Pyrylium ion ring opening

Pyrylium salts react with hydroxide ions in a complex series of equilibria involving the pseudo-base (207) and ring-opened forms (208) and (209). 

Pyrylium cations form pyridines with ammonia and pyridinium salts with primary amines (B-82MI 505-02). For example, 2,4,6-triphenylpyrylium cation (261 Z=0) yields 2,4,6-triphenylpyridine with ammonia, the corresponding 1-methylpyridinium salt with methylamine, and pyridine 1-arylimines with phenylhydrazine. Xanthylium ions (210), where ring opening cannot readily occur, form adducts (262) with ammonia, amines, amides, ureas, sulfonamides and imides. Similar adducts (e.g. 263) are formed by benzo[( ]pyrylium ions. 

Complex hydride reduction (NaBH4 or LiAlHJ of 1-methylquinolinium ions proceeds analogously to 1,2-dihydro compounds (e.g. 333). 1-Methyl- and 1-acylisoquinolinium ions (the latter with Bu3SnH (88CL913)) give the corresponding 1,2-dihydro compounds (334). Borohydride reduces pyrylium salts to mixtures of 2H- and 4/7-pyrans the former immediately ring opens to form the dienone (Scheme 33). 

Attack of the nucleophile at C-2 and formation of the 2-hydroxy-2/f-pyrans 2 occur first, followed by electrocyclic ring-opening, yielding the products 3 and 4. This ring-opening, which in the case of the unsubstituted pyrylium ion already occurs with H2O, is reversible, i.e. acids reconstitute the pyrylium system from 3 and 4. 

The retrosynthesis of pyrylium ions (Fig. 6.2) is based on the ring-opening reaction by hydroxide ions (see p 224) and leads to 1,5-dicarbonyl compounds 20-22. From these starting materials the synthesis of pyrylium ions can occur by cyclocondensation, their oxidation level is brought about by removal of a hydride ion or of a suitable leaving group ... 

Pyridinium ions 45 with iV-acceptor substituents also add O- and N-nucleophiles via C-2 to give 46. This is followed by ring opening, probably in an electrocyclic process, at N/C-2 resulting in the formation of 1-azatrienes 47 Zincke reaction, cf. the corresponding transformations of the pyrylium ion, see p 225) ... 

The chemistry of pyrylium and thiopyrylium ions is largely based on the sensitivity to nucleophiles. In this sense, they resemble pyridinium ions. The pyrylium ion is more reactive, because oxygen is more electronegative than nitrogen. The addition of nucleophiles such as alkyl lithiums, NaCN, NaOH, and so on, occurs readily and leads initially to pyran derivatives. Usually, these adducts undergo ring opening to dienals (Scheme 6.29), which are valuable in the synthesis of other compounds. 

Certain pyrylium salts react with primary amines to undeigo a process of ring opening and closing, which results in the replacement of oxygen by nitrogen. This can be a useful approach to pyridinium ions, as found in Scheme 6.31. 

Trisubstituted pyrylium ions 1 undergo ring-opening with aqueous alkali giving rise to the (tautomeric) enediones 3/4 ... 

This addition/ring-opening sequence - which in the case of the unsubstituted pyrylium ion (1, R = H) already occurs with H20-is reversible, that is, treatment of 3/4 with acid reconstitutes the pyrylium system. In the reaction of the 2,4,6-triphenylpyrylium ion (1, R = Ph) with methoxide, C-2-addition is accompanied by C-4-addition affording the methoxysubstituted 2H- and 4H-pyrans (5) and (6). 

Benzene derivatives, for example, 46/47, are obtained from the 2,4,6-trimethyl-pyrylium ion on ring-opening with hydroxide ions or with secondary amines and re-cydization via intramolecular aldol condensations ... 

The basis of a reusable colorimetric probe for cyanide ion in water is the opening of a polymer-bound pyrylium ring by the anion <05CC2790>. Oligomers derived from the reaction of a -phenylene-bis-4,4 -(2,6-diphenylpyrylium) salt with amines are rod-like and range in length from 2-9 nm <05JOC405>. 

The attack of the acid (154) on the readily polarizable 1,2-dithiafulvene (155) corresponds to the extremely ready addition of electrophilic reagents to the simple and vinylogous heptafulvene derivatives, which are iso-n-electronic with 155. The opening of the dithiole rings in 156 and 158 under the pressure of the carbanionoid electron pair liberated by the proton abstraction and of the free electron pair on the sulfur, as well as the elimination of elementary sulfur and the intramolecular electrophilic attack of the mercaptide ion (157) on the 5-position to form 158, are simply the typical reactions of 1,2-dithioles that have already been discussed (Section II, B, 3). The reactivity of the 3-methyl group in 154 finds many parallels in the ease of condensation of the methyl-substituted pyridinium, pyrylium, thiopyrylium, and tropylium salts, and particularly... 

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