[Development of Catalytic Cycloisomerization Reactions Enabling Construction of Polycyclic Frameworks Found in...

The transition-metal-catalyzed cycloisomerization of unsaturated compounds, such as alkynes, alkenes, allenes, and nitriles, is a powerful tool for constructing polycyclic frameworks found in many biologically active natural products and pharmaceuticals. However, this approach predominantly relies on precious transition metals, such as rhodium and iridium. Thus, the development of cycloisomerization reactions using less expensive, less toxic, and environmentally friendly transition metals that are abundantly found in the earth has attracted considerable attention in recent years. In this article, we reviewed our recent studies on the synthesis of various polycyclic compounds via cycloisomerization enabled by the cobalt/photoredox dual catalysis. Our research has demonstrated the excellent efficiency of the cobalt/photoredox dual catalyst system for the cycloisomerization of 1,6-diyne derivatives. We constructed tricyclic cyclohexadiene frameworks, which are found in many biologically active natural products such as 11-O-debenzoyltashironin, perforanoid A, and jiadifenolide, using 1,6,11-enediynes as substrates for the cobalt-catalyzed cycloisomerization. Using a chiral ligand, (S)-Segphos, we achieved an enantioselective reaction that allowed access to enantio-enriched tricyclic cyclohexadiene products. Furthermore, we discovered that a novel cascade cyclization of 1,6-diynyl esters, enabled by the cobalt/photoredox dual catalysis, provided various cyclic compounds via the formation of vinylallene intermediates.