The Diels–Alder reaction is a chemical reaction between a diene and a dienophile that forms a six-membered ring. This reaction is a type of cycloaddition and is widely used in organic chemistry to create complex molecules. It typically occurs under mild conditions and is valued for its ability to form new carbon-carbon bonds. In the Diels–Alder reaction, the diene must be in an electron-rich state, while the dienophile is usually electron-deficient. The reaction proceeds through a concerted mechanism, meaning that bonds are formed and broken simultaneously. This process is important in the synthesis of natural products and pharmaceuticals.