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Agricultural and Biological Sciences » "Flavonoids - From Biosynthesis to Human Health", book edited by Goncalo C. Justino, ISBN 978-953-51-3424-4, Print ISBN 978-953-51-3423-7, Published: August 23, 2017 under CC BY 3.0 license. © The Author(s).

Chapter 2

Isolation and Structure Identification of Flavonoids

By Weisheng Feng, Zhiyou Hao and Meng Li
DOI: 10.5772/67810

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Basic structure of flavonoids.
Figure 1. Basic structure of flavonoids.
UV spectrum of flavonoids.
Figure 2. UV spectrum of flavonoids.
Substitution modes of ring A.
Figure 3. Substitution modes of ring A.
Substitution modes of ring B.
Figure 4. Substitution modes of ring B.
MS fragmentation pathway of flavone.
Figure 5. MS fragmentation pathway of flavone.
MS fragmentation pathway of flavanone.
Figure 6. MS fragmentation pathway of flavanone.
ECD spectra for compounds 1a and 1b.
Figure 7. ECD spectra for compounds 1a and 1b.
Synthetic isoflavans (31a/b–36a/b).
Figure 8. Synthetic isoflavans (31a/b–36a/b).
CD spectra of isoflavans with oxygenation at both the A and B rings.
Figure 9. CD spectra of isoflavans with oxygenation at both the A and B rings.

Isolation and Structure Identification of Flavonoids

Weisheng Feng, Zhiyou Hao and Meng Li
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Flavonoids, which possess a basic C15 phenyl‐benzopyrone skeleton, refer to a series of compounds in which two benzene rings (ring A and B) are connected to each other through three carbon atoms. Based on their core structure, flavonoids can be grouped into different flavonoid classes, such as flavonols, flavones, flavanones, flavanonols, anthocyanidins, isoflavones and chalcones. Flavonoids are often hydroxylated in positions 3, 5, 7, 3′, 4′ and/or 5′. Frequently, one or more of these hydroxyl groups are methylated, acetylated, prenylated or sulfated. In plants, flavonoids are often present as O‐ or C‐glycosides. The O‐glycosides have sugar substituents bound to a hydroxyl group of the aglycone, usually located at position 3 or 7, whereas the C‐glycosides have sugar groups bound to a carbon of the aglycone, usually 6‐C or 8‐C. The most common carbohydrates are rhamnose, glucose, galactose and arabinose. This chapter mainly introduces the methods of isolation and structure identification of flavonoids.

Keywords: flavonoids, structures and classification, extraction, isolation, structure identification

1. Introduction

Flavonoids are important natural organic compounds of secondary metabolites that are produced during the long process of natural selection. They widely exist in the roots, stems, leaves, flowers and fruits of higher plants and ferns [1], which are of numerous categories and chemical structures. Because of the special chemical structures, flavonoids possess wide range of physiological and biochemical effects to the cells of mammal and other kinds of animals. Firstly, flavonoids possess strong chemical reactivity. For example, some flavonoids have antioxidant activity via scavenging free radicals in organism [2]. Furthermore, flavonoids possess various pharmacological activities of inhibiting the activity of enzymes, antitumor, antibiosis, antivirus, anti‐inflammatory and so on [39]. The potential treatment and prevention effects have been shown in degenerative diseases such as tumors, aging and cardiovascular diseases [1015]. Additionally, some compounds of flavonoids possess potential application prospects as weak hormones at treating menopausal syndrome of women [1417].

2. Structures and classification

Flavonoids generally refer to the natural products of C6‐C3‐C6 basic structure. Most of them are the chromone derivatives with the core structure of 2‐phenylchromone and made up of three rings of A/B/C as shown in Figure 1.


Figure 1.

Basic structure of flavonoids.

According to connection mode of ring A with B, the connection position of ring B, the oxidation level of C3 substructure and degree of polymerization, various type of flavonoids could be classified, as shown in Table 1.

Structure typeBasic structureStructure typeBasic structure

Table 1.

Main structure types of flavonoids.

The main factors of the structure diversity of flavonoids are as follows:

2.1. Change of ring system, degree of oxidation and number of core structure

Most of the flavonoids possess the core structure of C6‐C3‐C6; few of them are C6‐C1‐C6 (xanthone, for example). A few of them, such as homoisoflavones and rotenoids, possess C6‐C4‐C6 structure skeleton. In most cases, C3 part is formed to be a hexatomic or pentagon ring with C6 part. It could also be aliphatic chain, such as chalcone and dihydrochalcone. Supposing that the double bond of ring C was hydrogenated, dihydro derivative was formed, such as flavanone and flavononol. Commonly, ring B is connected to C‐2; it might be connected to C‐3 or C‐4 in a few cases, such as isoflavone and neoflavonoid. Most of the flavonoids have only one core structure; some of them possess two, however. We called them biflavonoids.

