3- Pyrylium oxides

For 3-pyrylium oxides, the Science of Synthesis (Houben-Weyl) series has dedicated a separate chapter [83], Substituted pyrylium-3-oxides are zwitterionic compounds and participate in intra- or intermolecular [3 + 2]-cycloadditions with alkenic double bonds, or dimerize by [3 + 3] cycloadditions. 

Ullman and Henderson have studied the photoisomerization of 2,3-diphenylindenone oxide, 46, to the isomeric pyrylium oxide, 47.471... 

Mono-enolisation of a 1,5-diketone, then the formation of a cyclic hemiacetal, and its dehydration, produces 4//-pyrans, which require only hydride abstraction to arrive at the pyrylium oxidation level. The diketones are often prepared in situ by the reaction of an aldehyde with two moles of a ketone (compare Hantzsch synthesis, 8.14.1.2) or of a ketone with a previously prepared conjugated ketone - a chalcone in the case of aromatic ketones/aldehydes. It is the excess chalcone that serves as the hydride acceptor in this approach. 

Diazoalkanes add to the carbon-carbon double bonds of 2,3-diphenylthiirene 1-oxide and 1,1-dioxide. The adducts lose SO or SO2 to give pyrazoles and related compounds (Scheme 103) (80CB1632). Mesoionic oxazolones (75CLH53), 4-methyl-5-phenyl-l,2-dithiolene-3-thione (80JOU395) and pyrylium betaines (72JOC3838) react similarly via intermediate adducts (Scheme 104). Enamines (Scheme 96) and ynamines add to the double bond of 2,3-diarylthiirene 1,1-dioxides to give acyclic and cyclic sulfones by a thermal. 

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... 

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. ... 

Since 1,5-enediones are usually obtained via pyrylium salts, syntheses of the type found in Section B, 2, a have a rather theroetical interest, save for a few special syntheses. There exist several direct syntheses of l,5-enediones, e.g., from j8-chlorovinyl ketones and j8-diketones or j8-keto esters special pathways to 1,5-enediones have also been described, namely, oxidation with lead tetraacetate or with periodic acid of cyclopentene-l,2-diols. ... 

An interesting application of this reaction was the use of macro-molecular anhydrides, namely, styrene-maleic anhydride or vinyl acetate-maleic anhydride copolymers in the presence of perchloric acid as catalyst, these copolymers acylate mesityl oxide or d rpnone to macromolecular pyrylium salts which, with aryl substituents, are fluorescent.No crystalline products could be obtained from succinic anhydride because of the solubility and ease of decarboxylation. 

Besides acetophenone, this reaction was also applied to p-chloro- andp-methoxyacetophenone, and even to an aliphatic ketone, acetone (although the yield was stated to be only half as large as that obtained from mesityl oxide, i.e., less than 30%, Dorofeenko and co-workers reported a 45% yield of 2,4,6-trimethylpyrylium perchlorate from acetone, acetic anhydride, and perchloric acid), and is the standard method for preparing pyrylium salts with identical substituents in positions 2 and 4. The acylating agent may be an anhydride in the presence of anhydrous or hydrated ferric chloride, or of boron fluoride, or the acid chloride with ferric chloride.Schneider and co-workers ... 

Formation of a 6-hydroxydihydropyran-3-one by the oxidative rearrangement of a furan followed by its conversion to a pyrylium ylide forms part of a synthesis of the taxane skeleton <96T14081>. 

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... 

Although cyclic azoalkanes are well known as biradical precursors [159] they have been used as 1,2- and 1,3-radical cation precursors only recently [160-164]. Apart from the rearrangement products bicyclopentane 161 and cyclopentene 163, the PET-oxidation of bicyclic azoalkane 158 yields mostly unsaturated spirocyclic products [165]. Common sensitizers are triphenyl-pyrylium tetrafluoroborate and 9,10-dicyanoanthracene with biphenyl as a cosensitizer. The ethers 164 and 165 represent trapping products of the proposed 1,2-radical cation 162. Comparison of the PET chemistry of the azoalkane 158 and the corresponding bicyclopentane 161 additionally supports the notion that the non-rearranged diazenyl radical cation 159 is involved (Scheme 31). 

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. 

Methylcarbazole reacted with 2,6-dimethyl-4-pyrone in the presence of phosphorus oxychloride generating the pyrylium salt 170. Xanthydrol and thiaxanthydrol condensed with carbazole and 9-methylcarbazole under acid catalysis and sulfuric acid oxidation to give the colored salts 171 (X = O or S) the process has been used for the colorimetric determination of carbazole. ... 

Method A From X -phosphorins by oxidation with H2O2. Method B From 2-hydro-phosphinic-90 with diazomethane. Method C From l.l-dialkoxy-X -phosphorins with BBr3. Method D From pyrylium sMts with phenyl phosphine... 

Photoinduced electron transfer (PET) has attracted considerable interest and has been intensively studied as a fundamental step in mechanistic and synthetic organic photochemistry and appears to be involved in key biological processes. Cyclore version of oxetans by PET is important for the photoenzymatic repair of the photoproducts of the DNA dipyrimidine sites by photolyase244. The oxidative version of this reaction has been achieved using cyanoaromatics, chloranil, or (thia)pyrylium salts as electron-transfer photosensitizers245 246. 

---