Benzofurans as Efficient Dienophiles in Normal Electron Demand [4 + 2] Cycloadditions

Abstract

Dearomatization of electron-poor benzofurans is possible through involvement of the aromatic 2,3-carbon-carbon double bond as dienophile in normal electron demand [4 + 2] cycloadditions. The tricyclic heterocycles thereby produced bear a quaternary center at the cis ring junction, a feature of many alkaloids such as morphine, galanthamine, or lunaridine. The products arising from the reaction have been shown to depend on different factors among which the type of the electron-withdrawing substituent of the benzofuran, the nature of the reacting diene, and the method of activation. In the presence of all-carbon dienes, the reaction yields the expected Diels-Alder adducts. When thermal activation is insufficient, a biactivation associating zinc chloride catalysis and high pressure is required to generate the cycloadducts in good yields and high stereoselectivities, for instance, when cyclohexadiene is involved in the process. The use of more functionalized dienes, such as those bearing alkoxy or silyloxy substituents, also shows the limits of the thermal activation, and hyperbaric conditions are, in this case, well-suited. The involvement of Danishefsky's diene induces a competition in the site of reactivity. The aromatic 2,3-carbon-carbon double bond is unambiguously the most reactive dienophile, and the 3-carbonyl unit becomes a competitive site of reactivity with benzofurans bearing substituents prone to heterocyloaddition, in particular under Lewis acid activation. The sequential involvement of both the aromatic double bond and the carbonyl moiety as dienophiles is then possible by using an excess of diene under high-pressure activation. In line with the experimental results, DFT computations suggest that the Diels-Alder process involving the aromatic double bond is preferred over the hetero-Diels-Alder route through an asynchronous concerted transition state. However, Lewis acid catalysis appears to favor the heterocycloaddition pathway through a stepwise mechanism in some cases.