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General procedure for the synthesis of methyl 2,5-dichloroisonicotinate from methanol and 2,5-dichloroisonicotinic acid: concentrated sulfuric acid (0.5 mL) was slowly added to a methanolic solution of 2,5-dichloroisonicotinic acid, the reaction mixture was heated to reflux and the reaction was maintained for 12 hours. Upon completion of the reaction, the mixture was cooled to room temperature, neutralized to pH neutral with aqueous sodium bicarbonate and subsequently extracted with ethyl acetate. The organic phases were combined and the solvent was removed by evaporation under reduced pressure to afford the target product methyl 2,5-dichloroisonicotinate (6-2) in 89% yield.

[1] Bioorganic and Medicinal Chemistry Letters, 2008, vol. 18, # 19, p. 5186 - 5189
[2] Patent: WO2008/141385, 2008, A1. Location in patent: Page/Page column 83
[3] Patent: WO2013/72825, 2013, A1. Location in patent: Page/Page column 55