The synthesis of seven- and eight-membered rings by radical strategies

Abstract

Seven- and eight-membered rings widely exist in natural products and drugs. Considering the transition state nonbonding interactions and torsional strains, the flexible conformations and dense multi-ring skeleton, the synthesis of seven- and eight-membered rings has been demonstrated to be challenging and is thus attracting continuous attention. The Ugi reaction, metal coordination, and rearrangement reactions are common methods for preparing seven- or eight-membered rings. Despite their power, the radical strategy shows significant potential as a powerful tool that avoids the poor selectivity and atom economy and the limitations of harsh reaction conditions. The metal-induced system, oxidant-initiated system, electrochemically promoted system, and visible light-induced system are four common platforms for the preparation of seven- or eight-membered rings by the radical strategy, which uses the multi-variable valence properties of metals, the redox ability of oxidants, and the energy of electrons or photons to induce the generation of radicals. The purpose of the present review is not only to illustrate recent significant progresses in the synthesis of seven- or eight-membered rings but also to provide insights into probing new reaction models in radical chemistry.