*2.1.6 Ethers*


When the molecule is optically active, the two enantiomers are very often present in different plants. For example, (+)α-pinene from *Pinus palustris*, (−) β-pinene from *Pinus caribaea* and from *Pinus pinaster*. Another example is linalool from coriander is (+); however, linalool from lavender oil is (−). In some cases, it is the racemic form which is the most frequently encountered. (±)citronellol is widespread, the form (+) is characteristic of *Eucalyptus citriodor*, the form (−) is common to the rose and geranium essential oils.

The sesquiterpenes are formed from the assembly of three isoprene units (C15). The extension of the chain increases the number of cyclisations which allows a great variety of structures. The structure and function of the sesquiterpenes are similar to those of the monoterpenes. Examples of plants containing these compounds are angelica, bergamot, caraway, celery, citronella, coriander, eucalyptus, geranium, juniper, lavandin, lavander, lemon, lemongrass, mandarin, mint, orange, peppermint, petitgrain, pine, rosemary, sage, thyme.

## **2.2 Aromatic compounds**

Derived from phenylpropane, the aromatic compounds occur less frequently than the terpenes. The biosynthetic pathways concerning terpenes and phenylpropanic derivatives generally are separated in plants but may coexist in some, with one major pathway taking over (e.g., cinnamom oil with cinnamaldehyde as major and eugenol as minor constituents, also clove oil, fennel, etc.).

Aromatic compounds comprise:

1.**Aldehyde**: e.g., cinnamaldehyde

2.**Alcohol**: e.g., cinnamic alcohol

3.**Phenols**: e.g., chavicol and eugenol

4.**Methoxyderivatives**: e.g., anethole, elemicine, estragole and methyleugenols

5.**Methylene dioxy compounds**: e.g., apiole, myristicine and safrole

The principal plant sources for these compounds are anise, cinnamon, clove, fennel, nutmeg, parsley, sassafras, star anise, tarragon, and some botanical families (Apiaceae, Lamiaceae, Myrtaceae, Rutaceae).

Nitrogenous or sulfured components such as glucosinolates or isothiocyanate derivatives (garlic and mustard oils) are also characteristic as secondary metabolites of diverse plants or of torrefied, grilled or roasted products.
