The endo rule for the Diels-Alder reaction

Reaction of hexa-2,4-diene and acrolein

In this case both the diene and dienophile have stereochemistry. Although there are four stereogenic centres in the product only two diastereoisomers can be formed. The diastereoisomers formed are known as exo and endo.

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Sn2Click the structures and reaction arrows in sequence to view the 3D models and animations respectively

The endo product is favoured, but which one is it? The easiest way to work this out is by drawing the reactants coming together in 3D. See one method for working this out.

J. Sauer, Angew. Chemie Int. Ed. English, 1967, 6, 16–33.