Pyrylium compounds

Because of the increased importance of inductive electron withdrawal, nucleophilic attack on uncharged azole rings generally occurs under milder conditions than those required for analogous reactions with pyridines or pyridones. Azolium rings are very easily attacked by nucleophilic reagents reactions similar to those of pyridinium and pyrylium compounds are known azolium rings open particularly readily. 

Near infrared absorbers include cyanin compounds, pyrylium compounds, phthalocyanine compounds, and dithiol metal complexes. Antistatic agents include long chain alkyl alcohols and fatty acid esters with polyhydric alcohols. Stearyl alcohol and behenyl alcohol are the especially preferred compounds. Anti-fogging agents include sorbitan fatty acid esters and glycerin fatty acid esters. 

Chloride ions are comparatively weak nucleophiles, and do not react with pyridines. In general, there is also no interaction with pyridinium and pyrylium compounds, but xanthylium chloride is in equilibrium with an appreciable amount of (269), this being a particularly favorable case with little loss of resonance energy on adduct formation. 

Much of the NMR work on pyrylium compounds has been carried out on the perchlorate salts, but measurements on the tetrafluoroborate and hexachloroantimonates show that the nature of the anion has little effect on the chemical shifts of the ring protons. A paramagnetic anion, such as FeCLT, may cause line broadening. 

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). 

Enamines are also potential ketones and accordingly have been used as the ketonic component in this type of pyrylium synthesis (63AG(E)394). The initial adducts, which form pyrylium salts on treatment with perchloric acid, are similar to those derived from secondary amines and pyrylium salts. The significant feature of this variant is that it gives access to pyrylium compounds which are unsubstituted at the 4- and 6-positions but which carry substituents at C-2 and C-5. More conventional enamines, such as 1-piperidinocyclohexene, afford bicyclic pyrylium salts (652), whilst polycyclic salts (653) result if cyclic chlorovinyl ketones are used. 

After the 1930s there was a relative lull in work on pyrylium salts until the mid-1950s. Two independent research groups almost simultaneously observed the formation of simple pyrylium compounds under strong acid conditions. 

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