Phytochemical and Pharmacological Properties of Flavonols


Flavonoids are polyphenolic compounds, which are ubiquitous in nature and have been categorised into flavonols, flavones, flavanones, isoflavones, catechins, anthocyanidins and chalcones according to their chemical structure. Currently more than 4000 flavonoids have been identified, which were isolated from fruits, vegetables and beverages (tea, coffee, beer, wine and fruit drinks). The flavonoids have aroused interest recently because of their beneficial effects on human health especially reported to have antiviral, antiallergic, anti‐inflammatory, antitumour and antioxidant activities. Flavonoids may enrich the immunogenic potential against such diseases of human body. Epidemiological studies have shown that flavonoid intake is inversely proportional to mortality rate because of coronary heart disease and heart attacks. However, recent studies have demonstrated that flavonoids found in fruits and vegetables may act as antioxidants. Current article mainly focuses upon the structure–function relationship of flavonols, one of the key components of flavonoids for their potential therapeutic use.

Key Concepts

  • Flavonols are phytochemical compounds, which are associated with beneficial health effects.
  • Antioxidants are molecules that inhibit the oxidation of other molecules in living organisms.
  • Enzymes are macromolecules biocatalysts, which accelerate the chemical reactions.
  • Metastasis is a pathogenic agent's spread from an initial position or primary site to a secondary site within the host cell.
  • Angiogenesis is the physiological process in which new blood vessels form from preexisting vessels.
  • Tumour is an abnormal mass of tissue or cell that may be solid or fluid‐filled. Tumours can be benign (not cancerous), premalignant (precancerous) or malignant (cancerous).
  • Apoptosis is a process of programmed cell death, which occurs in multicellular organisms.
  • Astrogliosis or astrocytosis is an abnormal increase in the number of astrocytes because of the destruction of nearby neurons from CNS trauma, infection, ischaemia, stroke, autoimmune responses and neurodegenerative disease.

Keywords: flavonols; structure–function; derivatives; biosynthesis; action mechanism; therapeutic

Figure 1. The basic carbon skeleton of flavonoid (a) as well as flavonol subclass (b) with chemical structure of flavonols (c) and some flavonol derivatives (d).
Figure 2. Schematic pathway of flavonol biosynthesis, starting with general phenylpropanoid metabolism of conversion of phenylalanine into 4‐coumaroyl‐CoA and its reaction with malonyl‐CoA resulting in the formation of tetrahydroxychalcone. The chalconaringenin converted into flavonols by various enzymes that have shown outside the black dotted line. The biosynthetic production pathway of fisetin from microorganism is given inside the black dotted line. The names of enzymes have been abbreviated as follows: phenylalanine ammonialyase (PAL); cinnamate‐4‐hydroxylase (C4H); 4‐coumarate:CoA ligase (4CL); chalcone synthase (CHS); chalcone reductase (CHR); flavanone 3‐hydroxylase (F3H); flavonoid 3′ hydroxylase (F3'H); flavonoid 3′,5′‐hydroxylase (F3'5'H); flavonol synthase (FLS); flavonoid 3′‐monooxygenase (FMO); cytochrome P450 reductase (CPR).
Figure 3. (A) General synthetic reactions scheme of flavonols via Kostanecki's approach; (B) synthesis of quercetin by using phloroacetophenone and veratraldehyde as starting materials; (C) synthesis of kaempferol involves the reactions of 2,4,6‐trimethoxyacetophenone and 4‐methoxybenzaldehyde; (D) fisetin synthesis involves 4‐ethoxy‐2‐hydroxyacetophenone and veratraldehyde as reactants.
Figure 4. (A) General Allan and Robison's method scheme of flavonols synthesis; (B) quercetin synthesis by condensation of ω‐methoxyphloroacetophenone, veratric anhydride and potassium veratrate; (C) synthesis of kaempferol involves condensation of ω‐benzoyloxyphloracetophenone, p‐acetyloxybenzoic anhydride and sodium p‐acetyloxybenzoate; (D) fisetin synthesised using ω‐methoxyresacetophenone, veratric anhydride and potassium veratrate as starting material; (E) synthesis of myricetin carried out by the condensation of ω‐methoxyphloroacetophenone, trimethylgallic anhydride and sodium trimethylgallate.
Figure 5. The structure of quercetin prodrug under clinical trial phase 1 and its synthetic derivatives were significantly effective against human oesophageal squamous cell EC109 as compared to 5‐florouracil.
Figure 6. Representation of flavonols mediated modulation of multiple cellular signalling pathways including apoptosis induction, cell cycle arrest, antimetastasis, antiangiogenesis, anti‐inflammatory and antioxidant.


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Further Reading

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Sharma, Ajay, Sharma, Pooja, Singh Tuli, Hardeep, and Sharma, Anil K(Jan 2018) Phytochemical and Pharmacological Properties of Flavonols. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0027666]