Flavanones aurones

I-3,II-8)-taxifolin to eriodictolyl (I-3,II-8)-taxifolin (125, 126, 295) (Table 7.6.3). A flavanone-aurone derivative, zehyerin (7A), has been isolated from a Phyllogei-ton species of the Rhamnaceae (371). Larixinol (7B), a (1-3,11-8) biflavonoid derived from unusual coupling of a dihydroflavonol with a flavan-3-ol was isolated recently from Larix gmelini (336). 

The mechanisms of the cyclisation of 2 -hydroxychalcone derivatives which can lead to flavanones, flavones and aurones have been reviewed <95MI1> and the formation of 3-hydroxy- chromanones and -flavanones from l-(2-hydroxyphenyl)-2-propen-l-ones via the epoxide has been optimised <96JOC5375>. 

We assayed a number of other phenolic compounds, including aurones, flavones, flavanones, flavanols, lignans and 5-hydroxycinnamic acid derivatives, but they all displayed little or no activity. The lack of activity of... 

In three of the volumes of The Flavonoids, Advances in Research, published between 1975 and 1993, flavanones and dihydroflavonols were part of the chapter on Minor Flavonoids, expertly written by Professor Bruce Bohm. These Minor Flavonoid chapters also included chalcones, dihydrochalcones, and aurones. The term Minor Flavonoids was first used by Harborne in 1967 to encompass not only flavanones, chalcones, and aurones, but also isoflavonoids, biflavonyls, and leucoanthocyanidins, because so few compounds belonging to each of these flavonoid classes were known at that time. For example, only about 30 flavanone and dihydroflavonol aglycones, 19 chalcones, and 7 aurones were known in 1967. The number of known minor flavonoids increased considerably in the next two decades, so that when the checklist for The Flavonoids, Advances in Research Since 1980 was published in 1988, 429 known flavanones and dihydroflavonols (including glycosides) were listed, 268 chalcones and dihydrochalcones, and 29 aurones. In the last 15 years, the total number of known compounds in these flavonoid classes has more than doubled, so that the term minor flavonoids is no longer appropriate. Consequently, it has been decided that separate chapters should be devoted to the flavanones and dihydroflavonols (this chapter), and chalcones, dihydrochalcones, and aurones (Chapter 16). 

FIGURE 16.1 Guide to the structures, ring labeling, and atom numbering of chalcones, dihydrochalcones, aurones, and auronols. The corresponding flavanone structure is shown for reference. 

Aurone-flavanone Campylopusaurone C30H20O12 Campylopus clavatus Dicranaceae 306... 

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]. 

Flavonoids are widely distributed colored phenolic derivatives. Related compounds include flavones, flavonols, flavanonols, xanthones, flavanones, chalcones, aurones, anthocyanins, and cat-echins (see Chapter 29). Plants containing flavonoids with anticancer properties include ... 

Flavonoids are a specific class of polyphenols. It is generally believed that flavonoids include a wide variety of phenolic compounds, such as flavones, flavonols, flavanones, flavanonols, anthocyanidins, flavan-3,4-diols, xanthones, flavan-3-ols, isoflavones, isoflavanones, chal-cones, dihydrochalcones, aurones, and homoisoflavones. Their separation poses special problems because there is often irreversible adsorption and even hydrolysis on solid supports. 

Figure 13. COMPLEX BIFLAVONOIDS (I-5 II-5)-Biaurone, (I-2 II-2)-Biaurone, (I-a,II-a)-Biaurone, (I-2,II-7)-Aurone-auronol dimers, (I-2 ,II-6)-Aurone-flavanone dimer, (1-5 ,11-6)-Aurone-flavanone dimer, (I-3,II-5)-Flavanone-auronol dimers, (I-3,II-7)-Flavanone-auronol dimers.

Hydroxychalcone is readily cyclized with hot aqueous alkali this is the basis of a widely applicable synthesis of flavanones [B-4]. In addition to the examples of dehydrogenative cyclization of chalcones described in Part 1 (pp. 191, 192), that produced by DDQ should be mentioned. In boiling dioxan, the reaction takes 20-72 h and produces flavones together with smaller amounts of flavanones and aurones. 

Sulfuric acid Flavones and flavonols dissolve into concentrated sulfuric acid giving a deep yellow solution. Chalcones and aurones produce red or red-bluish solutions. Flavanones give orange to red colors. 

Flavonoids can be classified according to the degree of oxidation of the three-carbon central segment. From lower to higher degree of oxidation, flavonoids are usually classified as catechins, chalcones, flavanones, isoflavones, flavan-3, 4-diols, flavones, aurones, flavonols and anthocyanins (Fig. (2)) [5],... 

X-Ray analysis of (-)-6-bromocryptostrobin (127 R Br, R2 Me) has confirmed the differentiation between crystostrobin (127 Rl=H, R2=Me) and strobopinin (127 R1=Me, R2=H). As a consequence of this work, the structures of several other natural products (lawin-al, unonal, etc) have had to be revised.135 The reaction of nucleophiles with 3-methanesulphonyloxyflavanones gives a mixture of two or more products which arise by elimination (to give flavones), substitution (to produce 3-substituted flavanones), ring contraction (to aurones) and additon-intramolecular substitution (to form 3,4-epoxides).136... 

Chalcones (e.g., 9) are key intermediates in the formation of several major groups of flavonoids (see Fig. 11.2). In some plants, they are converted into aurones (such as 11). In other instances, chalcones undergo reduction of the exocy-clic double bond to produce dihydrochalcones (such as 12). Chalcones and flavanones (e.g., 10) exist in equilibrium in in vitro systems. However, as only flavanones with a (25)-configuration are known to occur naturally, this interconversion appears to be enzymatically controlled and does not involve racemization. Desaturation of flavanones can yield flavones (such as 4 and 5), whereas introduction of oxygen at the 3-position gives dihydroflavonols (flavanonols) (such as 13) which, in turn, are the precursors of flavonols (e.g., 6-8), anthocyanins (e.g., 1-3), and condensed tannin precursors (Ebel and H lbrock, 1982 Hahlbrock, 1981 Haslam 1974) (Figs. 11.7 and 11.8). 

Most groups of flavonoids, e.g., chalcones, dihydrochalcones, flavanones, flavo-nols, aurones and isoflavones were shown to undergo a continuous turnover and may be degraded to products of primary metabolism (A 5). The degradation proceeds in two subsequent steps... 

Minor Flavonoids (Chalcones, Flavanones, Dihydrochalcones, and Aurones) ... 

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