Phenolic Glycosides 6-Flavonoids

Flavonoids-Distribution Important constituents of human diet: fruits, vegetables, nuts, seeds, stems, flowers & common drinks (tea). Common in higher plants: Polygonaceae, Rutaceae, Leguminosae, Umbelliferae & Compositae. Seeds of Leguminosae: rich in isoflavonoids. Plants of Polygonaceae: rich in flavan-3-ols & anthocyanins.

Flavonoids-Physiological role in plants Role in plant defense mechanism: limited due to low toxicity (≠ alkaloids). Pigments of flowers attracting birds & insects (Pollinators) Plant growth regulators.

Flavonoids-Therapeutic uses Flavonoid-containing plants are used as antioxidant, antibacterial & anti-inflammatory. Flavonoids possess antiviral, anticancer, spasmolytic, estrogenic, cardiotonic & hepatoprotective activities.

Flavonoids-Therapeutic uses Natural source Constituents Therapeutic use Crateagus spp Procyanidins Cardiotonic Thea sinensis (Green tea leaves) Catechins Antioxidant Licorice Flavonoids Antiviral Soya bean Isoflavonoids (Daidzin & Genistin) Anticancer Ruta spp Buchu leaves Flavonols (Rutin) Flavones (Diosmin) Reduction of capillary fragility Buchu leaves Broom tops Diuretic Saflower Quinochalcones (Tinctormine) Calcium antagonist Smilax spp Silybium marianum Flavanonols (Astilbin) Flavolignans (Silybin) Antihepatotoxic

Flavonoids-Chemistry Variety of structural forms Free aglycones or glycosides Skeleton contains 15 carbon atoms: C6-C3-C6 [2 phenyl rings linked by a three-carbon chain = diphenyl propane derivatives] The three-carbon chain (-C3-) may be included through an oxygen bond

Flavonoids - Classification According to type of oxygen bond: Flavonoids with oxygen bond present in a 6-membered (pyran) ring including compounds: with ring B attached to C-2: e.g. flavones, flavonols, flavanones, flavanonols, flavan-3-ols, anthocyanins etc….. with ring B attached to C-3: e.g. isoflavonoids. with oxygen atom of pyran ring carrying a +ve charge e.g. anthocyanins or anthocyanidins. Flavonoids within each group are differentiated according to: Oxidation state of C-3. Unsaturation between C-2 & C-3. Hydroxylation at C-3`& C-4`.

Flavonoids-Chemistry Diphenyl propane skeleton C6-C3-C6 Flavonoid skeleton

Flavonoids with oxygen bond present in a 5-membered (furan) ring: e. g Flavonoids with oxygen bond present in a 5-membered (furan) ring: e.g. aurones & dihydroaurones. Flavonoids with aliphatic oxygen bond: e.g. chalcones & dihydrochalcones. Aurones

Flavonoids - Effect of hydroxylation pattern on physico - chemical properties Increase of hydrophylic character & acidic nature. Enhancement of U.V. & visible light absorption  pigments with different colors (ivory-yellow, red or blue). Increase of chemical reactivity especially H-bonding capacity. Ortho-dihydroxyls  oxidation  quinones & polymeric melanins. Ortho-dihydroxyls + metals  modification of action.

Flavonoids-Isolation Extraction Glycosides: polar (due to sugar moiety) , extracted with alcohol, water or hydroalcoholic solutions. Aglycones: less polar, extracted with ether, chloroform & ethyl acetate Purification of crude extract: Several methods: Shaking with Na2CO3 or NaHCO3. Precipitation with Pb acetate. Chromatography (PC, TLC or CC).

