Ionic Hydrogenation of Oxyallyl Intermediates: The Reductive Nazarov Cyclization

Cyclization of tri- and tetrasubstituted dienones 1 under Lewis acidic conditions in the presence of triethylsilane led to formation of either silyl enol ethers 6 or cyclopentanones 7 in good to excellent yields, depending on work-up conditions. The proposed mechanism involves initial Nazarov cyclization to give oxyallyl intermediates 5, which are intercepted via intermolecular transfer of hydride. The reactions proceeded cleanly with as little as 2 equiv. of silane and in most cases catalytic amounts of Lewis acid could be used. Trapping with Et3SiD occurs at the less substituted terminus in unsymmetrical cases.