Aza-Diels–Alder reaction

The lowest-energy transition state for the concerted process places the imine lone pair (or coordinated Lewis acid) in an exo position.

[2] When the imine nitrogen is protonated or coordinated to a strong Lewis acid, the mechanism shifts to a stepwise, Mannich-Michael pathway.

In the enantioselective Diels–Alder reaction of an aniline, formaldehyde and a cyclohexenone catalyzed by (S)-proline even the diene is masked.

The second step, an endo trig cyclisation, is driven to one of the two possible enantiomers (99% ee) because the imine nitrogen atom forms a hydrogen bond with the carboxylic acid group of proline on the Si face.

Cycloadditions of these intermediates with dienes give single constitutional isomers, but proceed with moderate stereoselectivity.