Pyrrole, or pyrrol, is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. Substituted derivatives are also called pyrroles. For example, C4H4NCH3 is N-methylpyrrole. Porphobilinogen is a trisubstituted pyrrole, which is the biosynthetic precursor to many natural products.
The starting materials in the Piloty-Robinson pyrrole synthesis are 2 equivalents of an aldehyde and hydrazine. The product is a pyrrole with specific substituents in the 3 and 4 positions. The aldehyde reacts with the diamine to an intermediate di-imine (R–C=N−N=C–R), which, with added hydrochloric acid, gives ring-closure and loss of ammonia to the pyrrole.
In the second step, a [3,3]sigmatropic reaction takes place between two intermediates.
The resonance contributors of pyrrole provide insight to the reactivity of the compound. Like furan and thiophene, pyrrole is more reactive than benzene towards nucleophilic aromatic substitution because it is able to stabilize the positive charge of the intermediate carbanion. This is because the nitrogen can donate a lone pair into the ring by resonance
Pyrrole undergoes electrophilic aromatic substitution predominantly at the 2 and 5 positions, though the substitution product at positions 3 and 4 is obtained in low yields. Two such reactions that are especially significant for producing functionalized pyrroles are the Mannich reaction and the Vilsmeier-Haack reaction (depicted below) , both of which are compatible with a variety of pyrrole substrates. Reaction of pyrroles with formaldehyde form porphyrins.
In a 1994 report released by five top cigarette companies, pyrrole is one of the 599 additives to cigarettes.
Knorr-Rabe partial reduction of pyrroles: Application to the synthesis of indolizidine alkaloids.(Full Research Paper)
Jan 15, 2008; Authors: Brendon S Gourlay ; John H Ryan ; Jason A Smith (corresponding author)  Background The Birch reaction for the...