Heterocyclic Rings Pyrrole

Importance of Pyrroles Porphyrin Haem (iron (II) complex)

Chlorophyll

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Synthesis of pyrrole, Knorr Synthesis L. Knorr, Ber. 17, 1635 (1884); Ann. 236, 290 (1886); L. Knorr, H. Lange, Ber. 35, 2998 (1902). The original Knorr synthesis employed two equivalents of ethyl acetoacetate, one of which was converted to ethyl 2-oximinoacetoacetate by dissolving it in glacial acetic acid, and slowly adding one equivalent of saturated aqueous sodium nitrite, under external cooling. Zinc dust was then stirred in, reducing the oxime group to the amine.

Nitrosation of aliphatic carbon atoms The nitrosation reaction consists in the replacement of a hydrogen atom by the nitroso group with the formation of a nitroso or oximo derivative. With a few exceptions, the replacement of hydrogen on an aliphatic carbon atom requires the presence of an electron-attracting groups adjacent to the carbon to be nitrosated. Acyl, aroyl, carbonyl, carboxy, carbalkoxyl, nitrogen, cyano, imino, and aryl groups may serve as activators. Monoketones are readily converted into alpha-oximino ketones, whereas monoesters containing no other activating groups do not undergo the reaction. Victor Meyer discovered the reaction in 1873–1874.

Paal-Knorr Pyrrole Synthesis

The 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). Note: direct reaction of β-ketoesters (1) with α-haloketones (2) gives furan [Fiest-Benary furan synthesis], and this can be a troublesome side reaction.

The Hantzsch pyrrole synthesis

Reactions of Pyrrole Substitution at nitrogen, A) Metallation of Pyrrole

B) Formation of N-substituted pyrrole

Electrophilic Substitution (position 2 and 5) The aromatic five-membered heterocycles all undergo electrophilic substitution, with a general reactivity order: pyrrole >> furan > thiophene > benzene

The Vilsmeier Haack reaction

Cycloaddition Reactions with dichlorocarbene Reimer-Tieman Reaction

Reimer-Tieman Reaction

Ring Expansion

Diels-Alder Reactions of pyrrole Pyrroles normally do not undergo DA reactions, Exception

[2 +2] Cycloaddition