Final answer:
The Diels-Alder reaction favors the formation of the endo product due to secondary orbital interactions stabilizing the transition state, particularly when dealing with unsymmetrical reactants with electron-withdrawing groups on the dienophile. Steric and electronic factors can influence the regioselectivity, ultimately determining the major product.
Step-by-step explanation:
The Diels-Alder reaction typically favors the formation of the endo product under normal conditions, due to the secondary orbital interactions that stabilize the transition state. However, the dominant product also depends on the substituents present on the diene and the dienophile. In simple cases like the reaction between butadiene and ethene, the product will be a single six-membered cyclic compound without regioselectivity or stereoselectivity concerns because both reactants are symmetric. When unsymmetrical reactants are involved, the regioselectivity and the stability of the transition state determine the major product. Factors like steric hindrance and electronic effects can alter the outcome, and in cases where a dienophile is activated by electron-withdrawing groups, the endo rule usually applies leading to the formation of the endo product as the major isomer.