Terpenoids

Terpenoids
Presented To: Presented By:
Mrs. Nishi Gupta Vipin Singh
Assistant Professor M.Pharm 1st Year
United Institute of Pharmacy Pharmaceutical Chemistry

Contents
 Introduction
 Classification
 Characteristics
 Structural feature of Terpenoids ( Isoprene Rule, Special isoprene Rule , Gem-
Dialkyl Rule)
 Isolation
 General methods of structural elucidation of Terpenoids
 Strutural elucidation of Citral.

Terpenoids
 The terpenoids, also known as isoprenoids, are a large and diverse class of
naturally occurring organic chemicals derived from the 5-carbon compound
isoprene, and the isoprene polymers called terpenes.
 Terpenoids are the largest class of plant secondary metabolites, representing
about 60% of known natural products.
 Terpenoids are modified terpenes, wherein methyl groups have been moved or
removed, or oxygen atoms added. Some authors use the term "terpene" more
broadly, to include the terpenoids.

Classification on the basis of isoprene units

S.
N
TERPENOIDS NO. OF ISOPRENE
UNIT
MOLECULAR FORMULA EXAMPLES
1 Hemiterpene or Isoprene 1 C5H8 Prenol, isoprenol
2 Monoterpenes or Terpenes 2 C10H16 Camphor, Menthol, Citral
3 Sesquiterpenes 3 C15H24 Farnesol
4 Diterpenes 4 C20H32 Retinol, Phytol, Taxol
5 Sesterpenes 5 C25H40 Andrastin A
6 Triterpenes 6 C30H48 Squalene, Ginsenoside
7 Tetraterpenes or Carotenoids 8 C40H64 𝛽 − 𝐶𝑎𝑟𝑜𝑡𝑒𝑛𝑒
8 Polyterpenes or rubber n (C5H8)n Gutta-percha, natural rubber

Classification
Cont…
Each class of Terpenoids is further classified into following sub-classes on the basis
of the number of rings present in the molecule.
I. Acyclic : those having an open chain structure.
II. Monocyclic : those having one ring in their structure.
III. bicyclic : those having two rings in their structure.
IV. Tricyclic : those having three rings in their structure.

Characteristics
 Most of terpenoids are colourless liquids , few are solid which are lighter than
water.
 They are insoluble in water but soluble in organic solvents .
 Many of these are optically active .
 They are unsaturated compounds having one or more double bonds .
 Terpenoids undergo additions with hydrogen , halogens ,halogen acids
 On thermal decomposition, most of the terpenoids yield isoprene .

Structural features ofTerpenoids [ Isoprene Rule ]
 The most important structural feature of nearly all the terpenoids is their
derivation from one monomeric structural unit, isoprene.
 The empirical formula C5H8 of nearly all terpenoid hydrocarbons and their
thermal decomposition to yield isoprene as one product.
 Otto Wallach pointed out that skeleton structure of all naturally occurring
terpenoids can be built up of isoprene unit , this is called isoprene rule.
 Isoprene rule starts that the terpenoid molecules are constructed from two or
more isoprene unit .

Isoprene Rule
Cont…
The isoprene rule is further confirmed by the following facts .
 Isoprene may be dimerised in laboratory to give a widely distributed terpenoid,
dipentene.
2 C5H8 ( isoprene ) C10H16 ( dipentene )
 Isoprene may be polymerised in laboratory to give rubber ( polyterpenoid).
nC5H8 (isoprene) ( C5H8)n ( rubber)
 Destructive distillation or pyrolysis of rubber and other terpenoids affords
isoprene as one of the main products .
Terpenoid heat C5H8 ( isoprene )

Special isoprene rule
 Ingold in 1925 formulated this observation under another rule “ Special isoprene
rule” .
 According to which the isoprene units in terpenoids are usually joined in head to
tail linkage or 1,4- linkage .
 Examples

Gem–dialkyl rule
 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.”


Isolation
 As a terpenoids are of wide occurrence, there is no general method for their
isolation .
 But mono and sesqui-terpenoids have common source that is essential oils.
Their isolation is carried out in two steps:
I. Isolation of essential oils from plant parts.
II. Separation of terpenoids from essential oils.

Isolation
Cont…
Isolation of essential oils
 Plant conataing essential oils usually have the highest concentration at some particular time.
 Therefore better yield of essential oil plant material have to be collected at this particular
time. e.g. From jasmine at sunset.
 There are four methods of extractions of oils.
 a) Expression method
 b) Steam distillation method
 c) Extraction by means of volatile solvents
 d) Adsorption in purified fats( enflurage)

Isolation
Cont…
a) Expression method
 The plant material is crushed and the juice is screened to remove the large
particles.
 The screened juice is centrifuged in a high speed centrifugal machine when
nearly half of essential oil is extracted.
 The other half of the oil is generally not extracted and such residue is used for
inferior quality of oil.
 Citrus ,lemon and grass oils are extracted by this method.

