Flavonoids

Flavonoids, although phenolic compounds are discussed separately. Compared with the essential oils, the flavonoids of Origanum have received significantly less attention. It is only in recent years, that antioxidant compounds have become highly appreciated, hence more light is expected to be thrown on flavonoids that are always present, together with other phenolic compounds, in herbal infusion and other plant preparations (Vekiari et al, 1993; Moller et al, 1999). For example, Kanazawa et al. (1995) reported that galangin and quercetin extracted from Origanum have shown specific antimuta-genic activity against dietary carcinogens.

Two classes of flavonoids are recognized in plants of the Labiatae: free flavonoids and the flavonoid glycosides. Both classes are present in Origanum. Free flavonoids are often highly methylated. Methyl ethers of flavones, flavonols, flavanones and dihydroflavones are insoluble in water and are found on the plant surfaces, in the waxy layer or in the glands. The other class of flavonoids includes the O-glycosides and the C-glycosides, that are water soluble and are accumulated in the vacuoles of plants.

Free flavonoids

Free flavonoids have been identified in many members of the genus. Table 3.6 and Figures 3.16 show the free flavones, flavanones, dihydroflavonols and flavonols that have been found in Origanum species. As is characteristic of the Labiatae (Tomas-Barberan et al, 1988a), significantly more flavones (15) than flavonols (7) have been found. The amount of research work that has been carried out on Origanum does not yet offer sufficient information to make generalisations and draw conclusions on flavonoid distribution with regard to infrageneric taxonomic groups; however it is quite clear that there is an abundance of 6-substituted flavonoids; almost half of the flavones (7 out of 15) and half of the flavonols (3 out of 7) bear either a hydroxy- or a methoxy group in the sixth position. It also worth mentioning here the presence of 8-substituted compounds in the free flavone group (3 out of 15) of Origanum, which are uncommon elsewhere.

Tomas-Barberan et al. (1988b) report that within the genus Origanum species of the section Origanum do not accumulate external flavonoids. This, disagrees with other workers that report several free flavonoids of O. vulgare, from Russia (Antonesku et al, 1983; Peshkova and Mirovich, 1984; Mirovich, 1987), from Japan (Kanazawa etal, 1995), from China (Zheng et al, 1997) and from Greece (Vekiari et al, 1993); unfortunately in all these studies no subspecies are named. On the other hand Tomas-Barberan et al. (1988b) also state that species in the sections Amaracus, Anatolicon and Majorana do accumulate external flavonoids, and this is confirmed by other researchers (Passannanti etal, 1984; Voirin et al, 1984; Harvala and Skaltsa, 1986; Souleles, 1990). Inter-sectional hybrids seem to accumulate free flavones too (Palomino et al, 1997; Bosabalidis etal, 1998).

Not much information is yet available on the quantity of the various classes of free flavonoids accumulated in the plants, or on the quantitative and qualitative variation

Table 3.6 Free flavonoids found in Origanum spp.

Trivial t

Compound

Species

Flavones

Chrysin

Negletein

Mosloflavone

Apigenin

Genkwanin Acacetin

Apigenin-4,7-dimethylether

Cirsimaritin

Luteolin

Chrysoeriol Thymusin

6-Hydroxy-luteolin-7,3'

dimethylether

dimethylether Thymonin

5-Dismethylnobeletin

Flavonols

Galangin

Kaempferol

Penduletin

Quercetin

Axillarin

Chrysosplenol-D

Retusin

Flavanones Naringenin

Eryodictyol

5,7-di-OH-flavone 5,6-di-OH-7-OMe-flavone 5-OH-6,7-di-OMe-flavone 5,7,4' -di-OH-flavone

5,4' -OH-7-OMe-flavone 5,7-di-OH-4' -OMe-flavone 5-OH-7,4'-di-OMe-flavone 5,4'-di-OH-6,7-di-OMe-flavone 5,7,3' ,4' -tetra-OH-flavone

5,7,4'tri-OH-3'-OMe-flavone 5,6,4 '-tri-OH-7,8-tri-OMe-flavone

5,6-di-OH-7,3' ,4' -tri-OMe-flavone 5,6,4 '-tri-OH-7,8,3' -tri-OMe-flavone

3,5,7-tri-OH-flavone

O. vulgare O. vulgare O. vulgare O. dictamnus O. vulgare O. X majoricum O. X intercedens O. majorana O. pampanini O. akhadarensis O. dictamnus O. vulgare O. X majoricum O. X majoricum O. onites O. X intercedens O. boissieri O. vetteri O. acutidens O. leptocladum O. scabrum O. onites O. syriacum O. X intercedens O. onites

O. majorana O. boissieri O. X intercedens O. cordifolium O. saccatum O. vetteri O. acutidens O. leptocladum O. scabrum O. onites O. pampanini

O. vulgare O. vulgare O. majorana O. dictamnus O. majorana O. majorana O. vulgare

O. vulgare O. X majoricum O. dictamnus O. vulgare

98 Melpomeni Skoula and Jeffrey B. Harborne Table 3.6 (Continued)

Trivial name Compound Species

Dihydroflavonols Aromadendrin/

dihydrokampferol 3,5,7,4'-OH-dihydroflavonol O. compactum

Taxifolin/dihydroquercetin 3,5,7,3',4'-OH-dihydroflavonol O. vulgare within and between populations of Origanum species. Mirovich (1987) reported on the differences in the flavonoid content of O. vulgare with regard to the growing site, to the developmental stage and to the plant organ. Kanazawa et al. (1995) gave an indication of the flavonol content in O. vulgare, reporting that the galangin and the quercetin content were 3.4 mg and 10.9 mg respectively, in 100 g plant material. Palomino etal. (1997) found that the maximum content of flavonoid in O. X majoricum, from Balearic islands, occurs in May.

Flavonoid glycosides

A number of O-glycosides and C-glycosides have been found in Origanum (Table 3.7). Up to the present, 21 compounds have been reported, 14 of them being flavone

OH O chrysin

OH O chrysin

OH O mosloflavone

OH O mosloflavone

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