Application of Conjugated Carbonyls in the Synthesis of Heterocycles via Oxidative Cycloaddition and Cyclization Reactions

Synthesis of pharmaceutically active heterocycles is always appealing as the majority of the widely used drugs contain heterocyclic moieties as their core structure. So, the straightforward construction of heterocycles from readily available/accessible reagents is one of the prime targets of the synthetic chemists. In this context, C-H functionalization has emerged as an effective tool for the designing and synthesis of various heterocyclic moieties as it offers a straight-forward and step-economic pathway. On the other hand, the readily available/accessible conjugated carbonyls are well-known reagents for the construction of carbocycles and heterocycles over the years. However, the employment of C-H functionalization of the two C-H bonds of the conjugated alkene in carbocycle/heterocycle synthesis was not so explored. In the last decade, much focus has been paid on the synthesis of various pharmaceutically active heterocycles through C-H bond functionalization of alpha,beta-unsaturated aldehydes/ketones. These protocols have been developed through either oxidative coupling of conjugated carbonyls with suitable coupling partners or intramolecular C-H bond functionalization of conjugated carbonyls. In this review, we will discuss all the methodologies developed for the synthesis of heterocycles employing intermolecular C-H bond functionalization of conjugated carbonyls. The mechanistic pathways and usefulness of the methodologies will be also highlighted.