1. Squaric acid (1.0023 g) was dissolved in 30 mL of anhydrous ethanol and heated to reflux overnight. After the reaction was completed, the reaction solution was concentrated and purified by fast column chromatography to give 0.7430 g of diethyl ester of squaric acid in 50% yield.
2. The resulting diethyl (ethyl) squarate, 2-aminopyridine (100 mg, 0.588 mmol) and triethylamine (10 drops) were dissolved in dichloromethane (10 mL) and stirred at room temperature overnight. After completion of the reaction, the reaction solution was concentrated under reduced pressure and purified by fast column chromatography (petroleum ether/ethyl acetate, 2:1) to afford compound 6 as a white solid in 73% yield.
3. The NMR data of compound 6 were as follows: 1H NMR (400 MHz, acetone-d6) δ 10.26 (s, 1H), 8.50-8.29 (m, 1H), 7.98-7.78 (m, 1H), 7.68 (d, J = 8.3 Hz, 1H), 7.14 (dd, J = 6.9, 5.3 Hz, 1H), 4.85 (q, J = 7.1 Hz, 2H), 1.48 (t, J = 7.1 Hz, 3H); 13C NMR (100 MHz, acetone-d6) δ 186.42, 180.83, 170.33, 166.86, 152.22, 149.33, 139.43, 120.17, 113.63, 70.65, 16.06.
