Now we want to apply our knowledge of the cube method to get the
stereochemistry of the Diels-Alder reaction. There are two approaches, the
endo approach on the top and the exo approach on the bottom. Depending on what
you’re asked, if you’re asked, show me the endo approach and the stereochemistry
of the following reaction. You have to do the endo. If you’re asked for the exo
approach then you have to do the exo. So I’m going to teach you both. In the endo
approach, you have the secondary orbital overlap between C and this p-orbital and between D and this p-orbital. We’ve learned that. On the endo approach,
C and D are inside of the diene or pointing towards the inside of the
molecule. In the exo approach, C and D are outside. That’s why it’s called exo or
outside. You start with three pi bonds. One here, one here, and one here. In all
of them, it’s the same. You’re going to initiate your pi bonds rotation. So you do
one, two, and three. Also here you do the same thing. One, two, and three. Now you
can see that for the endo approach on the top, group C’s and D’s are on the left
side. Groups A and B are showing down here. Anything that is on the left side will be
down. It’s shown here for C and D. A and B are down. All of these groups will be
down in the endo approach. For the exo approach, A and B stay down because these
hydrogens are up. When we flatten this molecule to make it going from 3D to
2D, C and D are on the right side. So now they are shown as up.
