A sigmatropic reaction in organic chemistry is a pericyclic reaction. A sigmatropic reaction does not use a catalyst and involves a single molecule (an uncatalyzed intramolecular process). It changes one σ-bond into a different σ-bond. The name "sigmatropic" is the result of a compounding of the long-established "sigma" name for single carbon-carbon bonds and the Greek word "tropos", meaning turn. This is a rearrangement reaction which means that the bonds in a molecule shift between atoms without any atoms leaving or new atoms added to the molecule. In a sigmatropic reaction, a substituent moves from one part of a π-bonded system to another part in an intramolecular reaction with simultaneous rearrangement of the π system. True sigmatropic reactions usually do not need a catalyst. Some sigmatropic reactions are catalyzed by a Lewis acid. Sigmatropic reactions often have transition-metal catalysts that form intermediates in analogous reactions. The most well-known of the sigmatropic rearrangements are the [3,3] Cope rearrangement, Claisen rearrangement, Carroll rearrangement and the Fischer indole synthesis.