Diels–Alder reaction

In organic chemistry, the Diels–Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene derivative. It is the prototypical pericyclic reaction with a concerted mechanism; specifically, it is a thermally-allowed [4+2] cycloaddition with Woodward–Hoffmann symbol [_π4_s + _π2_s].

Simultaneously constructing two new carbon–carbon bonds, the Diels–Alder reaction reliably forms six-membered rings with good regio- and stereochemical control. Consequently, it is a powerful tool, widely applied to introduce chemical complexity in the synthesis of natural products and new materials.

The reaction was first described by Otto Diels and Kurt Alder in 1928. For its discovery, they were awarded the Nobel Prize in Chemistry in 1950.

The underlying concept has been generalized in several different directions. In the hetero-Diels–Alder reaction, π-systems involving heteroatoms, such as carbonyls and imines, furnish the corresponding heterocycles. Diels-Alder-like reactions occur for other ring sizes, although none matched the [4+2] reaction in scope and versatility. Because ΔH° and ΔS° are negative for a typical Diels–Alder reaction, the reverse of a Diels–Alder reaction becomes favorable at high temperatures. This retro-Diels–Alder reaction is of synthetic importance for only a limited range of Diels–Alder adducts, generally with some special structural features.