General procedure for the synthesis of 2'-trifluoromethyl diphenyl-4-carboxylic acid from 2'-(trifluoromethyl)-[1,1'-biphenyl]-4-carboxaldehyde: The product of step A (0.4 g, 1.6 mmol) was refluxed for 2 h under stirring with a suspension formed by potassium permanganate (KMnO4, 0.502 g, 3.2 mmol) in dioxane (15 mL). After completion of the reaction, the mixture was filtered through a silica gel column and the filter cake was washed with ethyl acetate (EtOAc, 30 mL). Subsequently, the solvent in the filtrate was evaporated to dryness to afford the target product 2'-trifluoromethyl diphenyl-4-carboxylic acid (0.35 g, 81.6% yield) as a white solid. The 1H-NMR (CDCl3) data of the product were as follows: δ 7.30-7.58 (m, 5H); 7.76 (d, 1H, J = 8.15 Hz); 8.11 (d, 2H, J = 8.03 Hz).
