Pyrylium-fused Systems

Pyrylium-fused Systems.—An extensive study on the condensation of orrAo-hydroxy- and or/Ao-alkoxy-thiophen aldehydes with methyl ketones has been carried out. Upon treatment with a hydrochloric acid-perchloric acid mixture, ring-closure to pyrylium salts occurred. Thus (638) gave 

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Three different fused systems have been reported in the literature thieno[3, 2 4,5]thieno[2,3-c]pyridine (f/r-thiophenes), thieno[2, 3 4,5]thieno[2,3-c]pyridine (tram- thiophenes) and thicno[3,2-g]thicno [3,2-d pyridine derivatives. So far, the parent thienothienopyridines have only been prepared via the corresponding pyrylium salts (see below) all others are benzo or extended carbocyclic derivatives, for example, thienothienoquinolines. 

In fused ring systems involving the pyran ring, protonation in the homocyclic rings is favoured by conjugation with the pyran ring to yield pyrylium salts, but alternative carbon protonation sites may arise. Systems [236] and [237] have been studied by nmr spectroscopy in trifluoroacetic acid and acetic acid with addition of 70%... 

Flavonoids are natural phenolic systems containing pyrylium and pyrone rings, and provide the most prominent examples. We have met some of these systems under antioxidants (see Box 9.2). Coumarins contain a 2-pyrone system. Note that all of these compounds are fused to a benzene ring and are strictly benzopyran or benzopyrylium systems. 

The action of mixtures of perchloric acid and an acid anhydride, or alternatively perchloric acid, a carboxylic acid, and polyphosphoric acid, on ketones, lactones, and carboxylic acids, mentioned briefly last year, has been utilized for the preparation of various fused pyrylium systems, including the bis-indeno[3,4 5,6]pyrylium salt (157), acyloxy-benzofuro[3,2-c]pyrylium systems (158), and other benzopyrylium salts. ... 

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