Proline-catalyzed aldol additions proceed via a six-membered enamine transition state according to the Zimmerman-Traxler model. Addition of 20-30 mol% proline to acetone or hydroxyacetone catalyzes their addition to a diverse set of aldehydes with high (>99%) enantioselectivity yielding diol products.
 Proline and proline derivatives have been implemented as organocatalysts to promote asymmetric condensation reactions. An example of such a reaction proceeding through a six membered transition state is modelled as follows.
Intramolecular aldolization reactions that are catalyzed by proline likewise go through six-membered transition states. These transition states can enable the formation of either the enolexo or the enolendo product.