Sodium triacetoxyhydroborate (12.3 mg, 0.058 mmol) was added to a solution of DIPEA (10 μL, 0.058 mmol), 3-(4-(2,4-difluorophenoxyl)piperidin-1-yl)-N-isopropyl-5,6,7,8-tetrahydropyrido[3,4-b]pyrazin-2-amine TFA salt (15.0 mg, 0.029 mmol) and formaldehyde (2 μL, 0.029 mmol) in MeOH (290 μL) at rt. After 30 min, the mixture was purified by HPLC Method A to give the title compound as a TFA salt (12.3 mg, 80%) as a yellow film. 1H NMR (400 MHz, methanol-d4) δ ppm 1.25 (d, J=6.6 Hz, 6H), 1.90-2.00 (m, 2H), 2.07-2.16 (m, 2H), 2.96-3.05 (m, 3H), 3.08 (br s, 4H), 3.33-3.42 (m, 2H), 3.43-3.85 (m, 2H), 4.14-4.38 (m, 3H), 4.44-4.51 (m, 1H), 6.84-6.91 (m, 1H), 6.95-7.02 (m, 1H), 7.17 (td, J=9.2, 5.6 Hz, 1H); ESI-MS m/z [M+H]+ 418.3.
