Boron-Based Dipolar Multicomponent Reactions: Simple Generation of Substituted Aziridines, Oxazolidines and Pyrrolidines

New multicomponent reactions of aldehydes, isocyanides, trialkylboron reagents and dipolarophiles have been developed as an efficient route to diverse scaffolds, including aziridines, oxazolidines and poly‐substituted pyrrolidines. This chemistry, inspired by a report by Hesse in 1965, is simple and involves mild conditions. Computational studies provide a basis to investigate the stereochemical features observed in the formation of oxazolidines and four‐component adducts, and permit identification of potential factors that might influence the outcome of the multicomponent reaction. Thus, a rational screening of all the components and reaction parameters is made to examine the manifold mechanistic pathways and establish the practical limits for standard applications. Finally, intramolecular and solid‐supported versions of these reactions bring new synthetic possibilities and practical protocols. Overall, the results describe a new family of multicomponent reactions valuable not only for organic reactivity, but also for combinatorial chemistry and drug discovery. One step, four components: An early report from the 1960s has been reformulated into a novel family of multicomponent reactions that leads to aziridine, oxazolidine and pyrrolidine scaffolds just by mixing the reactants (see picture). The scope and mechanistic course of these domino processes are determined. Solid‐phase protocols allow straightforward preparations and even permit chemo‐differentiation of pseudo‐equivalent inputs.