Prolines Substituted at the 4-Position

Rgure 24.9 Structure of catalysts 23-28 and their catalytic activity. 

4-Acyloxyprolines can also be modified with ionic liquid-tags. The use of the proper ionic liquid-tag enables the use of these catalysts in the presence of water and their recovery and reuse. Indeed, the solubility of ionic liquid-anchored orga-nocatalysts can be tuned by an accurate choice of cations and anions groups present in their structure, and the solubility of ionic liquids can be readily tuned with different cations and anions. This approach allows phase separation to be realized from organic as well as aqueous media. Following the proper extraction procedure, it could be possible to separate the catalytic molecule from the product and to allow its reuse. 

Quantum mechanical calculations have predicted the transition state geometries for the reaction of cyclohexanone enamine with benzaldehyde [55]. Transition states involving the Re attack on the anti-enamine are lower in energy than the 

As briefly discussed above, another approach for the modiflcation of the proline structure is its transformation into a prolinamide bearing an appropriate subshtu-ent. Apart from simple prolinamide, which catalyzed the self-aldol reaction of 

Further examples of prohnamide-based structures do not present any additional chiral center in their structures, but are based on the possibility of establishing an 

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