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

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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.