Actually.......The exo is the more stable, thermodynamic product that is obtained under conditions if higher heat is employed. However, most text books as well as exams such as the DAT want the product that is formed FASTER,,,,,the kinetic product. The endo is formed faster because of something called Secondary Orbital Overlap. When the diene and dienophile approach, optimal overlapping of orbitals occur when in the endo position. The Organic Chemistry text book written by Paula Yurkanis Bruice shows this wonderfully .Mait Jones of Princeton also does a great job in showing the orbital interaction in his text. Thus,,,,,the bottom, line is this..... Show the endo adduct if asked on any exam, and this is the MAJOR product due to better orbital overlap.
