Amine-Urea-Mediated Asymmetric Cycloadditions between Nitrile Oxides and o-Hydroxystyrenes by Dual Activation.

The first example of asymmetric 1,3-dipolar cycloadditions between nitrile oxides and o-hydroxystyrenes, mediated by cinchona-alkaloid-based amine-ureas is reported. The method is based on a dual activation involving both LUMO and HOMO activations. In addition to the stoichiometric asymmetric induction, a catalytic amount of amine-urea enables the cycloadditions to proceed in an enantioselective manner. Computational studies strongly support the HOMO activation of o-hydroxystyrenes and LUMO activation of nitrile oxides by hydrogen-bonding interactions with the Brønsted acid/base bifunctional catalyst.