Enantioselective 1,3-dipolar cycloadditions of unsaturated aldehydes promoted by a poly(ethylene glycol)-supported organic catalyst

We have developed a polymer-supported version of Mac-Millan's catalyst by anchoring a tyro sine-derived imidazolidin-4-one by means of a spacer to the monomethyl ether of poly(ethylene glycol) (M-w = 5000 Da). The supported organic catalyst was employed in some 1,3-dipolar cycloadditions involving alpha,beta-unsaturated aldehydes and nitrones. The products were obtained in enantiomeric excesses very similar to those observed with the non-supported catalyst, but the chemical yields were somewhat lower. Exploiting the solubility profile of the polymeric support, the catalyst was readily separated from the reaction products, recovered, and recycled. Catalyst recycling was accompanied by a very marginal erosion (if any) of the enantioselectivity and a more-substantial decrease in chemical efficiency. We ascribe the latter phenomenon mainly to the intrinsic instability of the catalyst under the reaction conditions, which is, however, a common feature of both the non-supported and supported catalysts.