A PRESENTATION ON TERPENOIDS

Presented by

Ayesha Choudhury

m.Pharm 2nd sem

Deptt. Of pharm. sciences

INTRODUCTION

TERPENOIDS:

Terpenoids are defined as the hydrocarbons of plant origin having the general formula -(C5H8)n where n is the

number of linked isoprene (2-methyl-1,3-butadiene) units (isoprenoids). Terpenoids can be thought of as modified

terpenes. However the term terpene is misnomer and restricted to the hydrocarbons C10H16 as the suffix “ene”

signifies unsaturated hydrocarbons and so the term terpene is in appropriate to compound having aldehyde, ketone &

alcohol functionality i.e., terpenoids.

OCCURANCE:

Terpenoids form a group of compounds, the majority of which occurs in plant kingdom.

• Mono and sesquiterpenoids are the essential oils obtained from sap and tissues of plants and trees.

• Di and Tri Terpenoids are obtained from plant and tree gums and resins.

• Tetra terpenoids form a group of compounds known as carotenoids.

• Polyterpenoid : Rubber .

CLASSIFICATION OF TERPENOIDS

Most natural terpenoids hydrocarbon have the general formula (C5H8)n. They can be classified on the basis of value of

‘n’ i.e., isoprene unit or number of carbon atoms present in the structure.

Each class can be further subdivided into subclasses according to the number of rings present in the structure:

i) Acyclic Terpenoids: They contain open structure e.g., citral

ii) Monocyclic Terpenoids: They contain one ring in the structure e.g., menthol

iii) Bicyclic Terpenoids: They contain two rings in the structure. E.g., camphor.

iv) Tricyclic Terpenoids: They contain three rings in the structure. E.g., abietic acid

v) Tetracyclic Terpenoids: contain four rings in the structure. E.g., gibberellic acid

ISOPRENE RULE

In 1887, Wallach pointed out the isoprene rule from the following facts:

• The empirical formula of almost all naturally occurring terpenoids is -(C5H8)n

• Thermal decomposition of almost all terpenoids gives isoprene as one of the products.

• Confirmed by:

2C5H8 heat , 280⁰c C10H16

nC5H8 polymerization -(C5H8)n

Special isoprene rule:

• Proposed by Ingold in 1925

• Isoprene units in terpenoids are linked in “ head to tail” fashion.

• The branched end (head) is isopropyl and other end being the (tail)

ISOLATION OF TERPENOIDS

The above four methods are used for isolating the essential oil containing mixture of terpenoids.

The next step is the separation of terpenoids from essential oil: it involves two methods

Chemical methods: involves treating the essential oil with some chemical agents like nitrosyl

chloride, phthalic anhydride etc.

Physical methods: involves fractional distillation method and gas chromatography(modern

method)

STRUCTURE ELUCIDATION OF SOME TERPENOIDS

General methods:

Molecular formula

Nature of oxygen atom present: generally terpenoids have alcoholic, carbonyl or carboxylic acid as oxygen function.

E.g.,

Unsaturation:

Dehydrogenation:

Oxidative degradation

Spectroscopic studies.

STRUCTURAL ELUCIDATION OF MENTHOL

Chemical formula: C10H20O, m.p. 34 °C, b.p. 213 °C

Menthol is an optically active compound & only the levo (-)form occurs naturally in

peppermint oil obtained from Mentha piperita.

It is a saturated compound under monocyclic monoterpenoids. MENTHOL

Molecular formula of menthol was determined as C10H20O. On treatment with phosphorus pentachloride and

phosphorus pentoxide menthol gave a chloride C10H19Cl and a hydrocarbon C10H18 respectively, inferring

that it is an alcohol.

Menthol was oxidized by chromic acid to a ketone, Menthone to

prove that menthol contained a secondary hydroxyl group.

Menthol was converted to p-cymene identified as

1-methyl-4-isopropylbenzene, which was also obtained by the

dehydrogenation of pulegone whose structure was established as

p-menth-4-(8)-en-3-one by chemical methods and confirmed by its

synthesis Therefore menthol must possess the same skeleton as

pulegone.

Pulegone on reduction yielded menthone, which on further reduction

gave Menthol. Correlation of pulegone with menthol proved the

structure of menthol

Stereochemistry:

Menthol has three asymmetric centers namely

C1, C3 and C4. In view of the three chiral centers,

eight optically active forms are possible and all

the eight enantiomers are known.

Four isomers are shown in Figure.

Finally the structure of menthol was proved

by its synthesis:

STRUCTURAL ELUCIDATION OF CAMPHOR

Camphor is solid ketone obtained from volatile oil of Cinnamomum camphora under the family Lauraceae,

mp: 180⁰c, bp: 204 ⁰c. optically active.

The molecular formula is C10H16O (bicyclic monoterpenoid)

Nature of oxygen atom: it is ketone as it forms oxime with NH2OH & phenyl hydrazine with

phenylhydrazine

Presence of bicyclic ring system: the parent hydrocarbon of camphor has molecular formula C10H18, which

corresponds to the molecular formula of bicyclic compounds(CnH2n-2)

Presence of six membered ring: when distilled with ZnCl2/P2O5 it forms p-cymene.

Presence of –COCH2- group in camphor is confirmed by its formation to isonitroso derivative with amyl nitrile &

HCl.

Camphor on oxidation with HNO3 gives camphoric acid & camphoronic acid .

Camphoronic acid is saturated tricarboxylic acid & on distillation at atm pressure gives:

Bredt suggested that camphoronic acid is 2,3-dimethyl-1,2,3-butanetricarboxylic acid, hence having 3 –CH3 group so

camphor must have 3-methyl group.

On the basis of the formula of camphoric acid “C10H16O4” it was found that camphoric acid is a

cyclo pentane derivative and the structure was proposed as:

Bredt proposed the following reaction to show the relationship between camphor, camphoric acid

& camphoronic acid and established the structure of camphor.

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