Introducing Small Rings into Farnesyl Pyrophosphates Paves the Way for the Enzymatic Generation of Unnatural Sesquiterpene Scaffolds.

New sesquiterpene skeletons are accessible when the geminal dimethyl group in farnesyl pyrophosphate (FPP) is exchanged by small strained rings, specifically cyclopropane, cyclobutane, and oxetane. When these new FPP derivatives are exposed to sesquiterpene synthases, the additional chemical reactivity installed in the strained rings can interact in a unique way with the carbocation intermediates in the active centers of sesquiterpene synthases BcBOT2, PenA, Omp7, and Cop4, which are known to be substrate promiscuous. As such, they can induce rearrangements and ring enlargements, which can yield completely new, previously unknown sesquiterpene carbon skeletons with additional carbon atoms embedded in the (oligo)cyclic backbones. A total of 17 new terpenoids are reported and structurally elucidated, 11 of which have so far unknown unnatural terpene backbones. Besides rearrangements of the small rings, we report on the nucleophilic involvement of the oxygen atom in the oxetane ring during the initial cyclization step. As an additional finding, the oxetane analogues of the two known sesquiterpenes africanene and pentalenene were isolated. Molecular modeling studies revealed that the FPP derivatives are optimally oriented for catalysis within the enzymes' active sites. The simulations unveiled alternative binding poses that facilitate divergent cyclization cascades, ultimately leading to the formation of previously uncharacterized molecular frameworks of sesquiterpenes.