Answer
The Diels–Alder reaction, a pericyclic reaction, is a concerted cycloaddition of a conjugated diene and a dienophile, resulting in the formation of a cyclohexene ring. It involves the interaction of four π electrons from the diene with two π electrons from the dienophile, leading to the formation of a new carbon-carbon bond. Substituents on the diene and dienophile can influence the reactivity and regio- and stereoselectivity of the reaction.