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With bis-triphenylphosphine-palladium(II) chloride In toluene at 20 - 90℃;
To a degassed stirred solution of 6-bromo quinoxaline (2.0 g, 9.50 mmol) in toluene (20 mL), 1- ethoxy vinyl tributyltin (3.8 g, 10.5 mmol) followed by Pd(PPh3)2CI2 (0.67 g, 0.95 mmol) were added at RT and stirred at 90 °C overnight. After completion of the reaction (monitored by TLC), the reaction mixture was cooled to RT, filtered through celite and the obtained filtrate was evaporated under vacuum. To the resulting crude mixture, 6 N HCI solution (20 mL) was added and the mixture was stirred at RT for 1 h. The solution was neutralized with sat. NaHC03 and the aqueous layer was extracted with DCM (2 x 100 mL). The combined organic layer was dried over Na2S04 and concentrated under vacuum. The resulting crude material was purified by flash chromatography (Biotage Isolera, eluent: 30percent EtOAc in hexane) to afford the title compound. Yield: 45percent (800 mg, brown solid). 1H NMR (400 MHz, DMSO-tf6): δ 9.06-9.04 (m, 2H), 8.70 (d, J = 2.4 Hz, 1 H), 8.28 (dd, J = 8.8, 2.8 Hz, 1 H), 8.16 (d, J = 8.4 Hz, 1 H), 2.97 (s, 3H). LCMS: (Method A) 173 (M+H), Rt. 2.2 min, 99.1 percent (Max).
With bromine In acetonitrile for 2.5 h; Reflux; Inert atmosphere
General procedure: Bromine was added dropwise to a magnetically stirred refluxing solution of quinoxaline (1) or tetrahydroquinoxaline 15 or 19 in the relevant solvent. The resulting reaction mixture was heated at reflux temperature. The reaction was monitored by TLC or 1H NMR spectroscopy. After the desired time, the resulting reaction mixture was allowed to cool to room temperature and the solvent was removed under reduced pressure. The mixture was diluted with a saturated solution of sodium carbonate (10mL) and the mixture was extracted with ethyl acetate (2×25mL). Combined organic layers were washed with water, dried over Na2SO4 and concentrated. The crude was purified appropriate method described in below.
With N-Bromosuccinimide; dibenzoyl peroxide; In chloroform; for 45h;Reflux; Inert atmosphere;
General procedure: A solution of quinoxaline (1) (390mg, 3.0mmol), NBS (390mg, 3.0mmol), and benzoyl peroxide (catalytic amount) in glacial acetic acid (10mL) was heated at reflux temperature for 20h. The reaction was monitored by TLC or 1H NMR spectroscopy. The resulting reaction mixture was allowed to cool to room temperature and the solvent was removed under reduced pressure. The mixture was diluted with a saturated solution of sodium carbonate (10mL) and the mixture was extracted with ethyl acetate (2×25mL). Combined organic layers were washed with water, dried over Na2SO4 and concentrated. 6-Bromoquinoxaline (10) (315mg, 50%) was obtained as a sole product. The reaction was repeated using DMF as a solvent at the same reaction condition and monobromide 10 was obtained in 51% yield.
With bromine; barium carbonate; In acetonitrile; for 25h;Reflux; Inert atmosphere;
Bromine (3.84g, 24mmol) was added dropwise to a magnetically stirred refluxing mixture of quinoxaline (1) (390mg, 3.0mmol) and barium carbonate (1.20g, 6.1mmol) in acetonitrile (20mL). The resulting reaction mixture was heated at reflux temperature for 25h and allowed to warm to room temperature. The solvent was evaporated and the mixture was diluted with a saturated solution of sodium carbonate (10mL). The mixture was extracted with ethyl acetate (3×25mL) and combined organic layers were washed with water, dried over Na2SO4 and concentrated. The residue was purified via column chromatography on silica gel (100g) by eluting with 15% EtOAc/n-hexane. The first fraction was 6-bromoquinoxaline (10) (195mg, 31%) data as before. The second fraction was 5,8-dibromoquinoxaline (12) (45mg, 5%) data as before. The third fraction was 6,7-dibromoquinoxaline (13) (50mg, 6%) data as before. The fourth fraction was 5,7-dibromoquinoxaline (11) (120mg, 14%) data as before. The fifth fraction was 5,6-dibromoquinoxaline (14) (240mg, 28%) data as before.