Isolation
Cont…
b) Steam distillation method
 In this method macerated plant material is steam distilled to get essential oils
into the distillate form these are extracted by using pure organic volatile solvents,
like light petroleum.
 Method should be used with a great care , since some essential oils are
decomposed during distillation.
 In such cases plant material is directly treated with light petrol at 50 degree and
solvent is then removed by distillation under reduced pressure.

Isolation
Cont…
d) Adsorption in purified fats( enflurage)
 The fat is taken in glass plate warm about to 50 degree then it’s surface is
covered with the petals and it is allow to be kept as for several days.
 The petals are then removed and fat is digested with ethyl alcohol when the
essential oils present in fat are dissolved in ethyl alcohol .
 The extract having ethyl alcohol and essential oils is distilled under reduced
pressure to remove the solvent .

Isolation
Cont…
Separation of terpenoids from essential oils.
 A number of terpenoids are present in essential oil obtained from the extraction.
 Definite physical and chemical methods can be used for the separation of
terpenoids.
 They are separated by fractional distillation.
 The terpenoid hydrocarbons distill over first followed by the oxygenated
derivatives.
 More recently different chromatographic techniques have been used both for
isolation and separation of terpenoids.

General methods of structural elucidation
1. Molecular formula: molecular formula is determined by usual quantitative
analysis and mol. wt determination methods and by means of mass
spectrometry. If terpenoid is optically active, its specific rotation can be
measured.
2. Nature of oxygen atom present: If oxygen is present in terpenoids its
functional nature is generally as alcohol aldehyde, ketone or carboxylic groups.
a) Presence of oxygen atom present: presence of –OH group can be determined
by the formation of acetates with acetic anhydride. Primary alcoholic group
undergo esterification more readily than secondary and tertiary alcohols.

cont…
b) Presence of >C=O group: Terpenoids containing carbonyl function form
crystalline addition products like oxime, phenyl hydrazone and bisulphite etc.
If carbonyl function is in the form of aldehyde it gives carboxylic acid on oxidation
whereas the ketone on oxidation yields a mixture of lesser number of carbonatoms

cont…
3. Unsaturation: The presence of olefinic double bond is confirmed by means of
bromine, and number of double bond determination by analysis of the bromide or
by quantitative hydrogenation.
Presence of double bond also confirmed by means of catalytic hydrogenation or
addition of halogen acids. Number of moles of HX absorbed by one molecule is
equal to number of double bonds present.

cont
4. Dehydrogenation: On dehydrogenation with sulphur, selenium, polonium or
palladium terpenoids converted to aromatic compounds. Examination of these
products the skelton structure and position of side chain in the original terpenoids
can be determined.
For example α-terpenol on Se-dehydrogenation yields p-cymene.

cont…
5. Number of the rings present: With the help of general formula of corresponding
parent saturated hydrocarbon, number of rings present in that molecule can be
determined.

cont…
6. Spectroscopic studies: All the spectroscopic methods are very helpful for the
confirmation of structure of natural terpenoids and also structure of degradation
products.
The various methods for elucidating the structure of terpenoids are;
a) UV Spectroscopy: In terpenes containing conjugated dienes or α,β-unsaturated
ketones, UV spectroscopy is very useful tool.
The values of λmax for various types of terpenoids have been calculated by
applying Woodward’s empirical rules.
b) IR Spectroscopy: IR spectroscopy is useful in detecting group such as hydroxyl
group (~3400cm-1) or an oxo group (saturated 1750-1700cm-1).

cont…
c) NMR Spectroscopy: This technique is useful to detect and identify double
bonds, to determine the nature of end group and also the number of rings present,
and also to reveal the orientation of methyl group in the relative position of double
bonds.
d) Mass Spectroscopy: It is now being widely used as a means of elucidating
structure of terpenoids.
Used for determining mol. Wt., Mol. Formula, nature of functional groups present
and relative positions of double bonds.
e)X-ray analysis: This is very helpful technique for elucidating structure and
stereochemistry of terpenoids.

Structural elucidation of Citral
 Citral is an acyclic monoterpenoid. It is a major constituent of lemon grass oil in
which it occurs to an extent of 60-80%.
 It is pale yellow liquid having strong lemon like odour and can be obtained by
fractional distillation under reduced pressure from Lemongrass oil
Constitution:
i) Mol. formula C10H16O, b.p-77oC
ii) Nature of Oxygen atom: Formation of oxime of citral indicates the presence
of an oxo group in citral molecule.

Cont…..
 On reduction with Na/Hg it gives an alcohol called geraniol and on oxidation with
silver oxide it give a monocarboxylic acid called Geranic acid.
 Citral reduces Fehling’s solution, further confirming the presence of aldehydic
group.
iii) It adds on two molecule of Br2, shows the presence of two double bonds.
iv) Citral on heating with potassium hydrogen sulphate, gives a well known
aromatic compound p-cymene and thus the carbon sekelton.

Cont….
v) The position of double bond and complete structure of citral is established by its
oxidation with alkaline permanganate followed by chronic acid to give acetone,
laevulic acid and oxalic acid .

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