[Synthesis]
General procedure: 4,5-dimethoxy-2-nitroaniline (1.0 mmol), 2,3-butanedione (1.0 or 2.0 mmol), indium powder (0.574 g, 5.0 mmol) and acetic acid (0.300 g, 5 mmol or 0.600 g, 10 mmol) or indium chloride (0.221 g, 1 mmol or 0.265 g, 1.2 mmol) were dissolved in methanol (5 mL) or toluene (5 mL). The reaction mixture was stirred at 50°C, 80°C or 50°C under nitrogen protection. After completion of the reaction, the reaction mixture was diluted with ethyl acetate (30 mL), filtered through diatomaceous earth, and the filtrate was poured into 10% NaHCO3 solution (30 mL) and extracted with ethyl acetate (30 mL x 3). The organic phases were combined, dried with anhydrous MgSO4, filtered and concentrated. The residue was purified by silica gel column chromatography with ethyl acetate/hexane (v/v 10/90) as eluent to give purified 6,7-dimethoxy-2,3-dimethylquinoxaline. The product structure was confirmed by 1H NMR, 13C NMR, FTIR and GC-MS characterization. For unknown compounds, elemental analysis is also required.
4.2.1. Synthesis of 6,7-dimethoxy-2,3-dimethylquinoxaline (5): 87% yield. Yellow solid, melting point 109-110°C (literature value 105°C).TLC (10% ethyl acetate/hexane) Rf 0.30; 1H NMR (400 MHz, CDCl3) δ 7.91 (dd, 2H, J=6.3, 3.5 Hz), 7.59 (dd, 1H, J=6.3, 3.5 Hz), 2.66 (s, 6H); 13C NMR (100 MHz, CDCl3) δ 153.4, 141.0, 128.7, 128.2, 23.1; IR (KBr) 3109, 3078, 3030, 2995, 2953, 2914, 1490, 1398, 1164 cm-1; GC-MS m/z (relative intensity) 158 (M+, 99) , 117 (100), 76 (28), 50 (12). |