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Pyrrole was first detected by F. F. Runge in 1834, as a constituent of coal tar. [8] In 1857, it was isolated from the pyrolysate of bone. Its name comes from the Greek pyrrhos (πυρρός, "reddish, fiery"), from the reaction used to detect it—the red color that it imparts to wood when moistened with hydrochloric acid. [9]
The Knorr pyrrole synthesis is a widely used chemical reaction that synthesizes substituted pyrroles (3). [ 1 ] [ 2 ] [ 3 ] The method involves the reaction of an α- amino - ketone (1) and a compound containing an electron-withdrawing group (e.g. an ester as shown) α to a carbonyl group (2) .
This difference is partly related to the lower symmetry of the individual pyridine molecule (C 2v vs D 6h for benzene). A trihydrate (pyridine·3H 2 O) is known; it also crystallizes in an orthorhombic system in the space group Pbca, lattice parameters a = 1244 pm, b = 1783 pm, c = 679 pm and eight formula units per unit cell (measured at 223 K).
In organic chemistry, the Bohlmann–Rahtz pyridine synthesis is a reaction that generates substituted pyridines in two steps, first a condensation reaction between an enamine and an ethynylketone to form an aminodiene intermediate, which after heat-induced E/Z isomerization undergoes a cyclodehydration to yield 2,3,6-trisubstituted pyridines.
Examples of basic aromatic rings are pyridine or quinoline. Several rings contain basic as well as non-basic nitrogen atoms, e.g., imidazole and purine. In the non-basic rings, the lone pair of electrons of the nitrogen atom is delocalized and contributes to the aromatic pi-electron system.
Pyrrole Ring. The Paal–Knorr reaction is quite versatile. In all syntheses almost all dicarbonyls can be converted to their corresponding heterocycle. R2 and R5 can be H, aryl or alkyl. R3 and R4 can be H, aryl, alkyl, or an ester. In the pyrrole synthesis (X = N), R1 can be H, aryl, alkyl, amino, or hydroxyl. [9]
The reaction is carried out in the liquid phase in a continuous tube- or tube bundle reactor, which is operated in the cycle gas method. The catalyst is arranged as a fixed-bed and the conversion is carried out in the downflow mode. The product is obtained after multistage purification and separation by extractive and azeotropic distillation. [5]
[1] [2] The initial reaction product is a dihydropyridine which can be oxidized in a subsequent step to a pyridine. [3] The driving force for this second reaction step is aromatization. This reaction was reported in 1881 by Arthur Rudolf Hantzsch. A 1,4-dihydropyridine dicarboxylate is also called a 1,4-DHP compound or a Hantzsch ester.