2.2. Various substituents at ring A and B

Generally, hydroxyl, methoxyl, methyl, isopentenyl, methylenedioxyl, benzyl, nitro groups and so on, could be substituted at ring A and/or B.

2.3. Glycosidation

Flavonoids are often glycosided to be O‐glycosides or C‐glycosides. During the glycosidation of flavonoids, almost every hydroxyl group could be the reaction position. However, the glycosidation reaction mostly occurred at 7‐OH of flavone, flavanone and isoflavone, 3‐ and/or 7‐OH of flavonol and flavanonol 3‐ and/or 5‐OH of anthocyanidin. The glycosyl group of C‐glycosides is often connected to C‐6 and/or C‐8.

2.4. Formation of complexes

Complexes of flavonoids could be formed with other types of compounds, such as phenylpropanoids, coumarins and alkaloids.

3. Extraction and isolation

3.1. Extraction

Traditional extraction methods of flavonoids often cause the problems of inefficiency, high energy consumption, more solvent consumption and so on. The new extraction methods and technologies occurred in recent years promoted the development of flavonoids. Because of the numerous types of flavonoids, the single extraction methods generally cannot meet the requirement. Traditional and modern methods should be applied together according to the extraction purpose.

3.1.1. Traditional extraction methods

Ethanol and methanol are frequently used to extract flavonoids. The common extraction methods include dipping, percolation, reflux, continuous reflux and so on. The alcohol of high concentration (90–95%) is applied to extract free flavonoids, and the alcohol at the concentration of about 60% is applied to extract flavonoid glycosides. For example, reflux method was applied to extract total flavonoids from leaves of Ginkgo biloba with 70% ethanol, and the product yield was significantly higher than the water decoction method [18].

Hot water extraction method is applied to flavonoid glycosides. It possesses the advantages of low cost, safety, simple equipment and could be applied in industrial production, but much water‐soluble impurities, such as proteins and saccharides might be mixed into the product.

Most of the flavonoids are acidic because of hydroxyl or carboxyl groups, so they could be extracted with alkaline water or alkaline dilute alcohol. The commonly used solvents include dilute sodium hydroxide, lime water, 5% sodium hydroxide dilute ethanol solution and so on. Water‐soluble impurities, such as tannins, pectins and mucilages, could be precipitated because of the formation of calcium salts during the extraction with lime water. It has often showed good results if 5% sodium hydroxide dilute ethanol solution was used. However, the product yield might be reduced because some flavonoids obtained after acidification might be dissolved in dilute ethanol solution. It should be noted that the concentration of alkali should not be excessive during the extraction, and the acidity of the solvent should not be excessive during acidification. Additionally, sodium borate should be used if adjacent phenolic hydroxyl groups are existed in the structures. Extraction of rutin from Flos Sophorae Immaturus is one example [19].

3.1.2. New extraction technologies Supercritical fluid extraction (SFE)

The goal of selected extraction, isolation or purification might be achieved via controlling temperature, pressure and regulating the type and consumption of cosolvent during the supercritical fluid extraction. Cosolvent (e.g. ethanol) is usually added to induce product yield. For example, the product yield of supercritical CO2 extraction of flavonoids from Licorice has been raised 2.2 times than the ordinary alcohol extraction [20]. Ultrasonic extraction

This method has been used in the quality analysis and small amount extraction of flavonoids. It’s still seldom used in industrial production, however. For instance, ultrasonic extraction was used in the extraction of flavone from the bud of Sophora japonica, and the product yield was higher than reflux extraction method [21]. Ultrasonic extraction is superior to reflux method from the perspectives of energy saving, time saving and technology. Microwave‐assisted extraction

It has obtained good results in the extraction of flavonoids. However, it is confined to laboratories so far. It also can be applied combined with other methods to induce product yield. For example, refluxing extraction was used after treatment with microwave for a short time during the extraction of flavonoids from Ophiopogon japonicus. The product yield was induced significantly [22]. Enzyme method

The impurities, such as starches, pectins and proteins, could be removed after enzymolysis. Long extraction time is the limitation of this method. However, the mild operational conditions could overcome the shortcomings that some bioactive components may be decomposed under high temperature. Macroporous adsorption resin

It has been used in the separation and enrichment of flavonoids. Suitable types should be chosen according to the nature of target constituents. Ultrafiltration

The molecules of different molecular weight are separated depend on the pressure difference between both sides of ultrafiltration membrane. Proteins, polypeptide, polymeric pigments and starches could be removed largely. It possesses advantages of simple operations such as no need to heat and destroy the molecular structures. It could remove 69.4% pectins and 66% proteins during the preparation of soybean isoflavones [23]. Aqueous two phase extraction (ATPE)