Flavonoids -Tests for identification All flavonoids + AlCl3  yellow color + UV light  different fluorescences Aurones Chalcones Flavanones Flavonols Flavones + AlCl3 Pale-brown Brownish- pink Yellow to Yellowish-green Green Fluorescence in UV All flavonoids + Pb sub acetate  yellowish ppt Flavanones & flavonols (Alcoholic solution) + Mg metal / HCl  orange, red or violet (Shinoda's test). Chalcones (Alcoholic solution) + SbCl5/ CCl4  red or violet. Chromatography (PC & / or TLC)

Flavonoids- Structural Elucidation Aglycones: by spectral analysis (UV, NMR & MS). Glycosides: by spectral analysis before & / or after hydrolysis. Acid hydrolysis: + 1 N HCl for 30-40 min at 100oC & extract aglycones with organic solvent. C-glycosides need more drastic acid conditions.

Flavonoids-Quantitative determination Spectrophotometric method: depends on measuring the absorbance of the aluminum chloride complex. Chromatographic methods: e.g. HPLC analysis of aglycones obtained after extraction & acid hydrolysis of the glycosides

Flavone glycosides - Apiin Source Leaves & seeds of parsley (Petroselinum sativum) & celery (Apium graveolens) & flowers of Roman chamomile (Anthemis nobilis) Hydrolysis + 0.5 - 1 % H2SO4  partial hydrolysis  apiose + glucoapigenin. + Mineral acid + reflux  complete hydrolysis  apiose + glucose + apigenin. Tests for identification + FeCl3  reddish brown color + alkali  yellow color + lead acetate  yellow precipitate

Flavone glycosides-Diosmin (Barosmin) Source Buchu leaves, Conium & Dahlia. Hydrolysis Acid hydrolysis rhamnose + glucose + diosmetin Uses Reduces capillary fragility: used as prophylactic & in treatment of varicose veins & acute hemorrhoids.

Flavonol glycosides-Rutin (Quercetin-3-rutinoside) Source Ruta graveolens, Nicotiana tabacum & Sophora japonicum. Isolation Extract with 80% ethanol, concentrate & filter. Shake filtrate with ether (remove lipoid impurities). Concentrate alcoholic layer  crystals of rutin Hydrolysis + 1 N H2SO4 + reflux  quercetin + glucose + rhamnose Tests for identification + FeCl3  green color. + PbAc  yellow precipitate. + Ammoniacal AgNO3  reduction. Uses Reduces capillary fragility, permeability & bleeding. Ruta graveolens

Acylated flavonol Glycosides from Gingko biloba

Acylated flavonol Glycosides from Gingko biloba Standardized extract of Gingko biloba leaf is used in treatment of cerebrovascular insufficiency & poor arterial circulation. Main active constituents: acylated flavonol glycosides of kaempferol, quercetin, & isorhamnetin. Ginko biloba glycosides are acylated with p-coumaric acid at C-6 of the glucose moiety.

Flavanone glycosides General characters Flavanones have a reactive C=O: with hydroxylamine  oximes (c.f. from flavones). On treatment with alkalis: flavanones  chalcones  flavones. In presence of acid: chalcones  flavanones. On treatment with strong alkalis: flavanones  benzaldehyde + HAC + phenol , while flavones produce cinnamic acid + phenol

Flavanone glycosides-Hesperidin- Hesperitin-7-rutinoside Source Peel of unripe Citrus fruits e.g. bitter orange & orange. Hydrolysis + acid + heat  hesperitin + rhamnose + glucose Isolation from orange peel Extract powder with ether (to remove volatile oil). Dry the marc & reflux with methanol for 3 hours. Concentrate  crude hesperidin is precipitated. Bitter orange

Flavanone glycosides-Hesperidin Properties Sparingly soluble in cold water, more soluble in hot water & alcohol. On treatment with alkali hesperidin chalcone. On acid hydrolysis  hesperitin + rhamnose +glucose Tests for identification + FeCl3 solution  wine-red color. Alcoholic solution + Mg metal + conc. HCl  violet color. Uses: Hesperidin is necessary for the absorption of vitamin C & it reduces capillary fragility

