Pyrylium special

Since 1,5-enediones are usually obtained via pyrylium salts, syntheses of the type found in Section B, 2, a have a rather theroetical interest, save for a few special syntheses. There exist several direct syntheses of l,5-enediones, e.g., from j8-chlorovinyl ketones and j8-diketones or j8-keto esters special pathways to 1,5-enediones have also been described, namely, oxidation with lead tetraacetate or with periodic acid of cyclopentene-l,2-diols. ... 

The first step involves the formation of a pyridinium ion by reaction of a pyrylium ion with a primary amine the second step (dequaternization) has been studied more extensively than the first (amine + pyrylium). This important work on dequaternization deserves special mention because, besides the value for synthesis and understanding of steric acceleration, it sheds new light on the mechanism of aliphatic nucleophilic substitution (84CSR47). 

Thiopyrylium, selenopyrylium, and telluropyrylium salts are reviewed by G. Doddi (Rome, Italy) and G. Ercolani (Catania, Italy). Whereas the chemistry of the analogous pyrylium salts was the subject of a special supplementary volume in our series in 1982, no exhaustive previous review of the other chalcogenopyrylium salts has been available. 

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