Chromone 3-hydroxy-, opening

However, chromones react differently, because the phenolic hydroxy group in the ring-opened intermediate is unreactive. Thus, isoxazoles (264) result from the reaction with hydroxylamine, and a pyrazole is formed with hydrazine. y-Pyrones also give pyrazoles with hydrazine. 

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. 

When a homoisoflavanone, for example 3-benzylidenechromanone (649), is heated with a base in DMF, two products are obtained. The main reaction is the migration of the exocyclic double bond to form 3-(3 -hydroxy-4 -methoxybenzyl)chromone (650) a skeletal rearrangement through a ring opening reaction accounts for the formation of the other minor product, 3 -hydroxy-4 -methoxy-3-methylflavone (651). Labelling with 14C showed... 

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). 

Reactions of chromones 230 with acetoacetamide and ethyl acetoacetate in ethanol in the presence of ammonium acetate proceed at the C-2 atom of the chro-mone system with pyrone ring-opening and subsequent cycUzation to 252. Similar reaction with p-aminocrotononitrile gave 5-hydroxy-2-methyl-5-(polylluoroalkyl)-5ff-chromeno[4,3- ]pyridine-3-carbonitriles (253a) [129]. Three-component reaction between chromones 230, dimedone, and AcONH, is accompanied by detrifluoroacetylation and leads to 254 in low yields [130] (Schane 82). 

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