Alkenes pyrylium salt formation

On treating diisobutene with acetic anhydride and anhydrous zinc chloride, A. C. Byrns and T. F. Doumani had isolated in 1943 a crystalline compound to which they had ascribed the structure of a zinc complex with a 1,3-diketone 40 the correct pyrylium chlorozincate structure was established by A. T. Balaban et al.41 in 1961, after extended investigation on the formation of pyrylium salts by alkene diacylation.42 This formation again had remained undetected for many decades during which alkenes had been acylated but only the water-insoluble monoacylation products had been investigated, whereas the water-soluble pyrylium salts went into the sink with the Lewis or Bronsted acid catalysts that had been used in the acylation. 

The acylation of alkenes gives rise to unsaturated ketones, which themselves may be further acylated under the same reaction conditions. This is particularly the case with the nonconjugated products that often are formed preferentially. The diacyl derivatives readily cyclize to form pyrylium salts when this can be accommodated, and the sequence represents one of the best strategies for the formation of pyrylium salts symmetrically substituted at the 2- and 6-positions. Friedel-Crafts acylation as a route to pyrylium salts has been reviewed, and compared with other synthetic strategies. ... 

Yields of pyrylium salts are often higher in diacylations of alkenes than in the acylation of an equilibrium mixture of unsaturated ketones. This may be attributed to the preferential formation of the nonconjugated isomer in the first acylation, whereas isomerization of the (usually) thermodynamically more stable isomer is required before an a,p-unsaturated ketone may acylate. 

The aliphatic version of the classical aromatic Friedel-Crafts acylation produces, by loss of proton, a non-conjugated enone which can then undergo a second acylation thus generating an unsaturated 1,5-diketone. Clearly, if the alkene is not symmetrical, two isomeric diketones are formed. Under the conditions of these acylations, the unsaturated diketone cyclises, loses water and forms a pyrylium salt. The formation of 2,4,6-trimethylpyrylium, best as its much more stable and non-hygroscopic carboxymethanesulfonate, illustrates the process. 

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