Medium-sized heterocycles (with 8 to 11 atoms) constitute important structural components of several biologically active natural compounds and represent promising scaffolds in medicinal chemistry. However, they are under-represented in the screening of chemical libraries as a consequence of being difficult to access. In particular, methods involving intramolecular bond formation are challenging due to unfavorable enthalpic and entropic factors, such as transannular interactions and conformational constraints. The present review focuses on the synthesis of medium-sized heterocycles by transition-metal-catalyzed intramolecular cyclization, which despite its drawbacks remains a straightforward and attractive synthesis strategy. The obtained heterocycles differ in their nature, number of heteroatoms, and ring size. The methods are classified according to the metal used (palladium, copper, gold, silver), then subdivided according to the type of bond formed, namely carbon–carbon or carbon–heteroatom.

The hetero-Diels-Alder reaction is one of the most powerful transformations in the chemistry toolbox for the synthesis of heterocycles. In this article are presented the recent developments of this reaction involving heterodienophiles such as imines, azenes, carbonyls, thiocarbonyls, and nitroso compounds as well as heterobutadienes such as azabutadienes, diaazabutadienes, oxabutadienes, thiabutadienes, nitrosoalkenes, and nitroalkenes.