Chromones 2- -, iodination

C-Substitution of coumarins and chromones has been observed in both rings in strongly acidic media, in which presumably it is a hydroxy-benzopyrylium cation that is attacked, substimtion takes place at C-6, for example nitration. This can be contrasted with the dimethylaminomethylation of chromone, iodin-ation of flavones or the chloromethylation of coumarin where hetero-ring substitution takes place, presumably via the non-protonated (non-complexed) heterocycle (CAUTION CH2O/HCI also produces some CICH2OCH2CI, a carcinogen). 

Numerous examples of stilbene-to-dihydrophenanthrene photocyclization incorporating oxygen and sulfur heterocycles have been reported. Oxidation to the phenanthrene is usually effected by added iodine or by oxygen. Thus, irradiation of 2,3-diphenylchromone (37) results in the formation of phenanthro[9,10 -2,3]chromone (38)29 analogous photocyclizations have... 

At a lower oxidation level, orffto-hydroxy-acyl-arenes undergo base-catalysed aldol condensations with aromatic aldehydes to give chalcones, °° which can be cyclised to 2,3-dihydro-chromones via an intramolecular Michael process the dihydro-chromones can in turn be dehydrogenated to produce chromones by a variety of methods, for example by bromination then dehydrobromination or by oxidation with the trityl cation, iodine, dimethyldioxirane or iodobenzene diacetate. ° ... 

The dehydrogenation of 2 -hydroxychalcones and flavanones to flavones, of l-(2-hydroxyaryl)alk-2-en-l-ones and chroman-4-ones to chromones and of thiochroman-4-ones to thiochromones can be accomplished using iodine in hot DMSO <97HCM223>. E-3-Styrylchromones result from the oxidative rearrangement of 5-aryl-l-(2-hydroxyphenyl)penta-2,4-dien-l-ones with thallium(lll) nitrate <97LA2065>. Dimethyldioxirane converts flavanones into flavones by way of the 2-hydroxyflavanone. This approach enables flavans to be converted to a variety of flavonoids <97TL4651>. 

Molecular iodine in the synthesis of chromone-type compounds 12COS 583. 

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