Synthesis of angular quinoid heterocycles from 2-(2-nitrovinyl)-1,4- benzoquinone

Reactions of 2-(2-nitrovinyl)-1,4-benzoquinone with furans, indoles, and endocyclic enol ethers form angular fused heterocyclic quinoid ring systems. The reaction proceeds by a formal inverse electron-demand [4 + 2] cycloaddition reaction of the nitrovinylquinone and a double bond of the heterocycle. The initial quinoid cycloadducts can be readily tautomerized to hydroquinoid species, which may be dehydrogenated to fully aromatic quinones. In the case of furans, the tautomerized adducts may be ring-opened to give 6,7-di- or 5,6,7-tri-substituted-1,4-naphthalenediols. In the case of endocyclic enol ethers, formation of benzofuran products competes with [4 + 2] cycloaddition.