Diastereodivergent and Enantioselective Access to Spiroepoxides via Organocatalytic Epoxidation of Unsaturated Pyrazolones

Readily available chiral amine–thioureas are effective catalysts for the first diastereo- and enantioselective epoxidation of unsaturated pyrazolones. The trans- or cis-spiroepoxides are preferentially obtained in good yield and high to excellent enantioselectivity using an appropriate organocatalyst and tert-butyl hydroperoxide as the oxidant. The epoxidation appears applicable to highly challenging β,β′-substituted unsaturated pyrazolones, giving access to spiroepoxides bearing two vicinal quaternary stereocenters. The reaction represents a unique example of Weitz–Scheffer epoxidation, where the catalyst-controlled ring-closure step is usefully exploited to prepare both enantioenriched diastereomeric epoxides.