Pyryliums from 1,5-dicarbonyl compound

Dicarbonyl compounds pyrylium salt synthesis from, 3, 861... 

The synthesis of pyrylium salts, discovered in a classical study by Dilthey (20JPR(l0l)l77>, involves the facile acid-catalyzed cyclization of a 1,5-dicarbonyl compound. The value of this route stems from the different structural features which are acceptable in the five-carbon unit and from the variety of methods available for their synthesis. The reaction is the reverse of the synthetically useful ring opening of pyrylium salts by nucleophiles. 

The retrosynthesis of pyrylium ions (Fig. 6.2) is based on the ring-opening reaction by hydroxide ions (see p 224) and leads to 1,5-dicarbonyl compounds 20-22. From these starting materials the synthesis of pyrylium ions can occur by cyclocondensation, their oxidation level is brought about by removal of a hydride ion or of a suitable leaving group ... 

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