Aqueous two phase system (ATPS) is formed when either two polymers and kosmotropic salt, or two salts (one chaotropic salt and the other a kosmotropic salt) are mixed at appropriate concentrations and at a particular temperature. The distribution coefficients are different in specific ATPS of different substances. The separation objective will be achieved via selective distribution between the two phases after adding substances into the system. It possesses advantages of time‐saving, simple operation, mild condition, being easy to expand process, large treatment capacity and so on. The commonly used ATPS are high polymer system (e.g. PEG‐Dextran system), high polymer‐inorganic salt system and PEG‐sulfate/phosphate system. The distribution characteristics of puerarin in the two‐phase aqueous systems of PEG/(NH4)2SO4 and acetone/K2HPO4 have been studied [24], and the best system has been determined.

3.2. Isolation

3.2.1. General methods

The isolation of flavonoids includes the separation of flavonoids and other kinds of compounds, and the obtaining of monomeric compounds. The choosing of isolation methods is made primary according to polarity, acidity, molecular weight difference and special structure. Chromatography is still the first choice to isolate flavonoids. Silica gel chromatography

Silica gel chromatography is the main method to isolate or identify flavonoids. It is applied to isolate low or medium polar constituents. Reversed phase silica gel (e.g. reversed phase C18 silica gel) is commonly used to isolate flavonoid glycosides. Polyamide chromatography

Polyamide is a good adsorbent to isolate flavonoids. The adsorption strength hinges on hydrogen bonding associated between polyamide and flavonoids, which depends on the number and positions of hydroxyl groups in the molecules of flavonoids. Polydextran gel chromatography

The most commonly used polydextran gel is sephadex LH‐20 during the isolation of flavonoids. Adsorption is the main mechanism during the isolation of free flavonoids, and the adsorption strength is mainly based on the phenolic hydroxyl groups. However, molecular sieve effect plays the leading roles during the isolation of flavonoid glycosides.

3.2.2. Application of new isolation technologies High‐performance liquid chromatography (HPLC)

This technology has been widely used in the isolation and quality analysis of flavonoids and other kinds of natural products. The determination of chromatographic condition is the key to achieve separation purpose. Choice of stationary phases

Silica gel and amino columns are mostly used during the operation of normal phase chromatography. In the reversed phase, HPLC (RP‐HPLC), C18, C8, C2, amino or phenyl columns could be applied, whereas C18 and C8 columns are mostly used among them. Choice of mobile phases

Methanol‐water and acetonitrile‐water system are commonly applied in RP‐HPLC. In order to improve separation performance, minute quantity of acid (e.g. trifluoroacetic acid) could be added into mobile phase. Detection

All of the flavonoids are able to absorb ultraviolet rays, so generally they could be detected by UV detectors. It is usually detected at 254–280 nm or 340–360 nm for flavones, flavonols and the corresponding glycosides, 520–540 nm for anthocyanidins and the corresponding glycosides, 250 nm for chromones. High‐speed counter current chromatography (HSCCC)

High‐speed counter current chromatography (HSCCC) has been applied successfully to the isolation of flavonoids. The method is simple and quick to operate, and could get product with high purity. Furthermore, it is suitable to industrial production. For example, an HSCCC system has been employed to separate seven flavonoids from a methanolic extract of the leaves of Oroxylum indicum by a one‐step isocratic elution using a chloroform‐methanol‐water (9.5:10.5) two‐phase system [25]. Molecular imprinting technology (MIT)

Molecular imprinting technology (MIT) has been applied in recent years to isolation and active screening of flavonoids. As the study [26] of Pakade et al., molecularly imprinted polymers (MIPs) targeting quercetin were prepared from 4‐Vinylpyridine and ethylene dimethacrylate (EDMA) under various solvent systems with the aim to form MIPs with high recognition for the quercetin molecule in aqueous systems at high temperature. The slopes for the effect of extraction time revealed that the mass transfer of the analytes was higher at 84°C than at 25°C. Also, the binding capacity for the most promising MIP and its corresponding NIP was higher at 84°C. The binding capacity for the MIP was similar to 30 μmol/g at 25°C and 120 μmol/g at 84°C, while for the corresponding NIP, it was similar to 15 and 90 μmol/g, at 25 and 84°C, respectively.

4. Structure identification of flavonoids

Generally, structure determination of flavonoids can be achieved easily because of the systematic research of their structures and the progress of spectroscopic technologies (Nuclear Magnetic Resonance spectroscopy, especially). Series of spectroscopic technologies, such as IR, UV, NMR, and MS, are often used during structure identification of flavonoids. In rare cases, total synthesis should be applied to verify the elucidated structures.

