The Diels-Alder reaction is concerted. The Diels-Alderreos The ste IM-M0C. 女- Theecnrnomrwec8naorim。ndosoctv e品o2 the r 7 7 The Diels-Alder reaction is concerted. Both new C-C single bonds and the new π bond form simultaneously, just as the three π bonds in the starting materials break. This type of one-step reaction is called concerted. The Diels-Alder reaction is stereospecific. The stereochemistry at the original double bond of the dienophile is retained in the product. The stereochemistry of the diene is also retained. Note that only one enantiomer is shown in each of the above reactions. The other enantiomer is equally probable and a racemic mixture is formed in each case. Diels-Alder cycloadditions follow the endo rule. In the reaction of 1,3-cyclopentadiene with dimethyl cis-2- butenedioate, two products are conceivable: an exo adduct and an endo adduct: The Diels-Alder reaction usually proceeds with endo selectivity. This is a kinetic effect known as the endo rule. As an aid to determining the positioning of the substituents in an adduct, use the following diagram: