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Combes quinoline synthesis
The Combes quinoline synthesis is a chemical reaction, which was first reported by Combes in 1888. It involves the condensation of unsubstituted anilines (1) with β-diketones (2) to form substituted quinolines (4) after an acid-catalyzed ring closure of an intermediate Schiff base (3).
Further studies and reviews of the Combes quinoline synthesis and its variations have been published by Alyamkina ''et al''., Bergstrom and Franklin, Born, and Johnson and Mathews.
The Combes quinoline synthesis is often used to prepare the 2,4-substituted quinoline backbone and is unique in that it uses a β-diketone substrate, which is different from other quinoline preparations, such as the Conrad-Limpach synthesis and the Doebner reaction.
== Mechanism ==
The reaction mechanism undergoes three major steps, the first one being the protonation of the oxygen on the carbonyl in the β-diketone, which then undergoes a nucleophilic addition reaction with the aniline. An intramolecular proton transfer is followed by an E2 mechanism, which causes a molecule of water to leave. Deprotonation at the nitrogen atom generates a Schiff base, which tautomerizes to form an enamine that gets protonated via the acid catalyst, which is commonly concentrated sulfuric acid (H2SO4). The second major step, which is also the rate-determining step, is the annulation of the molecule. Immediately following the annulation, there is a proton transfer, which eliminates the positive formal charge on the nitrogen atom. The alcohol is then protonated, followed by the dehydration of the molecule, resulting in the end product of a substituted quinoline.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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