Bischler–Möhlau indole synthesis

Bischler–Möhlau indole synthesis is a chemical reaction that involves the synthesis of indole derivatives from phenylhydrazine and an acyl derivative (such as an ester or carboxylic acid) in the presence of polyphosphoric acid (PPA). This reaction is an important method in the field of organic chemistry for the construction of indole rings, which are core structures in many natural products and pharmaceutical compounds. The reaction was independently discovered by August Bischler and Bernard Möhlau in the late 19th century.

Reaction Mechanism

The Bischler–Möhlau indole synthesis proceeds through several key steps. Initially, the phenylhydrazine reacts with the acyl derivative to form a hydrazone. Upon heating with polyphosphoric acid, a cyclization occurs, leading to the formation of an indole ring. The mechanism involves an electrophilic aromatic substitution followed by a dehydration step.

1. Formation of Hydrazone: The first step involves the reaction between phenylhydrazine and the acyl derivative to form a hydrazone. 2. Cyclization: The hydrazone then undergoes cyclization in the presence of polyphosphoric acid to form an intermediate. 3. Dehydration: Finally, dehydration of the intermediate leads to the formation of the indole derivative.

Applications

The Bischler–Möhlau indole synthesis is widely used in the synthesis of complex natural products and pharmaceuticals. Indoles are a key structural motif in many biologically active molecules, including tryptophan, serotonin, and various alkaloids. Therefore, the ability to synthesize indole derivatives efficiently is of great importance in medicinal chemistry and drug discovery.

Limitations

While the Bischler–Möhlau indole synthesis is a valuable tool for the synthesis of indoles, it has some limitations. The reaction conditions, particularly the use of polyphosphoric acid, can be harsh and may lead to side reactions or degradation of sensitive functional groups. Additionally, the reaction's scope is somewhat limited by the need for specific starting materials (phenylhydrazine and an acyl derivative).

See Also

• Fischer indole synthesis
• Reissert indole synthesis
• Madison indole synthesis