Flavones, naturally-occurring

Verschoyle R D, Greaves P, Cai H, Borkhardt A, Broggini M, D Incalci M, Ricci E, Doppalapudi R, Kapetanovic I M, Steward W P and Gescher A J (2005), Preliminary safety evaluation of the putative cancer chemopreventive agent tricin, a naturally occurring flavone , Cancer Chem Pharmacol, 57, 1-6. 

The stereoselective epoxidation of chalcones, followed by acid-catalysed ring closure and concomitant cleavage of the epoxide ring, provides a very efficient route to chiral flavon-3-ols and, subsequently, by borohydride reduction to produce flavan-3,4-diols [13, 14], It has been shown that diastereoselective reduction of the chiral flavon-3-ols by sodium borohydride in methanol yields the trans-2,3-dihydroxy compounds, whereas borohydride reduction in dioxan produces the cis-isomers [14] the synthetic procedure confirms the cis configuration of the 2,3-hydroxy groups of naturally occurring leucodelphinidins [14]. 

Meier B. (1995). Passiflora incarnata L.—Passion flower. Zeitschrift fur Phytotherapie. 16(2) 115-26. Medina JH, Paladini AC, Wolfman C, Levi de Stein M, Calvo D, Diaz LE, Pena C. (1990). Chrysin (5,7-di-OH-flavone), a naturally-occurring ligand for benzodiazepine receptors, with anticonvulsant properties. Biochem Pharmacol. 40(10) 2227-31. 

Keki, S., Deak, G., and Zsuga, M., Fragmentation study of rutin, a naturally occurring flavone glycoside cationized with different alkali metal ions, using post-source decay matrix-assisted laser desorption/ionization mass spectrometry, J. Mass Spectrom., 36, 1312, 2001. 

It might be of more value to check the substitution patterns for their chemosystematic significance, as had been done earlier in frequency analysis. According to current data, 6-substitution, both —OH and —OMe, appears to be more frequent than the corresponding 8-substitution in flavones. The number of their 6,8-diOMe derivatives is quite considerable though. By comparison, the number of the related 6,8-OH-flavones is restricted to a few compounds reported from natural sources (compounds 136, 222, 227, and 262 in Table 12.1). All of the other polyhydroxylated structures have so far not been found as natural products. A similar ratio between 6- and 8-substitution was found with the flavonols, but the number of naturally occurring 6,8-diOH flavonols is limited to two compounds only (compounds 239 and 289 in Table 12.2). Further accumulation trends of possible chemosystematic relevance have been discussed in the respective sections. 

The scarcity of reviews1 and recent interest in thiochromanones and related compounds calls for a comprehensive summary of this area of heterocyclic chemistry. The apparent similarity between these systems and the naturally occurring chromanones, chromones (flavones), chromenes, etc., is responsible for the continued importance of these sulfur heterocycles. Chemical Abstracts (through November, 1973) has been employed as the principal reference source and nomenclature guide for this review. 

The parent nucleus of the flavonoids is flavone ((58), 2-phenylchromone or 2-phenylbenzopyran-4-one). Flavone and isoflavone ((59), 3-phenylchromone, the parent nucleus of the isoflavonoids) are the simplest oxygen-containing naturally occurring compounds that possess the 2-phenylnaphthalene -type structure. The chalcones, represented by the nucleus (60), may be regarded as open-chain flavonoids and are usually hydroxylated. The interconversion of chalcone and flavonone catalyzed by chalcone isomerase is well known [326, 327, 331], Chalcones can be precursors of both the flavonoids and the isoflavonoids [326-332]. 

The conversion of androstenedione to estrone is catalyzed by aromatase. Inhibition of aromatase (human estrogen synthetase) by several naturally occurring flavonoids, including quercetin, chrysin, and apigenin, has been described. The synthetic flavone 7,8-benzoflavone was most active. Aromatization of androstenedione was affected by several flavonoids, of which 7-hydroxy-flavone and 7,4-dihydroxyflavone were the most potent. 

Utilizing this reaction, total syntheses of naturally occurring flavones, namely a phytoalexin, homopterocarpin (92), and chrysin (93) and luteolin (94), which are promising chemotherapeutics for AIDS, have been achieved. 

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