General procedure for the synthesis of tert-butyl (S)-2-carbamoylazetidine-1-carboxylate from 1-Boc-L-acridine-2-carboxylate: Boc-L-2-azetidinecarboxylic acid (compound 8a, 2.91 g, 14.45 mmol) and triethylamine (Et3N, 2.01 mL, 14.45 mmol) were dissolved in tetrahydrofuran (THF, 33 mL) in tetrahydrofuran (THF, 33) and cooled to -10 °C. Under stirring, pre-cooled ethyl chloroformate was slowly added dropwise, and stirring was continued for 20 min by keeping the reaction temperature at -10 °C. Subsequently, 28% ammonia (NH4OH, 3.21 mL) solution was added and the reaction system was gradually warmed up to room temperature. Upon completion of the reaction, the THF solvent was removed by distillation under reduced pressure. The residue was partitioned between ethyl acetate (EtOAc) and water, and the organic phase was separated, dried over anhydrous magnesium sulfate (MgSO4), filtered and the solvent was concentrated under reduced pressure to give compound 9a as a white crystalline solid (2.36 g, 82% yield). The product was characterized by 1H NMR (400 MHz, DMSO-d6): δ 1.34 (s, 9H), 1.96-1.98 (m, 1H), 2.30-2.39 (m, 1H), 3.75-3.80 (m, 2H), 4.37 (dd, J = 9.1, 5.5 Hz, 1H), 7.11 (s, 1H), 7.34 (s. 1H).
