Step 1. Synthesis of 5-bromo-2,3-dimethylpyridine
In a round-bottomed flask fitted with a water-cooled reflux condenser and a calcium chloride drying tube, 2,3-dimethylpyridine (6.79 mL, 60 mmol) was dissolved in fuming sulfuric acid (80 mL) and the reaction temperature was maintained at 150°C. The reaction temperature was kept at 150°C. Bromine (3.1 mL, 60 mmol) was added slowly and dropwise over a period of 2 hours. The reaction mixture took on a deep red color and after continued stirring for 16 hours, it was cooled to room temperature and allowed to stand overnight. The reaction mixture was carefully poured into about 400 g of ice and adjusted to pH 12 with base under cooling in an ice bath.The aqueous phase was extracted with ether and the organic phases were combined, washed with brine and dried over anhydrous magnesium sulfate. The solvent was removed by concentration under reduced pressure and the residue was purified by silica gel column chromatography (50 g silica gel, eluent 2-20% ethyl acetate in hexane solution) to give 2,3-dimethyl-5-bromopyridine (8.3 g) as a colorless oil.
