Synthesis
1. Mix 5-amino-2-chloro-4-nitrobenzotrifluoride (24.9 g) with 48% aqueous hydrogen bromide (400 ml) in an ice bath. Sodium nitrite (12.9 g) dissolved in a small amount of water was added slowly and dropwise. The reaction mixture was stirred at -5°C to 15°C for 4 h, followed by alkalinization with 6N aqueous sodium hydroxide. The precipitated brown solid was collected by filtration and washed with water. Purification by silica gel column chromatography (eluent: ethyl acetate-hexane, 1:50) afforded 5-bromo-2-chloro-4-nitrobenzotrifluoride (17.7 g, 56% yield).
2. A mixture of reduced iron (17.7 g), acetic acid (290 ml) and water (145 ml) was heated and stirred at 60 °C for 15 min, 5-bromo-2-chloro-4-nitrobenzotrifluoride (17.7 g) and 1,4-dioxane (100 ml) were added, and stirring was continued at 60 °C for 30 min. After the reaction mixture was cooled in an ice bath, ethyl acetate (700 ml) was added and stirred at room temperature. Filtered through diatomaceous earth, the filtrate was washed sequentially with 1N aqueous sodium hydroxide and saturated brine and dried over anhydrous sodium sulfate. Concentration under reduced pressure gave 2-bromo-5-chloro-4-(trifluoromethyl)aniline (17.1 g, 100% yield).
3. 2-Bromo-5-chloro-4-(trifluoromethyl)aniline (17.1 g), trimethylmethylsilylacetylene (14.8 ml), bis(triphenylphosphine)palladium dichloride (816 mg), and triethylamine (115 ml) were stirred for 9 hr. at 60°C. The reaction was cooled in an ice bath. After cooling in an ice bath, ether was added and filtered through diatomaceous earth. The filtrate was concentrated under reduced pressure to give a brown oil. Purification by silica gel column chromatography (eluent: ethyl acetate-hexane, 1:20 to 1:10) afforded 5-chloro-4-(trifluoromethyl)-2-[(trimethylsilyl)ethynyl]aniline (16.5 g, 92% yield).
4. 5-Chloro-4-(trifluoromethyl)-2-[(trimethylmethylsilyl)ethynyl]aniline (16.5 g), cuprous(I) iodide (20.4 g), and dimethylformamide (100 ml) were stirred at room temperature for 10 min, followed by stirring and heating at 100 °C for 2.5 h. The mixture was cooled and then heated for 2.5 h. The reaction was purified by adding ether. After cooling, ether was added and stirred at room temperature. Filtered through diatomaceous earth, the filtrate was washed with water and saturated brine and dried over anhydrous sodium sulfate. Concentrate under reduced pressure to obtain a brown oil. Purification by silica gel column chromatography (eluent: ethyl acetate-hexane, 1:5 to 1:3) afforded 6-chloro-5-(trifluoromethyl)-1H-indole (5.75 g, 46% yield).
5. Phosphoryl chloride (1.83 ml) was added dropwise to dimethylformamide (16 ml) under ice bath conditions and stirred at 0 °C for 20 min. 6-Chloro-5-(trifluoromethyl)-1H-indole (3.24 g) dissolved in dimethylformamide (10 ml) was added and stirred at 0°C for 3 hours. Water was added and stirred at room temperature for 3 h. The mixture was alkalized with 1N aqueous sodium hydroxide. The solid was collected by filtration, washed and dried with water to give 6-chloro-5-(trifluoromethyl)indole-3-carbaldehyde (3.25 g, 89% yield).
6. Nitromethane (20 ml) and ammonium acetate (1.6 g) were added to 6-chloro-5-(trifluoromethyl)indole-3-carboxaldehyde (3.25 g) and heated to reflux for 1 hour. After cooling, the solid was collected by filtration and washed and dried with methanol-water (1:1). The solid was washed with diethyl ether suspension, filtered and dried to give 6-chloro-3-[(E)-2-nitrovinyl]-5-(trifluoromethyl)-1H-indole (1.86 g, 49% yield).
7. 7-Chloro-6-(trifluoromethyl)-2,3,4,9-tetrahydro-1H-β-carboline (512 mg, 29% yield) was obtained from 6-chloro-3-[(E)-2-nitroethenyl]-5-(trifluoromethyl)-1H-indole (1.86 g), according to a similar operation as in Example 3 of the synthesis of raw materials.
