[Synthesis]
At room temperature, 8 g (40.58 mmol) of methyl 2-nitro-3-hydroxybenzoate was dissolved in 160 ml of a mixed solvent of acetic acid and ethanol (1:1 volume ratio). To the solution was added 9.971 g (178.55 mmol) of iron powder and the reaction mixture was heated to reflux and maintained for 2 hours. Upon completion of the reaction, the mixture was cooled to room temperature, diluted with 50 ml of water and subsequently extracted twice with 100 ml of ethyl acetate. The organic phases were combined, washed to neutrality with dilute sodium bicarbonate solution and dried over anhydrous sodium sulfate. The solvent was removed by concentration under reduced pressure to give 6.50 g (95% yield) of the target product methyl 2-amino-3-hydroxybenzoate. The product was characterized by 1H-NMR (400 MHz, DMSO-d6): δ 9.66 (s, 1H), 7.20 (d, 1H), 6.81 (d, 1H), 6.39 (t, 1H), 6.09 (br.s, 2H), 3.78 (s, 3H).
700 mg (4.19 mmol) of methyl 2-amino-3-hydroxybenzoate, 877 mg (4.19 mmol) of 2,4-dichlorobenzoyl chloride and 210 mg (0.838 mmol) of p-toluenesulfonic acid monohydrate were suspended in 10 ml of xylene in a microwaveable reaction vial. The reaction vial was sealed and heated at 160 °C for 25 min using a Biotage Initiator Sixty microwave reactor. At the end of the reaction, it was cooled to room temperature, the solvent was removed under reduced pressure, and the crude product was purified by column chromatography (eluent: n-heptane/ethyl acetate, 4:1→pure ethyl acetate) to afford 500 mg (35% yield) of the target product, methyl 2-(2,4-dichlorophenyl)-1,3-benzoxazole-4-carboxylate. The product was characterized by 1H-NMR (400 MHz, CDCl3): δ 8.21 (d, 1H), 8.08 (d, 1H), 7.81 (dd, 1H), 7.59 (d, 1H), 7.48 (t, 1H), 7.42 (dd, 1H), 4.05 (s, 3H).
450 mg (1.40 mmol) of methyl 2-(2,4-dichlorophenyl)-1,3-benzoxazole-4-carboxylate was dissolved in a solvent mixture of 10 ml of THF and 2 ml of water, and 0.838 ml of aqueous 2N sodium hydroxide was added. The reaction mixture was stirred at room temperature overnight and solid precipitation was observed. The reaction solution was acidified with 2N hydrochloric acid and the solid was collected by filtration and air dried to give 300 mg (66% yield) of the target product 2-(2,4-dichlorophenyl)-1,3-benzoxazole-4-carboxylic acid. The product was characterized by 1H-NMR (400 MHz, CDCl3): δ 11.62 (br.s, 1H), 8.23-8.18 (m, 2H), 7.88 (d, 1H), 7.67 (d, 1H), 7.60 (t, 1H), 7.49 (dd, 1H).
250 mg (0.81 mmol) of 2-(2,4-dichlorophenyl)-1,3-benzoxazole-4-carboxylic acid, 132 mg (0.97 mmol) of 1-hydroxybenzotriazole, 171 mg (0.89 mmol) of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and 10 mg of DMAP were dissolved in 5 ml of dichloromethane. After stirring for 15 minutes at room temperature, 1.78 ml of 0.5 M ammonia in 1,4-dioxane solution was added dropwise. Stirring was continued for 2 h. The reaction solution was sequentially washed twice with 0.5N hydrochloric acid and once with dilute sodium bicarbonate solution. The solvent was removed by concentration under reduced pressure and the residue was suspended with acetonitrile, treated with ultrasonic heating and then pumped and filtered, and the solid air-dried to give 120 mg (45% yield) of the target product 2-(2,4-dichlorophenyl)-1,3-benzoxazole-4-carboxamide. The product was characterized by 1H-NMR (400 MHz, DMSO-d6): δ 8.42 (br.s, 1H), 8.33 (d, 1H), 8.07-8.01 (m, 3H), 7.98 (d, 1H), 7.71 (d, 1H), 7.62 (dd, 1H). |