Organic Chemistry: Essential Oils and Taxol

Introduction to Terpenes (Terpenoids)

• A large, structurally diverse family of natural products (>35,000)
• Formed from C₅ isoprene units joined together head-to-tail or tail-to-tail
• Classification based on the number of isoprene units in the carbon skeleton

Isoprene Units

• Five-carbon unit (C₅)
• Basic building block of terpenes
• Chemical formula: C₅H₈
• Also known as 2-methyl-1,3-butadiene

Mevalonic Acid Pathway

• Pathway producing isopentenyl pyrophosphate (IPP)
• Precursor to isoprene units
• Starts with acetyl-CoA and utilizes ATP
• Key enzymes include: Mevalonate kinase, 5-phosphomevalonate kinase, 5-pyrophosphomevalonate decarboxylase, isopentenyl pyrophosphate isomerase

Classification of Terpenes

• Based on the number of isoprene units
• Hemiterpenes (C₅)
• Monoterpenes (C₁₀)
• Sesquiterpenes (C₁₅)
• Diterpenes (C₂₀)
• Sesterterpenes (C₂₅)
• Triterpenes (C₃₀)
• Tetraterpenes (C₄₀)
• Polyterpenes (>C₄₀)

Representative Monoterpenes

• α-Phellandrene (eucalyptus)
• Menthol (peppermint)
• Citral (lemon grass)

Representative Sesquiterpenes

• α-Selinene (celery)

Representative Diterpenes

• Vitamin A

Representative Triterpenes

• Squalene (shark liver oil)

Biological Isoprene Unit

• Isoprene units in terpenes come from isopentenyl pyrophosphate (IPP)
• IPP is derived from acetate (2 carbons) via mevalonate (6 carbons)

Carbon-Carbon Bond Formation

• The key process is the nucleophilic attack of the double bond of isopentenyl pyrophosphate on the allylic carbon of dimethylallyl pyrophosphate.
• This forms a new carbon-carbon bond.

After C-C Bond Formation

• The carbocation loses a proton to form a double bond.
• This forms geranyl pyrophosphate, a precursor to monoterpenes

From 10 Carbons to 15

• A chain elongation process that adds an isoprene unit to geranyl pyrophosphate
• The resulting 15-carbon compound is farnesyl pyrophosphate

From 15 Carbons to 20

• Another isoprene unit is added to farnesyl pyrophosphate to form geranylgeranyl pyrophosphate (a diterpene precursor)

Cyclization

• Formation of rings through intramolecular carbon-carbon bond formation
• Key examples including limonene and α-terpineol

Bicyclic Terpenes

• Examples: α-pinene and β-pinene

Volatile Oils (Essential Oils)

• Complex mixtures of volatile, aromatic compounds
• Common uses include: antimicrobial, antispasmodic, flavoring, aromatherapy, and other therapeutic applications.
• Typically found in glandular structures or cells of certain plants

Steam Distillation

• Common method for extracting essential oils
• Botanical material heated in a still with pressurized steam.
• Heat forces oils from pockets within the plant material through a tube to a condensation chamber.

Cold Pressing

• Method to extract oils from citrus fruits.
• Fruits are rolled and pressed to extract essential oils from the skin pouches.
• The oil rises to the top of the juice and is separated from pulp by centrifugation

Enfleurage

• Method to extract essential oils from delicate flowers (e.g., jasmine)
• Flower petals placed on trays into an odorless oil (like fat) which absorbs the essential oil. Trays are changed regularly until the oil is saturated.

Testing Essential Oil Purity

• Simple test to check if essential oils are pure, or have base oils added
• Apply 1-2 drops to a tissue, and observe if it completely evaporates leaving no oily residue.

