Quinoline Riehm synthesis

The Riehm synthesis involves the formation of quinolines via the reaction of an aniline 

In 1885 Riehm and Engler discovered that 2,4-dimethylquinoline could be synthesised by heating aniline hydrochloride with acetone over several days.  

The mechanism inevitably involves the loss of water and, remarkably, methane. The yields are usually low due to the high temperatures required for this transformation to occur. If the 1,2-dihydroquinoline is isolated the aromatization can be performed in a separate step under acidic or basic conditions.  

Vaughan, W. E. In Organic syntheses, Wiley New York, 19SS, Voi. ill, pp 329. 

The Skraup reaction involves the synthesis of quinoline 3 from the reaction of aniline 1 and glycerol 2 in the presence of a strong acid and an oxidant. 

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A number of dihydroquinolines have been prepared by treating aniline derivatives with acetone or mesityl oxide in the presence of iodine. In these cases aromatization to the fully unsaturated quinoline would require the loss of methane, a process known as the Riehm quinoline synthesis. Such Skraup/Doebner-von Miller-type reactions are often low yielding due to large amounts of competing polymerization. For example, aniline 37 reacts with mesityl oxide to give dihydroquinolines 39, albeit in low yield. ... 

Riehm quinoline synthesis. Formation of quinoline derivatives by prolonged heating of aryla-mine hydrochlorides with ketones with or without use of aluminum chloride or phosphorus pentachlo-ride. 

This reaction was first reported by Engler and Riehm in 1885. It is a synthesis of 2,4-dimethylquinoline from the thermal condensation of aniline hydrochloride and acetone or mesityl oxide along with the evolution of water and methane, and is known as the Riehm quinoline synthesis. This reaction can occur under the conditions with or without any condensation reagents and may evolve other hydrocarbons as well. ... 

Other references related to the Riehm quinoline synthesis are cited in the literature. ... 

Amides can also be obtained by AICI3 catalyzed ester amine exchange which proceeds primarily without racemization of chiral centers (eq 24). The reaction of phenols with /3-keto esters is known as the Pechmann condensation (eq 25). Aryl amines are used in the Riehm quinoline synthesis (eq 26) Aromatic systems may be coupled via the Scholl reaction (eq 27) and indole derivatives are prepared in the Stolle synthesis (eq 28). In the Zincke-Suhl reaction, phenols are converted to dienones (eq 29). 3 ... 

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