Fischer Indole Synthesis

The Fischer indole synthesis is one of the oldest and most reliable ways to produce indoles, and was first developed in 1883 by Fischer. A variety of indoles can be prepared from aryl hydrazines and substituted ketones or aldehydes in the presence of a Brønsted acid or Lewis acids. Hydrazine starting materials can be substituted but are generally limited to small alkyl groups, whereas ketones and aldehydes can withstand larger substituent variety and provide substitutions at 2 and 3 positions. Indoles remain one of the most important heterocyclic aromatic compounds and are ubiquitous in many natural products and pharmaceuticals, and this reaction is also useful to form pharmachologically important indole derivatives.