AN ASYMMETRIC CARCEPLEX AND NEW CRYSTAL-STRUCTURE YIELD INFORMATION REGARDING A 1-MILLION-FOLD TEMPLATE EFFECT

We report the incorporation of methyl ''feet'' as pendant groups in cavitands and carceplexes and their use in the creation of an asymmetric carceplex and the facilitation of the determination of the crystal structure of a carceplex. From the asymmetric carceplex, we have determined a 19 kcal/mol energy barrier to rotation of the guest pyrazine about the host's pseudo-C-2 axes, which demonstrates a high degree of complementarity between the shell of the carceplex and the guest pyrazine. The crystal structure reveals the extensive conjugation of the aryl ethers into the aromatic rings of the host when pyrazine is the guest. This result helps explain why pyrazine is, to date, the best template for the reaction to form the carceplex and thus provides insight to the powerful template effect that is the hallmark of this carceplex reaction. Thus, the investigation of this template effect provides a sensitive probe into noncovalent interactions in general and may be useful in the understanding of recognition in both natural systems such as enzyme/substrate complexes and in non-natural systems such as designed self-assembling structures.