Building complex chemicals from their simplest components in the laboratory relies on the tools available for the task, and for chemists these tools are the repertoire of reactions they have at hand. Using these reactions, synthetic organic chemists act as chemical construction engineers, gradually piecing together the correct molecules in the correct manner until the final product is created.

The 1950 Nobel Prize in Chemistry acknowledged one of the most powerful and versatile reactions chemists have in their toolbox. Otto Diels and his then student Kurt Alder noticed a peculiar interaction between two compounds – one molecule containing four carbon atoms linked by two double bonds separated by a single bond, known as a diene, and another molecule containing two carbon atoms linked by a double bond. Rather than creating a product that extends in a linear manner, the resulting product closes up on itself to form a ring of six carbon atoms, with only one pair of carbon atoms linked by a double bond. Other chemists had noticed reactions that could form this ring, but it was Diels and Alder who provided the first description of how this proceeds in practice, and consequently the reaction was named after them.

What helped to elevate the Diels-Alder reaction to its position as one of the most versatile tools for synthesizing chemicals is the remarkable ease with which the reaction occurs. This ring formation can be achieved without the use of powerful chemical reagents, and in some cases can occur almost spontaneously. Diels and Alder also showed that their reaction had broad application, enabling the creation of a host of ring structures found regularly in natural compounds. From morphine to plastics, the Diels-Alder reaction provides a tool for creating key structural features of these complex chemicals, which had been tricky or impossible to produce by other methods.