Example 12: Synthesis of 4-[dimethylamino)diazenyl]thiophene-2-carboxamide (Compound 12);.
Step a: Synthesis of 4-nitrothiophene-2-carboxylic acid: Concentrated sulfuric acid (3.0 mL, 5.505 g, 56.17 mmol) was slowly added dropwise to concentrated nitric acid (2.0 mL, 2.98 g, 49.6 mmol) at 0-10 °C in an ice bath. After the dropwise addition was completed, thiophene-2-carboxylic acid (2.8 g, 21.87 mmol) was added to the above nitrification mixture in batches over 15 minutes, maintaining the same temperature, followed by continued stirring of the reaction mixture for 1 hour. After completion of the reaction, the mixture was slowly poured into ice water and stirred for 30 minutes. The precipitated solid was filtered, washed with cold water and dried. The filtrate was extracted with ethyl acetate and the organic phases were combined, washed sequentially with water, saturated brine and dried over anhydrous sodium sulfate. After filtration, the organic phase was concentrated under reduced pressure. The resulting crude product was stirred with hexane (2 x 50 mL) and filtered to give an off-white solid product (2.8 g, 75%) with a melting point of 110-118 °C. HPLC and 1H NMR analyses showed the product to be a mixture of the two compounds, which was used directly in the next step of the reaction.
Step b: Synthesis of methyl 4-nitrothiophene-2-carboxylate: Thionyl chloride (6 mL, 78.6 mmol) was added slowly and dropwise to a solution of 4-nitrothiophene-2-carboxylic acid (6.8 g, 39.3 mmol) in methanol (50 mL) with stirring at room temperature. After the dropwise addition, the reaction mixture was heated to reflux for 2 hours and subsequently cooled to room temperature. The reaction mixture was poured into ice water and stirred for 15 minutes. The precipitated solid was filtered, washed with cold water and dried to give an off-white solid product (6.2 g, 85%).1H NMR analysis showed the product to be a mixture of the two compounds and the crude product was used directly in the next step of the reaction.
Step c: Synthesis of methyl 4-aminothiophene-2-carboxylate and methyl 5-aminothiophene-2-carboxylate: To a mixed solution of methyl 4-nitrothiophene-2-carboxylate (7 g, 37.43 mmol) in water (150 mL) and methanol (50 mL) was added concentrated hydrochloric acid (4.5 mL). To this solution, iron powder (10.5 g, 188 mmol) and ammonium chloride (10 g, 187 mmol) were added sequentially at room temperature. The reaction mixture was stirred and heated to 70 °C, maintained for 1 h and cooled to room temperature. The reaction mixture was filtered and the filtrate was alkalized with saturated sodium bicarbonate solution and extracted with chloroform (4 x 100 mL). The organic phases were combined, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure. The residue was separated by silica gel column chromatography using hexane-ethyl acetate (90:10 with a small amount of triethylamine) as eluent to first obtain methyl 4-aminothiophene-2-carboxylate (1.8 g, 31%) with a melting point of 76-78 °C. 1H NMR (400 MHz, CDCl3): δ 7.31 (1H, d, J=1.6 Hz), 6.40 (1H, d, J= 1.6 Hz), 3.85 (3H, s), 3.63 (2H, br s). Continued elution with the same solvent system afforded methyl 5-aminothiophene-2-carboxylate (0.5 g, 8.5%) with a melting point of 70-72 °C.1H NMR (400 MHz, CDCl3): δ 7.45 (1H, d, J=4.0 Hz), 6.09 (1H, d, J=4.0 Hz), 4.29 (2H, br s), 3.81 (3H, s ).
