To a solution of tert-butyl 6-[(ethoxycarbonyl)amino]-1,3,4,5-tetrahydro-2H-pyrido[4,3-b]indole-2-carboxylate (8.7 g, 24.3 mmol) in CH2Cl2 (40 mL) at rt was added TFA (20 mL) and stirred at rt for 3 hours before the solvent was removed by the nitrogen stream. The solution was diluted with water (30 mL) and extracted with CH2Cl2 (2×30 mL). The combined extracts were dried over magnesium sulfate, and concentrated to afford an crude residue that was re-dissolved in CH2Cl2 (40 mL) followed by saturated K2CO3 (30 mL), ethyl chloroformate (2.4 mL, 4.0 mmol) and stirred at 25° C. for 0.5 hour. The solution was diluted with water (30 mL) and extracted with CH2Cl2 (2×30 mL). The combined extracts were dried over magnesium sulfate, concentrated, and purified by flash chromatography to afford ethyl 6-[(ethoxycarbonyl)amino]-1,3,4,5-tetrahydro-2H-pyrido[4,3-b]indole-2-carboxylate (4.5 g, 67%) as a clear oil. MS [M+H]+332. 1H NMR (CDCl3, 300 MHz): δ 7.00 (t, 1H, J=7.7 Hz), 6.82 (m, 1H), 6.71 (m, 1H), 4.62 (s, 2H), 4.02-4.31 (m, 4H), 3.82 (t, 1H, J=5.5 Hz), 2.82 (t, 1H, J=5.5 Hz), 1.20-1.24 (m, 6H) ppm.
