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In a round-bottomed flask, 2-chloro-4-bromophenylacetonitrile (19.0 g, 82.4 mmol, 1.0 eq.) was dissolved in water (196 mL), followed by the addition of potassium hydroxide (KOH, 23.1 g, 412 mmol, 5.0 eq.). The reaction mixture was heated to reflux and stirred for 12-18 hours. After completion of the reaction, it was cooled to room temperature and washed with ethyl acetate (EtOAc) to remove organic impurities. The aqueous phase was acidified with 2N hydrochloric acid to pH < 2 and subsequently extracted with ethyl acetate (150 mL x 3). The organic phases were combined, washed with saturated sodium chloride solution (brine), dried over anhydrous magnesium sulfate (MgSO4), filtered and concentrated under reduced pressure to afford 2-(4-bromo-2-chlorophenyl)acetic acid (S3) as brown crystals (19.1 g, 93% yield). The product characterization data were in agreement with literature reports. [20]

[1] Tetrahedron Letters, 2016, vol. 57, # 3, p. 449 - 451
[2] Patent: WO2013/43232, 2013, A2. Location in patent: Paragraph 00502
[3] Patent: EP2017257, 2009, A1. Location in patent: Page/Page column 21-22
[4] Patent: JP2015/231988, 2015, A. Location in patent: Paragraph 0361; 0362