Chromone ring isoflavones

Chromones. 3-Substituted chromones, including isoflavones (3-arylchro-niones), can be prepared in fair to excellent yields by treatment of 2-hydroxy-phcnyl alkyl (arylalkyl) ketones (I) with BF3 etherate (exothermic reaction) in DMF. Metlianesulfonyl chloride in DMF (Vilsmeier reagent) is then added at 52 and (he reaction is warmed on a steam bath (90 minutes). Chromones (2) arc obtained in 65 96% yield. The function of BF , etherate is to deactivate the hydroxylalcd aromatic ring hy complex formation and thus prevent ring lormylalion. 

Dipole moments of flavone (22) and isoflavone (64) have been calculated as a function of the angle by which the phenyl ring is rotated out of the plane of the chromone moiety (B-77MI22200). Correlation of the observed and calculated moments occurs when the ring is turned through less than 30° in the case of flavone, but through approximately 50° for the other flavonoid, and it thus appears that neither molecule is planar. 

Chromones are readily oxidized by permanganate or dichromate with opening of the pyran ring and the formation of a salicylic acid (402). Flavones and isoflavones are also degradatively oxidized for example oxidation of munetone (403) with alkaline hydrogen peroxide yields 2-methoxybenzoic acid and 4-hydroxy-2-isopropylbenzofuran-5-carboxylic (isotubaic) acid. 

Under normal reaction conditions (heating the reactants with or without a solvent in an alkaline medium) the pyranone ring of chromones, flavones and isoflavones is opened at... 

Pyran-4-ones and their monobenzo derivatives undergo ring opening when treated with aqueous alkali. This reaction has been widely used in the investigation of structure of chromones, flavones and isoflavones but xanthones are little affected. The nucleophile attacks the 1,2-bond in a rate-determining step whose rate constant varies with the electron density in the ground state at C-2. A kinetic study of the reaction with several chromones supports the mechanism shown in Scheme 25 (79ACH(101)73). 

The rearrangements of phenols which are accompanied by hydroxy group transpositions are called the Wessely-Moser reaction (equations 50 and 51). In essence, these rearrangements are recyclizations of flavonoides 114 via the ring-opened form 115 to give the novel structures 116. Compounds that can participate in these rearrangements are flavones (114, R = H, R = Aryl), flavonoles (114, R = OH, R = Aryl), isoflavones (114, R = Aryl, R = H), chromones (114, R = H, R = Alkyl), chromonoles (114, R = OH, R = Alkyl), xanthones (114, R R = benzo) as well as benzopyrylium salts (e.g. see Reference 95). 

Isoflavones result from a Suzuki coupling reaction with 3-iodochromones (Scheme 11) <05TL3707> and ring expansion of chromones by treatment with Me3SiOTf and a diazoester affords 2,3-benzoxepins via the benzopyrylium salt and cyclopropane derivative <05TL4057>. 

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