Click the structures and reaction arrows in sequence to view the 3D models and animations respectively
Diels-Alder reactions occur between a conjugated diene and a dienophile (an alkene). This reaction shows a simple diene combining with an unsaturated aldehyde, which acts as the dienophile.
Notice how the diene has to rotate into an unfavorable s-cis coplanar conformation for the reaction to proceed and the carbonyl group occupies the endo position in the transition state.