Molecules2010157603-7698; doi:10.3390/molecules15117603  
_molecules_  
**ISSN 1420-3049**  
www.mdpi.com/journal/molecules  
_Review_  
**The Biosynthesis of Artemisinin (Qinghaosu) and the**  
**Phytochemistry of Artemisia annua L. (Qinghao)**  
**Geoffrey D. Brown**  
Department of Chemistry, The University of Reading, Whiteknights, Reading, RG6 6 AD, UK;  
E-Mail: g.d.brown@reading.ac.uk  
_Received: 13 September 2010 / Accepted: 17 October 2010 / Published: 28 October 2010_  
**Abstract:** The Chinese medicinal plant Artemisia annua L. (Qinghao) is the only known source of the sesquiterpene artemisinin (Qinghaosu), which is used in the treatment of malaria. Artemisinin is a highly oxygenated sesquiterpene, containing a unique 1,2,4-trioxane ring structure, which is responsible for the antimalarial activity of this natural product. The phytochemistry of _A. annua_ is dominated by both sesquiterpenoids and flavonoids, as is the case for many other plants in the Asteraceae family. However, _A. annua_ is distinguished from the other members of the family both by the very large number of natural products which have been characterised to date (almost six hundred in total, including around fifty amorphane and cadinane sesquiterpenes), and by the highly oxygenated nature of many of the terpenoidal secondary metabolites. In addition, this species also contains an unusually large number of terpene allylic hydroperoxides and endoperoxides. This observation forms the basis of a proposal that the biogenesis of many of the highly oxygenated terpene metabolites from _A. annua_ - including artemisinin itself - may proceed by spontaneous oxidation reactions of terpene precursors, which involve these highly reactive allylic hydroperoxides as intermediates. Although several studies of the biosynthesis of artemisinin have been reported in the literature from the 1980s and early 1990s, the collective results from these studies were rather confusing because they implied that an unfeasibly large number of different sesquiterpenes could all function as direct precursors to artemisinin (and some of the experiments also appeared to contradict one another). As a result, the complete biosynthetic pathway to artemisinin could not be stated conclusively at the time. Fortunately, studies which have been published in the last decade are now providing a clearer picture of the biosynthetic pathways in _A. annua_. By synthesising some of the sesquiterpene natural products which have been proposed as biogenetic precursors to artemisinin in such a way that they incorporate a stable isotopic label, and then feeding these precursors to intact _A. annua_ plants, it has now been possible to demonstrate that dihydroartemisinic acid is a late-stage precursor to artemisinin and that the closely related secondary metabolite, artemisinin acid, is not (this approach differs from all the previous studies, which used radio-isotopically labelled precursors that were fed to a plant homogenate or a cell-free preparation). Quite remarkably, feeding experiments with labeled dihydroartemisinic acid and artemisinin acid have resulted in incorporation of label into roughly half of all the amorphane and cadinane sesquiterpenes which were already known from phytochemicalal studies of _A. annua_. These findings strongly support the hypothesis that many of the highly oxygenated sesquiterpenoids from this species arise by oxidation reactions involving allylic hydroperoxides, which seem to be such a defining feature of the chemistry of _A. annua_. In the particular case of artemisinin, these _in vivo_ results are also supported by _in vitro_ studies, demonstrating explicitly that the biosynthesis of artemisinin proceeds via the tertiary allylic hydroperoxide, which is derived from oxidation of dihydroartemisinic acid.