25-Flavanones

Flavanones.—Most of the recent literature on flavanones concerns their natural occurrence, but the oxidation of 4-oximinoflavan with SeOi in aqueous dioxan has provided a means of reclaiming flavanone from its oxime some flavone and a little 3,3 -biflavonyl were also produced.A method has been described for separating flavanones and flavones, using commercial high-performance t.l.c. plates. The Rf values of the components varied considerably, according to the proportion of water in the eluting liquid. An extensive study has been made of the behaviour of flavanones in the mass spectrometer. 

Deschamps-Vallet, J. B. Ilotse, M. Meyer-Dayan, and D. Molho, Tetrahedron Lett., 1979, 1109. 

Delle Monache, G. B. Marini-Bettolo, D. L. B. Costa, and I. L. D Albuquerque, Gazz. 

Of the many phytochemical and biological reports on flavanones, the following are a selection of new compounds. A flavanone of unusual oxygenation pattern has been identified in the leaves of New Zealand spinach (Tetragonia expansa). The compound is 7,8-dimethoxyflavanone. Amongst several new prenyl-flavonoids obtained from the roots of Marshallia grandiflora is 8-prenyl-5,7,4 -trihydroxyflavanone.  

New derivatives of 5,7-dihydroxy-8-prenylflavanone have been extracted from the aerial parts of Helichrysum hydrocephalum one of the gem-dimethyl groups has been replaced by CH2OH or CHO. Flemiflavanones A (199), B, and C are new flavanones isolated from the roots of Flemingia stricta, and two new racemic 8-benzylated flavanones (200) and (201) have been identified in the root bark of Uvaria chamae. The cultivated mulberry tree. Morns alba L., is a fruitful source of benzopyrans a flavanone, kuwanon F (202), has recently 

Flavanones. - A simple method of synthesizing 5,3 -dihydroxy-4 -methoxy-flavanone (165) has been described.3-Nitroflavanones have been synthesized by heating 2 -hydroxy-2-nitroacetophenone, benzaldehyde, and 

The heterocycle of flavanones also contains a ketone group, but there is no unsaturated carbon-carbon bond. The A- and B-ring can be substituted analogous to the flavones, as in naringenin (1.35). 

Detailed experimental studies have established that it is possible to choose reaction conditions giving a more selective process. For instance, the other conditions that can yield 324 as the major product from the oxidation of 323 are (i) IBD-p-TsOH in acetonitrile, and (ii) iodosoben-zene-MsOH in dichloromethane or acetonitrile. 

Although substituents present in the flavanones generally do not affect the course of these reactions, the presence of hydroxyl or other oxidizable 

2-alkyl shift provides a convenient route for the synthesis of chromones 335 (Eq. 46), tetrahydroxanthones (336, n = 1) and higher homologs (336, n = 2) (Eq. 47). Both heat and ultrasonic conditions work well to effect this alkyl shift (94SC2637). 

Removal of the bitter taste of citrus juices and citrus fruit pulps is possible by enzymatic cleavage of the sugar moiety using a mixture of a-rhamnosidase and P-glucosidase. These enzymes are isolated from microorganisms such as Phomopsis citri, Cochliobolus miyabeanus or Rhizoctonia solanii  

A number of neutral or bitter flavanone glycosides can be converted through ring opening to sweet chalcones (11) which, by additional hydrogenation, can be stabilized as sweet dihydrochal-cones (III)  

Flavanones (Formula 18.28 R = H, R = OCH3 isosacuranetin R = H, R = OH naringenin R = OH, R = OCH3 hesperitin R R = OH erio-dictyol) occur mostly as glycosides in citrus fruits (Table 18.23)  

The presence of a free OH-group in position R or R is necessary for a sweet taste. Table 18.25 shows that the dihydrochalcone of naringin corresponds to saccharin in sweetness intensity, whereas the dihydrochalcone of neohesperidin is sweeter than saccharin by a factor of 20. Conversion of naringin to highly sweet neohesperidin dihydrochalcone (VII) is possible by al- 

A sweet compound can be obtained from the neutral-tasting hesperidin of oranges by first converting hesperidin to another neutraltasting compound, hesperidin dihydrochalcone. The latter can then be hydrolyzed, by acidic or enzymatic catalysis, to remove the rhamnose residue, yielding hesperidin dihydrochalcone glucoside, which is sweet. The use of dihydrochalcones as sweeteners is discussed in Section 8.8.11. 

