16.21 : Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry

While Diels–Alder reactions between an acyclic diene and a dienophile give cyclic products, cyclic dienes form bridged bicyclic compounds.

Unlike the open-chain dienes, cyclic analogs are locked in an s-cis conformation, leading to rapid Diels–Alder reactions.

Reactions between cyclic dienes and substituted dienophiles give stereoselective results based on the relative alignment of the two components.

When the dienophile aligns with its electron-withdrawing group under the diene, the approach leads to an endo product.

Alternatively, when the electron-withdrawing group is directed away from the diene, the approach results in an exo product.

The two stereoisomers differ in the orientation of the substituent relative to the double bond.

In general, the major product is endo because the activation energy to its transition state is lowered, given the additional interaction between the electron-withdrawing group of the dienophile and the diene. This interaction, called secondary orbital overlap, is absent in the exo transition state.

Consequently, the endo product is formed faster and is the kinetic product. Whereas, the exo product is sterically favored and is the thermodynamic product.