Flavonoids

Flavonoids List

Introduction

Flavonoids are a class of natural substances with variable phenolic structures found in many plant products. They are based on the 2-phenylchromen-4-one (2-phenyl-1-benzopyran-4-one) structure, with a 15-carbon skeleton that consists of two phenyl rings and a heterocyclic ring. In addition to rich antioxidant activity, flavonoids also have anti-inflammatory, anti-mutagenic, and anti-carcinogenic properties, resulting in various applications such as nutraceutical, pharmaceutical, medicinal and cosmetic. Flavones and catechins appear to be the flavonoids with the strongest antioxidant activity (Fig. 1) ^[1].

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Fig. 1 Structures of flavone and catechin.

Classification of Flavonoids

In the flavonoid backbone, the two phenyl rings are generally called ring A and ring B, while the heterocyclic ring which contains embedded oxygen is generally called ring C, as shown in Fig. 2 A. Flavonoid compounds can be subdivided into flavonoids, isoflavonoids and neoflavonoids depending on the carbon of the C ring on which the B ring is attached ^{[1], [2]}.

• Flavonoids: Flavonoids or bioflavonoids refer to a group of flavonoids in which the B ring is linked in position 2 of the C ring, with the skeleton being 2-benzopyrans. They can be further divided into different subclasses based on the degree of unsaturation and oxidation of the C ring, including flavones, flavonols, flavanones, flavanonols, flavanols or catechins, anthocyanins and chalcones.
• Isoflavonoids: Isoflavonoids refer to a group of flavonoids in which the B ring is linked in position 3 of the C ring, with the skeleton being 3-benzopyrans.
• Neoflavonoids: Neoflavonoids refer to a group of flavonoids in which the B ring is linked in position 4 of the C ring, with the skeleton being 4-benzopyrans.

Fig. 2 (A)Basic skeleton of flavonoid and (B) structures of three subclasses of flavonoids ^{[1], [2]}.

Representative Reactions

Flavonoids are involved in diverse chemical reactions. Here are some representative reactions involving flavonoids.

• Ring Cleavage: Flavonoids can undergo ring cleavage reactions, particularly under oxidative conditions. This can result in the formation of smaller phenolic compounds or fragmentation products with different chemical properties.
• Condensation Reactions: Flavonoids can undergo condensation reactions, where two or more flavonoid molecules react to form larger polymeric structures or complex heterocycles.

Activities of Flavonoids

As they possess a wide range of activities, flavonoids have created a great deal of interest. Their biological activities include antioxidant, antimicrobial, anti-inflammatory, anti-mutagenic, and anti-cancer activities ^[1].

• Antioxidant activity: The antioxidant properties of flavonoids are very prominent, which can protect cells from the oxidation of reactive oxygen species. Therefore, they can be used as potential drugs to prevent oxidative stress by maintaining a balance between reactive oxygen species and antioxidants.
• Antimicrobial activity: Some flavonoids have antifungal activities, resulting in them being used in the fields of nutrition, food safety and health. For example, quercetin has good antibacterial activity and may be a promising antibacterial agent for the treatment of periodontitis.
• Anti-inflammatory activity: Some flavonoids have anti-inflammatory activity and have a significant inhibitory effect on inflammatory factors, so they can be used to treat inflammatory diseases.
• Anti-mutagenic and anti-cancer activities: In addition to the above-mentioned activities, flavonoids also have anti-mutagenic and anti-cancer activities, which enables them to be used in various nutraceuticals or drugs to promote human health.

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References:

1. Panche, A. N.; et al. Flavonoids: an overview. J Nutr Sci. 2016, 5: e47.
2. Uivarosi, V.; et al. An overview of synthetic and semisynthetic flavonoid derivatives and analogues: perspectives in drug discovery. Studies in Natural Products Chemistry. 2019, 60: 29-84.