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Hantzsch pyrrole synthesis
The Hantzsch Pyrrole Synthesis, named for Arthur Rudolf Hantzsch, is the chemical reaction of β-ketoesters (1) with ammonia (or primary amines) and α-haloketones (2) to give substituted pyrroles (3).〔Hantzsch, A. ''Ber.'' 1890, ''23'', 1474.〕〔Feist, F. ''Ber.'' 1902, ''35'', 1538.〕 Pyrroles are found in a variety of natural products with biological activity, so the synthesis of substituted pyrroles has important applications in medicinal chemistry.〔Furstner, A. ''Angew. Chem. Int. Ed.'' 2003, ''42''¸ 3582-3603.〕 Alternative methods for synthesizing pyrroles exist, such as the Knorr Pyrrole Synthesis and Paal-Knorr Synthesis.
== Mechanism ==
Below is one published mechanism for the reaction:〔Li, J.J. ''Name Reactions''; 4th ed.; Springer-Verlag: Berlin, Germany, 2009; p. 276.〕
The mechanism starts with the amine (1) attacking the β carbon of the β-ketoesters (2), and eventually forming an enamine (3). The enamine then attacks the carbonyl carbon of the α-haloketone (4). This is followed by the loss of H2O, giving an imine (5). This intermediate undergoes an intramolecular nucleophilic attack, forming a 5-membered ring (6). Finally, a hydrogen is eliminated and the pi-bonds are rearranged in the ring, yielding the final product (7).
An alternative mechanism has been proposed in which the enamine (3) attacks the α-carbon of the α-haloketone (4) as part of a nucleophilic substitution, instead of attacking the carbonyl carbon.〔Wang, Zerong. ''Comprehensive Organic Name Reactions and Reagents, 3 Volume Set''; John Wiley & Sons, Hoboken, New Jersey, 2009; pp. 1326-1327.〕
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