[Synthesis]
The general procedure for the synthesis of methyl 4-bromopyrazolo[1,5-a]pyridine-3-carboxylate and methyl 6-bromopyrazolo[1,5-a]pyridine-3-carboxylate by flow chemistry using 1-amino-3-bromopyridin-1-ium 2,4,6-trimethylbenzenesulfonate and methyl propargylate was as follows: a Vapourtec R-series flow chemistry system equipped with three pumps. MSH 1 (2 g, 71% wet solids, 6.60 mmol) and 3-bromopyridine (1.04 g, 6.60 mmol) were co-dissolved in THF/H2O (1:1, 33 mL, 0.2 M). Triethylamine (0.92 mL, 6.60 mmol) was dissolved in THF (8.25 mL, 0.8 M) and mixed at a flow rate of 2.86 mL/min and 1.07 mL/min through the first Y-mixer (1.5 equiv. of triethylamine). Methyl propiolate (0.56 g, 6.60 mmol) was dissolved in THF (8.25 mL, 0.8 M) and introduced through the second Y-mixer at a flow rate of 1.07 mL/min (the amount of methyl propiolate was 1.5 equivalents). The system solvent was THF. the volumes of the PFA reactor coils were set to 2 mL and 10 mL, and the temperatures were set to 30 °C and 90 °C, respectively. The reaction mixture from the first two inlets was allowed to remain in the first reactor for 31 s, followed by 2 min in the second reactor. The operating pressure was 7.5 bar. Upon completion of the reaction, the reaction unit was rinsed using a MeOH solution containing 33% HCl (concentrated) followed by IPA. The total outlet flow rate was 5 mL/min. the outlet stream was collected (40 mL over 8 min), concentrated under vacuum to remove THF, and subsequently diluted with EtOAc (250 mL) and brine (100 mL). The organic layer was separated and the aqueous phase was washed twice with EtOAc (2 x 200 mL). The organic layers were combined, dried with anhydrous sodium sulfate and concentrated in vacuum to give a dark red oil. The crude product was purified by column chromatography on a 100 g silica gel column using a Biotage system and a 7% to 60% EtOAc/heptane gradient. First to elute from the column was 5a as a light red solid (0.43 g, collection time 8 min, yield 42%).1H NMR (400 MHz, d6-DMSO) δ 4.10 (3H, s, CH3), 7.79 (1H, d, J=8 Hz, ArH), 8.26 (1H, d, J=8 Hz, ArH), 8.73 (1H, s ArH), 9.56 (1H, s, ArH) ppm. 13C NMR (101 MHz, d6-DMSO) δ 51.2, 103.3, 108.1, 118.8, 130.3, 131.4, 138.7, 144.6, 162.6 ppm. HRMS (FAB) calculated value C9H8O2N2Br 254.97637, measured value 254.97636/256.97421.Subsequently eluting from the column was 5b as a pale yellow solid (0.14 g, collection time 8 min, yield 14%).1H NMR (400 MHz, d6-DMSO) δ 3.82 (3H, s, CH3), 7.06 (1H, dd, J=4 and 4 Hz. ArH), 7.87 (1H, d, J=4Hz, ArH), 8.50 (1H, s, ArH), 8.93 (1H, d, J=4Hz, ArH) ppm.13C NMR (101MHz, d6-DMSO) δ 51.3, 104.7, 109.8, 114.4, 129.8, 132.8, 137.6, 145.6, 161.9 ppm. HRMS (FAB) calculated value C9H8O2N2Br 254.97637, measured value 254.97638/256.97424. |