3-Hydroxy flavanone

Fig. 8 Biosynthesis of anthocyanins (39) via leucoanthocyanidins (flavan-3,4-diols, 37) from dihydroflavonols (3-hydroxy-flavanones, 31, 32)...

Flavan-3-ols are derived from flavanones, via 3-hydroxy-flavanones (or dihydroflavonols). Naiingenin (31) gives rise to 5,7-dihydroxyproanthocyanidins, whereas liquiritigenin (32) (Fig. 12.11) appears to be the precursor for 5-deoxy-proanthocyanidins, both via the intermediacy of flavan-3-ols and the same basic pathway as in Fig. 11.16 (Heller and Forkmann, 1988 Lewis and Yamamoto, 1989 Stafford, 1989). 

The biosynthetic correlations between different flavonoid classes (flavones, flavonols, flavanonols, anthocyanidins, flavan-3-ols, etc.) have been made mainly on the basis of genetic data. In many respects, however, the problems related to the formation of these differing categories is only partially solved and the pathways to some flavonoid compounds is based solely upon good chemical analogies. Plausible schemes have been elaborated Figure 6.3) and in these Grisebach and his associates " have accorded 3-hydroxy-flavanones (dihydroflavonols) a key role. Thus for example this... 

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>. 

The stereospecific cyclization of chalcones to (2S)-flavanones is a prerequisite for the synthesis of the majority of fiavonoid subclasses derived from this branch point metabolite. This reaction is catalyzed by chalcone isomerase (CHI, CFI EC 5.5.1.6). CHI exists in two forms, one that accepts only 6 -hydroxychalcones and another that accepts both 6 -hydroxy-(naringenin chalcone) and 6 -deoxychalcones (isoliquirgentin), the latter generally found in legumes. Although 6 -hydroxychalcones will spontaneously convert to a racemic flavanone mixture, the CHI-catalyzed reaction proceeds at a rate 36 million-fold faster and is highly stereoselective for the formation of (25)-flavanones [60]. Spontaneous isomerization of 6 -deoxychalcones does not substantially occur without enzyme catalysis. 

The condensation of 2-hydroxyacetophenone with benzaldehyde yielded exclusively 2 -hydroxy-chalcone, and the cyclization to flavanone was not observed. An investigation of the species adsorbed on the catalyst (289) suggested that CS condensation on the Ba(OH)2 surface occurs via a very rigid transition state, whereby the OH group of 2-hydroxyacetophenone is bonded to the catalyst surface and placed at great distance from the carbonyl carbon atom of the aldehyde, making the cyclization of 2 -hydroxy-chalcone to flavanone difficult. Deactivation of the catalyst was not observed in the presence of moderate amounts of organic acids, such as benzoic, acrylic, or trichloroacetic acid. 

Flavanones with Hydroxy, Methoxy, and Methylenedioxy Substituents Reported from 1992 to 2003... 

Six new flavanones having one or more C-benzyl groups attached to C-6 or C-8, or both, have been reported from members of the Annonaceae, a family from which many C-benzylated flavanones and dihydrochalcones had previously been described. The new compounds (212-217) are presented in Table 15.6. Macrophyllol (212) and macrophyllol A (213, Figure 15.6) have been isolated from the roots of Uvaria macrophylla. These consist of 5-hydroxy-6,7-dimethoxyflavanone with one 2-hydroxy-5-methoxybenzyl unit attached to C-8 and C-6, respectively. The roots of Xylopia africana yielded isouvarinol (214, Figure 15.6), in which one 2-hydroxybenzyl unit is attached to C-6 and two units to C-8. Its... 

Anam, E.M., 2""-FIydroxy-3" -benzyluvarinol, 2"" -hydroxy-5""-benzylisouvarinol-A and 2" -hydroxy-5"-benzylisouvarinol-B three novel tetra-C-benzylated flavanones from the root extract of Xylopia africana (Benth.) Oliver (Annonaceae), Indian J. Chem., 33B, 1009, 1994. 

Flavanone aglycones bearing hydroxy, methoxy, and methylenedioxy substituents only... 

Out of a series of 42 compounds isolated and characterized in our investigation on B. papyrifera [41], comprising benzofurans, coumarins, and various types of flavonoids (biphenylpropanes, chalcones, flavans, flavanones, and flavones), only certain representatives of the latter class of compounds showed potent aromatase inhibition activity (Table 2). Flavanone 39 (2S)-2, 4 -dihydroxy-2"-(l -hydroxy- 1-methylethyl)-... 

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