What are terpenoids?

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• Those organic compounds which are derived from a combination of two or more isoprene units are called terpenoids.
• Terpenoids form naturally occurring organic compounds, most of which occur in plants and some are obtained from other sources.
• These are volatile compounds that give odor and fragrances to plants and flowers.
• They act as flavoring agents.
• They are secondary metabolites and natural products.
• These compounds are also called isoprenoids.

What is the difference between terpenes and terpenoids?

• Terpenes are simple hydrocarbons while terpenoids are modified and complex forms of terpenes that contain functional groups and some rearrangements.
• Terpenes are used in the food industry, cosmetic industry, pharmaceutical industry, and biotechnology while terpenoids are used in traditional herbal medicines.

What is isoprene?

• Isoprene is a compound which contains five carbons.
• Its chemical formula is 2-methyl -1,3-butadiene.

• 

  According to the isoprene rule proposed by Leopold Ruzicka, terpenoids are derived from head-to-tail joining of isoprene units.
• Carbon 1 is known as the tail and carbon 4 is the head.
• For example
• Myrcene is a simple 10 carbons containing terpenoid which is organized according to the head-to-tail arrangement of two isoprene units as shown.

Classification of Terpenoids :

• The classification of terpenoids based on the number of isoprene units involved in the formation of such organic compounds is :

• Each above class of terpenoids is further sub-classified based on the number of rings present in their structures:

• 

General methods of structure elucidation of Terpenoids

1. Molecular formula:

The molecular formula is obtained using molecular weight analysis and mass spectrometry. When a terpenoid is optically active, its specific rotation is measured.

2. Presence of oxygen:

If oxygen is present in terpenoids then it may contain functional groups such as alcohol, aldehyde, ketone, or carboxylic groups.

• Confirmation of the presence of –OH group:

The presence of the –OH group is confirmed by:

1. Formation of acetate by acetic anhydride

1. Formation of benzoate by 3,5 dinitro benzoyl chloride

• Confirmation of the presence of carbonyl group (>C=O) :

The terpenoids having a carbonyl functional group give crystalline addition products such as oximes, phenyl hydrazones, bisulfites, etc.

• Confirmation of the presence of carboxylic acid group (–COOH) :

The presence of a carboxylic acid group in terpenoids is detected by NaHCO3.The carboxylic acid gives effervescence with NaHCO3.

3. Unsaturation:

1. The presence of olefinic double bonds and unsaturation in terpenoids are detected by bromination.

2. The number of double bonds is determined by catalytic hydrogenation.

3. The addition of halogen acids also determines the presence of double bonds.

4. The addition of nitrosyl chloride (Tilden’s reagent) gives an idea up to some extent the presence of double bonds in terpenoids.

5. Formation of epoxide with peracid also gives an idea about the double bonds in terpenoid molecules.

4. Dehydrogenation:

When terpenoids are dehydrogenated with Sulphur or Selenium then terpenoids are converted into aromatic compounds.

The skeleton structure and position of the side chain in the original terpenoid molecule may be detected by the examination of these products.

For example,

Para-cymene is formed on selenium dehydrogenation of α-terpinol.

5. Oxidative degradation:

Oxidative degradation is used to detect the position of double bonds and the presence of functional groups in the terpenoid molecule.

6. Relation between the general formula of the compound and the type of compounds:

For example, Limonene (molecular formula: C₁₀H₁₆) absorbs 2 moles of hydrogen to produce tetrahydro limonene (molecular formula: C10H20) according to the general formula ( CnH2n ). It indicates that limonene has a monocyclic structure.

7. Spectroscopic studies:

• UV spectroscopy is useful for the detection of the degree of unsaturation.
• IR spectroscopy is useful for the detection of functional groups.
• NMR and CMR are useful for the detection of several sets of protons and carbon atoms and finally molecular structure of a terpenoid molecule.
• Mass spectra are useful for the determination of molecular weight.
  

  8. Synthesis:

  The structure of a terpenoid molecule is also confirmed by the synthesis of a known compound.

FAQ

What are the most important terpenoids?

Some most important terpenoids are:

• Monoterpenes (C-10) such as carvone, geraniol, d-limonene, and perillyl alcohol
• Sesquiterpenes (C-15) such as Farnesol and Artemisinin
• which are the main source of the essential oils.
• While diterpenes (C-20) such as retinol, trans-retinoic acid and Forskolin
• Sesterterpenes (C-25) such as geranylfarnesol, Gascardic acid
• Triperpenes (C-30) such as betulinic acid, alpha-amyrin, oleanolic acid , ursolic acid, and lupeol
• Tetraterpenes (C-40) such as α-carotene, β-carotene and lutein
• and other terpenes are the source of balsams, resins, waxes, rubber, etc.

What is the basic unit of terpenoids?

An Isoprene unit containing five carbons in its structure is the basic unit of terpenoids.

What is the nature of terpenoids?

• All terpenoids are organic compounds.
• They are soluble in organic solvents and usually insoluble in water.
• Various terpenoids are optically active.
• They are acyclic or cyclic unsaturated compounds containing one or more double bonds.
• They also give hydrogenation and halogenation etc. addition reactions.

Describe some functions of terpenoids.

Terpenoids are used in some pharmacological activities like:

• Anti-viral
• Anti-bacterial
• Anti-inflammatory
• Anti-malarial
• Hypoglycemic activities
• Anti-cancer activities