Interrupted Ugi and Passerini Reactions: An Underexplored Treasure Island

The formation of the key short-lived intermediate nitrilium ion in the Passerini and Ugi reactions was recognized early in the discovery of these two-multicomponent transformations. Surprisingly, the idea of intramolecularly intercepting it, eluding the attack by the carboxylate, and thus interrupting the normal course of the reaction, was missed by chemists for several decades. In this review we describe, in an exhaustive manner, the reported synthetic approaches, which can be categorized as interrupted Passerini/Ugi reactions. Besides, the clear goal of this review is to show the potential of these transformations, whilst highlighting the underexplored combinations of reagents toward the identification of novel interrupted Passerini/Ugi reactions. 1 Introduction 2 The Interrupted Ugi Reaction 2.1 The Interrupting Functional Group is Present on the Amine Reactant 2.2 The Interrupting Functional Group is Present on the Isocyanide Reactant 2.3 The Interrupting Functional Group is Present on the Carbonyl Reactant 3 The Interrupted Passerini Reaction 3.1 The Interrupting Functional Group is Present on the Carbonyl Reactant 3.2 The Interrupting Functional Group is Present on the Isocyanide Reactant 4 Conclusions and Future Outlook.