A mixture of 30.0 grams (67.5 mmol) of di-(2,2,6,6-tetramethylpiperidin-4-yl) phthalate, 27.5 grams (214 mmol) of 70% aqueous tert-butyl hydroperoxide, 2.0 grams of molybdenum trioxide and 200 ml of cyclohexane is heated to reflux at 81° C. Water is collected in a Dean-Stark trap. After 75 minutes, the reaction mixture is red. Another portion of tert-butyl hydroperoxide (42.5 grams of 70% aqueous solution, 330 mmol) is added over 30 minutes. After additional water is collected in the trap, the reaction mixture is transferred to a Fischer-Porter bottle and heated at 140° C. for 4.5 hours. The nearly colorless reaction mixture is treated with 6.9 grams (69 mmol) of 90% aqueous tert-butyl hydroperoxide and heated at 140° C. for 90 minutes to remove the last traces of pink color. The reaction mixture is cooled, filtered and stirred with a solution of 43 grams of sodium sulfite in 530 ml of water for 2 hours. Methylene chloride (600 ml) is added and the organic layer is separated, dried over anhydrous magnesium sulfate and concentrated to a crude solid. Purification (Waters Prep. 500A HPLC, 25:1 heptane:ethyl acetate) affords 31.0 grams (72% yield) of the title compound as a white solid, melting at 149°-151° C.
