“When a mixture of 1,3-butadiene and ethene is heated in the gas phase, a remarkable reaction takes place in which cyclohexene is formed by the simultaneous generation of two new carbon – carbon bonds. This is the simplest example of the Diels-Alder reaction, in which a conjugated diene adds to an alkene to yield cyclohexene derivatives. The Diels-Alder reaction is in turn a special case of the more general class of cycloaddition reactions between psystems, the products of which are called cycloadducts.”