29681-38-7

67-56-1

589-93-5

29681-38-7

Steps for the synthesis of Intermediate 20-6: Methyl 5-methylpyridine-2-carboxylate: Selenium dioxide (4.66 g, 42.0 mmol, CAS 7446-08-4) was added to a solution of 2,5-dimethylpyridine (3.00 g, 28.0 mmol, CAS 589-93-5) dissolved in pyridine (15 mL). The reaction mixture was heated to reflux overnight. After completion of the reaction, it was cooled to room temperature, the solid precipitate was removed by filtration and washed sequentially with water and pyridine (5 mL each, 2 times). The filtrate was evaporated and the crude product obtained was redissolved in methanol (100 mL). Sulfuric acid (1.34 mL, 25.0 mmol) was added to this solution and the reaction mixture was then heated to reflux for 5 hours. At the end of the reaction, it was cooled to room temperature and the reaction mixture was alkalized with 20% NaOH aqueous solution. Methanol was removed by evaporation and diluted by adding water (50 mL). The mixture was extracted with diethyl ether (DEE, 3 x 100 mL). The organic phases were combined, dried over anhydrous sodium sulfate (Na2SO4), and the solvent was evaporated to give the title compound methyl 5-methylpyridine-2-carboxylate (2.11 g, 50% yield) as a light brown oil.1H NMR (400 MHz, CDCl3) δ 2.40 (s, 3H), 3.97 (s, 3H), 7.61 (d, 1H), 8.01 (d , 1H), 8.54 (s, 1H).