Diels-Alder: Z,Z (cis,cis) diene

Stereochemistry of the diene

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The stereochemistry of the diene must be taken into account to predict the stereochemistry of the product. Cyclopentadiene diene is cis,cis (Z,Z), and so has two sets of substituents - inside and outside. The inside one is the bridging CH2 group, which has to end up on one side of the molecule (in this case, sticking up in the products), while the two green hydrogens are outside and remain so. In the product, they end up below the six-membered ring. The acetylene dienophile is chosen to simplify the structure of the product.

Examples of sterochemistry in trans,trans (E,E) and cis,trans (E,Z) dienes.

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