Substituted norbornene, obtained by a Diels–Alder reaction of cyclopentadiene and ethylene, shows how two different substituted ethylenes, replaced by an electron-withdrawing group (EWG, e.g., -COOH, schematized with a green sphere), lead to two different results for the cyclopentadiene reaction. The exo products are obtained when the outside group (hydrogen) points down and the bond to the EWG points up, while the endo products result when they both point down. These are two isomers of substituted norbornene [2].

The interaction of ethylene substituents with double bonds causes the endo structure to be kinetically favored over exo. This is the opposite of the thermodynamic prediction. Since the reaction takes place under temperature conditions in which kinetic control prevails, the formation of the endo compound is favored, as shown in the graph with reaction coordinates.

Cyclopentadiene reacts with acetylene to form norbornadiene: the triple bond of acetylene turns into a double bond, leading to the formation of norbornadiene, which cannot isomerize [3].

After obtaining a norbornadiene, another reaction with an acetylene molecule forms a polycyclic compound named a deltacyclene. This is not classified as a Diels–Alder reaction. In fact, the two double bonds of norbornadiene are not conjugates, and the -electrons involved in the reaction are [2+2+2]; also in this case, a 6-membered ring is obtained [4].

You can use the "focus" button to show the characteristic 6-carbon ring of Diels–Alder reactions.