Aurones

Aurones (1.28) are formed by cyclization of chalcones, whereby the meta-hydroxyl group reacts with the a-carbon to form a five-member heterocycle. Aurones are also yellow pigments present in flowers. 

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In 1934 the transformation of 2 -hydroxychalcones to flavonols in the presence of hydrogen peroxide and sodium hydroxide was reported simultaneously by Algar and Flynn in Ireland and Oyamada in Japan However, many reports following the original disclosures showed that the Algar-Flynn-Oyamada reaction could lead to several products including aurones 4, dihydroflavonols 5, 2-benzyl-2-hydroxydihydrobenzofuran-3-ones 6, and 2-arylbenzofuran-3-carboxylic acids... 

It is also hypothesized that formation of 2-benzyl-2-hydroxydihydrobenzofuran-3-ones 6 and 2-arylbenzofuran-3-carboxylic acids 7 are derived from an intramolecular attack of the phenoxide at the P-position. Despite the complex mechanism and multiple products, general trends have emerged through experimental results. If the chalcone lacks a 6 -methoxy group but has a hydroxyl group at the C2 or C4 positions, flavonols are favored. However, if the 6 -methoxy group is present and no hydroxyl substituent is present at C2 or C4 aurones and flavonols are formed. Others have also shown that pH and temperature influence the product distribution. 

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>. 

Havonoids are made up of a number of classes of very similar groups in which two phenyl rings are connected by a three-carbon unit [10], The open structure members are yellow in color and termed as chalcones, and simple cyclization to a furanoid structure deepens the color to the orange aurones. The most usual flavonoids are, however, the pale-yellow flavones and flavonols, or 3-hydroxyflavones, which will be treated here, and the red-blue anthocyanins, which will be treated in the next section. Figure 13.4 shows examples of these main classes and their structural relationships. The natural compounds of all classes often occur as glycosides and as methyl ethers. 

A., Dobos, Z., Hollosy, F., Eros, D., Orfi, L., Keri, G., Idei, M. Comparison of measured and calculated lipophilidty of substituted aurones and related compounds./. Chromatogr. B 2004, 801, 229-235. 

An efficient synthesis of flavones, which occurs without formation of the corresponding aurone, involves the carbonylative cyclisation of oacetoxyiodophenols and arylalkynes (Scheme 48) . A large scale, one-pot synthesis of isoflavones has been described <00SC469>. 

Fig. 2.38. Basic chemical structure of flavones, flavonols and flavonoids (chalcones and aurones).

According to Imafuku (90MI1, 90MI2), 3-cinnamoyltropolones (75) on oxidation cyclize in two different directions Depending on their aryl substitution and on the reagents used (see above for 73), they form aurone-like... 

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... 

Seabra, R.M. et al., Methoxylated aurones from Cyperus capitatus. Phytochemistry, 45, 839,... 

The key enzymes involved in the formation of the hydroxycinnamic acids (HCAs) from phenylalanine and malonyl-CoA are now discussed in detail, while later sections address the branches of the flavonoid pathway leading to anthocyanins, aurones, flavones, flavonols, PAs, and isotlavonoids. This is followed by brief reviews of the regulation of flavonoid biosynthesis and the use of flavonoid genes in plant biotechnology. To assist the reader. Figure 3.1 presents the carbon numbering for the various flavonoid types discussed. 

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