Chalcones and Aurones

The two classes of yellow phenolic anthochlor pigments - the chalcones and aurones - are of restricted distribution in the plant kingdom (14, 15). Chalcones are reported from 25 families and aurones from about 10. The two classes are related in that aurones are formed from chalcones by an oxidative process and related chalcone-aurone pairs tend to be found together in the same plant source. The 

Type Structure (name) Genus, species (family) Location  

2 -hydroxy-4, 6 -dimethoxy (flavokawin B) 2, 4, 4-trihydroxy (isoliquiritigenin) Aniba riparia (Lauraceae) TW 

3-trihydroxy-4-methoxy (homobu-tein) Acacia spp. (Leguminosae) HW 

4 -tetrahydroxy (aureusidin) 6,3, 4 -trihydroxy-4-methoxy Melanorrhoea aptera (Anacardiaceae) HW 

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

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). 

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. 

Chalcones and aurones collectively are sometimes called anthochlors (Harbone, 1991). Techniques for the isolation and characterization of chalcones and aurones have been reviewed (Bohm, 1988, 1989). 

The color of many yellow flowers is due to the presence of carotenoids, although chalcones and aurones are responsible for the color in others, especially those of the family Asteraceae. Orange flowers are produced by carotenoids alone, or by pelargonidins and aurones (Harbome, (1988c). 

Methanolic extracts of Coreopsis spp.. Dahlia spp., and Helichrysum bracteatum containing chalcones and aurones have been tested on silica gel plates with ethyl acetate-formic acid-water (60 12 16) as the mobile phase. The plates are developed to 8.S cm (ca. 40 min), dried, and sprayed with NP/PEG 4000. The clear yellow zones turn to violet and red, and the red-to-orange fluorescent colors seen under longwave UV light confirm the presence of chalcone and aurone pigments. Ammonia vapor intensifies the colors after spraying. 

Numbering of chalcone and aurone ring systems is indicated in formulae 27 and 29. 

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