Flavonoid Oxygenation Patterns

Whereas some flavonoids may lack one or more hydroxyl groups from the usual 5,7,4 -oxygenation pattern of naringenin, others may acquire new hydroxyls through the action of some of the hydroxylases mentioned above. These commonly occur at positions 6, 8, 2, 3, and/or 5 and may be fiirther substituted. On the other hand, 2-hydroxylated flavonoids are of rare occurrence. The following examples chosen from the different flavonoid classes demonstrate both phenomena, although 

7-Hydroxyflavanone represents the simplest compound of this class and is found in many legumes.9 2-Hydroxyflavanones, which are expected to be quite labile and converted to their corresponding flavones, have also been identified. These include the 2,5,7-trihydroxyflavanone 7-0-glucoside (8) in Malus spp.,80 2,5-dihydroxy-7-methoxyflavanone in Populus nigra, 1 6,7,8-trihydroxy-5-methoxyflavanone (9) in Isodon oresbius,82 and 2,5-dihydroxy-7-methoxyflavanone and its 6-C- and 8-C-methyl isomers (10) in Friesodielsia enghiana.83 All of these exhibit a lack of B-ring hydroxylation. 

Apart from the common 4 -, 3, 4 - and 3, 4, 5 -oxygenated flavones and their flavonol analogs, the less common 5,7-dihydroxyflavone (11) and the more common 

7-trihydroxyflavone (11) represent those that lack B-ring oxygenation. A-ring hydroxylation of flavones and flavonols at positions 6 and or 8 is not uncommon, and may be associated with a 2, 4 -di- or 2, 4, 5 -trioxygenation (12) of 6-hydroxyquercetin (quercetagetin).84 

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). 

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The structural diversity of metabolites belonging to the different flavonoid classes, including their oxygenation patterns, glycosylation, sulfation, acylation, methylation, and/or prenylation are best illustrated in a recent text,9 as well as in several earlier monographs.10 14 These different substitution reactions, which are catalyzed by substrate-specific and position-oriented enzymes, contribute to the enormous diversity of flavonoid compounds that amount to >5000 chemical structures in Nature and hence, to the wide spectrum of functional roles they play in... 

Of the many phytochemical and biological reports on flavanones, the following are a selection of new compounds. A flavanone of unusual oxygenation pattern has been identified in the leaves of New Zealand spinach (Tetragonia expansa). The compound is 7,8-dimethoxyflavanone. Amongst several new prenyl-flavonoids obtained from the roots of Marshallia grandiflora is 8-prenyl-5,7,4 -trihydroxyflavanone. ... 

According to these, and earlier investigations, a specific role for dihydroflavones in flavonoid biosynthesis has been defined. Thus the biosynthetic pathway to quercetin (14) and cyanidin (19) in buckwheat has been formulat as shown in Figure 6.2. Nevertheless, the possibility that the oxygenation pattern of ring C may be determined by other kinds of control mechanism still exists, and it has been tentatively proposed " that these may represent some of the differences between the routes for anthocyanidin biosynthesis and that for the other fiavonoids and isoflavonoids. 

The most important types of flavonoids are collected in Table 55 and Fig. 314. With a few exceptions the oxygen-containing substituents in ring A are in m-position to each other, while the substitution pattern of ring C resembles that of cinnamic acid derivatives. 

The hydroxylation pattern of the chalcone is discriminating those with hydroxy-groups as in 5.71) are converted into 5,7-dihydroxyflavonoids [as (5.75)], those with hydroxy-groups at C-2 and C-4 only [i.e. one oxygen lost from C-6 during chalcone formation, see (5.7/)] afford selectively 7-hydroxy-flavonoids [see (5.75)]. The intact incorporation of chalcones into flavonoids has been proved using doubly labelled precursors [57] (for further discussion of this technique see Section 2.2.1). 

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