Studies on taxadiene synthase: interception of the cyclization cascade at the verticillene stage and rearrangement to phomactatriene

The cyclization of geranylgeranyl diphosphate (GGDP) to taxadiene catalyzed by taxadiene synthase has been suggested to proceed in stages, involving a transient bicyclic verticillyl carbocation intermediate, which also has been proposed in the biosynthetic pathway leading to phomactatriene by marine fungi of Phoma sp. On incubation with des-7-methylGGDP, which would be expected to decrease the stability of a carbocation produced by hydride migration, taxadiene synthase produced phomactatrienes as major products. This indicates that the verticillyl carbocation was indeed formed but underwent further skeletal rearrangements, diverging from the usual pathway taken by GGDP en route to taxadiene. Products were identified using GC–MS, one- and two-dimensional NMR, and X-ray crystallography.