Flavones and Isoflavones

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Slide 1: 

Mohammad Ovais Dar www.facebook.com/dar.ovais

Flavones: : 

Flavones: Flavone consists of benzo- gamma pyrone ring with phenyl substitution at position 2 of pyrone ring. 2,3 dihyroxy derivative of flavone is called flavonone.

Slide 3: 

These are yellow pigments which occur in plant kingdom either in the free state or as glycosides or associated with tannins. These are also known as anthoxanthins. When a flavone is hydrolysed with mineral acids, it yields anthoxanthidine (aglycon) and one or more sugars. The sugars are generally glucose, rhamnose, etc

Slide 4: 

However flavones may exist as C-glycosyl derivatives as well as O-glycosides, e.g. Vitexin

Properties of Flavones: : 

Properties of Flavones: Most flavones are yellow solids. Most flavones are soluble in water, ethanol and dilute acids and alkalis. Flavones are precipitated by lead salt. With ferric chloride, flavones give either a dull green or a red brown colour Flavones exhibit 2 absorbtion bands: Band I 330-350 nm and Band II, 250-270 nm

Natural flavones include : 

Natural flavones include Apigenin (4',5,7-trihydroxyflavone) Luteolin (3',4',5,7-tetrahydroxyflavone) Tangerirtin (4',5,6,7,8-pentamethoxyflavone) Scutellarein (5,6,7,4'-tetrahydroxyflavone) Synthetic flavones are Diosmin Flavoxate

Determination Of Structure Of Flavones : 

Determination Of Structure Of Flavones Flavones does not contain any hydroxyl group. On fusion with alkali, flavones are degraded to a phenol and aromatic acid.

Determination Of Structure Of Flavones : 

Determination Of Structure Of Flavones Structure proposed by degradation is finally proved by various synthesis some general methods of synthesis are Kostanecki method Robinson method  Baker-Venkataraman method Chalcone method Wheeler method

Flavones : 


Kostanecki method : 

Kostanecki method It is a method of condensing the ester of alkylated salicylic acid with acetophenone OR alkylated o-hydroxyacetophenone with the ester of an aromatic acid in the presence of sodium to form an intermediate which on reduction form flavone.

Kostanecki method : 

Kostanecki method

Robinson method : 

Robinson method It is a general method which involves heating o-hydroxyacetophenone with the anhydride and sodium salt of benzoic acid to form flavone

Robinson method : 

Robinson method

Baker-Venkataraman method : 

Baker-Venkataraman method In this method o-hydroxyacetphenone is benzoylated to give o-benzoyl- acetophenone which undegoes isomerisation with a base which on acidification forms flavone

Baker-Venkataraman method : 

Baker-Venkataraman method

Chalcone method : 

Chalcone method O-hydroxyacetophenone on condensation with benzaldehyde or hydroxybenzaldehyde to give chalcone Chalcone is converted to flavone by three different ways

Chalcone method : 

Chalcone method

Wheeler method : 

Wheeler method It is a general method for the synthesis of flavones in which ring expansion of 2-benzylidenecoumaran3-ones take place

Wheeler method : 

Wheeler method

Iso-flavone: : 

Iso-flavone: Isoflavone contains benzo-gamma-pyrone ring with phenyl substitution at position 3 of the pyrone ring. Isoflavones occur in nature either in the free state or as glycosides.

Slide 21: 

Some isoflavones, in particular soy isoflavones, when studied in populations eating soy protein, have indicated that there is a lower incidence of breast cancer and other common cancers because of its role in influencing sex hormone metabolism and biological activity through intracellular enzymes, protein synthesis, growth factor actions, malignant cell proliferations, differentation and angiogenesis

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A recent study has demonstrated that isoflavones have potent antioxidant properties, comparable to that of the well known antioxidant vitamin E. The antioxidant powers of isoflavones can reduce the long-term risk of cancer by preventing free radical damage to DNA. Genistein is the most potent antioxidant among the soy isoflavones, followed by daidzein.

Determination Of Structure Of Iso-flavones : 

Determination Of Structure Of Iso-flavones The structures of isoflavones have been elucidated in the same manner as that of flavones by employing the following methods: The number of hydroxyl groups and methoxyl groups present in isoflavones are estimated by employing the usual methods, i.e., Zerewitinoff’s and Zeisel’s method

Slide 24: 

Oxidation with alkaline hydrogen peroxide may also be employed for degrading isoflavones. This method is of limited use because this does not yield recognisable fragments. However, this method provides useful information about the substituents in the 3-phenyl nucleus, i.e., genistein (4’,5,7-trihydroxyisoflavone) yields p-hydroxybenzoic acid. The final confirmation of the structure of isoflavones is done by its synthesis. However, the various general methods for the synthesis of isoflavones are as follows:

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a) Spath et al. (1930) gave a general method for the synthesising of isoflavones from benzyl o-hydroxyphenyl ketone and ethyl formate in the presence of sodium dust.

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b) Wessely et al (1933) synthesised various isoflavones from substituted ketones, e.g., diadzein is synthesised from 2,4-dihydroxyphenyl p-hydroxybenzyl ketone and ethyl formate in the presence of sodium folloed by acidification.

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c) Baker-oillis synthesized isoflavones from the reaction of benzyl o-hydroxy-phenyl ketones with ethoxalyl chloride in pyridine at room temperature to yield the products which on alkaline hydrolysis followed by acidification and heating yield isoflavones. The method of Baker-Oillis is illustrated by synthesising diadzein.

Distinction of Isoflavones from flavones: : 

Distinction of Isoflavones from flavones: One can distinguish isoflavones from flavones by UV and NMR spectroscopy. The simple isoflavones exhibit intense UV absorption at 255-275 nm and generally a less intense band or inflection at 310-330 nm. The low intensity of absorption of the second band of isoflavones is a valuable diagnostic feature.

Slide 29: 

The NMR signal of the olefinic proton at C-2 in isoflavones appears as a characteristic down field singlet at 7.8 in DMSO as compared to 6.7 for C-3 proton in flavones. The importance of NMR on structure determination is most evident in complex isoflavones.

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