4.1. Preparation of Azidoisoxazole 1

Azidoisoxazole 1 was synthesized by following the previously reported procedure [12]. Sodium hydride (0.041 g, 1.7 mmol) was placed a 25 mL round bottom flask, washed three times with hexane (2 mL) and once with dichloromethane (2 mL), and then vacuum dried for 1 h. Dry THF (3 mL) was added to the round bottom flask and the reaction was cooled to −30 °C using a xylene–dry ice bath. A solution of 5-amino-3-phenyl isoxazole (0.16 g, 0.99 mmol) and tosyl azide (1.5 mL, 7.5 mmol) in dry THF (3 mL) was added dropwise to the cooled suspension over 10 min under an argon atmosphere. The xylene bath temperature was raised to room temperature and the black reaction mixture was stirred overnight, diluted with water (25 mL), extracted with diethyl ether (25 mL), and dried under reduced pressure. Azidoisoxazole 1 was purified by silica gel column chromatography eluted with 5% ethyl acetate in hexane. Recrystallized azidoisoxazole 1 crystals (diethyl ether; 50 mg, 17% yield) were characterized using ¹H-NMR and IR spectroscopy, and the obtained spectra corresponded well with the reported data [12]. ¹H-NMR (400 MHz, CDCl₃): 7.77–7.74 (m, 2H), 7.48–7.45 (m, 3H), 6.01 (s, 1H) ppm; IR (neat): 3129, 3059, 2121, 1598, 1570, 1469, 1368, 1168 cm⁻¹.