## 2 The Phytochemistry of _Artemisia annua_ L. (Qinghao)  
_Artemisia annua_ Linn. (sweet wormwood; Chinese wormwood) is a member of the Asteraceae family of plants (formerly Compositae). It is a native of China, originally found in the steppes of Chahar and Suiyuan Provinces, but is now widespread in many parts of the World, and is cultivated in countries such as Vietnam, Thailand, Burma, Madagascar, Malaysia, USA, Brazil, Australia (Tasmania), Holland, Switzerland, France and Finland. Most phytochemical investigations of _A. annua_ have employed the aerial parts (leaves and/or stems - sometimes also including the flowers) although one report has concentrated on the seeds [9 ]. The phytochemistry of _A. annua_ is dominated by terpenoids (in particular sesquiterpene lactones), flavonoids, coumarins and other shikimate metabolites, as is the case for many other members of the genus _Artemisia_. The sesquiterpene, artemisinin, is however unique to _A. annua_ - it has been searched for on several occasions in other species in the genus _Artemisia_, always without success [10 ; 11 ]. The artemisinin content from _A. annua_ is highly variable, ranging anywhere between 0.01% and 1%, depending on variety, and can even be as high as 1.4% in some cultivated strains. There have been several reviews of the phytochemistry of _A. annua_[12 - 15 ]. In this section, an attempt has been made to provide a comprehensive review of the original phytochemical literature concerning _A. annua_ up to 2009. Almost six hundred secondary metabolites are described, which have been divided into eight sections (Sections 2.1-2.8) and further sub-divided into more than forty Tables. The grouping of these natural products from _A. annua_ is based on the (largely biogenetic) classification adopted by a standard reference work, the Dictionary of Natural Products [16 ]. Thus, the structurally most simple group of metabolites - derivatives of aliphatic hydrocarbons (branched, unbranched, saturated or unsaturated) with varying levels of oxidation at C-1 (alcohol, aldehyde, ketone, acid or ester) - are described first in Section 2.1. The next major group to be covered in Section 2.2 comprises derivatives of simple aromatic hydrocarbons. The phenylpropanoids, in which a C\({}_{3}\) substituent is attached to the aromatic unit (C\({}_{6}\)), form a biosynthetically distinct group of aromatic metabolites, which is described in Section 2.3. The flavonoids, which are biosynthetically derived from this same C\({}_{6}\)C\({}_{3}\) precursor, constitute the second largest group of metabolites from _A. annua_, and are described separately in Section 2.4. By far the largest group of metabolites from _A. annua_ is the terpenoids, which are biosynthetically derived from a branched isoprenoid unit (C\({}_{5}\)). They have been further subdivided into monoterpencoids (C\({}_{10}\); _i.e._ 2 \(\times\) C\({}_{5}\)); sesquiterpenoids (C\({}_{15}\) ; _i.e._ 3 \(\times\) C\({}_{5}\)); and higher terpenoids (which include both diterpenes (C\({}_{20}\)), triterpenes (C\({}_{30}\)) and sterols (C\({}_{29}\)). These large groupings are covered in Sections 2.5, 2.6 and 2.7, respectively. Finally, a very small number of miscellaneous nitrogen-containing natural products appear in Section 2.8. The essential oil of _A. annua_ was first studied as long ago as 1917 [17 ; 18 ]. Researchers have continued to analyse the volatile constituents of this essential oil over the past century, primarily utilising gas chromatography (GC) and the hyphenated analytical technique of gas chromatography-mass spectrometry (GC-MS) [19 - 35 ]. GC/GC-MS studies of the essential oil of _A. annua_ have been reported from countries as diverse as France [36 ], Finland [37 ], Hungary [38 ], Romania [39 ], Kazakhstan [40 ], Iran [41 ; 42 ], India [43 - 45 ], China [46 - 50 ] and Vietnam [51 ]. A detailed study of the essential oils from Chinese and Vietnamese varieties of _A.