Taxadiene Synthase-Catalyzed Cyclization of 6-Fluorogeranylgeranyl Diphosphate to 7-Fluoroverticillenes

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Abstract
The mechanism of the taxadiene synthase-catalyzed cyclization of (E,E,E)-geranylgeranyl diphosphate (GGPP, 7) to taxadiene (5) is proposed to proceed through a verticillen-12-yl carbocation intermediate (8) that undergoes an 11 → 7 proton transfer leading to formation of the C ring. The substrate analogue 6-fluoroGGPP (17) was synthesized to elucidate the stereochemistry of the putative verticillenyl intermediate. It was expected that the inductive electron-withdrawing effect of the fluoro substituent would prevent the critical proton transfer to the Δ7 double bond and thereby derail the cyclization at the bicyclic stage. Incubation of the fluoro analogue with recombinant taxadiene synthase yielded a mixture of three major and two minor fluoro diterpenes according to GC/MS analyses. The three major products were identified as the exocyclic, endocyclic, and 4(20)-methylene 7-fluoroverticillenes, i.e., Δ3,7,12 (18), Δ3,7,12, and Δ4(20),7,11 isomers (22, 23, and 24) on the basis of 1H NMR analyses and comparisons with the parent bicyclic diterpenes. The H1β, H11α (1S,11R) configurations at the bridgehead positions of 22 were established by means of NOE experiments and CD spectra. The absolute configuration of (+)-verticillol (4) was revised after the anomalous dispersion X-ray analysis of (+)-verticillol p-iodobenzoate. Of particular note, all absolute configurations of verticillane diterpenes in the literature should be reversed. This work affords compelling evidence supporting the H11α (11R) stereochemistry of the verticillen-12-yl+ ion intermediate in the taxadiene synthase-catalyzed reaction and illustrates the capability of vinyl fluoro analogues to intercept complex cyclization cascades.