The Ugi reaction is a multi-component reaction in organic chemistry involving a ketone or aldehyde, an amine, an isocyanide and a carboxylic acid to form a bis-amide. The reaction is named after Ivar Karl Ugi, who first reported this reaction in 1959. The Ugi reaction is exothermic and usually complete within minutes of adding the isocyanide. High concentration (0.5M - 2.0M) of reactants give the highest yields. Polar, aprotic solvents, like DMF, work well. However, methanol and ethanol have also been used successfully. This uncatalyzed reaction has an inherent high atom economy as only a molecule of water is lost, and the chemical yield in general is high. Several reviews have been published. Due to the reaction products being potential protein mimetics there have been many attempts to development an enantioselective Ugi reaction, the first successful report of which was in 2018. Reaction mechanism One plausible reaction mechanism is depicted below: Amine 1 and ketone 2 form the imine 3 with loss of one equivalent of water. Proton exchange with carboxylic acid 4 activates the iminium ion 5 for nucleophilic addition of the isocyanide 6 with its terminal carbon atom to nitrilium ion 7. A second nucleophilic addition takes place at this intermediate with the carboxylic acid anion to 8. The final step is a Mumm rearrangement with transfer of the R4 acyl group from oxygen to nitrogen. All reaction steps are reversible except for the Mumm rearrangement, which drives the whole reaction sequence. In the related Passerini reaction (lacking the amine) the isocyanide reacts directly with the carbonyl group but other aspects of the reaction are the same. This reaction can take place concurrently with the Ugi reaction, acting as a source of impurities. Variations Combination of reaction components The usage of bifunctional reaction components greatly increases the diversity of possible reaction products. Likewise, several combinations lead to structurally interesting products. The Ugi reaction has been applied in combination with an intramolecular Diels-Alder reaction in an extended multistep reaction. A reaction in its own right is the Ugi–Smiles reaction with the carboxylic acid component replaced by a phenol. In this reaction the Mumm rearrangement in the final step is replaced by the Smiles rearrangement. Another combination (with separate workup of the Ugi intermediate) is one with the Buchwald–Hartwig reaction. In the Ugi–Heck reaction a Heck aryl-aryl coupling takes place in a second step. Combination of amine and carboxylic acid