Solid‐supported and pegylated H–Pro–Pro–Asp–NHR as catalysts for asymmetric aldol reactions

Abstract

H-Pro-Pro-Asp-NH2 is a highly active and selective catalyst for asymmetric aldol reactions. Here, the versatility of H-Pro-Pro-Asp-NH2 has been further improved by immobilization on a solid support and functionalization with a short polyethylene glycol linker at the C-terminus. The development, synthesis, and the catalytic properties in aldol reactions of H-Pro-Pro-Asp-resin and H-Pro-Pro-Asp-Ahx-NH(CH2CH2O)3CH3 are described. For the solid-supported catalyst, TentaGel with a loading of 0.1-0.2 mmol g(-1) proved to be the optimal support. The solid-supported catalyst can be recycled at least three times without a significant drop in the catalytic activity or selectivity. Using the pegylated catalyst H-Pro-Pro-Asp-Ahx-NH(CH2CH2O)3CH3, only 0.5 mol % are necessary to obtain aldol products in up to 96% yield and 91% enantiomeric excess. In all cases, enantioselectivities are comparable to those obtained with the parent catalyst H-Pro-Pro-Asp-NH2. Thus, immobilization of H-Pro-Pro-Asp-NH2 on Tentagel as well as pegylation led to catalysts with selectivities comparable to the nonmodified catalyst, exhibiting additional distinct advantages such as facile reusability, ease of handling, higher solubility, and thereby greater versatility. handling, higher solubility, and thereby greater versatility.