To a mixture of aluminum chloride (40.0 g, 300 mmol) in anhydrous dichloromethane (1000 cm3) is added a suspension of phthalic anhydride (14.8 g, 100 mmol) in anhydrous dichloromethane (200 cm3). The suspension is stirred for 30 minutes, cooled to 5° C. and then a solution of dibenzothiophene (20.0 g, 110 mmol) in anhydrous dichloromethane (200 cm3) added drop wise under cooling with ice. After addition, the mixture is allowed to stir at 23° C. for 4 hours. The reaction mixture poured into a solution of water (1000 cm3) and concentrated hydrochloric acid (400 cm3). The product extracted with dichloromethane (2 200 cm3) and the organic extracted with aqueous sodium hydroxide (5%, 500 cm3). The basic layer acidified and the oily precipitate is isolated by decanting away the acidic solution. Water (200 cm3) is added, which solidified the oil and the solid collected by filtration, washed with water (500 cm3) and dried under vacuum to give 2-(o-carboxybenzoyl)dibenzothiophene as a pale yellow solid (35.64 g, 99%). 1H NMR (300 MHz, CDCl3) 8.47-8.52 (m, 1H), 7.99-8.15 (m, 2H), 7.60-7.86 (m, 4H), 7.35-7.57 (m, 4H).
