Chemistry of Natural Products

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CHEMISTRY OF NATURAL PRODUCTS
Terpenoids

Sameena Bano Department of Chemistry Faculty of Science Jamia Hamdard New Delhi-110062

(24.09.2007) CONTENTS Introduction Classification of Terpenoids Isolation of mono and sesquiterpenoids General properties of Terpenoids General methods of structure elucidation Terpenoids Citral Menthol Camphor Eugenol

Keywords Terpenes, isoprene, citral, menthol, camphor and eugenol

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Introduction 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 sesqui terpenes are chief constituent of the essential oils obtained from sap and tissues of certain plant and trees. The di and tri terpenoids 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 include hydrocarbons and their oxygenated derivatives. However the term terpene is being used these days by some authors to represent terpenoids. By the 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 give isoprene as one of the product. Otto Wallach pointed out that terpenoids can be built up of isoprene unit. Isoprene rule stats that the terpenoid molecules are constructed from two or more isoprene unit.

isoprene unit

Further Ingold suggested that isoprene units are joined in the terpenoid via ‘head to tail’ fashion. Special isoprene rule states that the terpenoid molecule are constructed of two or more isoprene units joined in a ‘head to tail’ fashion. head

H3C CH3

H3C

tail

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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In applying isoprene rule we look only for the skeletal unit of carbon. The carbon skeletons of open chain monotrpenoids and sesqui terpenoids are, CH3 CH3

head tail ~~~~

head tail ~~~~ tail CH
3

tail
~~ ~~

head
CH3

H3C

H3C

CH3

H3C

head

CH3

head

Examples.
CH2 CH3

HOH2C CH3 H3C CH3

H3C

CH3

Myrcene (monoterpene)

Farnesol (Sesquiterpene)

Ingold (1921) pointed that a gem alkyl group affects the stability of terpenoids. He summarized these results in the form of a rule called ‘gem dialkyl rule’ which may be stated as "Gem dialkyl group tends to render the cyclohexane ring unstable where as it stabilizes the three, four and five member rings.” This rule limits the number of possible structure in closing the open chain to ring structure.Thus the monoterpenoid open chain give rise to only one possibility for a monocyclic monoterpenoid i.e the p-cymene structure. CH3 CH3

~~~~
CH3

~~~~
~ ~~~

H3C

CH3

H3C

CH3

P-cymene structure

Bicyclic monoterpenodis contain a six member and a three member ring. Thus closure of the ten carbon open chain monoterpenoid gives three possible bicyclic structures. CH3 O CH3 CH3

Camphor (6+5) system

Pinane (6+4) system

Carane (6+3) System

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Classification of Terpenoids Most natural terpenoid...
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