4-Hhydroxyproline has been anchored to a polystyrene resin through click chemistry, and the resulting catalyst has been successfully applied to the direct aldol reaction in water. The high hydrophobicity of the resin and the presence of water are key to ensuring high stereoselectivity, whereas yield can be increased by using catalytic amounts of DiMePEG. This effect has been further demonstrated by the inefficiency of a homogeneous, more polar analogue.
Polystyrene-Supported Hydroxyproline: An Insoluble, Recyclable Organocatalyst for the Asymmetric Aldol Reaction in Water
Org. Lett. 2006, 8, 4653-4655.