4.1. Ultraviolet spectrum (UV)

The positions, types and number of substituents in the conjugated systems could be speculated via means of UV spectrum. Most of the flavonoids in methanol possess two main absorption bands. Band I is at 300–400 nm, which is caused by electron transition of cinnamoyl group. Band II is at 240–280 nm, which is caused by electron transition of benzoyl group, as shown in Figure 2. The structure types and oxygen‐bearing substituent types of flavonoids could be determined by the peak locations, shapes and strengths of band I and II, as shown in Table 2 [27].


Figure 2.

UV spectrum of flavonoids.

Structure typeBand II (nm)Band I (nm)
Flavonol (3‐OH is substituted)250–280328–357
Flavonol (3‐OH is free)250–280358–385
Isoflavone245–270310–330 (shoulder peak)
Flavanone and flavanonol270–295300–330 (shoulder peak)
Chalcone220–270 (weak peak)340–390
Aurone230–270 (weak peak)370–430

Table 2.

The spectral characteristics of UV‐VIS spectrum of flavonoids.

The locations and shapes of Band I and II will be affected by the substituents attached to rings A and B. Normally, red shift of band I increases accordingly when the number of hydroxyl groups located at ring B increases. Similarly, red shift of band II increases accordingly when the number of hydroxyl groups located at ring A increases, but it has trifling impact to band I, with the exception of 5‐OH. The corresponding bands will be violet shifted 5–15 nm if the particular hydroxyl is glycosided. Furthermore, the influence of the hydroxyl groups will almost disappear if they are acetylated.

4.2. Infrared spectrum (IR)

It is used mainly to determine the types of functional groups, substitution modes of aromatic rings and so on. The all functional groups, such as carbonyl, phenolic hydroxyl, phenyl and glycosyl, have possessed corresponding IR absorptions. The absorption band of hydroxyl groups are in the 3200–3650 cm−1 region, carbonyl groups are in 1660–1680 cm−1 region and the vibrations of benzene rings are at about 1500, 1580 and 1600 cm−1.

4.3. Nuclear magnetic resonance spectrum (NMR)

Nuclear magnetic resonance spectrum (NMR) is the most powerful method to elucidate the structures of flavonoids. Kinds of solvents, such as CDCl3, DMSO‐d6, C5D5N, (CD3)2CO and CD3OD, could be employed while performing NMR experiments. DMSO‐d6 is the optional solvent among them to perform NMR to flavonoids. Almost all kinds of flavonoids could be well dissolved in DMSO‐d6, and the resonance signals of flavonoids are rarely overlapped by solvent peaks (about δ2.5). Furthermore, NMR signals of phenolic hydroxyl groups could be displayed clearly with DMSO‐d6 as the solvent. The drawback of this solvent is high boiling point, which leads to difficulty in sample recovery.

4.3.1. 1H‐NMR spectrum

It provides information of chemical shifts, coupling constants and proton number. The types of flavonoids, substituted modes, number and configurations of glycosyls and so on, could be determined via 1H‐NMR spectrum. Protons on ring C

1H‐NMR characteristics of protons on ring are shown in Table 3 [28].

Flavanoneδ6.3– 6.8 (s)The signals maybe overlapped by H‐6 or H‐8.
FlavonolNone signal.
Isoflavoneδ7.6–7.8 (s)The signal is at rather low field because of influence of oxygen atom at position 1 and carbonyl at position 4.
Chalconeα‐H: δ6.7–7.4 (d, J = 17 Hz)
β‐H: δ7.3–7.7 (d, J = 17 Hz)
C‐2 and C‐3 form a trans double bond.
Flavanoneδ5.0–5.5 (dd, J = 11, 5 Hz)δ2.3– 2.8 (2H) (dd, J=17, 11 Hz); (dd, J = 17, 5 Hz)H‐2 is coupled by two protons of position 3.
Flavanonol4.8–5.0 (d, J = 11 Hz)δ4.1–4.3 (d, J = 11 Hz)Configurations of both C‐2 and C‐3 are R.
Flavanone‐3‐O‐glycosideδ5.0–5.6δ4.3–4.6After glycosidation of 3‐OH, resonance signals of both H‐2 and H‐3 shift to low field.
AuroneExocyclic proton: δ6.5–6.7 (s)

Table 3.

Chemical shifts and coupling constants of ring C of common flavonoids. Protons on ring A

The ordinary substitution modes are 5,7‐dioxygenation, 7‐oxygenation, 5,6,7‐trioxygenation and 5,7,8‐trioxygenation, See Figure 3.


Figure 3.

