The Diels-Alder reaction is an addition reaction between a conjugated diene and a substituted alkene, known as the dienophile, to form a cyclohexene system. It is widely used in organic synthesis because it forms cyclic compounds in a stereoselective, regioselective, and often stereospecific manner, which allows for the construction of complex molecular architecture from simpler substances. This reaction occurs under thermal conditions, generally without the need for a catalyst, and the resulting adducts have a high degree of stereochemical control, providing an efficient method for creating six-membered rings, which are common structural motifs in natural products and pharmaceuticals.