1,5-Diketones pyrylium salts

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 ... 

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. ... 

Tri- butylpyrylium can be obtained by dehydrogenating the corresponding 1,5-diketone with triphenylmethyl fluoroborate, It was shown by Farcasiu that 1,5-diketones can also he dehydrogenated and dehydrated to pyrylium salts by triphenylmethyl hexachloroantimonate generated in situ from chlorotriphenyl-methane and antimony pentachloride. Even pentaphenylpyrylium may thus be prepared at room temperature. ... 

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 salts with a free a- or -position react in a similar way without ring fission, e.g. flavylium ions (220) add dimethylaniline and the product aromatizes to give (297) xanthylium ions (210) form adducts at the 9-position with (3-diketones, 3-keto esters and malonic esters e.g. 298). 

Dioxaborinium salts, isoelectronic with pyrylium salts, have been prepared from 1,3-diketones and boron compounds such as arylboronic acids of diarylborinic acids or esters thereof. A number of 1,3,2-dioxaborinium perchlorates (202) have been isolated (70RRC635). 

Furopyrylium salts (43, 55) have been repeatedly mentioned in the foregoing subsections. They may be prepared from acetonylfurans (42,54) and carboxylic acid anhydrides in the presence of perchloric acid. Benzofuro[3,2-6]pyrylium salts have been synthesized from /3-coumaranone and /3-diketones, e.g. equation (15) (B-78MI31704). 

The series of equilibria allow a change in the anion of a pyrylium salt to be accomplished. The readily available pyrylium perchlorates and tetrafluoroborates are converted into the pseudobases on treatment with weak bases. Reaction of these diketones with a wide variety of acids leads to the new pyrylium salt generally in high yield (Table 9) (80T679). In some... 

The reaction between acetophenone and acetic anhydride alone yields 2-methyl-4,6-diphenylpyrylium (23CB1012). It seems possible that a 1,3-diketone is formed by the acetylation of acetophenone (540R(8)59), which subsequently reacts with unchanged ketone. Alternatively, an intermediate enone derived from the condensation of two moles of acetophenone (53JA626) may be acylated to the pyrylium salt. There is supporting evidence for both reaction schemes and it is not possible to discard either alternative (Scheme 255). 

Chlorovinyl ketones are potential 1,3-diketones and provide a useful extension to the above method. In the presence of Friedel-Crafts catalysts they react with ketones to give pyrylium salts which are unsubstituted in the 4-position. The example shown in Scheme 257 illustrates the synthesis of a more complex pyrylium salt (63ZC266). 

More recently, salts of the triphenylmethyl cation Ph3C+ have been shown to be effective in converting the diones into pyrylium salts (61BSF538). The cation may be generated in situ, for example from triphenylmethyl chloride and antimony(V) chloride (69T1209). The r-butyl cation has been used for the same purpose (67T4001). Earlier workers considered that the synthesis involved dehydration of the diketone to a 4//-pyran followed by rapid oxidation to the pyrylium salt. In view of the successful application of Ph3C, the reaction... 

Although the value of this route has been improved by developments in the synthesis of 1,5-diketones, it is often easier to generate the diketone in situ. Provided that the reaction is carried out in the presence of a hydride acceptor, a direct synthesis of pyrylium salts is available from simple precursors. 

The interaction between ammonia and l-R1-3-aryl-substituted ben-zo[c]pyrylium salts 30, as well as 3,4-diaryl-substituted indeno[e l,2-]-benzo[c]pyrylium, and other substituted cations having a similar fragment is more complicated, especially in hydroxyl-containing solvents. In these cases, mixtures in different ratios are obtained that contain isoquinolines 138, 3-hydroxy-3,4-dihydroisoquinolines 137, OR-adducts 109, anhydro-bases 119, diketones 29, a-naphthylamines 140, and a-naphthols 141 (88UP2). 

At the same time, 1,3-diarylbenzo[c]pyrylium salts 156 with electron-donor groups in the aryl substituents and in the annelated benzenoid ring give only diketones 158 under the same conditions (70TH1). This fact may be explained by a low rate of heterocyclization, and as a consequence, the hydrolysis of intermediate 157 leads to 158. 

The formation of acyl-chrysenes 270 from 1-methyl-substituted ben-zo[c]pyrylium salts 266 occurs not only on heating in alkaline solutions, but also in acidic nucleophilic media (cf. Section III,C,4,b,i). However, in the latter case, together with acylchrysenes 270, their deacylated analogs 273 are formed (85KGS910). Under these conditions, the same results were obtained for conversions of diketones 29, therefore it is difficult to conclude which compound (anhydrobase 267 or diketone 29) is the intermediate in the formation of chrysenes 270 and 273 from salts 266 in acidic nucleophilic medium. It was not possible to trap or detect the dimeric pseudobase 269 under these conditions. However, the latter compound, under the described conditions or on heating in acetic acid, forms the mixture of the same products (270 and 273). 

Besides saturated 1,5-diketones, unsaturated 1,5-diketones can also, in some cases, be converted into thiopyrylium salts. The reaction of aryl substituted 2,4-dichloro-2-pentene-1,5-diones (87) with HjS and HCIO4 in a mixture of AcOH and AC2O leads to the formation of 3-chlorothiopyrylium perchlorates 88-90. It should be noted that under the same conditions l,3,5-triphenyl-2-pentene-l,5-dione is converted quantitatively into the corresponding pyrylium salt 8. The pentenediones not containing chlorine atoms in the molecule evidently do not react with H2S under the conditions of acid catalysis as a result of the fact that the cyclization rate for them significantly exceeds the addition rate of H2S (90ZOR1904). 

The oxidation of some 1,5-diketones to pyrylium salts has been reported [47] this reaction is discussed in Chapter 16. 

Synthesis of heterocycles. The 1,5-diketone (1) on reaction with trityl perchlorate undergoes dehydrogenative cyclization to the pyrylium salt (2), which is transformed by reaction with ammonium acetate into the pyridine derivative (3). ... 

Like trityl perchlorate and trityl fluoroborate (1, 1256-1258 2, 454), this salt can be used for hydride abstraction one advantage over the perchlorate salt is that it is not explosive. It has been used for the conversion of cycloheptatriene into tropylium hexachloroantimonate1 and for conversion of 1,5-diketones into pyrylium salts.2... 

---