Bicyclic cyclopentenones found in many natural products are often prepared by the Pauson–Khand reaction (PKR). While the intramolecular PKR affords 3,4-fused bicyclopentenones from tethered enynes, the intermolecular PKR that accesses isomeric 4,5-fused bicycles has severe scope limitations. During our synthesis of (+)-ryanodol, we discovered unusual oxidative reactivity mediated by selenium dioxide (SeO2). Treatment of intramolecular PKR products with SeO2 under either aqueous or anhydrous conditions provided highly oxidized, transposed 4,5-fused cyclopentenones. We present our development of these peroxidations to construct oxidation patterns elusive via canonical PKRs and provide valuable synthetic building blocks.
We found that treatment of simple PKR products with SeO2 in the presence of excess water leads to selective dioxidation, whereas the exclusion of water selects for trioxidation. The reaction is general to 5/5-bicycles, tolerating various geminal di- and heterocyclic substitution. The 4,5-fused bicyclopentenones are afforded as single diastereomers and can be readily elaborated via Pd-catalyzed cross-coupling.
Identification and characterization of fleeting intermediate species led us to propose two possible pathways that account for the water-dependent product selectivity. We present Hammett analyses and study of the enantiospecificity of reaction steps that probe these mechanistic possibilities and have resulted in an improved reaction protocol and scope.

SeO2-Mediated Oxidative Transpositions of Pauson–Khand Products

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