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. Cation contractions The cationic rearrangement contraction proceeds through the loss of a leaving group and the migration of an endocyclic bond to the carbocation. Pinacol type rearrangements are often used for this type of contraction. Like the expansion reaction this proceeds with an electron donating group aiding in the migration. Contraction reactions of one ring can be coupled with an expansion of another to give an unequal bicycle from equally sized fused ring. These cationic rearrangements have found use to synthesize the cores of complex molecules. Carbenoid contractions The Wolff rearrangement can be used to perform ring contractions. It proceeds through a carbenoid generated from decomposition of a diazoketone, which inserts into the carbon-carbon bond on the opposite side of the ketone. References Rearrangement reactions