Molecular Capsules Catalyze Terpene Cyclization

The many molecules within the terpene family include natural rubber, menthol flavoring, and several drug classes, such as steroids and artemisinins. Enzymes are known to elegantly assemble terpenoid compounds, characterized by their five-carbon subunits, by stabilizing carbocation cascade reactions. Getting the same transformations to take place in a reaction flask, however, has proven considerably more difficult, often requiring “brute force” methods and leading to a messy mix of products. Now, Konrad Tiefenbacher and Q. Zhang of Germany’s Technical University of Munich have come up with a molecular capsule that catalyzes the tail-to-head cyclization of terpenes (Nat. Chem. 2015, DOI: 10.1038/nchem.2181). The capsule is made of six resorcinarene units and eight water molecules. It acts as a Brønsted acid, stabilizing a carbocation long enough for a 1,2-rearrangement to take place, for example, in the cyclization of geranyl acetate to α-terpinene (shown). Reactions within the capsule shed light on how enzymes catalyze such cyclizations, Tiefenbacher and Zhang note.