Development of regioselective inverse-electron-demand [4+2] cycloaddition with electron-rich arylalkynes for access to multi-substituted condensed oxapolycyclic compounds

Condensed oxygen-heteropolycyclic compounds are one of attractive synthetic targets in organic synthetic chemistry. Among these useful compounds, chromenes and benzofurans are important structural motifs, which are found in a wide range of biologically active compounds and photochemical materials. Therefore, a number of research groups have developed methodologies to synthesize these compounds. ortho-Quinone methides are key reactive intermediates with a wide range of applications in organic synthesis. In this account, our recent research on acid-promoted inverse-electron-demand [4+2] cycloaddition reaction of electron-rich arylalkynes with ortho-quinone methides is described. This reaction can be applied to a variety of arylalkynes, affording high regioselective cycloadducts. The present reactions provide versatile access to functionalized multi-substituted chromenes and benzofurans, that would be a useful tool for the synthesis of biologically and photochemically active molecules.