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Scheme 2. Diastereoselectivity observed in the [2+2+1] cycloaddition.
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in acetone for 20 min, and then the solution was stirred in an
atmosphere of carbon monoxide at room temperature for the
remainder of the reaction. A solution of CAN (5 equiv) in
acetone was added over 3.0 h by syringe pump to the reaction
mixture. At the conclusion of the slow addition, the reaction
mixture was quenched with saturated aqueous NaHCO3 and
then concentrated under vacuum (to remove the acetone). The
aqueous solution was extracted with CH2Cl2, the combined
organic layers were washed with brine, dried over MgSO4, and
the organic solvent was removed under vacuum. The cyclo-
addition product was then purified by chromatography on silica
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Angew. Chem. Int. Ed. 2005, 44, 4929 –4932