Intramolecular Anodic Olefin Coupling Reactions: A New Approach to the Synthesis of Angularly Fused, Tricyclic Enones

A number of intramolecular anodic olefin coupling reactions have been examined in order to determine the feasibility of using a sequential electrochemical oxidation-aldol condensation approach for constructing polycyclic enones. Initially, substrates comprised of an aldehyde enol ether tethered to a ketone enol ether were examined. In a model study, it was shown that such a substrate could lead to a successful electrochemical oxidation-aldol condensation sequence. However, the difficulties associated with both the synthesis and the poor stability of the ketone enol ethers greatly reduced the potential utility of this approach. This problem was circumvented by taking advantage of the compatibility of allylsilane groups with the electrolysis reaction. In these examples, ozonolysis of the electrochemical cyclization product afforded the 1,4-dicarbonyl substrate needed for the aldol condensation. The use of the less reactive allylsilane group in the electrochemical reaction still allowed for the construction of quaternary carbons. Finally a pair of angularly fused, tricyclic enones were synthesized. The regiochemistry of the enone in the product could be controlled by the proper choice of the substrate for the electrolysis and manipulation of the resulting cyclized product.