The fragmentation of a 5-membered heteroaromatic ring to afford a conjugated ene–ene–yne skeleton, and the corresponding reverse process, cyclization of the hetero-ene–ene–yne motif to generate a variety of heterocyclic systems, are the subject of this review. These synthetically useful reactions, which proceed through a coarctate/pseudocoarctate mechanistic pathway, are unique in that they involve the generation of either a carbene or nitrene intermediate, and provide access to hard to obtain heterocyclic or ene–ene–yne structures. While fragmentation of heteroaromatic rings containing a exocyclic carbene or nitrene has been well documented in the literature for over 40 years, the use of hetero-ene–ene–yne precursors to synthesize heterocycles is a relatively new approach that is generating much interest in the literature. This review highlights both the synthetic and mechanistic aspects of these unique reactions.
