Pyrylium salts Friedel-Crafts acylation

Friedel-Crafts acylation of t-butanol (Scheme 6) followed by treatment with ammonia gives a mixture of 2,7-naphthyridines (379 R = Me, Et, Pr", Pr, Bu) and pyridines (380) (38-91% yield) <84BSF449>. Heterocycles (379) are the major products in these reactions and pyrylium salts have been proposed as reaction intermediates. Unsymmetrical naphthyridines (382) have been obtained as the major products of the reaction of alkenones (381) under similar conditions (Scheme 6) <84BSF454>. 

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. ... 

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. 

Russian workers have advocated the use of orthoformates in the synthesis of pyrylium salts and their benzologues. In the presence of acids, alkyl orthoformates are converted into dialkoxycarbocations, which are efficient C-acylating agents. Even in the absence of a Friedel-Crafts catalyst and under mild conditions, acetophenone is converted into the pent-2-ene-l,5-dione (648), which cyclizes to the 2,6-diphenylpyrylium salt (Scheme 254) (71CHE147). Yields decrease with increasing alkyl chain length in the orthoformate. There is evidence that a 3-alkoxymethylpyrylium salt is the initial product. 

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