NatureΓs terpenoids, with over 35 000 known members, constitute compounds that are either essential for life (namely, cholesterol, vitamins) or represent secondary products, such as chemical attractants, defense compounds, and antibiotics. Until recently, terpenoids were assumed to be formed exclusively by the mevalonate pathway. [1] It has now been shown that an alternative metabolic route exists in plastids of higher plants and in the majority of bacteria. This pathway leads from pyruvate (1) and d-glyceraldehyde-3phosphate (2) via 1-deoxy-d-xylulose phosphate (3, DXP, Scheme 1) and the intermediates 4 Β± 7 to the key metabolites isopentenyl diphosphate (9, IPP) and dimethylallyl diphosphate (10, DMAPP), which are essential to all organisms. [2] The cyclic diphosphate 7 has been proven to be a precursor to 9 and 10 in the alternative pathway and thus to plastidic isoprenoids, mainly phytoene (11). [3] This reaction involves a threefold, possibly stepwise, dehydroxylation at carbon atoms C-2, C-3, and C-4 of 7.
On comparative phytochemical grounds, we postulated that (E)-4-hydroxy-3-methylbut-2-enyl diphosphate (8, Schemes 1 and 2) is a likely intermediate in the deoxyxylulose phosphate pathway between 7 and 9/10. [4] This hydroxylated hemiterpene is seen biogenetically in numerous plant-derived products, such as the plant hormone 13, the glucoside of (E)-2-(PPh 3 )) has been successfully used in RCM and other ruthenium(ii)-catalyzed reactions. In all cases, the reactions proceeded in high yields, and the catalyst was recovered quantitatively by simple filtration and reused. Further investigation to apply P,S-RuCl 2 (PPh 3 ) to other ruthenium(ii)catalyzed reactions is now in progress.