To a vigorous stirred suspension of 4-amino-butyric acid benzyl ester toluene-4-sulfonic acid (0.730 g, 2.00 mmol) and triphosgene (0.200 g, 0.667 mmol) in dichloromethane (40 mL) at −78° C. under nitrogen atmosphere, was added triethylamine (1.0 mL, 7.188 mmol, in 10 mL dichloromethane) dropwise via an additional funnel over a period of 2 hours. The cooling bath was removed and the resulting reaction was allowed to stir at room temperature for 1 hour, after which time 4-dimethylaminobenzylamine dihydrochloride (0.446 g, 2.00 mmol) was added to the reaction mixture, followed by triethylamine (0.67 mL, 4.8 mmol). The resulting reaction was allowed to stir for about 18 h, then the reaction mixture was diluted with brine (20 mL), transferred to a separatory funnel and the aqueous layer was extracted with dichloromethane. The organic layer was dried over sodium sulfate and concentrated in vacuo to provide a crude residue which was purified by flash column chromatography (silica gel 60, CH2Cl2/EtOAc=4:1 to 2:1) to afford 4-[3-(4-Dimethylamino-benzyl)-ureido]-butyric acid benzyl ester (0.686 g, 93% yield). 1H NMR (300 MHz, CDCl3) δ (ppm) 7.31 (m, 5H), 7.09 (d, J=9.0 Hz, 2H), 6.63 (d, J=9.0 Hz, 2H), 5.20 (t, J=5.4 Hz, 1H), 5.09 (t, J=5.4 Hz, 1H), 5.05 (s, 2H), 4.14 (d, J=5.7 Hz, 2H), 3.09 (dt, J=6.6, 6.3 Hz, 2H), 2.86 (s, 6H), 2.32 (t, J=7.5 Hz, 2H), 1.73 (m, 2H). 13C NMR (75 MHz, CDCl3) δ (ppm) 173.3, 158.5, 149.8, 135.8, 128.5, 128.15, 128.09, 127.0, 112.6, 66.2, 43.9, 40.7, 39.5, 31.5, 25.4.
