Flavonols, occurrence

Table 2.16 Occurrence of flavonols in leaf exudates of Notholaena standleyi (after Seigler and Wollenweber, 1983)...

These substances are present in small amounts in tea leaf but there is little quantitative data on their occurrence. Figure 3 shows the structure of several of these that have been identified. They occur both as free flavonols and glycosides.24 They may enter into the oxidative reactions that take place during tea manufacture.50... 

Hollman PCH and Arts ICW. 2000. Flavonols, flavones and flavanols nature, occurrence and dietary burden. J SciFood Agric 80 1081-1093. 

Stewart, A.J. et ak. Occurrence of flavonols in tomatoes and tomato-based products, J. Agric. Food Chem., 48, 2663, 2000. 

The occurrence of the diester structures of the malonic acid moiety in natural anthocyanin pigments has so far been reported in pigments from flowers of Eichhornia crassipes and chive. Allium schoenoprasum, where the anthocyanin-flavone and anthocyanin-flavonol disubstituted malonate structures were exhibited, respectively (Figure 10.8 and Figure 10.9). In some anthocyanins from flowers of Anemone coronaria, malonic acid is esterified with galactose in one end and tartaryl in the other end. ... 

Data on this type of flavonols are summarized in Table 12.4. In contrast to the corresponding flavones, the number and complexity of derivatives is smaller. This concerns particularly the formation of furano-, pyrano- and other cyclic flavonols. There is a remarkable number of 0-prenylated flavonols known to date, contrasting to only very few flavones exhibiting this substitution pattern (see Table 12.3). Similar trends have been earlier documented in the review of Barron and Ibrahim. The occurrence of a series of glycosides based on C-prenylated structures is considerable. This substitution trend concerns also some of the dihydroflavonols, thus indicating specific enzyme activities probably dependent on the presence of a 3-OH group. 

The occurrence of acylated flavones and flavonols still appears interesting enough to justify a short paragraph on this subject (for compilation see Table 12.5). Of the flavones, only three compounds are known to date, with one newly reported isobutyrate flavone from leaf exudates of Asarina procumbens (Scrophulariaceae). One further compound, the 5 -benzoate of 8,2, 5 -trihydroxyflavone, was isolated recently from the exudate of Primula palinuri (linuma and Wollenweber, unpublished). 

By comparison, a series of mostly monoacylated flavonols is known to date and recent reports increased the number slightly. Four new products came from Pseudognaphalium robustum and Tanacetum microphyllum (both Asteraceae), and from Adina cordifolia (Rubia-ceae). A diacetylated compound (3,5-diacetyltambulin) was recently isolated from the bark of Zanthoxylum integrifoliolum (Rutaceae). Since most of the flavonols are monoacylated, the accumulation of quercetin tetraacetate in Adina cordifolia is a remarkable result. Altogether, the newly reported compounds occur scattered in the plant kingdom their occurrence is so far of little chemosystematic value. Aerial parts of Tanacetum microphyllum (Asteraceae) yielded a derivative, which is structurally not an ester. It is, indeed, a carbo-methoxy derivative of 6-hydroxyluteolin-4 -methyl ether (compound 34 in Table 12.5). No other flavonoid of this type is known so far. 

Ogundipe, 0.0., Moody, J.O., and Houghton, P.J., Occurrence of flavonol sulfates in Alchornea laxiflora, Pharm. Biol (Lisse, Netherlands), 39, 421, 2001. 

The concentration of 4-oxo-flavonoids depends on the plant, environmental conditions, the part of the plant consumed, the degree of ripeness as well as on the food processing. Flavonoids are preferentially located in the epidermis solubilized in the vacuolar sap (especially flavones and flavonols glycosides) or in the epicuticular zone [20]. More detailed information about the occurrence of the different compounds is given in each subchapter. 

Fragmentation patterns of standards can help to determine whether observed peaks are the result of in-source fragmentation or natural occurrence in the sample. Flavonol glycoside standards are available from Extrasynthese and Indo Fine Chemical. 

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