Synthesis
1. To a solution of N-methylimidazole (1.64 g, 19.97 mmol) and sodium acetate (25 g, 300 mmol) in acetic acid (180 mL) was added dropwise a solution of bromine (9.6 g, 60.07 mmol) in acetic acid at room temperature. The reaction mixture was stirred at room temperature for 2.5 hours. Upon completion of the reaction, the acetic acid was removed by vacuum distillation and the residue was suspended in 500 mL of water and stirred for 10 minutes at room temperature. The precipitate was collected by filtration, washed with water and dried under high vacuum to afford 2,4,5-tribromo-1-methyl-1H-imidazole (1.82 g, 29% yield, part of the product was retained in the mother liquor) as a light yellow powder, which could be used in the next reaction without further purification.
2. 2,4,5-tribromo-1-methyl-1H-imidazole (1.82 g, 5.71 mmol) was suspended in 45 mL of water, sodium sulfite (13 g, 103 mmol) was added, and the reaction mixture was stirred under rapid reflux for 24 hours. After cooling to room temperature, the organic phase was extracted with ether (3 x 75 mL), the organic phases were combined, dried with magnesium sulfate, filtered and concentrated to give 1.61 g of a mixture of tribromo, dibromo and monobromo imidazole.
3. The above mixture was dissolved in 15 mL of a 3:1 water/acetic acid solvent mixture, the same amount of sodium sulfite was added, placed in a sealed container and heated at 130 °C for 60 hours. After cooling to room temperature, the pH of the reaction mixture was adjusted to 9-10 with 2N sodium hydroxide solution. the organic phase was extracted with ether (3 x 50 mL), the organic phases were combined, dried with magnesium sulfate, filtered and concentrated to give the crude 4-bromo-1-methyl-1H-imidazole (571 mg, yield about 62%), which could be used for the next reaction without further purification.
4. using 4-bromo-1-methyl-1H-imidazole (571 mg, about 3.53 mmol) as a raw material, 4-butyl-1-methyl-1H-imidazole (95 mg, 22% yield) was synthesized by substituting hexylboronic acid with propylboronic acid (372 mg, 4.24 mmol) according to the method of Example 3.1 , which was used in the next reaction without further purification.
5. Butyllithium (0.34 mL, 2.5 M hexane solution) was added dropwise to 2 mL of anhydrous tetrahydrofuran solution of diisopropylamine (0.13 mL, 0.918 mmol) at -0 °C. The reaction was carried out with stirring over a period of 20 min. With stirring, the solution was warmed to -20 °C over 20 min and then cooled to -78 °C. A 2 mL solution of anhydrous tetrahydrofuran of 4-butyl-1-methyl-1H-imidazole (95 mg, 0.765 mmol) was added dropwise and stirred for 40 min at -78 °C. Dimethylformamide (0.24 mL, 3.06 mmol) was added, stirred and warmed to room temperature. The reaction mixture was poured into 15 mL of 1N hydrochloric acid and stirred for 5 minutes. The pH was adjusted to 7-8 with saturated sodium bicarbonate solution, the organic phase was extracted with dichloromethane (3 x 20 mL), the organic phases were combined, dried with magnesium sulfate, filtered and concentrated. The crude product was purified by silica gel column chromatography with gradient elution (5-50% hexane solution of ethyl acetate) to afford 1-methyl-4-propyl-1H-imidazole-2-carboxaldehyde (9 mg, 8% yield) as an off-white solid, which can be used without further purification.