Substitution modes of ring A. 5,7‐Dihydroxyl substituted

5,7‐Dihydroxyl flavonoids are most common. For this type of flavonoids, the signals of H‐6 and H‐8 are shown at δ5.7–6.9 as doublets, and the signal of H‐6 is always at the higher field than H‐8. The signals of both H‐6 and H‐8 shift to lower field after glycosidation of 7‐OH. 7‐Hydoxyl substituted

Signal of H‐5 is shown to be a doublet since vicinal coupling exists between H‐5 and H‐6. Additionally, the chemical shift is at rather low field (about δ8.0) because of the shielding effect of carbonyl at position 4. H‐6 is affected by H‐5 and H‐8, so it has showed a double‐doublet (dd, J=2.0, 8.0 Hz). H‐8 is showed to be a doublet (J=2.0Hz) because of the vicinal relationship with H‐6. Signals of both, H‐6 and H‐8 are at δ6.3–7.1. The chemical shifts of protons on ring A are shown in Table 4 [28].

5,7‐DihydroxylFlavone, flavonol, isoflavoneδ6.0–6.2 dδ6.3–6.5 d
7‐O‐Glucosides of above compoundsδ6.2–6.4 dδ6.5–6.9 d
Flavanone, flavanonolδ5.75–5.95 dδ5.9–6.1 d
7‐O‐Glucosides of above compoundsδ5.9–6.1 dδ6.1–6.4 d
7‐Hydroxyl flavonoidsFlavone, flavonol, isoflavoneδ7.9–8.2 dδ6.7–7.1 dδ6.7–7.0 d
Flavanone, flavanonolδ7.7–7.9 d5.7–6.0 dδ5.9–6.1 d
5,6,7‐Trihydroxyl flavonoidsδ6.95 s
5,7,8‐Trihydroxyl flavonoidsδ6.3 s

Table 4.

Chemical shifts of protons on ring A. Protons on ring B

There are a variety of substituted modes of ring B, such as non‐substitution, 4’‐oxygenation, 2’‐oxygenation, 3’,4’‐dioxygenation, 2’,4’‐oxygenation, 3’,4’,5’‐trioxygenation and 2’,4’,5’‐trioxygenation, as shown in Figure 4. Generally, signals of protons on ring B are showed at slightly lower field, and the chemical shifts are usual at δ6.7‐8.1. The substitution modes and structural information could be determined via  the  chemical  shifts  and  coupling  constants  of  ring B.


Figure 4.

Substitution modes of ring B. None substituent on ring B

For this mode, there are five protons on ring B. Signals of H‐2’ and H‐6’ are shown at lower field than H‐3’, H‐4’ and H‐5’ because of the shielding effect of ring C. Furthermore, the peak shapes of all of the protons are complicated because of the coupling effects of the vicinal‐ and meta‐coupling. The signals of H‐2’ and H‐6’ are usually at δ7.1–7.6 and of H‐3′, H‐4′ and H‐5′ are at δ7.9–8.2. 4′‐Oxygenation

In this circumstance, ring B is a symmetrical substructure. One AA’BB’ coupling system is formed by four aromatic protons. The spectral characteristics are show in Table 5 [28].

TypeH‐2′, H‐6′H‐3′, H‐5′

Table 5.

Chemical shifts of protons on ring B of 4′‐oxygenated flavonoids. 3′,4′‐Dioxygenation

In this circumstance, one ABX coupling system is formed by three aromatic protons, and three groups of signals are displayed as H‐2′(1H, d, J ≈ 2.0Hz), H‐5′(1H, d, J ≈ 8.0Hz) and H‐6′(1H, dd, J ≈ 2.0, 8.0Hz). The chemical shifts of protons on ring B are shown in Table 6 [28].

TypeSubstituent modeH‐2′H‐5′H‐6′
Flavone3′,4′‐OH and 3′‐OH, 4′‐OCH3δ7.2–7.3δ6.7–7.1δ7.3–7.5
Flavonol3′,4′‐OH and 3′‐OH, 4′‐OCH3δ7.5–7.7δ7.6–7.9
3′‐OCH3, 4′‐OHδ7.6–7.8δ7.4–7.6
3′,4′‐OH, 3‐O‐glcδ7.2–7.5δ7.3–7.7
Isoflavone, flavanone, flavanonol3′,4′‐OHδ6.7–7.1δ6.7–7.1

Table 6.

Chemical shifts of protons on ring B of 3′,4′‐dioxygenated flavonoids. 2′‐Oxygenation

ABCD coupling system is formed by the rest protons of ring B. The peak shapes are rather complicated. Signals of H‐3’ and H‐5’ are usually displayed at δ6.8–6.9, H‐4’ at about δ7.2 and H‐6’ at δ7.4–7.5. 3′,4′,5′‐Trioxygenation

If identical substituents are attached to C‐3’ and C‐5’, which allows the formation of a symmetrical substructure of ring B, H‐2’ and H‐6’ will display to be a singlet at δ6.5–7.5. 2′,4′,5′‐Trioxygenation

In the cases of this substituent mode, either of the two protons on ring B displays to be a singlet. Generally, signals of H‐6’ in flavones and flavonols are showed at δ7.2–7.5, H‐3’ at δ6.4–6.6. Signals of H‐6’ are shown at slight higher field. 2′,4′‐Dioxygenation

In the cases of this mode, H‐3′ will be showed at δ6.00–6.6 (d, J ≈ 2.0 Hz), H‐5′ at δ 6.6–6.5 (dd, J ≈ 2.0, 8.0 Hz) and H‐6′ at δ7.0–7.4 (d, J ≈ 8.0 Hz). See Table 7.

