Benzopyrones Chromones, Coumarins, and Isocoumarins

Addition to carbonyl oxygen of a proton produces a hydroxybenzopyrylium salt chromones undergo this protonation more easily than the coumarins, for example passage of hydrogen chloride through a mixture of chromone and coumarin in ether solution leads to the precipitation of only chromone hydrochloride (i.e. 4-hydroxy-1-benzopyrylium chloride). (9-Alkylation requires the more powerful alkylating agents.  

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

Non-phenolic coumarins are relatively stable to oxidative conditions. Various oxidative methods have been extensively used in the structure determination of natural flavones.  

Flavones and isoflavones (3-arylchromones) are quantitatively converted into 2,3-epoxides by exposure to dimethyl dioxirane such intermediates have obvious synthetic potential, the former oxides, for example, being quantitatively converted by acid into 3-hydroxyflavones, which are naturally occurring.  

Coumarins (and isocoumarins) are quantitatively hydrolysed to give yellow solutions of the salts of the corresponding cis cinnamic acids (coumarinic acids) which cannot be isolated since acidification brings about immediate relactonisa-tion prolonged alkali treatment leads to isomerisation and the formation of the trans acid (coumaric acid) salt. 

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