Cascade Michael addition/cycloketalization of cyclic 1,3-dicarbonyl compounds: important role of the tethered alcohol of α,β-unsaturated carbonyl compounds on reaction rate and regioselectivity.

Reactions of α,β-unsaturated aldehydes and cyclic 1,3-dicarbonyl compounds proceed primarily by cascade Knoevenagel condensation/six-π-electron electrocyclization (K6EC, formal [3 + 3] cycloaddition), while α,β-unsaturated ketones usually react with cyclic 1,3-dicarbonyl compounds in a 1,4-addition manner. This paper discloses our findings that under acidic conditions, α,β-unsaturated carbonyl compounds (ketones and aldehydes) with a tethered alcohol react with cyclic 1,3-dicarbonyl compounds in a highly regioselective 1,4-addition fashion via in situ generation of a hypothetical α-methylene cyclic oxonium ion as the reactive Michael acceptor. Our studies uncovered the important effect of the tethered alcohol on the reaction rate and/or efficiency and some new mechanistic aspects of the cascade Michael addition/cycloketalization. Finally, the substrate scope was examined, and 43 analogues of penicipyrone and tenuipyrone were prepared in good to excellent yields.