Phthalic anhydride (20.0 g ; 0.135 moles) & tert-butyl methyl ether (111.0 g, 1.259 moles) was added to a 1l, 4 necked flask bearing a mechanical stirrer, addition flask and thermometer, set up on a cooling tub at room temperature. The reaction mixture was cooled to 0°C with ice salt mixture under stirring. Conc. H₂SO₄ (98%; 23.8 g; 0.243 moles) was added slowly through a dropping funnel keeping the reaction temperature < 5°C in 1-2 h. The temperature of the reaction mixture was maintained between 0°-5°C for 4 h. The reaction mixture was allowed to come to room temperature and was stirred overnight before adding to to 50% Na₂CO₃ solution (200 ml) . The reaction mass was extracted with CH₂Cl₂ (2 X 200 ml ) and dried over Na₂SO₄. The dichloromethane was concentrated to obtain a light yellow oily liquid (23 g, 70%).