An enantiomerically pure hydrogen-bonded assembly

Chiral molecules have asymmetric arrangements of atoms, forming structures that are non-superposable mirror images of each other. Specific mirror imag es ('enantiomers') may be obtained either from enantiomerically pure precur sor compounds, through enantioselective synthesis, or by resolution of so-c alled racemic mixtures of opposite enantiomers, provided that racemization (the spontaneous interconversion of enantiomers) is sufficiently slow. Non- covalent assemblies can similarly adopt chiral supramolecular structures(1, 2), and if they are held together by relatively strong interactions, such a s metal coordination(3), methods analogous to those used to obtain chiral m olecules yield enantiomerically pure non-covalent products. But the resolut ion of assemblies formed through weak interactions, such as hydrogen-bondin g, remains challenging, reflecting their lower stability and significantly higher susceptibility to racemization. Here we report the design of supramo lecular structures from achiral calix[4]arene dimelamines and cyanurates, w hich form multiple cooperative hydrogen bonds that together provide suffici ent stability to allow the isolation of enantiomerically pure assemblies. O ur design strategy is based on a non-covalent 'chiral memory' concept(4,5), whereby we first use chiral barbiturates to induce the supramolecular chir ality in a hydrogen-bonded assembly(6), and then substitute them by achiral cyanurates. The stability of the resultant chiral assemblies in benzene, a non-polar solvent not competing for hydrogen bonds, is manifested by a hal f-life to racemization of more than four days at room temperature.