Flavanones reactions

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... 

Dauzonne has reported a simple synthesis of flavanones by radical denitration and dehalo-genation of 3-chloro-2,3-dihydro-3-nitro-2-aryl-4T/- I-benzopyran-4-ones,92 which are readily prepared by the reaction of salicylaldehydes with l-chloro-l-nitro-2-arylethenes (Eq. 7.73).93... 

An improved version of the Knoevenagel reaction between acetophenones and aldehydes allows direct access to trarcr-2,3-disubstituted chroman-4-ones, examples of which show high anti-estrogenic activity <00T1811>. Reduction of flavanones by NaBUi leads to the 2,4-cis-flavan-4-ols from which 4-methoxyflavans can readily be obtained detailed H and 13C NMR data are provided <00T6047>. 

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. 

Utility of Cmc/iona-alkaloid derived chiral thionreas were nsed in Scheidt s gronp [82] for the enantioselective syntheses of flavanones. The qninoline-tethered thionrea catalyst 140 displayed better stereodirecting properties than the corresponding thionrea that is tethered to a chiral cyclohexadiamine (139). Under optimized reaction conditions, flavanone 142 was obtained in 92% yield and 97 3 er the best er nsing catalyst 139 was 90 10 nnder similar conditions (Scheme 32). 

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

In a reaction that establishes the flavonoid heterocyclic C-ring, chalcone isomerase (CHI) catalyzes the stereospecific isomerization of chalcones to their corresponding (2S)-flavanones, via an acid base catalysis mechanism. Almost 40 years ago, the first flavonoid enzyme to be described was CHI (in the adopted hometown of the authors of this chapter). Since then CHI has been analyzed in great detail, and surprisingly, it shows little similarity to other known protein sequences, although CHI-like sequences have recently been reported from plants and other organisms. ... 

With 6 -hydroxychalcones, such as naringenin chalcone, the isomerization reaction can readily occur nonenzymically to form racemic (2R,2S) flavanone. This occurs easily in vitro and has been reported to occur in vivo to the extent that moderate levels of anthocyanin can be formed. However, 6 -deoxychalcones are stable under physiological conditions, due to an... 

A desaturation reaction forming a double bond between C-2 and C-3 of the C-ring is involved in the formation of both flavones and flavonols, and the respective substrates involved, (25)-flavanones and (2R,3R)-DHFs, differ only in the presence or absence of the 3-hydroxyl (Figure 3.2). 

Hashim, M.F. et al.. Reaction mechanism of oxidative rearrangement of flavanone in isofiavone biosynthesis. FEBS Lett., 271, 219, 1990. 

Since the early contributions of Willstatter and Robinson, several alternative approaches following mainly two routes have been considered for synthesis of anthocyanins.One of the routes includes condensation reactions of 2-hydroxybenzaldehydes with acetophenones, while the other uses transformations of anthocyanidin-related compounds like flavonols, flavanones, and dihydroflavonols to yield flavylium salts. The urge for plausible sequences of biosynthetic significance has sometimes motivated this latter approach. In the period of this review, new synthetically approaches in the field have also predominantly been following the same general routes however, some new features have been shown in synthesis of pyranoanthocyanidins. 

Among the most intensively investigated of all the chalcone Diels Alder adducts are a group obtained solely from Morus species in which the diene component of the reaction is a dehydroprenylflavanone. The structures of several such compounds published prior to 1992 have now been revised on the basis of new spectroscopic and chemical data. Among the most important of the techniques used were two-dimensional NMR and circular dichroism spectroscopy. The revised structures listed in Table 16.5 are those of sanggenons C (210), D (211), E (212), and O (213). In these compounds, the flavanones show the common feature... 

The isolation of flavonoids from the methanol extract of G. uralensis was carried out under non-basic conditions, because some flavonoids isomerize under basic conditions, e.g. racemization of flavanones and isoflavanones, ring-open reaction of flavanones etc. Bioactive fractions were separated by some chromatographic methods and each step was monitored with anti-H. pylori activity with the paper disk method. Eighteen compounds were isolated from these bioactive fractions and... 

Fig. (1). Schematic view of some branches of phenylpropanoid metabolism. Solid arrows indicate enzymatic reactions with the respective enzyme indicated on the right. PAL, phenylalanine ammonia-lyase C4H, cinnamate 4-hydroxylase 4CL, 4-coumarate CoA ligase CHS, chalcone synthase CF1, chalcone flavavone isomerase F3H, flavanone 3-hydroxylase DFR, dihydroflavonol reductase CHR, chalcone reductase. Broken arrows indicate metabolic branches towards several classes of phenylpropanoids, or several subsequent enzymatic steps. In some cases the enzymes indicated are also involved in other reactions, not shown.

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