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The Fischer indole synthesis is one of the most important classical methods for the preparation of indole and its substituted derivatives. It was developed by Emil Fischer and involves the acid-catalysed conversion of an arylhydrazone, usually a phenylhydrazone, into an indole derivative. The reaction is particularly useful because indoles are important structural units in many natural products, pharmaceuticals, dyes and biologically active compounds. The general reaction may be represented as: Aldehyde or ketone + phenylhydrazine → phenylhydrazone → indole derivative For example, acetone reacts with phenylhydrazine to form acetone phenylhydrazone, which on treatment with a strong acid and heating gives a substituted indole. Similarly, acetophenone phenylhydrazone gives a methyl-substituted indole derivative. The first step of the Fischer synthesis is the formation of an arylhydrazone. An aldehyde or ketone reacts with phenylhydrazine under mildly acidic conditions. The carbonyl oxygen is protonated, making the carbonyl carbon more electrophilic. Phenylhydrazine then attacks the carbonyl carbon, followed by proton transfers and elimination of water. The resulting compound is the corresponding phenylhydrazone. The second stage is acid-catalysed rearrangement of the hydrazone. Strong acids such as hydrochloric ________ _______ __________ ___ _____ __________ _______.
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