Throughout history, Niementowski quinoline synthesis has been a topic of constant interest to humanity. From ancient times to the modern era, Niementowski quinoline synthesis has captured the attention and curiosity of people of all cultures and nationalities. In this article, we will explore in depth all facets of Niementowski quinoline synthesis, from its origins to its relevance today. Throughout the pages that follow, we will discover the importance of Niementowski quinoline synthesis in different contexts and how it has influenced the way we perceive the world around us. So join us on this journey through the fascinating world of Niementowski quinoline synthesis.
Niementowski quinoline synthesis | |
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Named after | Stefan Niementowski |
Reaction type | Ring forming reaction |
The Niementowski quinoline synthesis is the chemical reaction of anthranilic acids and ketones (or aldehydes) to form γ-hydroxyquinoline derivatives.[1][2][3][4]
In 1894, Niementowski reported that 2-phenyl-4-hydroxyquinoline was formed when anthranilic acid and acetophenone were heated to 120–130 °C. He later found that at higher heat, 200 °C, anthranilic acid and heptaldehyde formed minimal yields of 4-hydroxy-3-pentaquinoline.[5] Several reviews have been published.[6][7]
The temperatures required for this reaction make it less popular than other quinoline synthetic procedures. However, variations have been proposed to make this a more pragmatic and useful reaction. Adding phosphorus oxychloride to the reaction mixture mediates a condensation to make both isomers of an important precursor to an important α1-adrenoreceptor antagonist.[8] When the 3 position of an arylketone is substituted, it has been shown that a Niementowski-type reaction with propionic acid can produce a 4-hydroxyquinoline with 2-thiomethyl substitute.[9] The method has also been altered to occur with a catalytic amount of base,[10] or in the presence of polyphosphoric acid.[11]
Because of the similarity of these to the reagents in the Friedlander quinolone synthesis, a benzaldehyde with an aldehyde or ketone, the Niementowski quinoline synthesis mechanism is minimally different from that of the Friedländer synthesis. While studied in depth, two reaction pathways are possible and both have significant support.[5] The reaction is thought to begin with the formation of a Schiff base, and then proceed via an intra-molecular condensation to make an imine intermediate (see below). There is then a loss of water that leads to a ring closing and formation of the quinoline derivative. Most evidence supports this as the mechanism in normal conditions of 120–130 °C. Alternatively, the reaction begins with an intermolecular condensation and subsequent formation of the imine intermediate.[12] The latter has been shown to be more common under acidic or basic conditions. A similar pathway has been proposed for the Niementowski quinazoline synthesis.[13]
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