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.