The endo rule for the Diels-Alder reaction

How to determine which product is the endo product

In the reaction between hexa-2,4-diene and acrolein two stereoisomers may be formed - endo and exo. The endo product is favoured, but how can we tell which one this is? The easiest way to work this out is by drawing the reactants coming together in 3D. Click on the structures below for a method of doing this.

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