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Ring expansion and ring contraction
Cyclic compounds, ubiquitous in organic compounds, are chains of atoms are covalently bonded to form a closed ring. These rings are primarily composed of carbon, but can also contain heteroatoms such as nitrogen, oxygen, and sulfur (called heterocycles). In the course of organic synthesis a chemist often needs to form a new or alter an existing ring. By expanding or contracting an existing ring it is possible to access structures that would be difficult to nearly impossible with a single cyclization reaction.
== Ring expansion 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 (seven member and larger rings).〔Casadei, M.A.; Calli, C.; Mandolini, L., J. Am. Chem. SOC. 1984, 106, 1051-1056〕 Classifying ring expansions by the mechanism of expansion and the atom(s) added allows one to see the similarities between different expansions methods and different incorporated atoms. The broadest classification comes by the mechanism of expansion. The rings can be expanded by attack of the ring onto an outside group already appended to the ring (a migration/insertion), opening of a bicycle to a single larger ring, or coupling a ring closing with an expansion.〔Kantorowski, E.J.; Kurth, M.J. Tetrahedron 2000 4317-4353〕 These expansions can be further broken down by what type of atom they incorporate (a carbon or a heteroatom) into the expanded ring.
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