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.