Pyrylium salts benzene ring

The addition of ammonia or primary and secondary amines to pyrylium salts is usually accompanied by a rapid ring opening without detection of the expected 2//-pyran intermediates. The exceptional behavior of the salt 160a was observed in its reaction with Af./V-dimethylaniline, affording 85% of 4H-pyran 210, whereas analogous addition products from toluene and methoxy-benzene were too unstable to be isolated.214... 

Pyrylium salts, especially the 2,4,6-triaryl substituted, react with a wide variety of nucleophiles some of which lead to the formation of rings other than benzene or pyridine. Sodium sulphide, for instance, attacks the pyrylium ring at C-2 to produce the blue-coloured dienone (78) which then cyclizes on treatment with add to the thiopyrylium salt (79) (56HCA207). 

Ammonia and amines open pyran-2-one rings and the acyclic products may cyclize again on acidification to pyridones or a benzene ring, a reaction reminiscent of those of pyrylium salts (Section 2.23.2.3). Under mild conditions, unsaturated amino acids such as (275) are formed but at higher temperatures and longer reaction times, a pyridone (276) or benzamide (277) is formed. The probable course of benzamide formation is as shown in Scheme 14 (74JOU852). 

Benzene derivatives can also be obtained from pyrylium salts 43 which are substituted by R-CH2 in the 2- or 6-position. This is effected by C-2 addition of hydroxide ions, leading to a base-catalysed intramolecular aldol condensation. The reaction produces highly substituted phenols 44, which incorporate the R-CH2 carbon in the benzene ring ... 

Benzene ring form pyrylium salts Nitro compds. 

Benzene ring from pyrylium salts by intramolecular Wittig reaction... 

Scheme 37. Conversion of pyrylium and thiopyrylium salts to substituted benzene ring...

Russian workers have worked out another approach to benzo-[bjselenophens starting from selenophen derivatives. Acylation of (514) gave (515), which upon treatment with perchloric acid gave the pyrylium salts (516). Upon treatment with secondary amines, (516) gave amino-substituted benzo[b]selenophens, e.g. (517). From the pyrylium salt (518), only (519) was obtained. On treatment of (518) with aqueous hydroxide, (520) was obtained. Other benzo[b]selenophens hydroxylated in the benzene ring have been prepared similarly. Through the condensation of... 

The substitution of a CH unit in benzene by 0+ (the oxonia group) gives rise to the pyrylium cation (3). Since this ring still possesses 6 ir-electrons, it may be expected to exhibit aromatic properties. As the oxygen is primarily tricovalent, the pyrylium ring may be formally regarded as a cyclic oxonium ion. However its enhanced stability relative to aliphatic and alicyclic oxonium salts is doubtless due to its aromatic nature. 

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