General procedure for the synthesis of 2,5,6-trichloronicotinic acid from 2,3,6-trichloro-5-methylpyridine: 2,3,6-trichloro-5-methylpyridine (18.8 g, 60.0 mmol) was suspended in water (400 ml) and heated to 100 °C. Subsequently, potassium permanganate (KMnO4, 28.5 g, 180.2 mmol) was added in batches over 12 hours. The reaction mixture was stirred continuously at 100 °C for 2 days, during which additional potassium permanganate (10 g) was added. After confirming the complete consumption of raw materials by TLC monitoring, the reaction mixture was filtered while hot and washed with hot water (2 x 75 ml). The combined filtrates were cooled to room temperature and extracted with ethyl acetate (EtOAc, 3 x 100 ml). The aqueous phase was concentrated to about 50 ml, cooled to 0 °C and the pH was adjusted to 1-2 with 6.0 M hydrochloric acid.The precipitated solid was collected by filtration, washed with cold water and dried to afford the target product 2,5,6-trichloronicotinic acid (2.5 g, 18% yield), which was used in subsequent reactions without further purification.
