The Diels–Alder reaction, inverse electronic demand Diels–Alder reaction, as well as the hetero-Diels–Alder reaction, belong to the category of [4+2]-cycloaddition reactions, which are concerted processes. The arrow pushing here is merely mnemonic. It is postulated that a class of enzymes, known as Diels–Alderases, are involved in biosynthesis to promote [4 + 2]-cycloaddition reactions during the biosynthesis of natural products.