CYCLIZATION OF GERANYLGERANYL DIPHOSPHATE TO TAXA-4(5),11(12)-DIENE IS THE COMMITTED STEP OF TAXOL BIOSYNTHESIS IN PACIFIC YEW

The biosynthesis of taxol (paclitaxel) and related tax aids in Pacific yew (Taxus brevifolia) is thought to involve the cyclization of geran ylgeranyl diphosphate to a taxadiene followed by extensive oxygenation of this diterpene olefin intermediate, A cell-free preparation from s apling yew stems catalyzed the conversion of [1-H-3]geranylgeranyl dip hosphate to a cyclic diterpene oIefin that, when incubated with stem s ections, was converted in good radiochemical yield to several highly f unctionalized taxanes, including 10-deacetyl baccatin III and taxol it self. Addition of the labeled olefin to a yew bark extract, followed b y radiochemically guided fractionation, provided sufficient product to establish the structure as taxa-4(5),11(12)-diene by two-dimensional NMR spectroscopic methods, Therefore, the first dedicated step in taxo l biosynthesis is the conversion of the universal diterpenoid precurso r geranylgeranyl diphosphate to taxa-4(5),11(12)-diene, rather than to the 4(20),11(12)-diene isomer previously suggested an the basis of th e abundance of taxoids with double bonds in these positions. The very common occurrence of taxane derivatives bearing the 4(20)-ene-5-oxy fu nctional grouping, and the lack of oxygenated derivatives bearing a 4( 5)-double bond, suggest that hydroxylation at C-5 of taxadiene with al lylic rearrangement of the double bond is an early step in the convers ion of this olefin intermediate to taxol.