16.20 : 狄尔斯–阿尔德反应形成环状产物：立体化学

The simplest example of a Diels–Alder reaction is between 1,3-butadiene and ethene, forming cyclohexene.

This reaction is syn-stereospecific, proceeding via a transition state in which the diene's HOMO interacts with the dienophile's LUMO in a suprafacial manner.

Due to the concerted nature of the reaction, both components remain locked in their original configuration, giving rise to stereospecific outcomes.

For example, cis and trans dienophiles form products with syn and anti stereochemistry. The same applies to substituted dienes.

Interestingly, for a diene to undergo a Diels–Alder reaction, it must adopt an s-cis conformation.

Recall that 1,3-butadiene exits as an equilibrium mixture of s-cis and s-trans conformers.

Although the s-trans is more stable, the p orbitals on the terminal carbons are too far apart to simultaneously overlap with the dienophile. This is not the case in the s-cis conformation, making it more reactive.

Lastly, dienes where the s-cis form exhibits severe steric strain or dienes locked in an s-trans form will not undergo Diels–Alder reactions.