Pyiylium salts

However, radical intermediates cannot be definitely excluded, at least not in the reaction of pyiylium salts in pyridine. Steuber showed that such pyrylium salts as 2.4.6-triphenyl-pyrylium-or 2.4.6-tri-tert-butyl-pyrylium-tetrafluoroborate can be reduced to stable pyryl radicals 32 by pyridine this reduction proceeds particularly smoothly if traces of copper powder are added. 

Chroman-4-ones have been converted into [l]benzopyrano[4,3-ft][l]benzopyrans (26) via either the 3-benzylidene compound or the pyiylium salt <97F13>. 

Pyrylium salts are prone to nucleophilic attack. Reaction of pyiylium salts 339 with ammonia or anunonium salts undergo ring-opening/ring-closing reaction sequences to afford the corresponding pyridine derivatives 340. This reaction was also called the Baeyer pyridine synthesis. ... 

Pyrans. Grigoard reagents react with pyiylium salts to give either 2ff-pyrans (248), 4/r-pyrans (249), or a mixture of (248) and (249), or, depending... 

In the presence of 70% perchloric acid at 100 °C, the acetophmiones Ar COMe (Ar = C Hs or CeFj) react with the benzaldehydes Ar CHO (Ar = C Hs or C(Fg) during 10 min to give the chalcones (72), but reaction during 2 h yields the pyiylium perchlorates (74 Ar = Ar ), which may also be obtained from the chalcones and perdiloric add or, in better yield, from the chalcones and the appropriate acetophenone Ar COMe (Ar = CfHg or C Fs) in the presence of perchloric acid. A mechanism for these condensations has been outlined (Scheme 19), and the pyrylium salts yield triaryl-pyridines (73) on treatment with ammonium carbonate in acetic acid. ... 

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