Cyclic Imines in Ugi and Ugi-Type Reactions.

Ugi four-component reactions (U-4CRs) are widely recognized as being highly efficient for the synthesis of pseudopeptides. However, the products of these reactions are not so interesting as drug candidates because they are not conformationally restricted enough for a potent interaction with biological targets. One possible way to overcome this problem is to replace amine and oxo components in the U-4CRs with cyclic imines in so-called Joullie-Ugi three-component reactions (JU-3CRs). This approach provides a robust single-step route to peptide moieties connected to N-heterocyclic motifs that are found as core skeletons in many natural products and pharmaceutical compounds. JU-3CR reactions also provide much better diastereoselectivity than their four-component analogues. We survey here the re-design of many synthetic routes for the efficient preparation of a wide variety of three-, five-, six-, and seven-membered heterocyclic compounds connected to the peptide backbone. Additionally, in the Ugi reactions based on the cyclic imines, α-acidic isocyanides or azides can be replaced with normal isocyanides or acids, respectively, leading to the synthesis of N-heterocycles attached to oxazoles or tetrazoles, which are of great pharmaceutical significance. This review includes all research articles related to Ugi reactions based on the cyclic imines to the year 2020, and will be useful to chemists in designing novel synthetic routes for the synthesis of individual and combinatorial libraries of natural products and drug-like compounds.