Limonoids and terpenoids: d-limonene, α-terpineol, cucurbitacins MCQs With Answer

Introduction

This quiz collection is designed for M.Pharm students studying Advanced Pharmacognosy I and focuses on limonoids and terpenoids — specifically d-limonene, α-terpineol, and cucurbitacins. Questions cover chemistry, biosynthesis, analytical techniques, pharmacology, toxicology and quality control practices relevant to these natural products. The MCQs aim to deepen your understanding beyond basic definitions, testing structural features, reaction behavior, extraction and identification methods, and therapeutic/toxic effects that are important in drug development and phytochemical analysis. Use these items to evaluate your mastery of both theoretical concepts and practical applications commonly encountered in pharmacognosy and natural products research.

Q1. Which structural classification best describes d-limonene?

• Monocyclic monoterpene hydrocarbon
• Sesquiterpene alcohol
• Tetracyclic triterpenoid
• Tetranortriterpenoid limonoid

Correct Answer: Monocyclic monoterpene hydrocarbon

Q2. The optical activity of d-limonene (d- or (+)-limonene) arises from chirality at which position of the carbon skeleton?

• The isopropenyl terminal carbon
• The allylic methyl-bearing carbon (C4) adjacent to the ring double bond
• The tertiary alcohol carbon
• The quaternary carbon in the ring junction

Correct Answer: The allylic methyl-bearing carbon (C4) adjacent to the ring double bond

Q3. The direct biosynthetic precursor for monoterpenes such as d-limonene is:

• Oxidosqualene
• Geranyl pyrophosphate (GPP)
• Sedoheptulose phosphate
• Mevalonic acid methyl ester

Correct Answer: Geranyl pyrophosphate (GPP)

Q4. α-Terpineol is best described chemically as:

• Acyclic monoterpene aldehyde
• Cyclic monoterpene tertiary alcohol
• Triterpenoid ketone
• Tetranortriterpenoid lactone

Correct Answer: Cyclic monoterpene tertiary alcohol

Q5. The most appropriate industrial route to obtain α-terpineol in high yield is:

• Ozonolysis of d-limonene
• Acid-catalyzed hydration of α-pinene (from turpentine)
• Saponification of terpenoid esters
• Oxidative cleavage of geraniol

Correct Answer: Acid-catalyzed hydration of α-pinene (from turpentine)

Q6. Cucurbitacins are principally classified as which type of natural product?

• Monoterpenes
• Sesquiterpenes
• Triterpenoids (cucurbitane-type)
• Alkaloids

Correct Answer: Triterpenoids (cucurbitane-type)

Q7. A prominent pharmacological action attributed to several cucurbitacins is:

• Selective β2-adrenergic agonism
• Strong cytotoxic and antiproliferative activity often mediated via JAK/STAT pathway inhibition
• Cholinesterase inhibition
• Sweetening effect on taste receptors

Correct Answer: Strong cytotoxic and antiproliferative activity often mediated via JAK/STAT pathway inhibition

Q8. For routine isolation of volatile terpenes (d-limonene and α-terpineol) from plant material with minimal decomposition, the preferred technique is:

• Acid hydrolysis at reflux
• Steam distillation or hydrodistillation
• Supercritical CO2 extraction with no temperature control
• Direct Soxhlet extraction with water

Correct Answer: Steam distillation or hydrodistillation

Q9. The most definitive analytical technique to both identify and confirm minor constituents of essential oils such as d-limonene and α-terpineol is:

• Thin-layer chromatography with iodine spray
• Gas chromatography coupled with mass spectrometry (GC-MS)
• Ultraviolet spectroscopy
• Melting point determination

Correct Answer: Gas chromatography coupled with mass spectrometry (GC-MS)

Q10. Which reagent would selectively convert the double bonds of d-limonene to the corresponding epoxide(s) under mild conditions?

• m-CPBA (meta-chloroperoxybenzoic acid)
• Ozone at -78°C followed by reductive workup
• Potassium permanganate in neutral water
• Hydrogenation with Pd/C

Correct Answer: m-CPBA (meta-chloroperoxybenzoic acid)

Q11. Which of the following pharmacological effects is commonly associated with α-terpineol?

