Fischer Indole Synthesis

One of the oldest and most powerful methods of indole development is the Fischer Indole Synthesis. It was first patented in 1883 by Fischer. A variety of indole can be prepared from aryl hydrazine and substituted ketones or aldehydes in the presence of Brønsted or Lewis acids. It is a chemical reaction that generates the aromatic heterocyclic indole from a (substituted) phenylhydrazine and an aldehyde or ketone, under acidic conditions.

Fischer indole synthesis (FIS) is said to have a wide range of applications. This includes the synthesis of indole rings that are often present in the overall synthesis of natural products as a framework. Those are particularly found in the alkaloid domain, which comprises a ring system known as an indole alkaloid.

[Image will be Uploaded Soon]

The ability to use a wide range of alcohols instead of their oxidized equivalents is an obvious advantage of this process. The sequence can be performed in one pot, tolerates substitution on both hydrazine and alcohol, and gives moderate to excellent yields to the indoles. Synthesis is often done by subjecting the equimolar mixture of aryl hydrazine and aldehyde or ketone directly to the conditions of idolization without hydrazone isolation. Similarly, aryl hydrazone, prepared by a reduction of the connected aryl diazonium salt or N-nitroso arylalkylamine salt, can be directly subjected to idolization conditions in the presence of carbonyl without aryl hydrazone isolation.

The Fischer Indole Synthesis’s Main Features include:

1. The indole formation can be carried out in one pot, as the intermediate aryl hydrazones need not be isolated.

2. Two region-isomeric 2,3-disubstituted indoles are provided by unsymmetrical ketones with region-selectivity depending on medium acidity, hydrazine substitution, and steric effects.

3. 1,2-diketones can provide both mono and bis-indoles, usually forming mono-indoles in refluxing alcohols with strong acid catalysts.

Fischer Indole Synthesis Mechanism

Fischer indole synthesis converts aldehyde or ketone aryl hydrazones into aryl hydrazones in the presence of an acid catalyst, Indoles.

The arylhydrazone, prepared from the condensation of the aryl hydrazine and carbonyl compound, is protonated and isomerized to the receiver of the enamine. An irreversible electrocyclic rearrangement is then subjected to the protonated enamine tautomer, which is [3,3]-sigmatropic rearrangement, where the N-N bond is broken.

A cyclic amino acetal (or aminal) forms the resulting imine, which removes NH3 under acid catalysis, resulting in an energetically favorable aromatic indole. The Fischer indole synthesis in which an aromatic phenylhydrazone is heated in acid is the most useful route to the indoles. The condensation product of phenylhydrazine and an aldehyde or a ketone is phenylhydrazone. A cyclic rearrangement mechanism is involved in ring closure.

[Image will be Uploaded Soon]

Fischer Indolization

A convergent approach known as Fischer Indole Synthesis has been developed to access the fused indoline ring system found in a multitude of bioactive molecules. The approach involves the condensation of hydrazines with latent aldehydes via an interrupted Fischer indolization sequence. This eventually delivers indoline-containing products. The method is convergent, mild, easy to operate, broad in reach, and can be used for accessing enantioenriched goods.

The first catalytic Fischer asymmetric Indolization has been made. 4-substituted cyclohexanone-derived phenylhydrazones undergo strongly enantioselective indolization in the presence of a 5 mol percent loading of a novel spirocyclic chiral phosphoric acid. The addition of a weakly acidic cation exchange resin, which removes the ammonia produced, has achieved efficient catalyst turnover. The reaction can be performed under mild conditions and gives different genera of 3-substituted tetrahydro carbazoles. 

An effective way to create fused indoline ring systems present in a variety of natural alkaloids is the interrupted Fischer indolization. In alkaloids and perophoramidine, a succinct approach to the complete synthesis of communes is given. The technique is based on the use of the disrupted Fischer indolization to create the natural products’ tetracyclic indoline nucleus. Studies will be presented to test the reach and limitations of this strategy.

The disrupted Fischer indolization reaction could also be used to complete the formal overall synthesis of the natural product pyrrolidinoindoline debromoflustramine B (5) Pyrrolidinone was generated using a standard two-step sequence to hemiaminal 40.

Fischer Indole Synthesis Procedure

A heterocycle of considerable significance to biological systems is Indole. In proteins, one of the main charge carriers involved in electron transfer is the redox-active indole side-chain of tryptophan 1,2. The optical properties of indole make tryptophan one of the principal intrinsic fluorophores in the study of protein fluorescence. Indole is generated via the intermediate molecule indole pyruvic acid by reductive deamination of tryptophan. Tryptophanase catalyzes the deamination mechanism from which the amine (-NH₂) group of the tryptophan molecule is removed. Indole, pyruvate, ammonium and water are the final reaction components.

The Fischer indole synthesis in which an aromatic phenylhydrazone is heated in acid is the most useful route to the indoles. An achiral molecular unit is the parent indole; the creation of chiral products using a FIS is by applying alpha-branched carbonyl molecules.