Terpenoids

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Chapter: Pharmacognosy and Phytochemistry : Drugs Containing Volatile Oils

There are many different classes of naturally occurring compounds. Terpenoids also form a group of naturally occurring compounds majority of which occur in plants, a few of them have also been obtained from other sources. Terpenoids are volatile substances which give plants and flowers their fragrance. They occur widely in the leaves and fruits of higher plants, conifers, citrus and eucalyptus.


TERPENOIDS

 

 

There are many different classes of naturally occurring compounds. Terpenoids also form a group of naturally occurring compounds majority of which occur in plants, a few of them have also been obtained from other sources. Terpenoids are volatile substances which give plants and flowers their fragrance. They occur widely in the leaves and fruits of higher plants, conifers, citrus and eucalyptus.

 

The term ‘terpene’ was given to the compounds isolated from terpentine, a volatile liquid isolated from pine trees. The simpler mono and sesquiterpenes is the chief con-stituent of the essential oils obtained from sap and tissues of certain plant and trees. The di and triterpenoids are not steam volatile. They are obtained from plant and tree gums and resins. Tertraterpenoids form a separate group of compounds called ‘Carotenoids’.

 

The term ‘terpene’ was originally employed to describe a mixture of isomeric hydrocarbons of the molecular formula C10H16 occurring in the essential oils obtained from sap and tissue of plants and trees. But there is a tendency to use more general term ‘terpenoids’, which includes hydrocarbons and their oxygenated derivatives. However, the term terpene is being used these days by some authors to represent terpenoids.

 

According to modern definition, ‘Terpenoids are the hydrocarbons of plant origin of the general formula (C5H8)n as well as their oxygenated, hydrogenated, and dehydrogenated derivatives.’

 

Isoprene Rule

 

Thermal decomposition of terpenoids gives isoprene as one of the product. Otto Wallach pointed out that terpenoids can be built up of isoprene unit. Isoprene rule states that the terpenoid molecules are constructed from two or more isoprene unit.

 

 

Special Isoprene Rule

 

It states that the terpenoid molecules are constructed of two or more isoprene units joined in a ‘head to tail’ fashion.

 


 

But this rule can only be used as guiding principle and not as a fixed rule. For example carotenoids are joined tail to tail at their central, and there are also some terpenoids whose carbon content is not a multiple of five.

 

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