Of pyrylium salts

The normal pattern of coupling constants for aromatic six-membered rings is found in the heterocyclic aza systems, except that the ortho coupling to a proton a to a heterocyclic nitrogen is reduced from 7-8 Hz to 4.5-6 Hz. The J2.3 of pyrylium salts is still lower... 

The commonly employed benzenelike oxonium formula (la) will bo used in preference to other proposed formulations (15 or 16), although all reactions of pyrylium salts involve carbonium structures (lb or Ic). 

Under the influence of acids, cw-2-ene-l,5-diones (20) or their enol forms (19) are converted by dehydration back into pyrylium salts. Accordingly, any synthetic method resulting in 1,5-enediones is, in fact, a method for the synthesis of pyrylium salts. 

As mentioned previously, pyrylium salts are dehydration products of the conjugate acids of as-2-ene-l,5-diones, and therefore, most syntheses of pyrylium salts are, in fact, syntheses of 1,5-enediones. Dilthey has devised his new pathways to pyrylium salts by analogy... 

The syntheses of pyrylium salts will be classified into three groups, according to the number of building blocks required to form the ring in a one-step process (see Scheme 1). 

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. 

A 2,4)6-trisubstituted 2H or 4/f) pyran (38, R = R = Ph) was reported to result in low yield by catalytic reduction of 2,4,6-triphenyl-pyrylium salts by oxidation or by treatment with concentrated sulfuric acid it regenerated the triphenylpyrylium cation. There was no subsequent confirmation of this reaction. The reduction of pyrylium salts with sodium borohydride affords 1,5-diones by way of 4H-pyrans and 2,4-dien-l-ones by way of 2H-pyrans. ... 

Dilthey wished to adapt, for pyrylium, syntheses which with ammonia were known to yield pyridines. Indeed, there is a marked similarity, and probably a common mechanism between certain syntheses of pyrylium salts and of p5rridines, e g., Chichihabin s and... 

In comparing his three main S5mtheses of pyrylium salts (see Sections II,B,2,f C,2,g and D, l,b) besides his less general syntheses (see Sections II,B,2,e C,2,a and D,2,a), Dilthey stated that the type in Section D, l,b is the most convenient. This is the standard method for preparing 2,4,6-triarylpyTylium salts with identical 2- and 6-substituents, in particular 2,4,0-triphenylpyrylmm. 398,389 vepage yields are SQ% in the presence of BF3-Et.20 they can be raised to 40%. The reaction can be applied to substituted... 

Tetramethylethylene behaves in the AlCIs-catalyzed diacetylation as 155 (Aik = iso-Pr)affording2,6-dimethyl-4-isopropylpyrylium. Although the olefin acylation had been investigated by many chemists beginning with Kondakov (cf. Nenitzescu and Balaban and Balaban and Nenitzescu ), the formation of pyrylium salts had escaped notice because they are water-soluble and had been discarded after hydrolysis of the reaction mixture. Only in the study of the ZnCla-catalyzed acetylation of diisobutene had a crystalline product been observed by Byrns and Doumani its reaction... 

TABLE 30. Dienes and polyenes from ring-opening of pyrylium salts... 

On treating diisobutene with acetic anhydride and anhydrous zinc chloride, A. C. Byrns and T. F. Doumani had isolated in 1943 a crystalline compound to which they had ascribed the structure of a zinc complex with a 1,3-diketone 40 the correct pyrylium chlorozincate structure was established by A. T. Balaban et al.41 in 1961, after extended investigation on the formation of pyrylium salts by alkene diacylation.42 This formation again had remained undetected for many decades during which alkenes had been acylated but only the water-insoluble monoacylation products had been investigated, whereas the water-soluble pyrylium salts went into the sink with the Lewis or Bronsted acid catalysts that had been used in the acylation. 

The present procedure is an improved modification of that described by Balaban for the corresponding perchlorate. 2,4,6-Triphenylpyrylium tetrafluoroborate has also been prepared from the corresponding tetrachloroferrate with fiuoboric acid, from acetophenone and boron trifluoride, and from acetophenone, benzaldehyde, and boron trifluoride etherate. Additional methods for the preparation of pyrylium salts have been reviewed. ... 

The chemistry and transformation of pyrylium salts have been reviewed. ... 

A general method for the synthesis of pyrylium salts is the cyclodehydration of 1,5-dicarbonylalkanes (Scheme 4.4). Acetic anhydride is commonly used as both solvent and reagent, but since the initial product is a 4/f-pyran, an oxidant such as the triphenylmethyl (trityl) cation in the form of triphenylmethyl hexachloroantimonate is added (Ph3C + [H ] -> Ph3CH). In certain cases, however, it is advantageous to isolate the pyran and to oxidize it in a separate step. 

J. M. Herbert carried out his PhD research with associate professor P.D. Woodgate and Dr. W.A. Denny at the University of Auckland, into the synthesis and reactions of perimidine derivatives. He then carried out postdoctoral research with Dr. R.J.K. Taylor at the University of East Anglia on the reactions of pyrylium salts, before moving into Industry. He is presently a team leader in the Isotope Chemistry and Metabolite Synthesis Department of sanofi-aventis in Alnwick, UK. 

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