Chiral crystallization: freezing a rapid Cope rearrangement in the solid state.

Homotropylidene and its derivatives undergo a fast Cope rearrangement. A homotropylidene system (cyclooctatetraene dimer) is reported that racemizes rapidly by such a valence isomerization in solution at room temperature. In the solid state, however, this rearrangement is frozen, and the single crystals precipitating from the racemic solution contain only one enantiomer. Since very large crystals can be grown, the racemic solution in principle can be converted into a single enantiomer by crystallization.