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 in 1886
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. For instance, the compound benzoylacetanilide forms the 2-hydroxyquinoline in a large excess of polyphosphoric acid 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 acetophenone. Aniline reacts with another equivalent of benzoylacetanilide before forming the 4-hydroxyquinoline.
A 2007 study revised the reaction mechanism and based on NMR spectroscopy and theoretical calculations favors an O,O-dicationic intermediate over the N,O dicationic intermediate. For preparative purposes triflic acid is recommended:
