A mixture of 3,3',5,5'-tetrakis(methoxycarbonyl)biphenyl (12) (0.96 g, 2.5 mmol) was dissolved in water (40 mL) with THF (40 mL) and NaOH (1.6 g, 40 mmol) and reacted at reflux for 1 hour. Subsequently, the organic solvent was evaporated under reduced pressure and the remaining aqueous solution was continued to reflux for 4 hours. After completion of the reaction, the reaction mixture was cooled and acidified with 50% H2SO4 until the precipitation was complete. The precipitate was collected by filtration and dried to give the target product 3,3',5,5'-biphenyltetracarboxylic acid (9) as a white powder in 89% yield. Product melting point > 350°C. 1H-NMR (400 MHz, DMSO-d6): δ = 8.44 (d, 4H, J = 1.5 Hz), 8.53 (t, 2H, J = 1.5 Hz), 13.48 (bs, 4H). 13C-NMR (100 MHz, DMSO-d6): δ = 129.5, 131.4, 132.3 , 139.2, 166.3. MS-EI, m/z (%): 330 (100) [M+], 313 (40), 285 (15), 240 (5).
