Pseudo-multicomponent reactions.

Classical multicomponent reactions (MCRs) are domino-type one-pot processes in which three or more different reactants are combined sequentially in the same reactor to synthesize compounds containing all or almost all atoms coming from the reactants. Besides, pseudo-MCRs are also domino-type one-pot processes involving combinations of at least three reactants but in which at least one of them takes part in two or more reaction steps. In consequence, the products synthesized through pseudo-MCRs contain also all or almost all atoms but coming from two or more identical reactants. Thus, pseudo-MCRs differ from classical MCRs because the first ones appear to involve an assembly of a higher number of different components than those that are being truly assembled. However, pseudo-MCRs are also useful synthetic tools to generate libraries of complex compounds in few experimental steps, and although the repeated reactants may make them appear less diverse than classical MCRs, this can be offset by the higher number of reactants that can participate in this type of reaction. Overall, there are two types of pseudo-MCRs. The first are those in which the duplicated reagents participate in different steps of the corresponding reaction mechanism. The second kind of pseudo-MCRs are those in which one or more components react simultaneously with a main reagent containing two or more identical functional groups. These latter are known as repetitive pseudo-MCRs. Thus, the aim of the present review is to cover for the first time selected works mainly published in the last two decades about pseudo-MCRs and their repetitive versions toward the synthesis of novel, complex, and highly symmetrical molecules, often including their interesting applications in various fields of science and technology. The manuscript has been categorized considering the number of reagents participating in the corresponding pseudo-MCRs, aiming to give readers novel insights for their future investigations.