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A solution of crude 3 (3.46 mmol) in dichloromethane (60 mL) was treated with manganese dioxide (85%, 5.3 g, 15 equiv) and this suspension was stirred at reflux overnight. The solids were filtered out and washed with ethyl acetate. The solvent was removed in a rotary evaporator and the residue was chromatographed on silica (85 g) with 2% ethyl acetate in heptane (5 L). Pure 4 (0.48 g) as well as slightly impure 4 (1.01 g) were isolated (total yield 91%).

1. temperatureat reflux overnight
2. filtrationThe solids were filtered out
3. washwashed with ethyl acetate
4. customThe solvent was removed in a rotary evaporator
5. customthe residue was chromatographed on silica (85 g) with 2% ethyl acetate in heptane (5 L)