Substituent modeH‐2′H‐3′H‐4′H‐5′H‐6′
None substituent on ring Bδ7.1–7.6 (m)δ7.9–8.2 (m)δ7.9–8.2 (m)δ7.9–8.2 (m)7.1‐7.6 (m)
2′‐Oxygenatedδ6.8–6.9 (m or dd)δ7.2 (m or dd)δ6.8–6.9 (m or dd)7.4–7.5 (m or dd)
4′‐Oxygenatedδ7.1–8.1 (2H, d, J≈8.0Hz)δ6.5–7.1 (2H, d, J≈8.0Hz)δ6.5–7.1 (2H, d, J≈8.0Hz)7.1–8.1 (2H, d, J≈8.0Hz)
2′,4′‐Dioxygenationδ6.00–6.6(d, J≈2.0Hz)δ6.30–6.50 (1H, dd, J≈8.0,2.0Hz)7.0–7.4(d, J≈8.0Hz)
3′,4′‐Dioxygenationδ7.2–7.8(1H, d, J≈2.0Hz)δ6.7–7.1(1H, d, J≈8.0Hz)6.7–7.9(1H, dd, J≈2.0, 8.0Hz)
3′,4′,5′‐Trioxygenationδ6.5–7.5 (2H, s, H‐2′,6′) as identical oxygen‐bearing substituents are connected to position 3′,4′ and 5′.

Table 7.

Chemical shifts of protons on ring B of various substituent modes [28]. Common substituents

The proton chemical shifts of common substituents of flavonoids are shown in Table 8 [28].

Proton typeChemical shift
Phenolic hydroxyl5‐OH (δ12.0–14.0),7‐OH (δ10.8–11.0), 4′‐OH (δ9.8–10.6), 3′‐OH (δ9.2–10.4)
MethylC‐6,8 (δ2.0–2.5); rha‐CH3 [δ9.2–10.4 (d, J≈6.5Hz)]
CH3CO [glc: δ1.65–2.10 (3H,s); aromatic ‐CH3:δ2.3–2.5 (3H,s)
Methoxylδ3.5–4.1 (3H,s)
IsopentenylδCH2 (3–3.4); CH (5.2);CH3 (δ1.7–1.8)
Terminal protons of glycosylδ4.5–5.5

Table 8.

Chemical shifts of the protons on common substituents.

4.3.2. 13C‐NMR spectrum

Strong regularities are also shown in 13C‐NMR spectra of flavonoids. The types of flavonoids, number and connection positions of glycosyls could be elucidated from 13C‐NMR spectra. Identification of skeleton structures of flavonoids

The core structures are difficult to be elucidated by resonance signals of aromatic protons. However, the characteristic signals of carbons in ring C allowed the identification of different types of flavonoids, see Table 9.

Flavonol (3‐OH is free)δ145.0–149.5δ135.6–139.0δ172.0–178.0
Flavonol (3‐OH is free)δ155.7–157.0δ133.2–134.3δ172.0–178.0

Table 9.

Chemical shifts of carbons in ring C of flavonoids [28]. Determination of substituent modes of flavonoids

The substituent modes of core structures of flavonoids could be determined by the signals of aromatic carbons. The chemical shifts of carbons in ring A and B, if they are not substituent, are shown in Table 10 [28]. Signal characteristics of ring A

Usually, the substituents, such as hydroxyl, methoxyl and isopentenyl groups, are attached at position 5 or/and 7 of ring A, which leads to the changes of chemical shifts of other carbons in ring A. It is shown in Table 11 [28].

Ring AChemical shiftRing BChemical shift
C‐8δ116.5–119C‐1′ (flavone, flavonol, isoflavone)δ130–133
C‐9δ154–156C‐1′ (flavanone, flavonol)δ138–139

Table 10.

Chemical shifts of carbons in ring A and B if they are not substituent.

Substituent modeTypeC‐5C‐6C‐7C‐8C‐9C‐10
7‐OxygenatedFlavone, flavonolδ126δ114δ162–164δ100–102δ157δ114–116
Flavanone, flavanonolδ128δ109.5–110δ165–166δ100δ162–164δ114–116
5,7‐DioxygenationFlavone, flavonol, isoflavoneδ160–162δ97–100δ162–165δ93–95δ155–158δ103–109
Flavanone, flavanonolδ162–163δ93–97δ165–167δ93–97δ161–163δ100–104

Table 11.

