## 2 The Phytochemistry of _Artemisia annua_ L. (Qinghao)
### Monoterpenoids
#### 2.5.4. Bicyclic Monoterpenes  
Further cyclization of the carbocation which produces the \(p\)-menthane skeleton results in bicyclic monoterpenes. In the camphane monoterpenes listed in Table 26, the second cyclization has produced an additional five-membered ring. The most abundant member of this class of monoterpenoids is camphor (**341**) [25,29,37,45], which can account for up to 10-30% of the essential oil [19,22,41,42]. Borneol (**334**) [46,73] and bornyl acetate (**335**) [29] are sometimes reported together with camphor (**351**) as significant constituents of the essential oil. **Table 26.** Camphane Monoterpenoids (1,7,7-Trimethylbicyclo-[2.2.1]heptane, 9 CI). \begin{tabular}{|l|l|l|l|} \hline
**Name** & **Alternative Name(s)** & **CAS Number** & **References** \\ \hline Bormel **(334)** & & [464-43-7] & [19, 25, 29, 30, 31, 34, 40, 41, \\  & & [507-70-00] & 43, 45, 72, 73, 111, 112] \\  & & [124-76-5] & \\ \hline Bornyl acetate **(335)** & & [76-49-3] & [25, 31, 40, 45] \\  & & [92618-89-8] & \\ \hline Bormel isobutyrate **(336)** & & [24717-86-0] & [45] \\ \hline Bornyl valerate **(337)** & Bornyl pentanoate & [7549-41-9] & [31] \\ \hline
2-Butenoic acid, 3-methyl- & & [91404-82-9] & [31] \\ (1,2,8,45)-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl ester & & & \\ (**338)** & & & \\ \hline Cyclopentancarboxylate acid, 3- & & [74793-59-2] & [31] \\ methylene-, 1,7,7-trimethylbicyclo- & & & \\ \hline Isobornyl acetate **(340)** & 2-Bornanol acetate & [125-12-2] & [19] \\ \hline Camphor **(341)** & 1,7,7-Trimethylbicyclo[2.2.1] & [76-22-2] & [19, 23, 25, 26, 29, 32, 34, \\  & heptan-2-one & [464-48-2] & 36-38, 40-45, 48, 49, 51, 53, \\  & & & 66, 72- 74, 112, 113] \\ \hline _endo_-Dehydronorbornel **(342)** & Bicyclo[2.2.1]hept-5-en-2-ol & [694-97-3] & [23] \\ \hline \end{tabular}   
Other miscellaneous bicyclic monoterpenoids which contain both 5- and 6-membered rings are: camphene (2,2-dimethyl-3-methylenebicyclo[2.2.1]heptane [79-92-5]) **(343)**[19,23,25,26,28,29,31,32, 34,36,37,40,41,43,45,51,73,74,130]; camphene hydrate ([465-31-6]) **(344)**[19]; fenchyl alcohol; 1,3,3-trimethylbicyclo[2.2.1]heptan-2-ol [1632-73-1]) **(345)**[66]; and _trans_-pinene hydrate (2-norbornanol, 2,7,7-trimethyl-, endo-; bicyclo[2.2.1]heptan-2-ol, 2,7,7-trimethyl-, endo-; [3247-40-3]) **(346)**[25]. Camphene **(343)** has been obtained as both its (+)- and (-)-enantiomeric forms [37]. \begin{table}
\begin{tabular}{|l|l|l|l|} \hline
**Name** & **Alternative Name(s)** & **CAS Number** & **References** \\ \hline Bormel **(334)** & & [464-43-7] & [19, 25, 29, 30, 31, 34, 40, 41, \\  & & [507-70-00] & 43, 45, 72, 73, 111, 112] \\  & & [124-76-5] & \\ \hline Bornyl acetate **(335)** & & [76-49-3] & [25, 31, 40, 45] \\  & & [92618-89-8] & \\ \hline Bornyl **(336)** & & [24717-86-0] & [45] \\ \hline Bornyl valerate **(337)** & Bornyl pentanoate & [7549-41-9] & [31] \\ \hline
2-Butenoic acid, 3-methyl- & & [91404-82-9] & [31] \\ (1,2,8,45)-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl ester & & & \\ (**338)** & & & \\ \hline Cyclopentancarboxylate acid, 3- & & [74793-59-2] & [31] \\ methylene-, 1,7,7-trimethylbicyclo- & & & \\ \hline Isobornyl acetate **(340)** & 2-Bornanol acetate & [125-12-2] & [19] \\ \hline Camphor **(341)** & 1,7,7-Trimethylbicyclo[2.2.1] & [76-22-2] & [19, 23, 25, 26, 29, 32, 34, \\  & heptan-2-one & [464-48-2] & 36-38, 40-45, 48, 49, 51, 53, \\  & & & 66, 72- 74, 112, 113] \\ \hline _endo_-Dehydronorbornel **(342)** & Bicyclo[2.2.1]hept-5-en-2-ol & [694-97-3] & [23] \\ \hline \end{tabular}
\end{table} 
Table 26. Camphane Monoterpenoids (1,7,7-Trimethylbicyclo-[2.2.1]heptane, 9 CI). In the pinane monoterpenes (Table 27), the second cyclization of the C\({}_{10}\)_p_-menthane precursor has produced an additional four-membered ring. \(\alpha\)-Pinene (**347**) [40,42,43] is the most abundant pinane monoterpene from _A. annua_ (up to 10%) [33], and sometimes co-occurrs with significant amounts of either \(\beta\)-pinene (**356**) [101] or pinocarvone (**360**) [42].