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L‐Proline‐L‐valine dipeptide derivatives, which self‐assemble in toluene, have been studied as stereoselective catalysts in the conjugate addition of cyclohexanone to trans‐β‐nitrostyrene. Remarkable effects on the stereoselectivity are observed associated to the aggregation of the catalyst. Outstanding differences were observed between the catalytic activity of compound 1, which forms supramolecular gels in toluene, and compound 2, which is not a gelator. In the former case, the enantioselectivity of the reaction was almost insensitive to changes in catalyst concentration and temperature, but in the case of compound 2, the catalytic activity was very much affected by those variables. Structural studies indicate that the results can be rationalized by taking into account significant conformational changes experienced by the catalytic L‐proline derivatives associated with the aggregation process. The results highlight …