page_title: Ring expansion and contraction
section_title: Anionic contractions
hierarchical_section_title: Ring expansion and contraction / Ring contraction reactions / Anionic contractions
caption_attribution_description: English: A generalized mechanism of Favorskii rearrangement for a ring contraction.
context_page_description: Ring expansion and ring contraction reactions in the course of organic synthesis refer to a set of reactions which can lead to the expansion or contraction of an existing ring. This often makes it possible to access structures that would be difficult if not impossible to synthesise with single cyclization reactions. Ring expansions are valuable because they allow access to larger systems that are difficult to synthesize through a single cyclization due to the slow rate of formation. Ring contractions are useful for making smaller, more strained rings from larger rings. Expansions are classified by the mechanism of expansion and the atom added; contractions are characterized simply by the reactive intermediate which performs the contraction.
context_section_description: The Favorskii rearrangement is the poster child of the anionic ring contractions. It proceeds through a carbanion which attacks an endocyclic carbon and expels a leaving group (a halide) forming a bicyclic molecule with rings smaller than the original. The bicycle is then opened by nucleophilic attack on the ketone to give the contracted product.
  An alternative to the standard Favorskii rearrangement, is to perform what can be thought of as a negative pinacol rearrangement where an anionic group encourages a bond aligned with a leaving group to migrate and expel the leaving group, which has been used in several syntheses. It should also be noted that the so-called "quasi-Favorskii rearrangement" proceeds without an additional nucleophile to form the final contracted product.
caption: A generalized mechanism of the Favorskii rearrangement to give a ring contracted product. Note that anion formation has been omitted.
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