxalyl chloride (8.8 g, 74 mmol) and 1 mL of DMF were added, and the mixture was concentrated after stirring for 6 hours at room temperature to give 3 g of the title compound, 3-oxo-2,3-dihydro-[1 ,2,. 4]triazolo[4,3-a]pyridine-6-carbonyl chloride as a gray solid, which was used directly in the next reaction.
