Methoxylated flavones

Flavone dyes having these stmctures are hydroxylated and methoxylated derivatives. The degree of hydroxylation varies from two in chrysin [480-40-0] (17) to six in gossypetin [489-35-0] (18). Those dyes containing not more than three hydroxyls are generally termed flavones whereas those containing up to and including six are flavonols. 

Phuong, N.M. et al., Methoxylated flavones from Adenosoma capitatum, Pharmazie, 52, 647,1997. 

Brandao, M.G.L. et al., Two methoxylated flavone glycosides from Bidens pilosa. Phytochemistry, 48, 397, 1998. 

Similarly to L. densiflora, L. heteromorpha is unusual for its biosynthesis of methoxylated flavonoids glycosides (with the methoxyl groups at C-4 of ring B and C-7 of ring A). The production of such flavonoids could be a response to local environmental factors. In comparison to the other Licania species, L. heteromorpha predominantly yields flavonol glycosides instead of flavones or flavanones. 

Flavones contribute to plant tissue color provided that they occur in high concentrations or are complexed with metal ions. Some flavones participate in taste for example, the highly methoxylated aglycones nobiletin, sinensetin and tangeretin are responsible for the bitter taste of citrus peel. On the other hand, some glycosylated flavones (for instance neodiosmin and rhoifolin) reduce the bitterness of some substances (limonin, naringin, caffeine, quinine) [2]. 

Flavones (Figure II. 3.7) are less commonly found in plants, but often occur in citrus. Poly-methoxylated flavones, which include no-biletin, sinensetin, and tangeretin, are the characteristic features in citrus plants. Flavones are generally detected at 360 or 370 nm. The elution order, when analyzed as described in the Basic Protocol, is rhoifolin, luteolin, and apigenin (Table 11.3.1). 

JM Sendra, JL Navarro, L Izoquierdo. C18 Solid-phase isolation and high-performance liquid chro-matography/ultraviolet diode array determination of fully methoxylated flavones in citrus juices. J Chromatogr Sci 26 443-448, 1988. 

Recent observations reveal that when flavones are methoxylated, metabolic stability and membrane transport in the intestine/liver dramatically increases,... 

Walle T. 2007. Methoxylated flavones, a superior cancer chemopreventive flavonoid subclass Semin Cancer Biol 17 354-362. 

Certain bioflavonoids may play a preventive role against cardiovascular diseases. Some citrus and other bioflavonoids have been demonstrated to reduce serum cholesterol levels and to affect fatty acid metabolism (70,71,72). The strong antiadhesive action on red cells and platelets of highly methoxylated flavones such as nobiletin, which also demonstrates antithrombogenic activity (73), indicates an important role in blood rheology and tissue perfusion. The antiadhesive action may indicate a preventive role in atherosclerosis since there is evidence that reduced perfusion of the vascular wall may interact with serum lipids to promote atherogenesis (74). 

In the early work on bioflavonoids, citrus extracts were used, and later research has revealed the presence of highly active methoxylated flavones in citrus. The amount of these flavones recovered along with the more abundant hesperidin, naringin, eriodictyol varies with the methods of extraction and purification used. Thus, bioflavonoid preparations by the many workers may have varied widely in activity. 

Tatum et al. (29) used the presence of various methoxylated flavones from leaf extracts to distinguish between nucellar and zygotic seedlings. They also identified the predominant flavanone glycoside and three unidentified coumarins in their samples. Ting et al. (51) showed that there were quantitative as well as qualitative Hlfferences in the methoxylated flavone content from the juices of different citrus varieties. They further showed that certain blends of juices could be distinguished based on the amounts of methoxylated flavones found. 

Phenolics. The phenolics of citrus represent a varied and widely studied class of compounds. They range from the lipid-soluble methoxylated coumarins and psoralens to the water-soluble glycosides of the flavanones and flavones. They include the intensely bitter naringin and the highly insoluble hesperidin. Different classes within the group have characteristic UV spectra which have been used to detect adulterations in juices and oils. 

Several TLC methods have been widely used to quantitatively estimate the flavonoids for quality control purposes rather than to detect adulteration. The potential exists, however, for testing authenticity. Naringin is an important compound in grapefruit juice, since it is largely responsible for the bitter character of the juice. Fisher et al. (121) developed a TLC procedure for naringin estimation. This was later modified by Tatum and Berry (122). Swift (123) developed a TLC-spectrophotometric assay for the neutral methoxylated flavones in orange peel. The method was subsequently expanded to the determination of these compounds in orange juice (124). 

Isoflavones exhibit the same oxygenation pattern as flavones, and range from the simple (5-deoxy) to more complex structures where nearly all positions are substituted with hydroxyl, methoxyl, and/or prenyl groups (13).8, 86 Although isoflavanones are smaller in number than isoflavones, their structural complexity is in no way reduced, as appears in (14). 

Yang, S.-L. Roberts, M. R Phillipson, J. D. Methoxylated flavones and coumarins from Artemisia annua. Phytochemistry, 1989, 28(5) 1509-1511. 

Takanaga, H. Ohnishi, A. Yamada, S. Matsuo, H. Morimoto, S. Shoyama, Y. Ohtani, H. Sawada, Y. Poly-methoxylated flavones in orange juice are inhibitors of P-glycoprotein but not cytochrome P450 3A4. J. Pharmacol. Exp. Ther. 2000, 293 (1), 230-236. 

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