• Strong opioid receptor agonism
• Local anesthetic and sedative properties with antimicrobial activity
• Direct β-lactamase inhibition
• Hypoglycemic insulin-mimetic action

Correct Answer: Local anesthetic and sedative properties with antimicrobial activity

Q12. For visualization of cucurbitacins on a TLC plate, which spray reagent typically yields characteristic color development of triterpenoids?

• Ninhydrin reagent
• Anisaldehyde–sulfuric acid reagent (or vanillin–sulfuric acid)
• Dragendorff’s reagent
• UV light at 254 nm only

Correct Answer: Anisaldehyde–sulfuric acid reagent (or vanillin–sulfuric acid)

Q13. Limonoids are best defined as:

• Monoterpene glycosides found exclusively in Apiaceae
• Tetranortriterpenoids derived biosynthetically from triterpene precursors with loss of four carbon atoms
• Alkaloid derivatives of indole precursors
• Polyacetylenes formed from acetyl-CoA condensation

Correct Answer: Tetranortriterpenoids derived biosynthetically from triterpene precursors with loss of four carbon atoms

Q14. The primary biosynthetic precursor from which limonoids are formed in plants is:

• Geranyl pyrophosphate (GPP)
• Squalene/oxidosqualene leading to triterpene skeletons
• Shikimic acid
• Acetylsalicylic acid

Correct Answer: Squalene/oxidosqualene leading to triterpene skeletons

Q15. Limonin, a major limonoid in citrus, is pharmacognostically notable because it is:

• Responsible for the aromatic citrus scent at trace levels
• An intensely bitter compound contributing to off‑taste and reported to have cholesterol‑lowering and antifeedant effects
• A volatile monoterpene that evaporates on heating
• Unreactive and insoluble in organic solvents

Correct Answer: An intensely bitter compound contributing to off‑taste and reported to have cholesterol‑lowering and antifeedant effects

Q16. Human toxicity associated with accidental ingestion of cucurbitacin-laden gourds typically results in:

• Neurogenic muscular paralysis without gastrointestinal symptoms
• Acute gastrointestinal distress including nausea, vomiting and diarrhea (food poisoning)
• Chronic nephropathy only after months of exposure
• Immediate hypoglycemia and coma

Correct Answer: Acute gastrointestinal distress including nausea, vomiting and diarrhea (food poisoning)

Q17. Which structural feature is characteristic of the cucurbitacin family?

• They are C10 isoprenoids forming monocyclic hydrocarbons
• They are 30‑carbon tetracyclic triterpenoids based on a cucurbitane skeleton often heavily oxygenated
• They are linear diterpenes with multiple conjugated double bonds
• They are aromatic alkaloids containing a pyridine ring

Correct Answer: They are 30‑carbon tetracyclic triterpenoids based on a cucurbitane skeleton often heavily oxygenated

Q18. In essential oil quality control of sweet orange oil, which marker and analytical technique combination is most commonly used to detect adulteration?

• Aspartame content by UV spectroscopy
• High d-limonene content quantified by GC-FID or GC-MS and comparison of terpene ratios
• Total nitrogen by Kjeldahl method
• Lead contamination by flame photometry

Correct Answer: High d-limonene content quantified by GC-FID or GC-MS and comparison of terpene ratios

Q19. Which classical chemical test is frequently used for preliminary detection of triterpenoids and steroids in plant extracts?

• Liebermann–Burchard test (acetic anhydride + sulfuric acid)
• Biuret test for proteins
• Bronsted-Lowry titration
• Benedict’s test for reducing sugars

Correct Answer: Liebermann–Burchard test (acetic anhydride + sulfuric acid)

Q20. Under strong acid catalysis, α-terpineol can undergo dehydration to produce which aromatic hydrocarbon commonly observed in terpene transformations?

• p-Cymene
• Benzoic acid
• Geraniol
• Squalene

Correct Answer: p-Cymene

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