Pyrylium, 2-alkoxy

Baeyer and Piccard were the first to prepare crystalline monocyclic pyrylium salts without hydroxy or alkoxy substituents, from y-pyrones and Grignard reagents in 1911. They ascribed a correct structure to these salts, although the bonds in the ring and the valency of the oxygen heteroatom remained contested topics for the next 20 years. The discussions around the formula of pyrylium... 

It will be observed that most syntheses yield pyrylium salts in which positions 2,4, and 6 are substituted. Since according to formulas Ib-lc these positions have a partial positive charge, it can readily be understood why electron-donating substituents (hydroxy, alkoxy, alkyl, or aryl) in these positions stabilize the pyrylium salts. Only three pyrylium salts which do not have substituents in either a-position have been reported and few unsubstituted in y or in one a-position they are less stable toward hydrolysis, and in the case of perchlorates they explode more easily, than 2,4,6-trisubstituted compounds. In fact, the former are secondary, the latter tertiary carbonium ions. This fact also explains why the parent compound (1) was prepared only in 1953. 

The route offers the possibility of further simplification, since aromatic hydrocarbons are readily acylated under Friedel-Crafts conditions. Thus, the acetylation of toluene in the presence of perchloric acid yields the trisubstituted pyrylium compound (27CB716). Alkoxy-benzenes behave in a similar manner and anisole gives a good yield of 2,4-di-(4-methoxyphenyl)-6-methylpyrylium perchlorate (51JCS726). 

The presence of a functional substituent such as carbalkoxy in position 3 of benzo[c]pyrylium salts 49, or alkoxy in compound 52, or acylamino in compound 50 determines the formation of mono- or bis-condensed sys-... 

The first pyrylium salt without hydroxy or alkoxy substituents made by the protonation or alkylation of 4-pyrones was obtained in 1911 by Baeyer and Piccard fi"om 4-pyrones and Grignard reagents. Isolation of these compounds was often difficult, and their characterization was confiised by a lack of a clear understanding of their electronic structure. However, there was much interest in benzopyrenes (coumarins, flavones, and chromones) and benzopyrylium (flavylium, chromylium) salts isolated fi om plants (anthocyanins, anthocyanidines) to which they conferred a large diversity of colors. 

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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