Azetidine-1,3,3-tricarboxylic acid 1-tert-butyl ester 3-ethyl ester (1 g; 3.67 mmol), triethyl-amine (0.52 ml; 3.7 mmol) and diphenylphosphoryl azide (0.8 ml; 3.67 mmol) in toluene (6 ml) are heated to 80° C. for 4 hours. tert-Butanol (6 ml) is added and heating continued for 4 hours. The reaction mixture is poured on 2N Na2CO3, extracted with TBME three times, the organic phases are combined, dried over Na2SO4 and evaporated to dryness. Purification via chromatography (SiO2; TBME/hexanes 2:8>1:0) yields the desired compound as colorless crystals. 1H-NMR (400 MHz; DMSO-d6: 8.04 (s, 1H); 4.14 (q, 2H); 4.09 (bd, 2H); 3.81 (bd, 2H); 1.39 (s, 18H); 1.19 (t, 3H). MS (m/z) ES+: 345 (MH+).
