Here’s an important facet of the Diels Alder reaction that causes a headache for many students.
Recognizing endo and exo Diels Alder products can be tricky. Here’s one way to do it that works.
Look at your diene. Make sure it’s in the s-cis conformation (both alkenes are on the same side of the C2-C3 single bond). Now identify the “outside” substituents.
Look at the dienophile. Identify the “electron withdrawing group” – usually contains a C=O or CN group (although if those aren’t present you’ll pick the “highest priority group” on the dienophile).
Now connect the diene and dienophile to make the cyclohexene (3 bonds break, 3 bonds form). Now we’ll need to draw in the stereochemistry.
The ENDO product is the one where the “outside” groups on the diene are on the SAME side of the 6-membered ring as the electron withdrawing group (EWG).
The EXO product is the one where the “outside” groups are on the OPPOSITE side of the ring as the 6-membered ring.
Note how the Endo and Exo products are related – they’re diastereomers
Here’s some examples.
PS – check out this page on the Reaction Guide: The Diels Alder Reaction
Questions? Leave a comment!