Flavanone glycosides-Naringin (naringenin-7-neohesperidoside) Source Peels of grape fruit [mainly immature] (Citrus paradisi, Fam. Rutaceae) Main bitter constituent of grape fruit juice. Hydrolysis Naringin (intensely bitter glycoside) + Naringenase enzyme (hydrolysis)  naringenin (non-bitter aglycone) Peel of grape fruit

Flavanone glycosides-Naringin (naringenin-7-neohesperidoside) Properties Very bitter in taste. Sparingly soluble in cold H2O, soluble in hot H2O Naringin (intensely bitter glycoside) + alkali (e.g. NaOH) + reduction  naringin dihydrochalcone (intensely sweet derivative). Uses Bitter stomachic & antiinflammatory. Starting material for semisynthesis of naringin dihydrochalcone (used as substitute for sucrose in diabetes,1000 times sweeter than sucrose).

Flavanone glycosides-Liquiritin- (Liquiritigenin-4`-glucoside) Source Licorice roots & rhizome Main constituents of DGL (Deglycyrrhizinated Licorice extract) almost free from Glycyrrhizin (triterpenoid saponin of licorice) Uses Liquiritin & its aglycone are used in gastric & duodenal ulcers, & as antiinflammatory.

Flavanonol glycosides-Astilbin & Neoastilbin Source Roots of Smilax glabra (Sarsaparilla), Vitis vinifera & Astilbe spp. Hydrolysis: Taxfiolin from Astilbin, Neotaxifiolin from Neoastilbin Action & Uses Antioxidants, hepatoprotectives & prevention of cataract in diabetics

Chalcone glycosides - C-glucosyl quinochalcones-Tinctormine & Carthamin

Chalcone glycosides - C-glucosyl quinochalcones- Tinctormine & Carthamin Source Both are isolated from the petals of Carthamus tinctorius. Tinctormine is a yellow pigment while Carthamin is a red pigment. Pharmacological action & uses Tinctormine: calcium antagonist used in treatment of heart diseases. Carthamin: natural red dye.

Isoflavonoid glycosides- Daidzin & Genistin Source Daidzin in soya beans Genistin in Lupinus & Prunus spp. Pharmacological action & uses Isoflavones are considered as dietary phytoestrogens. They are cancer-protective, effective in preventing breast & prostate cancer. They also aid in the treatment of osteoporosis. Daidzin is used as antioxidant.

Anthocyanidins & their glycosides-General characters Structurally related to flavones Hydroxylated cations. Keto group at C-4 is absent. Glycosides are known as anthocyanins. Colored compounds named after the Greek words antho = flower & kyanos = blue.

Anthocyanidins & their glycosides-General characters Color is pH dependent. Example: cyanidin 3, 5-diglucoside: blue in corn flower [pH of cell sap alkaline] red in roses [pH of cell sap acidic] Soluble in water, insoluble in ether, acetone & chloroform. Stable in acidic medium.

Flavan-3-ols - General characters Do not possess a keto group at C-4. Source Green tea (fresh leaves of Camelia sinensis), rhubarb & black tea. Pharmacological action & uses Mainly used as antioxidant, anticancer, antiviral & antibacterial.

Biflavones - Amentoflavone Source Chrozophora spp. (black haw) bark & Amentotaxus spp. Pharmacological action & uses Uterine antispasmodic. Delay diabetic cataract (astilbin) & diabetic neuropathies. Cardiotonic. Chrozophora Amentotaxus

Flavonoid-containing compounds Flavonolignans - Silymarin Silydianin

Flavonoid-containing compounds Flavonolignans-Silymarin Source Silybum marianum fruits Structure Silymarin is a mixture of various flavanonol & flavonol derivatives Main components of silymarin are: silybin, silychristin & silydianin Uses Hepatoprotective prophylactic in: Liver damage caused by metabolic toxins. Liver dysfunction after hepatitis. Chronic degenerative liver conditions, e.g. liver cirrhosis & fatty liver. The oxeran ring is responsible for the antihepatotoxic effect of silybin, opening of this ring results in loss of activity.