Terpene cyclization catalysed inside a self-assembled cavity.

In nature, complex terpene natural products are formed by the so-called tail-to-head terpene (THT) cyclization. The cationic reaction cascade is promoted efficiently in complex enzyme pockets, in which cationic intermediates and transition states are stabilized. In solution, the reaction is hard to control and man-made catalysts able to perform selective THT cyclizations are lacking. We herein report the first example of a successful THT cyclization inside a supramolecular structure. The basic mode of operation in cyclase enzymes was mimicked successfully and a catalytic non-stop THT was achieved with geranyl acetate as the substrate. The results presented have implications for the postulated reaction mechanism in cyclase enzymes. Evidence indicates that the direct isomerization of a geranyl cation to the cisoid isomer, which so far was considered unlikely, is feasible.