Chapter 22 Terpenes

Terpenes are largely found as constituents of essential oils. They are mostly hydro-carbons. The building block is a five-carbon isoprene (CH₂=C(CH₃)—CH=CH₂) unit (Figure 22.1). Terpene hydrocarbons have a molecular formula of (C₅H₈)n; the n dictates the number of units involved. Terpene hydrocarbons are classified according to the number of isoprene units:

Figure 22.1 The chemical structure of the isoprene building block of terpenes and its three-dimensional representation.

Terpenes are found in:

Monoterpenes

• Two isoprene units.

General properties of monoterpenes (Figure 22.2):

• Volatile oils: can therefore be administered as inhalations. Have very characteristic odours and are used in perfumes.

• Easily penetrate membranes as non-polar (and therefore lipophilic) compounds. This quality is utilized in aromatherapy.

• Used as flavourings.

• Generally, monoterpenes are antimicrobial (Gram positive and Gram negative; Trombetta et al 2005) and antifungal.

• Antispasmodics.

• Some are glycosides (see Chapter 24 ‘Glycosides’, p. 181) e.g. nerol, citronellol, geraniol.

Figure 22.2 The formation of terpenes from isoprene units. This is an oversimplification of the process, but gives a general idea of the components of each type of terpene.

Menthols

Menthol (Figure 22.3) is a local anaesthetic and reduces irritation.

Figure 22.3 Menthol and thujone.

• Examples of Actions

• Menthol: found in Mentha spp. (mint, Bo He) is a carminative.

• Thujone: is the toxic agent found in Artemisiae annuae (wormwood, Qing Hao).

• The Artemisias are an old remedy in Western herbal medicine for worming.

Iridoids

General properties:

• Derivatives of monoterpenes.

• Most occur as glycosides.

• Have a bitter taste known as ‘bitter principles’.

• Cardiovascular, stomachic, choleretic, antihepatotoxic activities and insect attractant and repellant properties.

• Examples of Actions

• Aucubin (Figure 22.4) is found in Eucommiae ulmoides (Du Zhong); harpagoside is found in Harpagophytum procumbens (devil’s claw). Both have anti-inflammatory properties.

• Valtrate is thought to be one of the compounds responsible for the sedative properties of Valeriana officinalis (valerian). It belongs to the class of iridoids associated with valerian and referred to as valepotriates.

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