General procedure for the synthesis of tert-butyl 3-(4-nitro-1H-pyrazol-1-yl)azetidine-1-carboxylate from 4-nitropyrazole and N-Boc-3-hydroxyazetidine: DIAD (3.92 mL, 19.9 mmol, 1.5 eq.) was slowly added dropwise under nitrogen protection to a stirring mixture of 4-nitropyrazole (1.5 g, 13.27 mmol), N-Boc-3-hydroxyazetidine, and triphenylphosphine (5.22 g, 19.9 mmol, 1 eq.). 13.27 mmol), N-Boc-3-hydroxyazetidine (2.3 g, 13.27 mmol, 1 eq.) and triphenylphosphine (5.22 g, 19.9 mmol, 1.5 eq.) in a solution of THF (30 mL) and the reaction mixture was placed in an ice bath. The reaction was stirred at 0 °C for 10 min, then gradually warmed to room temperature and continued stirring for 16 h. The reaction was carried out at 0 °C for 10 min, then gradually warmed to room temperature and continued stirring. After completion of the reaction, the reaction mixture was diluted with ethyl acetate (100 mL) and washed sequentially with water (40 mL) and brine (30 mL x 2). The organic layers were combined, dried over anhydrous sodium sulfate and concentrated. The crude product was purified by silica gel column chromatography (petroleum ether/ethyl acetate = 1/10) to afford tert-butyl 3-(4-nitro-1H-pyrazol-1-yl)azetidine-1-carboxylate as a light yellow solid (3 g, 85% yield).ESI-MS (M + H-56): 213.1.1.1H NMR (400 MHz, CDCl3) δ: 8.28 (s 1H), 8.16 (s, 1H), 5.07-5.04 (m, 1H), 4.44-4.40 (m, 2H), 4.34-4.30 (m, 2H), 1.47 (s, 9H).
