Flavanones, skeleton

Only one major compound (SF-X) was present in the estrogenically active fractions, and this was isolated using semipreparative HPLC. The principles of separation are the same as for qualitative HPLC (see Section 25.4.1.2), with the difference that higher amounts of material can be loaded onto the ODS-column (Alltech, Econosil, Cig lOp, 250x22mm). For the identification of SF-x, a combination of spectroscopic techniques was used. Electrospray ionization in the mass spectrometer (HPllOO LC/MSD, Hewlett-Packard) with positive ionization mode gave a pseudo-molecular ion with m/z=439. H-NMR, C-NMR, DEPT, HMQC, and COSY spectra were recorded on a Varian-300 (300MHz) spectrometer. Analysis of the COSY spectram showed the presence of a lavandulyl (5-methyl-2-isopropenyl-hex-4-enyl) side chain. From the HMQC spectrum and a DEPT experiment, it appeared that SF-x possesses a disubstituted flavanone skeleton. 

The flavanone skeleton is present in a wide range of synthetic or naturally occurring products exhibiting various interesting pharmacological activities [86] such as antioxidant [87], antitumoral [88], and antiproliferative [89], etc. 

As in all other flavonoids, there is structural variation in flavanones and dihydroflavonols because of variation in hydroxylation, methoxylation, methylation, prenylation, benzylation, glycosylation, etc. of suitable carbon atoms in the skeleton, i.e., C-5, C-6, C-7, and C-8 of the A-ring, C-2, C-3, C-4, C-5, and C-6 of the B-ring, and C-2 of the C-ring in both flavanones... 

FIGURE 15.1 Skeletons of a (25)-flavanone and a (27 ,37 )-dihydroflavonol, showing the numbering of the carbons and naming of the rings. 

The farinose exudate of the frond of the fern Pityrogramma calomelanos (Adiantaceae) has been the source of complex flavonoids characterized by a novel Ce-Cs-Ce-Cs-Ce skeleton (Table 15.7). This group includes four flavanones, calomelanols G (238), H (239), I (240), and J (241) (Figure 15.10). In these compounds, a molecule of p-coumaric or cinnamic acid appears to be fused with the A-ring of the flavanone. Biosynthetic pathways for these complex flavanones and related flavones, chalcones, and dihydrochalcones in P. calomelanos and other Pityrogramma species have been proposed by the authors. 

Chalcones are unsaturated and, along with dihydrochalcones, contain an open pyronic cycle and a carbon skeleton numbered in a way different from other flavonoids, Fig. (8, 9). Native chalcone glycosides tend to transform into flavanone glycosides during extraction procedures. Chalcones per se are therefore of restricted occurence in foods [35]. 

Flavonoids constitute a large class of polyphenols found in fruits and vegetables that share a common skeleton of phenylchromane. This basic structure allows a large number of substitution patterns leading to several subclasses of flavonoids, such as flavonols, flavones, flavanones, flavanols, anthocyanidins, isoflavones, dihydroflavonols, and chalcones. Among the diverse flavonoid subclasses, flavonols (especially quercetin) and flavanols (catechins) are the most abundant in our food. Flavonols are present in foods as diverse glycosides, whereas flavanols are usually found as aglycones. 

Prenylated flavonoids contain one or two isoprenyl, geranyl, dimethylallyl, and lavandulyl on the skeleton. They have limited distribution and are mostly isolated from the Moraceae family. Chi et al. isolated 19 prenylated flavonoids from six different genera of Chinese medicinal plants and evaluated inhibition activities on eicosanoid metabolisms with multiple cell line models [189]. Two 8-lavandulylated flavanones, kurarinone and sophoraflavanone G, were discovered to be dual inhibitors. They possessed the most potent COX-1 inhibitory activities with IC50 less than 1 pM comparable with that of indomethacin. Sophoraflavanone G isolated from the roots of Sophora flavescense Art (Fabaceae) also had potent 5-LOX inhibition activity with an IC50 below 0.25 pM. 

The main flavonoid skeleton derives from the stepwise condensation of three molecules of malonyl CoA with one molecule of 4-coumaroyl CoA, a reaction catalyzed by chalcone synthase (CHS) to form naringenin (2, 4,4 ,6,-tetrahydroxy) chalcone, the common intermediate in the formation of all flavonoids with 5,7-dihydroxy (flavone numbering) A-ring substitution. In some plants, however, an NADP-dependent chalcone-ketide reductase coacts with CHS to form 6 -deoxychalcone, the precursor of 5-deoxyflavonoids. The resulting chalcones undergoe a stereospecific cyclization to the corresponding (2S) flavanones, the... 

The basic structural model of flavanones is the 2-phenylbenzopiran-4-one skeleton [6], The flavanones are compounds of great interest due to the fact that they are a compulsory step in the metabolic pathway of the other flavonoids. Their metabolic precursors are the chalcones, and the flavones, the dihydroflavonols, and the isoflavones are biosynthesised from the flavanones. 

Fig. (1). Main skeletons of flavonoids studied. Note that flavones, flavonols, flavanones and chalconesare 1, 3-diphenylpropane derivates, whereas isoflavones and rotenoids are 1,2-diphenylpropane derivates.

A large number of phenolics are derived from the C15 flavonoid skeleton, which is synthesised via the chalcone synthase (CHS) catalysed condensation of p-coumaroyl-coenzyme A and three molecules of malonyl-CoA. In most plant families, the initial CHS product is a tetrahydroxychalcone, which is further converted to other flavonoid classes, such as flavones, flavonols, flavanones, flavan-3-ols, isoflavones and anthocyanins (Fig. 2). Structural diversity among the phenylpropanoids arises from a variety of modifications, including... 

---