Quinone methide

A quinone methide is a type of conjugated organic compound that contain a cyclohexadiene with a carbonyl and an exocyclic methylene group or extended alkene unit. It is analogous to a quinone, but having one of the double bonded oxygens replaced with a carbon. The carbonyl and methylene are usually oriented either ortho or para to each other. There are some examples of transient synthetic meta quinone methides.

A quinone dimethide is a related derivative in which both carbonyl groups of a quinone are replaced by methylene groups. A well studied example is tetracyanoquinodimethane.

Quinone methides are more polar than quinones, and therefore more chemically reactive in a variety of situations. Simple quinone methides are short lived reactive intermediates that are not stable enough to be isolated under normal circumstances but quickly react with nucleophiles and other reactants. Some quinone methides have structural (e.g. steric hindrance) or electronic characteristics that stabilize them enough to be isolated.

Quinones methides and their derivatives are common constituents of biological systems. Quinone methanide itself arises by the degradation of tyrosine, leading ultimately to p-cresol. Various quinone methides are directly involved in the process of lignification (creation of complex lignin polymers) in plants.

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