Aug 30, 2017 · Today we’ll introduce a reaction I consider to be the most useful and powerful reaction in all of organic chemistry: the Diels-Alder reaction. In the Diels-Alder reaction, a diene combines with a pi bond (often called a “dienophile”) to give a new six-membered ring. Two C-C sigma bonds and a C-C (pi) bond are formed, and three C-C pi ...