Final answer:
The Diels-Alder reaction primarily occurs in the endo fashion when it is kinetically controlled.
Step-by-step explanation:
The Diels-Alder reaction can occur in both endo and exo fashion, depending on the reaction conditions. However, when the reaction is kinetically controlled, it primarily occurs in the endo fashion.
In the endo fashion, the dienophile approaches the diene from the same face, resulting in the addition of the dienophile to the less hindered face of the diene. This leads to the formation of a cis product.
In contrast, in the exo fashion, the dienophile approaches the diene from the opposite face, resulting in the addition of the dienophile to the more hindered face of the diene. This leads to the formation of a trans product.