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Abstract
Non-stop carbocationic polycyclizations of isoprenoids have been called the most complex chemical reactions occurring in nature. We describe a strategy for the initiation of tail-to-head polycyclization that relies on the sequestration of the counteranion away from the carbocation, which allows full propagation of the cationic charge. If the anion is mobile, Coulombic forces hold this species in close proximity to the carbocation and cause preemptive termination through elimination. Anion sequestration is crucial for effecting the biomimetic synthesis of complex and unstable terpenes, including the highly strained funebrenes. This study illustrates the deleterious role of the counterion in tail-to-head carbocationic polycyclization reactions, which to the best of our knowledge has not been rigorously explored. These observations are also expected to find use in the design and control of cationic polycyclization along biosynthetic pathways that have previously been inaccessible in bulk solvent. Sesquiterpenes are biosynthesized from linear isoprenols through the intermediacy of multiple, high-energy carbocations. Here a strategy is demonstrated for mimicking these reactions in bulk solvent to yield strained, acid-labile terpenes. Key to the success of these reactions is the sequestration of the counteranion away from the reactive carbocation, a strategy that should enable further study of challenging polycyclizations.Cite this article
Pronin, S., Shenvi, R. Synthesis of highly strained terpenes by non-stop tail-to-head polycyclization. Nature Chem 4, 915–920 (2012). https://doi.org/10.1038/nchem.1458 