Enantioselective organocatalytic cycloadditions for the synthesis of medium-sized rings

Optically active medium-sized cyclic compounds are often found in natural products and are therefore attractive targets in organic synthesis. However, generating these cyclic entities with specific stereochemistry is far from trivial owing to unfavourable entropic factors and competing pathways that favour the formation of rings of lesser size. As a result, conventional ring-forming strategies can be challenging, and alternative methods, such as organocatalytic cycloadditions, have emerged to address these issues. Enantioselective synthesis of medium-sized rings by organocatalytic cycloadditions is a rapidly growing field of research offering opportunities that are complementary to metal-catalysed cycloadditions. Several organocatalytic approaches are available, including enamine/iminium-ion activation, along with catalysis using Lewis and Brønsted acids, hydrogen-bond donors, N-heterocyclic carbenes, and nucleophilic phosphines and amines. Here we discuss the ability of organocatalytic cycloadditions to synthesize stereodefined medium-sized ring architectures, critically evaluate current synthetic strategies, and highlight avenues for further development. Enantioselective synthesis of medium-sized rings by organocatalytic cycloadditions offers opportunities that are complementary to metal-catalysed cycloadditions. This Review discusses the ability of organocatalytic cycloadditions to synthesize stereodefined medium-sized ring architectures, critically evaluates current synthetic strategies, and highlights avenues for further development.