Proline-calixarene thiourea host-guest complex catalyzed enantioselective aldol reactions: from nonpolar solvents to the presence of water

A proline-calix[4]arene thiourea host-guest complex catalyzed intermolecular aldol reaction of aromatic aldehydes with cyclohexanone has been developed. The anti-configured products were obtained in good yields and with high enantioselectivities. The reaction is proposed to work via a modified Houk-List model, where the carboxylate part of the praline constitutes as a supramolecular system with the thiourea. The outcome of the study indicates the influence of the calix[4]arene thiourea on both the reactivity and selectivity in a non-polar reaction medium, even in the presence of water at moderate temperatures. (C) 2014 Elsevier Ltd. All rights reserved.