Common Properties of Volatile Oils

• Liquid at room temperature
• Colourless or slightly yellowish
• Low solubility in H₂O
• Soluble in organic solvents
• Optically active
• Higher refractive index
• Usually lighter than water
• Oxidized by O2 from air (resinification)
• Do not stain filter paper (fixed oils do)
• Evaporate at room temperature.
• Obtained by steam distillation, water distillation, expression (citrus oils), organic solv. extraction

Phenylpropane-rich Volatile Oils

• Aromatic compounds
• Formed via shikimic acid
• Contain C₆ phenyl ring and C₃ propane side chain (examples: cinnamon, anise, fennel, clove, nutmeg)

Cinnamon Oil

• Obtained from the dried bark of the Cinnamomum tree.
• Main component: cinnamaldehyde
• Uses: carminative, flavoring agent, and antiseptic.
• Warning: Taking high doses of cinnamon oil is possibly unsafe.

Anise Oil

• Obtained from the dried ripe seeds of Pimpinella anisum,
• Main component: anethol
• Uses: antispasmodic, carminative, parasiticidal, and flavoring agent

Star Anise

• Similar in appearance to Japanese but smaller with milder scent.
• Has toxic compounds and should never be ingested if Japanese.

Fennel Oil

• Obtained from the seeds of Foeniculum vulgare
• Main components: trans-anethole and fenchone
• Uses: remedy for digestive complaints, such as flatulence, constipation, and colic

Wormwood (Artemisia annua)

• Family: Asteraceae
• Contains absinthin, anabsinthin, and artemisinin (volatile oil)
• Artemisinin used to treat malaria and some cancers

Feverfew

• Family: Asteraceae (Chrysanthemum parthenium)
• Contains parthenolide (used to treat migraine)

Camphor Oil

• Extracted from the wood of camphor trees using steam distillation
• Main component: D-camphor

Terpene Derivatives

• Include compounds derived from the Acetate-Mevalonate pathway

Polyterpenes

• Composed of many isoprene units (>40)
• Often large, complex molecules

Phytol

• A 20 carbon diterpene
• Found in chlorophyll

Taxanes

• Pharmaceutically significant diterpenes
• Including paclitaxel, used as an anticancer drug

Ginkolides

• Produced from extensive modification of diterpenes
• Found in Ginkgo biloba (e.g. ginkgolides A, B, C, J, M and bilobolide)
• Used traditionally for memory loss and as a blood flow improver

Steroidal Saponins

• Less widespread than pentacyclic triterpenoid saponins
• Commonly used in synthesis of sex hormones (such as testosterone and oestrogen),

Cucurbitacins

• Tetracyclic terpenes found in plants of the Cucurbitaceae family (e.g., pumpkins and cucumbers)
• Possess steroidal structures

Saponin Glycosides

• Organic compounds producing persistent froth in water
• Cause haemolysis (destruction of red blood cells) when injected intravenously

Licorice

• Dried, unpeeled root of the herb Glycyrrhiza glabra
• Contains Glycyrrhizic acid (sweetener) and other bioactive compounds
• Has anti-inflammatory and antiviral effects, but in high doses, can have negative effects.

Essential Oils: Uses and Precautions

• Vary in properties, and must be used in mixtures
• Different oils have different properties.
• Some may be irritating or toxic if not used correctly.

Monoterpene-rich Volatile Oils

• Examples: peppermint oil, lavender oil, lemon oil, camphor, thyme oil, turpentine oil, rosemary oil

Terpenoid Components of Volatile Oils

• Most monoterpenes are flavoring agents.
• Some are rubefacients, sedatives, or insecticides.
• Classified by degree of oxidation (hydrocarbons, alcohols, aldehydes, ketones, phenols).

How to Dilute Essential Oils

• Essential oils should not be used undiluted
• Dilutions should use an odorless carrier oil
• Common carriers include sweet almond oil and coconut oil Important: Dilute essential oils according to recommended proportions given in the provided presentations before use.

Other Compounds

Other important information regarding each compound/plant has been extracted where possible from the presentation pages.

Test your knowledge on organic chemistry, focusing on essential oils extraction methods and the structure and classification of Taxol. This quiz covers key concepts such as bond formation, reaction mechanisms, and the significance of different molecular components. Enhance your understanding of these crucial topics in organic chemistry.