Expedient recycling of peptide organocatalysts using a nanocrystalline hydroxyapatite catching system

Peptide catalysts represent versatile alternatives to metal-based catalysts and other organocatalysts, due to facile synthesis, modularity, and often high activity and stereoselectivity. In particular, in tripeptides the distance between the catalytic groups is expected to promote 1,4-addition reactions. Immobilization of the catalyst allows easy recovery, but this strategy requiresthe modification of the sequence by introducing a suitable linker, with the risk of losing catalyst effectiveness. Another option is the inclusion or adsorption on an appropriate material. Herein, we describe the expedient use of nanocrystalline hydroxyapatite (HAp) as a biocompatible, inert peptide-catching system. In contrast to the immobilization strategy, HAp can be utilized with the native sequences. Encouraging results have been obtained in the capture andrecycle of the peptide-acid Pro-Pro-AspNH2, utilized for the asymmetric conjugate addition of aldehydes to nitrostyrene.