To a solution of 3-azido-2,2-bis(3-bromophenyl)propyl methanesulfonate (1.3 g, 2.66 mmol) in anhydrous toluene (10 mL) and anhydrous THF (5 mL) under N2 was added triethyl phosphite (0.49 mL, 2.79 mmol) at 25° C. The mixture was stirred for 18 h. The reaction was concentrated by rotary evaporation in a dried apparatus. The residue was dried in vacuo, and used in the next reaction without further purification. The crude triethyl phosphorimidate was dissolved in anhydrous m-xylene (5 mL) under N2 and heated in an oil bath at 150° C. for 12 h. After cooled to room temperature, the solvent was removed by rotary evaporation (vacuum pump assisted) to give a thick light orange oil which was purified by prep-TLC (eluent with EtOAc: dichloromethane=1:5) to afford 960 mg of diethyl 3,3-bis(3-bromophenyl)azetidin-1-ylphosphonate (1.9 mmol, 71% after two steps). LC/MS: [M+1]=504. 1HNMR (CDCl3), 400 MHz: δ 1.25-1.36 (m, 6H), 4.05-4.39 (m, 4H), 4.40 (d, 2H, J=5.2
