Synthesis of Benzopyryliums, Chromones, Coumarins and Isocoumarins

There are three important ways of pntting together 1-benzopyryUnms, conmarins and chromones all begin with phenols. The isomeric 2-benzopyryUnm and isoconmarin nnclei reqnire the construction of an ortho-carboxy- or ortfto-formyl-arylacetaldehyde (homophthalaldehyde). 

Subject to the restrictions set out below, phenols react with 1,3-dicarbonyl compounds to produce 1-benzopyryliums or coumarins, depending on the oxidation level of the 1,3-dicarbonyl component. 

The simplest reaction, that between a diketone and a phenol, works best with resorcinol, for the second hydroxyl facilitates the cyclising electrophilic attack. This synthesis can give mixtures with unsymmetrical diketones, and it is therefore well suited to the synthesis of 1-benzopyryliums with identical groups at C-2 and C-4, however diketones in which the two carbonyl groups are appreciably different in reactivity can produce high yields of single products.  

Acetylenic ketones, synthons for 1,3-keto-aldehydes, also take part regiose-lectively in condensations, as do chalcones, though of course an oxidant must be incorporated in this latter case.  

For hetero-ring-unsubstituted targets the bis-acetal of malondialdehyde can be employed in this variant a heterocyclic acetal-ether is first obtained, from which two mol equivalents of ethanol must then be eliminated.  

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