Ring Synthesis of 2-Benzopyryliums

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 also produce high yields of single products.  

Acetylenic ketones, synthons for 1,3-keto-aldehydes, also take part regioselectively in condensations, as do chalcones (a chalcone has the form ArCH=CHC(0)Ar), though of course an oxidant must be incorporated in this latter case. Hexafluorophosphoric acid is recommended for the condensation of phloroglu-cinols and alkynyl-ketones  

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 mole equivalents of ethanol must then be 

Salicylaldehydes can be condensed, by base or acid catalysis, with ketones that have an a-methylene. When base catalysis is used, the intermediate hydroxy-chalcones can be isolated, but overall yields are often better when the whole sequence is carried out in one step, using acid. It is important to note that because this route does not rely upon an electrophilic cychsation onto the benzene ring, 1-benzopyryliums free from benzene ring (activating) substituents can be produced. 

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