Flavone 2,3-epoxides

The fragrance industry has made use of small molecule epoxy furans. The rose flavone epoxide 78 and rose isoflavone epoxide 79 are components of a perfume that is mild and gives refreshing feels <1996JPP8092588>. Perfumes containing these components can be used in the manufacture of cosmetics, air fresheners, and other products. 

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 first enantioselective synthesis of cis- and trans- 3-hydroxyflavanones is based on the Lewis-acid-catalysed reaction of phenylmethanethiol with chalcone epoxides <96CC2747>. Further support for the intermediacy of epoxides in the Algar-Flynn-Oyamada flavone synthesis has been provided by the isolation of epoxides in the corresponding preparation of 3-hydroxy-2-phenylquinol-4-ones <96JCS(P2)269>. 

The stereoselective epoxidation of chalcones, followed by acid-catalysed ring closure and concomitant cleavage of the epoxide ring, provides a very efficient route to chiral flavon-3-ols and, subsequently, by borohydride reduction to produce flavan-3,4-diols [13, 14], It has been shown that diastereoselective reduction of the chiral flavon-3-ols by sodium borohydride in methanol yields the trans-2,3-dihydroxy compounds, whereas borohydride reduction in dioxan produces the cis-isomers [14] the synthetic procedure confirms the cis configuration of the 2,3-hydroxy groups of naturally occurring leucodelphinidins [14]. 

Isoflavonoids are of limited distribution (Fabaceae, bean family) and are probably formed through the epoxidation and rearrangement of the enol form of a flavone (e.g., the conversion of naringenin to genistein Fig. 16). Several isoflavonoids are associated with strong estrogenic activity, and there is interest in their potential in the prevention of hormone-dependent... 

Scheme 3 presents substrates which carry both electron-donating and electron-withdrawing substituents. These systems, in comparison with the previous set of substrates, were considerably less reactive so that longer reaction times and excess oxidant were necessary for complete epoxidations. In the rosette are displayed the oxidation of p-oxo enol phosphates [13] 13, dihydrofuranone [17] 14, p-alkoxycyc-lohexenones [17] 15, alkoxymethylenecyclohexanones [17] 16, benzalphthalide [12a] 17, aurones [18] 18, flavones [19] 19, and isoflavones [18] 20. Many of these epoxides, which have become available for the first time, constitute valuable building blocks for natural product chemistry. 

Flavones and isoflavones (3-aryl-chromones) are quantitatively converted into 2,3-epoxides by exposure to dimethyl dioxirane flavone oxides are quantitatively converted by acid into 3-hydroxy-flavones, which are naturally occurring. ... 

X-Ray analysis of (-)-6-bromocryptostrobin (127 R Br, R2 Me) has confirmed the differentiation between crystostrobin (127 Rl=H, R2=Me) and strobopinin (127 R1=Me, R2=H). As a consequence of this work, the structures of several other natural products (lawin-al, unonal, etc) have had to be revised.135 The reaction of nucleophiles with 3-methanesulphonyloxyflavanones gives a mixture of two or more products which arise by elimination (to give flavones), substitution (to produce 3-substituted flavanones), ring contraction (to aurones) and additon-intramolecular substitution (to form 3,4-epoxides).136... 

The flavones themselves are almost colorless. Yellow colors in plant tissues are usually due to carotenoid pigments, especially the xantho-phylls and their epoxides, a group of pigments beyond the scope of the... 

Flavones are built by oxidation. The enzyme catalyzing the reaction needs molecular oxygen. It is not yet elucidated whether a chalcone epoxide is an intermediate. 

Dihydroflavonols [as (5.75)] are important intermediates in the biosynthesis of other flavonoids. This is illustrated for the biosynthesis of cyanidin 5.78) and quercitin (5.77) in Scheme 5.10 dihy-drokaempferol (5.75) was an excellent precursor and its role as an intermediate was confirmed in experiments with a cell culture [60, 61]. In the biosynthesis of taxifolin 5.76) chalcone incorporation was with retention of the C-2 proton [see (5.75)]. This excludes a flavone intermediate and formation of (5.75) can very reasonably be thought to occur via the epoxide 5.81) [62]. 

Adam W, Rao PB, Degen HG, Levai A, Patonay T, Saha-Moller CR. Asymmetric Weitz-Scheffer epoxidation of iso-flavones with hydroperoxides mediated by optically active phase-transfer catalysts. J. Org. Chem. 2002 67(l) 259-264. 

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