Reactions of Phthalide Ortho Esters with Dienophiles: Synthesis of Naphthalenes and Homoquinones

Abstract

<p>A new reaction between phthalide ortho esters with a series of dienophiles has been studied. It was found that treatment of 1,1-diethoxyphthalan or 5,6-dimethoxy-1,1-diethoxyphthalan with activated acetylenes yielded tetrasubstituted naphthalenes. Reaction of 1,1-diethoxyphthalan with diethyl maleate afforded a mixture of enol ethers and a trisubstituted naphthalene. The enol ethers were converted into a disubstituted naphthol under acidic conditions. Treatment of phthalide ortho esters with 1,4-benzoquinone or 1,4-napththoquinone gave p-homoquinones instead of the expected linear tricyclic and tetracyclic aromatic compounds.</p> <p>The formation of the compounds derived from the reaction of phthalide ortho esters with dienophiles has been rationalized in terms of a mechanism that involves l-ethoxyisobenzofurans as intermediates.</p>