Pyrylium reactions

There are a small number of pyrylium reactions which fall into the category of nucleophilic substitutions. 4-Pyrones react with acetic anhydride at carbonyl oxygen to produce 4-acetoxypyryliums, in situ, allowing nucleophilic substitution at C-4 the reaction of 2,6-dimethylpyrone with methyl cyanoacetate is typical. Phosphoryl chloride likewise converts 4-pyrones into 4-chloropyryliums. ... 

Because of the increased importance of inductive electron withdrawal, nucleophilic attack on uncharged azole rings generally occurs under milder conditions than those required for analogous reactions with pyridines or pyridones. Azolium rings are very easily attacked by nucleophilic reagents reactions similar to those of pyridinium and pyrylium compounds are known azolium rings open particularly readily. 

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. 

H-l-Benzopyran-4-ones — see Chromones 4H-l-Benzopyran-4-ones, 2-phenyl — see Flavones Benzopyranopyrazoles synthesis, S, 317, 341 Benzopyrano[4,3-c]pytazol-4-one synthesis, 3, 712 Benzopyrano[4,3-6]pyridine synthesis, 3, 712 Benzopyrano[4,3-ii]pyrimidine synthesis, 3, 712 Benzopyrans nomenclature, 1, 23 pyrylium salt synthesis from, 3, 873 reactions... 

Pyrylium perchlorate, 2,4,6-triphenyl-hydrolysis, 3, 741 nitration, 3, 649 reactions with alkali, 3, 652 with methoxides, 3, 652 with piperidine, 3, 655 synthesis, 3, 869... 

Pyrylium salts, 3-acetyl-2,4,6-trimethyl-crystallography, 3, 625 Pyrylium salts, 3-alkoxymethyl-synthesis, 3, 865 Pyrylium salts, alkyl-deprotonation, 2, 51 reactions, 2, 50 Pyrylium salts, 2-amino-reactions, 2, 55 Pyrylium salts, 4-amino-deprotonation, 2, 55... 

Pyrylium salts, 2,3,4,6-tetramethyl-synthesis, 3, 868 Pyrylium salts, 2,4,6-trialkyl-oxidation, 3, 650 Pyrylium salts, 2,4,6-triaryl-synthesis, 3, 870 Pyrylium salts, 2,4,6-trimethyl-deuteration, 3, 649 synthesis, 3, 861, 868, 873 Pyrylium salts, 2,4,6-trimethyl-4-styryl-reactions... 

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 vinylogous 3,5-hexadien-2-one (16) adds in a 1,4 cycloaddition with zl -dehydroquinolizidine (17) to form compound 18 (26). A similar 1,4-cycloaddition reaction takes place between pyrylium salts and the pyrrolidine or morpholine enamines of cycloalkanones (26a). 

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). 

The first chemists to prepare a monocyclic pyrylium salt were von Kostanecki and Rossbach who in 189 6 described the fluorescence in dilute aqueous solution of the reaction product obtained from l,3,5-triphenjdpontane-l,5-dione (benzylidene-diacetophenone) and sulfuric acid. However, they failed to isolate the compound which caused the fluorescence and did not suspect that it was a pyrylium salt. It was only in 1916-1917 that Dilthey recognized that this fluorescence resulted from 2,4,6-triphenylpyrylium (3). 

For the preparation of larger amounts of p3rrylium salts the method of Baeyer and Villiger was unsuitable. Therefore, the discovery by Dilthey" of new convenient methods for preparing aryl-substituted pyrylium salts in 1916 was an important event. By the studies of Dilthey, and a little later, of Schneider, many reactions of aryl-substituted pyrylium salts were discovered. 

Recent important developments consist in the synthesis of the unsubstituted pyrylium cation by Klagcs and Trager, the preparation of pyrylocyanines by Wizinger, the development of simple syntheses for alkyl-substituted pyrylium salts by Balaban and Nenitzescu, Praill, Schroth and Fischer, Schmidt, and Dorofeenko, the discovery of a variety of reactions by Dimroth and Hafner, and the study of physical properties by Balaban. 

First, mention should be made of the metathetical reaction, replacing an anion of a pyrylium salt by another-, when the solubility of the latter salt is lower than that of the former, the conversion is easy. In the opposite case, one has to find a solvent in which the solubilities are reversed (perchlorates are less soluble in water than chloroferrates or iodides, but in concentrated hydrochloric or hydroidic acids, respectively, the situation is reversed For preparing chlorides which are usually readily soluble salts, one can treat the less soluble chloroferrates with hydrogen sulfide or hydroxylamine. Another method is to obtain the pseudo base in an organic solvent and to treat it with an anhydrous acid. 

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... 

In the hands of Collie and Tickle in 1899 this reaction gave the first crystalline pyrylium salts. The salt character of the compounds was proved by conductivity measurements the basicity of 2,6-dimethylpyrone was found to be a little higher than that of urea. Basicities of other pyrones decrease in the order 2,6-dimethyl-> 2-phenyl-6-methyl-> 2,6-diphenylpyrone, paralleling the dipole moments. These hydroxypyrylium salts hydrolyze in water to pyrones. " The formation of salts of 2,6-dimethylpyrone with organic acids was investigated by Kendall,and with mineral acids by Cook. 11 ... 

So far, this reaction finds application for the conversion of 4-pyrone into 4-substituted pyrylium salts with free a-positions, for which few... 

In principle, complex hydrides (NaBHj, LiAlH ) ought to react similarly with 4-pyrones and lead after treatment with Bronsted or Lewis acids to 4-unsubstituted pyrylium salts. This reaction has not been reported the reduction of 2-pyrones with LiAlH4 results in ring opening. " ... 

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. ... 

A convenient method leading to pyrans (38) consists in the nucleophilic addition of R anions to 2,6-disubstituted pyrjdium salts, in which the y-position (secondary carbonium ion) is more reactive than the a-positions (tertiary carbonium ions), in opposition to the reactivity of 2,4,6-trisubstituted pyrylium salts.Krohnke and Dickore as well as Dimroth and WolH showed that 2,6-diphenyl-pyrylium salts add the anions R of nitromethane, 1,3-diketones, malonodinitrile, ethyl cyanoacetate, and benzoylacetonitrile. Similar reactions are known in the flavylium series. -Nonactivated R ... 

Despite the increasing information on the photochemistry of 2,4-dienones and other unsaturated ketones, as well as on the ring-chain valence isomerism of halogen-substituted pyran and dihydi opyran systems,the data are still very scarce. The intermediate formation of pyrans valence-isomeric with unsaturated carbonyl compounds in the pyridine syntheses based on reactions of ammonia with aldehydes or ketones, advocated by various authors (cf. Section II,B,2,f), is still rather speculative. (See also Section II,B,2,e for the valence isomerism of 5-chloro-2,4-dienones with pyrylium chlorides.)... 

Such methyenepyrans afford still another possibility for obtaining new pyrylium salts, namely, electrophilic alkylation or acylation at the exocyclio methylene carbon atom. Thus, 2,6-diphenyl-4-iso-propylidene-4/I-pyran is converted into 2,6-diphenyl-4-i-butyl-pyrylium iodide on refluxing with methyl iodide (see Scheme 3). Unlike the protonation of methylenepyrans, this reaction is no longer... 

---