The Construction of Five-Membered Heterocycles by Transition Metal-Catalyzed Cyclization of Propargylic Compounds

Heterocyclic compounds, five-membered components in particular, are of great significance in organic and pharmaceutical chemistry. This review highlights the substantial progress and examples, which have been made in the last decade to construct fivemembered heterocycles by transition metal-catalyzed cyclization of propargylic compounds. Propargylic compounds are versatile threecarbon building blocks in heterocyclic chemistry. They can be easily and efficiently prepared by nucleophilic addition of alkynyl anions to carbonyl compounds and the successive substitution reactions. Current trends have focused on and expanded the flexible applications of propargylic compounds in many aspects of organic syntheses. Heterocyclic syntheses involving transition metal complexes have become of common use in past decades because a transition metal-catalyzed reaction has a potential to directly build complicated molecules from readily accessible starting materials under mild conditions. Transition metal elements encompassing Ag, Au, Cu, Fe, Zn, Hg, Ru, Re, Ir, Rh, Ni, Pd, Pt, Sc and the like demonstrate intriguing catalytic activity in the aforesaid transformations. They are able to drive reactions either or both as Lewis acids and most commonly triple-bond activation agents by the formation of σ- or π-complexes. As a consequence, a variety of stages are set for new diversity-oriented routes to five-membered heterocycles such as furans, pyrroles, thiophenes, oxazoles, imidazoles, thiazoles, benzofurans, benzothiophenes, indoles and the like. Owing to the structural importance in biologically active natural and unnatural compounds, some of the nonaromatic five-membered heterocyclic compounds are also included with this review.