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Synthesis, Reactions and Medicinal Uses of Quinoline

Skraup synthesis - A mild oxidizing agent is used to produce quinoline from aniline, sulfuric acid, and glycerol.

Quinoline

Synthesis

Skraup synthesis - A mild oxidizing agent is used to produce quinoline from aniline, sulfuric acid, and glycerol. When glycerol is dehydrated to acrolein using concentrated sulfuric acid, the reaction begins. 1, 2-dihydroquinoline is then formed by adding aniline to it.


Further oxidation results in the formation of quinoline from 1,2-dihydroquinoline. Through Michael's addition, aniline is added to acrolein to produce aniline propanal (I). Benzene rings are substituted in substituted anilines to give quinoline derivatives.

Friedlander synthesis - In alcoholic sodium hydroxide solution, o-aminobenzaldehyde or o-aminoacetophenone condenses to quinoline. This reaction occurs because aldehyde or ketone (which must contain the α-methyl group) must form an active bond.



In general, this method is used to prepare 2-substituted quinoline derivatives.

Pfitzinger synthesis - The conversion of isatin to isotonic acid in the presence of a base results in quinoline-4-carboxylic acid being synthesized by combining a ketone with the isotonic acid. Pyrolysis with calcium oxide can be used to remove the carboxylic acid group and yield substituted quinolines.



Combes synthesis - The condensation of 1,3-dicarbonyl compounds with arylamines gives a cyclized β-amino enone, which is then cyclized with heat to afford quinoline.



Doebner Miller synthesis - In the presence of HCl, two moles of aniline and two moles of acetaldehyde form Schiff's base. This Schiff's base is a quinoline derivative formed by combining two molecules.


Pictet method -


Conrad – Limpach – Knorr synthesis - The reaction between anilines and β-keto esters (e.g., ethyl acetoacetate) occurs at a lower temperature and gives a β-amino acrylate, which after cyclization gives a 4-quinolone. At high temperatures, 2-quinolones are formed by cyclizing β-ketoester anilides.



In the presence of tetra-alkyl ammonium salts, Indole and chloroform react together to produce chloroquinoline.


Reactions

Addition to nitrogen - An extra bond can be formed using the N-atom's lone pair of electrons. Quinoline reacts with acidic solutions or metallic ions to form quaternary salts.


Electrophilic substitution - In positions 5 and 8, electrophilic substitution is preferred.




Acylation and alkylation - Despite being deactivated by the N-atom, the quinoline ring is still active. The alkylation and acylation of rings occurs therefore only when they have electron-donating substituents.


Oxidation - Quinoline has high resistance to oxidation. KWnO4 opens the benzene ring when treated with alkaline KMnO4, but the pyridine ring remains intact under vigorous conditions.
reduction - Using catalytic hydrogenation in methanol, quinoline is converted to 1, 2, 3, and 4 – tetrahydroquinoline.


Certain conditions can result in 1, 4 -dihydroquinoline being produced by lithium in liquid ammonia. It is possible to selectively reduce benzene rings in an acidic medium.

Medicinal Uses

Among other things, Quinoline is present in many different drugs such as chloroquine (antimalarial), papaverine (smooth muscle relaxer), quinapril (antihypertensive), Singulair (anti-asthma), hydroxychloroquine (antimalarial), emetine (emetic: vomiting inducer), dimethisoquin (anesthetic), etc.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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