Reactions of phthalans with dienophiles. I. Synthesis of naphthalenes from phthalans and activated acetylenes

Abstract

Diethoxyphthalans and activated acetylenes react at temperatures of 140–160 °C to yield substituted naphthalenes. It is proposed that the reaction occurs through the intermediacy of 1-ethoxyisobenzofurans generated thermally from the phthalans. The transient 1-ethoxyisobenzofurans are postulated to add to the acetylenes in a cycloaddition reaction to yield tricyclic intermediates that are transformed to naphthalenes at the elevated temperature of the reaction. Reactions with unsymmetrical acetylenes provided information on the regioselectivity of the reaction.