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.