Chemical shifts of carbons in ring A of flavonoids. Signal characteristics of carbons on ring B

The signal characteristics of carbons on ring B are shown in Table 12 [28].

Substituent modeTypeC‐1′C‐2′C‐3′C‐4′C‐5′C‐6′
4’‐OxygenatedFlavone, flavonol, isoflavoneδ121–123δ130δ115δ157–161δ115δ130
Flavanone, flavanonolδ128–130
3′‐OH,4′‐OCH3 (or 3′,4′‐dihydroxyl)Flavone, flavonol, isoflavoneδ121–125δ113–114δ145–147δ149–151δ112–116δ118–122
3′‐OH,4′‐OCH3 (or 3′,4′‐dihydroxyl)Flavanone, flavanonolδ128–129δ111–115δ144–147δ146–148δ112–116δ118–120

Table 12.

Chemicals shifts of carbons on ring B of flavonoids. Signal characteristics of common substituents

The carbon chemical shifts of common substituents are shown in Table 13 [28].

SubstituentChemical shift
Isopentenyl [‐CH2CH=CHCH3]CH2(δ21–22); CH(δ122–124; CH(δ129–131); CH3(δ17–27)
Terminal carbon of glycosylsO‐glycosidesδ95–105

Table 13.

Chemical shifts of carbons of common substituents on flavonoids.

4.3.3. Glycosides of flavonoids

In plants, flavonoids are often present as O‐ or C‐glycosides. The O‐glycosides have sugar substituents bound to a hydroxyl group of the aglycone, usually located at position 3 or 7, whereas the C‐glycosides have sugar groups bound to a carbon of the aglycone, usually 6‐C or 8‐C. The most common carbohydrates are rhamnose, glucose, galactose and arabinose.

Generally, the chemical shifts of terminal protons of glycosyls are at δ4.5–5.5 in 1H‐NMR. The terminal carbons of O‐glycosides are at δ95–105 and at δ71–78 for C‐glycosides. Furthermore, the number of glycosyls could be determined by combined analysis of 1H and 13C‐NMR spectra. It is an effective method to determine the connection positions of glycosyls by glycosylation shifts, as shown in Table 14 [28].

Glycosylation position23456789101′2′3′4′5′6′

Table 14.

Glycosylation shifts (average values) of flavonoids in 13C‐NMR spectrum.

The configurations of glycosyls should be determined. The relative configurations of some glycosyl groups could be determined sometimes by coupling constants of terminal protons in 1H‐NMR spectra. The absolute configurations, however, should be determined by chemical methods and gas chromatography.

As for the spectral method, the types and configurations could be speculated by the chemical shifts of glycosyl carbons in 13C‐NMR spectra, as shown in Table 15 [28].

Type of glycosylC‐1′′C‐2′′C‐3′′C‐4′′C‐5′′C‐6′′
α‐ D‐ galδ100.1δ69.2δ70.5δ70.2δ71.6δ62.2

Table 15.

Carbon chemical shifts of common glycosyls.

4.4. Mass spectral characteristics of flavonoids

ESI‐MS and FAB‐MS are widely applied in the studies of flavonoids. While the positive ion mode is employed, quasi‐molecular ion peaks such as [M+H]+, [M+Na]+, [M+K]+ and [M+NH4]+ will be displayed. [2M+H]+, [2M+Na]+ and so on will also be shown if the sample is concentrated. The MS fragmentation pathways of flavone and flavanone are shown in Figures 5 and 6.


Figure 5.

MS fragmentation pathway of flavone.


Figure 6.

MS fragmentation pathway of flavanone.

4.5. Determination of absolute configuration

The absolute configuration should be determined if chiral atoms are existed in the structures. The main methods to elucidate absolute configuration include circular dichroism (CD), optical rotatory dispersion (ORD) and X‐ray single crystal diffraction. Circular dichroism and ORD are mainly introduced here.

4.5.1. Optical rotatory dispersion (ORD)

For the flavonoids possess chiral centers, their optical activities (589.0 nm, Na‐D light source) are correlative with spatial configurations, as shown in Table 16 [27].

TypeOptical activitySpatial structure
Flavanonol (trans‐form)+2R,3R
Epicatechin (cis‐form)2R,3R
Epicatechin (cis‐form)+2S,3S
Catechin (trans‐form)+2R,3S
Pterocarpin (cis‐form)6aR, 11aR
6a‐Hydroxyl pterocarpin (cis‐form)6aS, 11aS
6a‐Hydroxyl pterocarpin (trans‐form)+6aR, 11aR

Table 16.

Optical activities of flavonoids.

