Enantioselective Helicene Synthesis by Rhodium-Catalyzed 2+2+2 Cycloadditions

Abstract The highly enantioselective synthesis of functionalized helicenes and helicene-like molecules have been achieved via rhodium-catalyzed 2+2+2 cycloaddition reactions. The rhodium-catalyzed enantioselective intramolecular 2+2+2 cycloaddition of 2-naphthol-linked triynes afforded 7helicene-like molecules in good yields and ee values. The more sterically encumbered reaction, the rhodium-catalyzed enantioselective double intramolecular 2+2+2 cycloaddition of a 2-naphthol-linked hexayne, also proceeded to give a 11helicene-like molecule with high ee value, although the product yield was low. Not only intramolecular cycloaddition reactions but also intermolecular ones were accomplished by combinations of electron-rich tetraynes and electron-poor diynes to give 7- and 9helicene-like molecules and 7helicenes in varying yields and ee values.