[Synthesis]
a) Synthesis of dimethyl 2-(4-bromo-5-fluoro-2-nitrophenyl)-malonate/2-(2-bromo-5-fluoro-4-nitrophenyl)-dimethyl malonate mixture A: Suspension of NaH (60% dispersed in mineral oil, 20.2 g, 504 mmol) in dioxane (233 ml) and cooling to 11 °C. A solution of 1-bromo-2,4-difluoro-5-nitrobenzene (50 g, 26.5 ml, 210 mmol) and dimethyl malonate (33.3 g, 28.9 ml, 242 mmol) in dioxane (467 ml) was slowly added at 11 °C. The temperature was raised to 14°C in 45 min (note the gas release). After addition, the reaction mixture was kept at 12 °C for 1 h and then raised to room temperature.After 16 h, the reaction mixture was cooled to 10 °C and 100 ml of saturated aqueous ammonium chloride solution was added. The reaction mixture was diluted with tert-butyl methyl ether, water and saturated aqueous ammonium chloride solution. The aqueous phase was extracted with tert-butyl methyl ether and the combined organic phases were washed with saturated aqueous ammonium chloride solution and brine and dried over sodium sulfate. The solvent was evaporated and the residue was purified by silica gel chromatography using ethyl acetate/heptane as eluent. 6-Bromo-5-fluoroindolin-2-one was obtained as a yellow liquid (53.7 g) as a 2.6:1 mixture and was used directly in the next reaction.
b) Hydrolysis reaction: Dimethyl 2-(4-bromo-5-fluoro-2-nitrophenyl)-propanedioate/2-(2-bromo-5-fluoro-4-nitrophenyl)-propanedioic acid dimethyl ester (2.6:1 mixture, 53.7 g, 153 mmol) was heated and refluxed with 6 M aqueous hydrochloric acid (767 ml) for 7 h. It was cooled down to 5 °C. The precipitate was precipitated with water and n-pentylamine. The precipitate was filtered, washed with water and n-pentane, then co-evaporated with toluene three times to give 25.9 g of 6-bromo-5-fluoroindolin-2-one as a white solid. The mother liquor was extracted with ethyl acetate and the combined organic phases were dried over sodium sulfate. The solvent was evaporated and the residue was ground with n-pentane and then co-evaporated with toluene to give 11.42 g of 6-bromo-5-fluoroindolin-2-one as an off-white solid. The two batches were combined to give a total of 37.32 g of 6-bromo-5-fluoroindolin-2-one as a 2.6:1 mixture, which was used directly in the next step of the reaction.
c) Reduction reaction: A solution (671 ml) of (4-bromo-5-fluoro-2-nitrophenyl)-acetic acid/(2-bromo-5-fluoro-4-nitrophenyl)-acetic acid (2.6:1 mixture, 37.3 g, 134 mmol) and iron powder (30.0 g, 537 mmol) in acetic acid was heated and kept at 100 °C for 7 h. The solution was cooled to room temperature. The remaining iron powder was removed with a magnet. Ice water (900 ml) was added to the reaction mixture. The precipitate was filtered, washed four times with water and then suspended in cold water with 25% HCl (300 ml) and concentrated hydrochloric acid (50 ml).After 10 min, the precipitate was filtered and washed four times with water. The precipitate was suspended in a mixture of 1 M Na2CO3 aqueous solution (400 ml) and 0.1 M NaOH (100 ml) and stirred for 40 min. The precipitate was filtered and washed four times with 0.1 M NaOH aqueous solution, three times with water and once with diisopropyl ether to give 6-bromo-5-fluoroindolin-2-one (20.5 g) as a light gray solid.MS ESI (m/z): 228.0/230.0 [(M+H)+].1H NMR (DMSO-D6, 400MHz): δ (ppm) = 10.47 (bs, 1H), 7.31-7.28 (m, 1H), 7.01-6.99 (m, 1H), 3.49 (s, 2H). |