Flavanones reduction

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... 

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>. 

Reports on the antiviral activity of flavanones isolated from medicinal plants have also been found in the literature. Two new antiviral flavanone derivatives were isolated from a methanol extract of leaves of Pithecellobium clypearia (Jack.) Benth (Fabaceae), guided by antiviral assays.The structures were characterized as 7-0-galloyltricetifavan (Fig. 14) and 7,4 -di-0-galloyltricetifavan. CPE reduction assay showed that both compounds possess antiviral activity against respiratory syncytial virus, influenza A HlNl virus, CVB3 and HSV-1. Examples of other... 

The biological activity of flavonoids has attracted much interest in the part twenty years and a few compounds of this class have been shown to have AChEI effects. The flavanone naringenin (74) from Citrus junos (Rutaceae) ameliorated scopolamine-induced amnesia in mice, which may be related to an antiAChE effect, since naringenin was shown to inhibit AChE in vitro dose dependently. A recent theoretical study has shown that flavonoids and xanthones exhibit polyvalent effects such as antioxidant, amyloid reduction and cholinesterase inhibition, which made them interesting candidates for further studies. 

A series of flavan-4-ols, e.g., 108, was conveniently prepared by metal hydride reduction of the corresponding flavanone. The flavan-4-ols were converted into the 4-methoxyflavans, e.g., 109, by acid-catalyzed solvolysis in methanol. Both these classes of compounds are currently evaluated as anticancer drugs. Enantiomerically enriched cA-flavan-4-ols have been prepared by lipase-catalyzed kinetic resolution of racemic counterparts. ... 

The probutinidins (see Section 11.2) represent a second class of proanthocyanidins with flavan chain-extension units. Only five members of this class of compounds have been identified (Table 11.14). Their structures and absolute configurations were also confirmed by synthesis via reduction of the flavanone, butin, followed by acid-catalyzed condensation with the appropriate flavan-3-ol. A notable feature of the synthetic studies was the apparent preference for (4 8) bond formation reported by both groups of authors. 

Flavanones arise from flavones after reduction of the double bond in the heterocycle (position C2/C3), Fig. (7). 

Whilst the reduction of flavones to flavanones is of little practical significance, isoflavones are easily reduced to isoflavanones. However, carefully controlled conditions are necessary if further reduction to isoflavanols and even isoflavans is to be avoided (7lJCS(C)1994). 

FMCa Flavonols Flavones Flavanones 24-26-28 9131 M-F Reduction of total mortality, lung cancer, prostate cancer arthritis, asthma, coronary heart disease (161-163)... 

Horowitz, R. 1957. Detection of flavanones by reduction with sodium borohydride. J. Org. Chem. 22 1733-1735. 

Figure 4.2 Chemical structures of various flavonoidsThe flavone heterocyclic ring can be reduced or oxidized in various ways. Reduction of the double bond leads to a flavanone. Additional loss of the carbonyl oxygen yields a flavan.Jhe flavone can be hydroxylated to form a flavonol. This can be reduced to a flavanol. The flavan, flavanol, and flavanone each have a chiral center. The biological isomers are not known.

There is evidence that acyl protection of an o-phenolic group improves the yield of the corresponding methylene compound, and this method has been utilized for the reduction of flavanones to flavans. ... 

The naturally occurring flavan-4-ols have been reviewed in Ferreira et al. and Porter. The flavan-4-ols may be chemically, and presumably also biosynthetically, derived by a single reduction step from a flavanone, for example, the conversion of (25)-naringenin (72) into the 2,A-tmns- and 2,4-ar-flavan-4-ols (73) and (74), respectively (Scheme 10). A majority of the naturally occurring analogues possess (2S) absolute configuration, while the configuration at C-4 may vary. 

Perillartine is the semi-synthetic a-,s> r/-oxime of perillaldehyde, a major constituent of the volatile oil of Perilla frutescens (L.) Britton (Labiatae), and is used in Japan as a replacement for maple syrup or licorice in the sweetening of tobacco (33). Neohesperidin dihydrochalcone, prepared by the sequential alkaline hydrolysis and catalytic reduction of neohesperidin, a flavanone constituent of Citrus aurantium L. (Rutaceae), is permitted for use In chewing gum and certain beverages in Belgium and elsewhere (31). 

Chalcones (e.g., 9) are key intermediates in the formation of several major groups of flavonoids (see Fig. 11.2). In some plants, they are converted into aurones (such as 11). In other instances, chalcones undergo reduction of the exocy-clic double bond to produce dihydrochalcones (such as 12). Chalcones and flavanones (e.g., 10) exist in equilibrium in in vitro systems. However, as only flavanones with a (25)-configuration are known to occur naturally, this interconversion appears to be enzymatically controlled and does not involve racemization. Desaturation of flavanones can yield flavones (such as 4 and 5), whereas introduction of oxygen at the 3-position gives dihydroflavonols (flavanonols) (such as 13) which, in turn, are the precursors of flavonols (e.g., 6-8), anthocyanins (e.g., 1-3), and condensed tannin precursors (Ebel and H lbrock, 1982 Hahlbrock, 1981 Haslam 1974) (Figs. 11.7 and 11.8). 

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