Pyrylium structure

Another interesting reaction of the pyrylium salt (396) has been reported (73TL2195). With nitrous acid in alcohol, (396) gave an intermediate (402) which on heating in acetic acid gave the diacylisoxazole (403). The structure of (402) was determined by X-ray crystallography. These ring interconversions are shown in Scheme 96. 

Selenolopyrylium salts, 4, 1034—1036 Selenolo[2,3-c]pyrylium salts synthesis, 4, 969 Selenolo[3,2-b]pyrylium salts synthesis, 4, 1035 Selenolo[3,2-c]pyrylium salts synthesis, 4, 969 Selenoloseknophenes electrophilic substitution, 4, 1057 NMR, 4, 13 synthesis, 4, 135 UV spectra, 4, 1044 Selenoloselenophenes, alkyl-synthesis, 4, 967 Selenolo[2,3-b]selenophenes ionization potentials, 4, 1046 Selenolo[3,2- bjselenophenes dipole moments, 4, 1049 ionization potentials, 4, 1046 structure, 4, 1038, 1039 Selenolo 3,4-f)]selenophenes H NMR, 4, 1042 synthesis, 4, 1067 Selenolo[3,4-c]selenophenes non-classical reactions, 4, 1062 synthesis, 4, 1076 Selenolothiophenes electrophilic substitution, 4, 1057 H NMR, 4, 1041 UV spectra, 4, 1044 Selenolo[2,3- bjthiophenes... 

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). 

Baeyer and Piccard were the first to prepare crystalline monocyclic pyrylium salts without hydroxy or alkoxy substituents, from y-pyrones and Grignard reagents in 1911. They ascribed a correct structure to these salts, although the bonds in the ring and the valency of the oxygen heteroatom remained contested topics for the next 20 years. The discussions around the formula of pyrylium... 

The pyrylocyanine obtained by Strzelecka and Simalty from 2,6-diphenyl-4-phenacylpyrylium and orthoformic ester (cf." Section II, B, 1, a) has the structure of a pseudo base. Accordingly, its protonation is accompanied by dehydration leading to a triple pyrylium cation (see Scheme 4). 

Presumably, jS-chloro ketones could also react similarly with methyl(ene) ketones. Another logical extension is the possibility of synthesizing pyrylium salts by dehydrogenative condensation of -chlorovinyl ketones with oleflns like styrene, in the presence of stannic chloride (the olefins so far tested, like isobutene, are not suitable structurally). 

A more complex cycloaddition type is observed when diphenyl cyclopropenone and its thio analogue are reacted with the pyrylium betaine 451276 and the products obtained were assigned structures 450 and 452, respectively. 

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. 

Pyrylium 3-oxides 54 (Scheme 25) should be considered as heteroaromatic six-membered mesomeric betaines. A diketo-oxepine 44 (Scheme 26) has two equivalent identically polarized limiting structures... 

The pyran-2-one ring system 174 (Scheme 68) is a potentially aromatic species, due to the contribution of the pyrylium-2-olate structure 174b, but facile cleavage of the ring by nucleophiles makes it most likely a lactone rather than an aromatic system. 

The pyrylium cation is isoelectronic with pyridine it has the same number of electrons and, therefore, we also have aromaticity. Oxygen is normally divalent and carries two lone pairs. If we insert oxygen into the benzene ring structure, then it follows that, by having one electron in a p orbital contributing to the aromatic sextet, there is a lone pair in an sp orbital,... 

Dimerization of pyridinium-3-olates (86) is also induced photochemically. In accord with theory the structure of the photodimers differs from that of the thermal dimers. A-Phenylpyridinium-3-olate (101) upon irradiation gives dimer 103 together with the bicyclic valence tautomer 102 (Scheme 4). This valence tautomerism (101 - 102) is analogous to that of pyrylium-3-... 

The amount of a given isomer in reaction mixtures is affected by substitution patterns of the starting pyrylium ions and by the structure of a reagent, in accordance with quantum chemical interpretations using various LCAO-MO methods.201-2028 Almost all additions to 2,6-disubstituted pyrylium ions occur at position 4 to give 4//-pyrans. Unsubstituted or 2,4,6-trisubstituted substrates may be attacked, on the other hand, at a or y positions providing 2H- and/or 4//-pyrans. Limited information on tetrasubstituted and penta-substituted pyrylium ions leads to the conclusion that the former afford AH-pyrans, whereas the latter tend to be attacked by nucleophiles only at positions 2 or 6. 

The regioselectivity of the addition of organometallics to 2,4,6-trisub-stituted pyrylium ions 166 (R2 = R4 = H) is mainly determined by the structure of the entering group R. Thus 2,4,6-trimethylpyrylium perchlorate 177 was found to react with MeMgl, MeLi, or MeNa exclusively to give 2,2,4,6-tetramethyl-2Z/-pyran (176),212,220 237 238,246 whereas i-PrMgX, /-PrLi,... 

The two features of interest in the structure of 6-(2-hydroxyprop-l-enyl)-2,4-dimethyl-pyrylium cation (179) are the enolic nature of the 6-substituent and the shortness of the C(6)—C(l ) bond. The latter is considered to indicate a significant contribution from the methylenepyran structure (180) (75CC284). 

The synthesis of pyrylium salts, discovered in a classical study by Dilthey (20JPR(l0l)l77>, involves the facile acid-catalyzed cyclization of a 1,5-dicarbonyl compound. The value of this route stems from the different structural features which are acceptable in the five-carbon unit and from the variety of methods available for their synthesis. The reaction is the reverse of the synthetically useful ring opening of pyrylium salts by nucleophiles. 

Dorofeenko and his coworkers have extended this route to the synthesis of a range of cyclic analogues of pyrylium salts. Thus, cyclohexenylacetophenone affords the reduced isobenzopyrylium salt (635) on treatment with acetic anhydride (67MI22402) and 2-(indol-2-yl)cyclohexanone yields the indolo[2,3-rf]pyrylium salt (636) in the same way (69ZOB716). The yields in these reactions are much improved, a feature which may be attributable to the conformational preference for the structure shown for the enone rather than a conjugated arrangement of double bonds. 

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