Malvidin-3-0-(6 -0-p-acetyl)glucoside Malvidin-3-0-(6 -0-p-coumaroyl)glucoside 

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Glycosides, particularly of phenoHc compounds, are widely distributed in plant tissues (2,10). Glycosides of anthocyanidins, flavones, flavanols, flavanones, flavanonols, stilbenes and saponins, gaUic acid derivatives, and condensed tannins are all common. 

Sesquiterpenes and flavonoids (flavones, flavanones, flavanes) are two elasses of natural substan-ees which occur frequently in plants and whieh have 15 C atoms in their framework. The nine... 

Dduzorme has reported a simple synthesis of flavanones by radical denitratiQn and dehalo-genadon of 3-chlQrQ-2,3-dihydrQ-3-nitrQ-2-aryl-4//-l-ben2Qpyran-4-ones, which are readily prepared by the reacdon of salicylaldehydes with l-chlQrQ-l-nltrQ-2-arylethenes fEq. 7.73. ... 

Bases frequently impede hydrogcnolysis, but hydrogenation of the flavanone 15 in aqueous potassium hydroxide over palladium gave 16 in high yield without reduction of an easily reduced aromatic ketone (48). 

Palladium, platinum, and Raney nickel 7,126) all have been used successfully under mild conditions for hydrogenation of the azido function. In especially sensitive molecules, subambient temperature may prove advantageous. Reduction of methyl 3, 5-dihydroxy-4 -methoxy-7-(3-azido-3-carboxypropoxy)flavanone (32) in aqueous alkali proved capricious, The major product (33) was contaminated by several other products when reagents were mixed and hydrogenated at room temperature or above, but by the... 

Treatment of dibromides 2 with sodium azide in N,N-dimethylformamide (DMF) at room temperature resulted in the formation of two products, 3-(a-azidobenzyOchromones 2a-c,g or -1-thiochromones 2d-f and the 3-arylidenechromanones la-c,g,h or -1-thiochromanones Id-f, respectively (eqn. 2). As shown by yield data given in Table 2, the substituent at position 2 plays decisive role in the product ratio. Dibromides unsubstituted at position 2 tended to give almost exclusively azides 3a-f and only a small amount of 1 was obtained. On the contrary, the reaction of flavanone derivatives 2gjh gave 3-arylideneflavanones... 

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>. 

Of this class of naturally occurring, sweet compounds, the flavanone glycosides found in citrus fruits have achieved considerable interest, owing to the systematic studies of Horowitz and Gentili " (see Fig. 20). 

In terms of taste, the flavanone glycosides can be divided into two groups. The first group consists of compounds that are bitter. These are glycosides... 

The conversion of the bitter flavanone glycosides into those of the corresponding chalcones by alkali-catalyzed fission of the pyrone ring, and of the dihydrochalcone glycosides by hydrogenation thereof (see Scheme 1)... 

It thus appears that the taste of the dihydrochalcones is not solely controlled by the sugar moiety, and subsequent studies 220 confirmed this. The flavanone 78 and the non-glycosidic dihydrochalcone 81 are intensely sweet. Furthermore, replacement of the bulky glycosyl residue by carboxyalkyl " (82) or sulfoalkyl (83) substituents did not significantly... 

For example, the rigid, planar flavone 84 is completely tasteless. Phyllodul-cin (77) and the flavanones 79 and 80, on the other hand, can exist in a conformational equilibrium between a planar form (with the B-ring in the quasiequatorial disposition) and a bent form (with the B-ring assuming a quasiaxial orientation). Fig. 23,i presents the best binding to the receptor site. 

In addition to the bitter acids and essential oils, the flowers of hops offer a rich array of polyphenolic compounds, primarily chalcones and their accompanying flavanones, many of which are prenylated derivatives (Stevens et al., 1997,1999a, b). The most prominent flavonoid in all plants studied was xanthohumol [342] (3 -prenyl-6 -0-methylchalconaringenin chalconaringenin is 2, 4, 6, 4-tetrahydroxychalcone) (see Fig. 4.11 for structures 342-346). Several additional chalcones—variously adorned with 0-methyl and/or C-prenyl functions—were also encountered, along with their respective flavanones. Three new compounds were described in the Stevens et al. 

The chemical formulae for a variety of plant phenols are given in Fig. 16.2, including examples of simpler phenols, such as cinnamic acid derivative, and of tocopherols, flavonoids, flavonoid glycosides and anthocyanidins. The flavonoids include the following subclasses flavanones (taxifolin), flavones (luteolin), flavonols (quercetin) and flavanols (catechin/epicatechin). The... 

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