All of the thousands of sesquiterpenes derive from one common precursor, farnesyl diphosphate. A plethora of enzyme activities catalyze loss of the diphosphate group, after which the farnesyl cation or its C2-C3 cis-trans isomer, the nerolidyl cation, undergoes a variety of chemical transformations. The majority of these transformations involve one of three cyclizations: C1-C6 closure produces the bisabolyl cation; C1-C10 closure produces the germacryl cation; and C1-C11 closure produces the humulyl cation. In each case, the enzyme activity responsible for each preliminary cyclization (so characterized because, depending on the enzyme, much more extensive transformation may follow) must organize the linear farnesyl/nerolidyl precursor to advantage one cyclization over the other possibilities. One focus of the research in our laboratories is the elucidation of detailed mechanisms by which a broad array of sesquiterpenes is synthesized in Nature from the common precursor, farnesyldiphosphate, using computational and chemical methods. The study presented here provides the preliminary details of the energy surfaces for the preorganized farnesyl and nerolidyl cations that result in each of the three observed cyclizations. We investigate the structures of the various isomers of these species using quantum chemical calculations in the gas phase, and compare our results to the structures of a subset of known sesquiterpene natural products. In addition, we present evidence for a variety of models by which the carbenium ion center in each of these precursors may be stabilized.

Price of admission: Exploring the transition from straight chain to first cycle in sesquiterpene biosynthesis using quantum chemical calculations

249th National Meeting (2015)

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