Pyrylium ring

According to Chemical Abstracts, the pyrylium ring is numbered as shown in formula 14. Positions 2 and 6 may also be denoted by a, positions 3 and 5 by j8, and position 4 by y, as in pyridine. 

Second, there exists a number of reactions starting from pyrylium salts and involving substitution at the pyrylium ring, or modification of existing substituents. These reactions will be described here briefly they will be discussed in more detail in the forthcoming second part of this review. 

Only one electrophilic substitution at the pyrylium ring has been described, namely the j8-deuteration of 2,4,6-trimethylpyrylium or 2,4,fi-triphenylpyrylium perchlorates on prolonged reflux in CHjCOOD possibly, however, this reaction proceeds via the pseudo base. The nitration of 2,4,6-triphenylpyrylium does not affect the pyrylium ring, but attacks the 2- and 6-phenyl groups in the meta-, and the 4-phenyl group in the am-position, as was proved by... 

Less reactive electrophilic reagents like those involved in acylation or alkylation apparently do not react with phenyl-substituted pyrylium salts the p-acylation of a phenyl group in position 3 of the pyrylium salt obtained on diacylation of allylbenzene (Section II, I), 3, a), and the p-l-butylation of phenyl groups in y-positions of pyrylium salts prepared by dehydrogenation of 1,5-diones by means of butyl cations (Section II, B, 2, f) probably occur in stages preceding the pyrylium ring closure. 

When 2,4,6-triarylpyrylium perchlorates and the 4,4 -dimethylbispyridinium dibromides 18 were heated in the presence of tiiethylamine/acetic acid in boiling ethanol, a double pyrylium ring transformation occurred to give high yields of the 4,4 -bis(2,4,6-triarylphenyl)bispyridinium salts 19 <96JHC(33)783>. 

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. 

Benzo-fusion to a pyrylium ring can give rise to two isomers, namely the 1- and 2-benzopyrylium ions (6) and (219) both of which are formally aromatic 107r- electron systems. 

Some nucleophiles open the pyrylium ring to give intermediates which are capable of spontaneous or add-induced cyclization to a new ring. The conversion of pyrylium salts into pyridines is the most thoroughly studied example and was first described in 1911 (B-64MI22300). Ammonia, ammonium acetate or carbonate react with pyrylium salts such... 

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). 

Alkyl groups RCH2, where R is a hydrogen or an alkyl or aryl group, attached to the 2- or 4-positions of pyrylium rings are more reactive than those of the corresponding pyran or pyridine. The methylene hydrogen atoms are acidic, that is they are easily abstracted with consequent formation of an anionic centre, for example in (105). The neutral molecule... 

A formal and rather remote analogy to the method previously described for salts 30 can be seen in another C2 + C3 synthesis of the Cs-chain, which cyclizes to yield the pyrylium ring of l-aminobenzo[c]pyrylium bromide 80 (78JOC3817). 

Since compounds 10 and 30 are rather complicated, the authors could not attempt to determine the normal frequencies of the molecule, but had to restrict the analysis of spectra to the determination of characteristic frequencies. Thus, six frequencies have been found that can be used to identify 2-benzopyrylium cations. Band I appears between 1650-1610 cm-, and the pyrylium ring is responsible for this vibration [8a band according to Wilson s notation (34MI1)]. The position of this band and its intensity are dependent on the nature and position of substituents in the cation, and these changes are similar to data of monocyclic pyrylium salts [82AHC(Suppl)]. 

Similar to aroxyl-benzo[b]pyrylium systems (78ZOR1643), there is no detectable delocalization of the unpaired electron on the benzo[c]pyrylium ring in 301-303 (87RRC417). The stability of the radical cation is determined in this case by the position of the aroxyl substituent in the cation, and the most stable is 301. The formation of diradical species 303 together with the monoradical cation, was indicated on oxidation of the corresponding 2-benzopyrylium salt. 

Pyrylium ions are six-membered heterocycles in which a positively charged sp2-hybridized oxygen replaces the nitrogen in pyridine. The pyrylium ring appears in many naturally occurring flower pigments. 

Figure 19.3 Schematic model of biomimetic material M3 for amine signaling. Changes in color are due to the reaction between the pyrylium ring and the primary amine. Short amines are too hydrophilic and do not enter into the hydrophobic pores, whereas long-chain amines tend to clog in the pore openings.

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