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Under nitrogen, 344 mg (1.31 mmol) of triphenylphosphine and 170 μl (1.08 mmol) of diethyl azodicarboxylate were added to a solution of Compound (39a) (503 mg, 0.875 mmol) in 35 ml of dry benzene, and the mixture heated to reflux for 25 min. After cooling to room temperature, the same amounts of triphenylphosphine and diethyl azodicarboxylate were added to the reaction mixture. The mixture was then heated to reflux for 25 min, and concentrated under reduced pressure to a half volume. The residue was purified by a silica gel column chromatography (silica gel 55 g; n-hexane:ethyl acetate=6:1) to give 434 mg (89%) of Compound (40a).

1. temperaturethe mixture heated
2. temperatureto reflux for 25 min
3. temperatureThe mixture was then heated
4. temperatureto reflux for 25 min
5. concentrationconcentrated under reduced pressure to a half volume
6. customThe residue was purified by a silica gel column chromatography (silica gel 55 g; n-hexane:ethyl acetate=6:1)