4.5.2. Circular dichroism (CD)

It is the most used method to elucidate the absolute configurations of flavonoids via cotton effect (CE) of CD spectra. Flavanone

Most of the protons of flavanones at position 2 are axial (J ≈ 11.0Hz). The characteristics of CE are shown in Table 17.

CEAbsolute configuration of C‐2
270–290 nm320–330 nm

Table 17.

Relationship between CE and absolute configurations of flavanones.

As reported in literature [29], the absolute configurations of the enantiomeric flavanone pair (2S)‐6‐formyl‐5,7‐dihydroxyflavanone (1a) and (2R)‐6‐formyl‐5,7‐dihydroxyflavanone (1b) were assessed via their chiroptical data. The ECD curves of compound (1a) showed sequential positive and negative cotton effects near 310 and 280 nm for the n→π* and π→π* electronic transitions, respectively. These cotton effects are reminiscent of flavanones exhibiting P‐helicity of the conformational flexible heterocycle with a C‐2 equatorial B ring and, hence, (2S) absolute configuration. The mirror image related to ECD spectrum of 1b accordingly confirmed its (2R) absolute configuration. It is shown in Figure 7.


Figure 7.

ECD spectra for compounds 1a and 1b. Flavanonol

Four possible structures are existed in nature because of the existence of two chiral centers (C‐2 and C‐3), while 2R, 3R configurations are commonest. The relative configuration could be determined by coupling constant between H‐2 and H‐3 and then CD spectrum is employed to elucidate the absolute configuration, as shown in Table 18 [30].

Relative configurationCotton effect (300–340)Absolute configuration
trans+2R, 3R
2S, 3S
cis+2R, 3S
2S, 3R

Table 18.

Relationship between absolute configurations of flavanonol with CE. 3‐Hydroxyl flavans

Similarly, C‐2 and C‐3 are also the chiral centers of 3‐hydroxyl flavans. The characteristics are shown in Table 19 [31].

Relative configurationCEAbsolute configuration
240 nm280 nm
Trans+2R, 3S
+2R, 3R
Cis+2S, 3R
++2S, 3S

Table 19.

Relationship between absolute configurations of 3‐hydroxyl flavans with CE. 4‐Hydroxyl flavans

The relative configuration could be determined by coupling constant of H‐2 and H‐4 combined with NOE spectra and then CD spectrum could be employed to elucidate the absolute configuration, as shown in Table 20 [32].

Relative configurationCEAbsolute configuration
240 nm280 nm
Trans2S, 4S
++2S, 4R
Cis2S, 4S
++2R, 4R

Table 20.

Relationship between absolute configurations of 4‐hydroxyl flavans with CE. 3,4‐Dihydroxyl flavans

More absolute configurations are existed because of three chiral centers, as shown in Table 21 [33].

Relative configuration between C‐2 and C‐3Relative configuration between C‐3 and C‐4Cotton effect (280 nm)Absolute configuration

Table 21.

Relationship of absolute configurations of 3,4‐dihydroxyl flavans with CE. Flavans

The cotton effects of flavans are show in Table 22 [34].

Cotton effect (280 nm)Absolute configuration

Table 22.

Cotton effects of flavans. Isoflavans

The CE characteristics are shown in Table 23.

CEAbsolute configuration
260–320 nm220–260 nm

Table 23.

Cotton effects of isoflavans.

Versteeg et al. [35] synthesized six isoflavans and their enantiomers (31a/b‐36a/b), and used authentic 3S‐ and 3R‐vestitol (30a and 30b) derivatives to establish the absolute configuration at C3 of the synthetic isoflavans (Figure 8). (3S)‐Isoflavans with oxygenation at both the A‐ and B‐rings (34a, 35a and 36a) display positive and negative CEs in the 240 (1La) and 270–280 nm (1Lb) regions, respectively, and conversely for the 3R‐enantiomers (34b, 35b and 36b) (Figure 9).


Figure 8.

Synthetic isoflavans (31a/b–36a/b).


Figure 9.

CD spectra of isoflavans with oxygenation at both the A and B rings. Pterocarpins

The spectral characteristics are shown in Table 24 [36].

Relative configuration of between C‐6a and C‐11aCEAbsolute configuration
260–310 nm220–250 nm
Trans+6aS, 11aR
6aR, 11aS
Cis+6aR, 11aR
+6aS, 11aS

Table 24.

Relationship of absolute configurations of pterocarpins with CE.

The relationships between the CE and absolute configurations will change after a hydroxyl group is attached to position 6a, as shown in Table 25 [37].

Relative configuration of between C‐6a and C‐11aCEAbsolute configuration
260–310 nm220–250 nm
Trans6aS, 11aR
++6aR, 11aS
Cis+6aS, 11aS
+6aR, 11aR

Table 25.

Relationship of absolute configurations of 6a‐hydroxyl pterocarpins with CE.


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