Knorr quinoline synthesis
- Knorr quinoline synthesis
The Knorr quinoline synthesis is an intramolecular organic reaction converting a β-ketoanilide to a 2-hydroxyquinoline using sulfuric acid. This reaction was first described by Ludwig Knorr (1859 - 1921) in 1886 ["Synthetische Versuche mit dem Acetessigester" Justus Liebig's Annalen der Chemie Volume 236, Issue 1-2, Date: 1886, Pages: 69-115 Ludwig Knorr DOI|10.1002/jlac.18862360105]
The reaction is a type of electrophilic aromatic substitution accompanied by elimination of water. A 1964 study found that with certain reaction conditions formation of a 4-hydroxyquinoline is a competing reaction ["The Conversion of Benzoylacetanilides into 2- and 4-Hydroxyquinolines" B. Staskun J. Org. Chem. 1964; 29(5); 1153-1157. DOI|10.1021/jo01028a038] . For instance, the compound benzoylacetanilide (1) forms the 2-hydroxyquinoline (2) in a large excess of polyphosphoric acid (PPA) but 4-hydroxyquinoline 3 when the amount of PPA is small. A reaction mechanism identified a N,O-dicationic intermediate A with excess acid capable of ring-closing and a monocationic intermediate B which fragments to aniline and (ultimately to) acetophenone. Aniline reacts with another equivalent of benzoylacetanilide before forming the 4-hydroxyquinoline.
A 2007 study ["Knorr Cyclizations and Distonic Superelectrophiles" Kiran Kumar Solingapuram Sai, Thomas M. Gilbert, and Douglas A. Klumpp J. Org. Chem. 2007, 72, 9761-9764 DOI|10.1021/jo7013092] revised the reaction mechanism and based on NMR spectroscopy and theoretical calculations favors a O,O-dicationic intermediate (a superelectrophile) over the N,O dicationic intermediate . For preparative purposes triflic acid is recommended:
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