To a 50-mL round-bottomed flask was added (R)-ethyl 5-(benzyloxy)-4-((tert-butoxycarbonyl)amino)-3-oxopentanoate (612) (0.51 g, 1.19 mmol), K2CO3 (0.41 g, 2.97 mmol), 2-bromo-1-(2-methyl-3-((1-methylcyclopropyl)amino)quinoxalin-5-yl)ethanone (611) (0.63 g, 1.90 mmol), and DMF (4 mL). The reaction mixture was stirred at RT for 1 h. The reaction mixture was diluted with sat NH4Cl (5 mL) and extracted with EtOAc (10 mL). The organic extract was washed with water (5 mL), dried over Na2SO4, and concentrated. Ethyl 5-(benzyloxy)-4-((tert-butoxycarbonyl)amino)-2-(2-(2-methyl-3-((1-methylcyclopropyl)amino)quinoxalin-5-yl)-2-oxoethyl)-3-oxopentanoate (355a) was obtained as a light yellow oil and used without further purification. m/z (ESI, +ve) 619.3 (M+H)+. To a 15-mL glass tube was added ethyl 5-(benzyloxy)-4-((tert-butoxycarbonyl)amino)-2-(2-(2-methyl-3-((1-methylcyclopropyl)amino)quinoxalin-5-yl)-2-oxoethyl)-3-oxopentanoate (355a), crude from previous step, NH4OAc (0.37 g, 4.75 mmol), EtOH (5 mL), and AcOH (1.5 mL). The tube was sealed and heated at 45° C. for 16 h. The reaction mixture was cooled to RT and was concentrated to half of its volume. The mixture was diluted with EtOAc (10 mL) and water (5 mL). The organic extract was washed with NaOH (1 N, 5 mL), water (5 mL), brine (5 mL), dried over Na2SO4, and concentrated. The crude material was absorbed onto a plug of silica gel and purified by chromatography through a Redi-Sep pre-packed silica gel column (40 g), eluting with a gradient of 10-30% EtOAc in hexanes to give ethyl 2-(2-(benzyloxy)-1-((tert-butoxycarbonyl)amino)ethyl)-5-(2-methyl-3-((1-methylcyclopropyl)amino)quinoxalin-5-yl)-1H-pyrrole-3-carboxylate (355b) (0.28 g, 40% yield) as a yellow solid. 1H NMR (400 MHz, DMSO-d6) δ 11.75 (br. s., 1H), 8.06 (d, J=1.96 Hz, 1H), 7.92 (dd, J=1.17, 7.63 Hz, 1H), 7.62 (dd, J=1.17, 8.02 Hz, 1H), 7.49 (s, 1H), 7.39 (t, J=7.82 Hz, 1H), 7.24-7.31 (m, 5H), 7.12 (br. d, J=9.20 Hz, 1H), 5.61-5.72 (m, 1H), 4.44-4.58 (m, 2H), 4.23 (q, J=7.04 Hz, 2H), 3.62-3.67 (m, 2H), 2.49 (s, 3H), 1.58 (s, 3H), 1.41 (s, 9H), 1.29 (t, J=7.04 Hz, 3H), 0.97-1.02 (m, 2H), 0.84-0.89 (m, 2H). m/z (ESI, +ve) 600.2 (M+H)+.
