| Abstract
| - The mechanism of the taxadiene synthase-catalyzed cyclization of (E,E,E)-geranylgeranyldiphosphate (GGPP, 7) to taxadiene (5) is proposed to proceed through a verticillen-12-yl carbocationintermediate (8) that undergoes an 11 → 7 proton transfer leading to formation of the C ring. The substrateanalogue 6-fluoroGGPP (17) was synthesized to elucidate the stereochemistry of the putative verticillenylintermediate. It was expected that the inductive electron-withdrawing effect of the fluoro substituent wouldprevent the critical proton transfer to the Δ7 double bond and thereby derail the cyclization at the bicyclicstage. Incubation of the fluoro analogue with recombinant taxadiene synthase yielded a mixture of threemajor and two minor fluoro diterpenes according to GC/MS analyses. The three major products wereidentified as the exocyclic, endocyclic, and 4(20)-methylene 7-fluoroverticillenes, i.e., Δ3,7,12 (18), Δ3,7,12, andΔ4(20),7,11 isomers (22, 23, and 24) on the basis of 1H NMR analyses and comparisons with the parentbicyclic diterpenes. The H1β, H11α (1S,11R) configurations at the bridgehead positions of 22 wereestablished 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 particularnote, all absolute configurations of verticillane diterpenes in the literature should be reversed. This workaffords compelling evidence supporting the H11α (11R) stereochemistry of the verticillen-12-yl+ ionintermediate in the taxadiene synthase-catalyzed reaction and illustrates the capability of vinyl fluoroanalogues to intercept complex cyclization cascades.
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