A solution of crude 24 (50 mmol) in dichloromethane (200 mL) and a solution of potassium bromide (0.7 g) in water (10 mL) were mixed together and stirred in an ice bath. TEMPO (120 mg) was added and a solution of 37% aqueous sodium hypochlorite (48 mL), sodium bicarbonate (5 g) and water (to 100 mL) was added in portions at such a rate that the temperature did not exceed 5° C. Stirring was continued on the ice bath for 20 min. The reaction mixture was diluted with dichloromethane (200 mL) and washed with water (300 mL), 1N hydrochloric acid (300 mL), water (300 mL) and brine (300 mL). The organic layer was dried over sodium sulfate, filtered and the solvent was removed on a rotary evaporator. The crude product (19 g) was purified by chromatography on silica (350 g) with 2% ethyl acetate in heptane (7 L). Pure 25 (16.1 g, 88%) was isolated.
