Dissolved 16E (3.0 g, 7.118 mmol) prepared in the previous step in 30 mL of DMF and added EDC (2.39 g, 12.46 mmol) and HOBt (1.65 g, 12.01 mmol). The mixture was stirred for at least 15 minutes. Next, DIEA (2.76 mL, 21.35 mmol) was added along with tert-butyl piperazine-1-carboxylate (1.39 g, 7.47 mmol). This reaction mixture was then stirred at RT for about four hours. After the disappearance of 16E, this mixture was poured into 300 mL of water, a white precipitate was formed. The white solid was filtered, rinsed with water and dried under vacuum to provide tert-butyl 4-(1-(1-(benzyloxycarbonyl)piperidin-3-yl)-2-oxo-5-phenyl-2,3-dihydro-1H-imidazole-4-carbonyl)piperazine-1-carboxylate (16F) (3.2 g, 76%). 1H NMR (400 MHz, DMSO-d6) δ ppm 1.34 (s, 9H) 1.40 (s, 2H) 1.65-1.80 (m, 4H) 2.34 (br. s., 1H) 2.83 (br. s., 4H) 3.13-3.19 (m, 4H) 3.94-4.07 (m, 2H) 5.03 (s, 2H) 7.23-7.50 (m, 10H) 10.73 (s, 1H). ESI-MS: m/z 590.1 (M+H)+.
