Prenyl flavonols

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. 

Species of the genus Vellozia have been extensively studied for their flavonoid complement in relation to chemosystematics. In addition to a series of C-methylflavonols and two C-prenylated flavonols, derivatives of vellokaempferol and velloquercetin are accumulated in whole plants, leaves, and leaf exudates. The basic structure of these compounds is characterized by 7,6-isopropenylfurano substitution, based upon kaempferol, quercetin, and their 0-methyl ethers. In addition, 8-C-methyl derivatives of these compounds were also identified from leaves of V. stipitata " So far, species of this genus are the only reported sources of these compounds, which in parts have been proved to be accumulated externally. ° Structures are exemplified by Figure 12.10. 

The great majority of prenylated dihydroflavonols are of the 2R,3R configuration, and only a few have the 2R,3S cofiguration, with prenylation at positions 8>6>6,7>7,8>3 and rare substitutions at other B-ring positions.149 About one-third of the compounds are (/-methylated at positions 5>4 >7=3. Prenylated flavonols, on the other hand, have a significant proportion of furano- and dimethylchromeno side attachments, but with a few di- and triprenyl derivatives, with prenylation at positions 8>6>4 >3 however, alkylation at both A and B rings is... 

The 5-OH signal of prenylated flavonols (3-hydroxyflavones) is observed between 5 11.9 and 12.6 in acetone-ds when the 3-hydroxyl group is free (276). The substituent parameters of flavonols are listed in Table 8. It should be noted that the 5-OH signal of flavonols is sometimes shifted due to chelation with metals. When a flavonol fluoresces under irradiation with UV light, the sample needs further purification. The shift upon prenylation at C-6 or C-8 of flavonols (parameters i and ii) is almost identical with that of the other flavonoids. Methylation of 3-OH causes a downfield shift of 0.64-0.67 ppm. However, the 5-OH signal of 3-0-methylflavonols appears still further upfield (about 0.2 ppm) than that of flavones having the same A and B rings. 

For compilation and arrangement of compounds, earlier reviews and surveys were taken as the basis. In comparison to the previously published reviews, the increasing number and complexity of structures observed is striking. Thus it became quite difficult to list all of these structures in a logical sequence, particularly prenylated derivatives with additional cyclized substituents. Substitution patterns used for grouping of the flavone and flavonol derivatives are as follows. 

Glycosides of Prenylated Flavones and Flavonols, and of Pyrano and Methylenedioxyflavonols... 

The major types of bioactive constituents reported from Broussonetia papyrifera are the prenylated flavonoids, which include compounds of the diphenylpropane, chalcone, flavan, flavanone, flavone, flavonol, and aurone classes (Table 2), Fig. (7). An early study on B. papyrifera resulted in the isolation of two diphenylpropanes, broussonins A (29) and B (30), and a coumarin, marmesin (52), with antifungal activity [39]. Also, a diprenylated diphenylpropane derivative, kazinol F (31) [40], was reported as an antioxidant and tyrosinase inhibitory constituent [34]. 

Silybin (52) is a naturally occurring flavanolignan from milk thistle. De-hydrosilybin (53) and isoprenoid dehydrosilybins were also recognized as potent inhibitors of P-gp [211], These compounds were tested for their binding affinity toward domain NBD2 of P-gp. Oxidation of silybin to flavonol dehydrosilybin (53) increased the affinity three times. Prenylation and ger-anylation led to a further increase in affinity [207]. C-isoprenylated deriva-... 

Eriosema compound B (prenylated xanthone) Fisetin (= 5-Deoxy-quercetin 3,7,3, 4 -Tetrahydroxyflavone) (flavonol)... 

Losses of small molecules and/or radicals from [M+H]" are observed in the positive-ion mode losses of HjO, CO, and HjC=C=0, and combinations thereof. The loss of a methyl radical CHj is characteristic for O-methylated isoflavones, flavones, and flavonols [13]. Loss of C4H8 indicates the presence of prenyl substituents (at 3-, 6-, and/or 8-position) [t4]. Kuhn et al. [t5] demonstrated the differentiation between isomeric flavones and isoflavones by means of positive-ion MS-MS in an ion-trap instmment. tsoflavones show an abundant fragment ion due to the loss of 2 CO molecules from the C-ring, while flavones only show a single CO loss. 

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