Pyrilium

The name pyrylium has been singularly contested by those who first studied it in the past. This name, pyrylium (which has been written pyrilium of pyryllium), was proposed by Decker and von Fellenberg ... 

This system gave results that were further improved upon by the addition of a photosensitizer (e.g., a thiopy-rilium, pyrilium, and selena-pyrilium salt) to bring the absorption of light into the visible region. 

A. Preparation.—The preparation of phosphorins from pyrilium salts and tris(hydroxymethyl)phosphine has been extended to include phosphorins with indolyl and pyrrolyl groups as substituents. ... 

The pKj, value (2.43) of the hydration constant of the cyanidin was found to be lower than the pKj, values of glycosylated and acylated cyanidins, meaning lower resistance of the anthocyanidin to hydration. The stability of nonacylated 3,5-diglucosides was lower compared to the 3-glucoside because the 5 position markedly lowered the hydration constant due to decreased electron density of the pyrilium ring that favors nucleophilic attack by water, enhancing hemiacetal formation. ... 

Since unsaturated ketones themselves are highly reactive compounds, they may react with excess of acid chloride in an overall diacylation process. The unsaturated diketones readily undergo cyclization to form pyrilium salts. In fact, this is often the best way to synthesize pyrilium salts.116... 

Cyclohexane can be acylated with acetyl chloride and AICI3 to yield l-acetyl-2-methylcyclopentene in 37% yield.127 Alkanes, such as 2-methylbutane, cyclohexane, methylcyclopentane, and methylcyclohexane, are easily acylated with a,p-unsaturated acyl chlorides in the presence of A1C13 and a hydride acceptor to afford mono- or bicyclic products after secondary transformations.128 Isoalkanes (isopentane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane) undergo diacylation and eventually form pyrilium salts under Friedel-Crafts acylation conditions with acetyl chloride or acetic anhydride.129... 

The heterocycle of anthocyanidins is a pyrilium kation. Anthocyanidins are typically not found as free aglycones, with the excepton of the following widely distributed, colored compounds Pelargonidin (orange-red 1.45), cyanidin (red 1.46), peonidin (rose-red 1.47), delphinidin (blue-violet 1.48), petunidin (blue-purple 1.49), and malvidin (purple 1.50). A convenient mnemonic is PCP-DPM. The most common anthocyanidin is cyanidin. These compounds are present in the vacuoles of colored plant tissues such as leaves or flower petals. The color of the pigment depends on... 

A six-membered aromatic ring containing sulfur or oxygen must carry a positive charge. Pyrilium (26) is just such an example. It is unattractive to electrophiles, but nucleophiles readily attack the 2-, 4-, and 6-positions. 

A related matrix effect of considerable analytical interest is the enhancement in absolute ion yield sometimes observed under conditions of high dilution in a solid matrix (22). Comparison of the SIMS spectrum of a neat pyrilium salt with that of the same salt diluted 1000-fold in NH Cl shows that the intact cation signal is observed in about three times greater abundance for the NH Cl-diluted sample. The threefold increase is observed even when the absolute amount of salt analyzed in the dilute sample is one thousand times less than that in the neat sample. An additional aspect of this experiment is the persistence of the enhanced signal. Ion bombardment yields products for one day in the NH Cl matrix, but for only about one hour in the neat sample under identical conditions. Effective desorption of ammonium chloride, which entrains analyte, is one way of accounting for these observations. 

Other workers have used pyrilium cations [137] as sensors for anions such as ATP or sulphate. Charge-transfer complexes between calixpyrroles and chlo-... 

The pyrilium cation 9.1, 2-pyrone 9.2, 4-pyrone 9.3, and their benzo-fused analogues the benzopyrilium cation 9.4, coumarin 9.5, chromone 9.6, are the parent structures of a series of six-membered ring heterocycles containing one oxygen atom. The impetus for research in this area comes from the enormous number of plant-derived natural products based on the benzopyrilium, coumarin, and chromone structures. 

A typical pyrilium salt synthesis is illustrated by the preparation of salt 9.12. The precursor to 9.12 is pyran 9.11, available by dehydration of 1,5-diketone 9.10. Note the similarity of this sequence to the Hantzch pyridine synthesis, Chapter 5. Also, the dehydrative cyclisation of a diketone to oxygen heterocycle 9.11 is reminiscent of furan synthesis, Chapter 2. 

The reaction of pyrilium salts with nucleophiles may involve electrocyclic ring-opening of the intermediate dienes, as in the formation of ketone 9.27. 

Likewise, isothiochroman derivatives can be prepared via a (thia)pyrilium salts-promoted photocycloaddition between thiobenzophenone and arylalkenes (Scheme 9.32) [53]. 

Hydrazido-complexes [WCl2(NNH2)(L)(PMe2Ph)2] (L = CO, C2H4) react with 2,4,6-trisubstituted pyrilium tetrafluoroborate salts to form (1 -pyr-idinio)imido-complexes 26 (L = CO, C2F14, R2 = R4 = R6 = Me L = CO, C2H4, R2 = R6 = COOEt, R4 = Ph) (96IC5118). 

Nitro-2,l-benzisoxazolyl-3)pyrilium perchlorates have been obtained from the corresponding oxaspiroindolines (Scheme 2.68) [448],... 

The pyrilium salt 30a was obtained from benzo-9-crown-3 in 29% yield in two steps by formylation with hexamine in the presence of CF3CO2H, followed by reaction with 2 equiv of acetophenone in the presence of POCI3 <2002JOC2065>. In the same manner, the Vilsmeier formylation of the /V-phenyl dithiazonine and the subsequent condensation reaction with 2-aminobenzenethiol resulted in substituted benzothiazole 43 in 38% yield < 1999J(P2)1273>. Benzo-9-crown-3 ether trimerizes in the presence of fed and aqueous sulfuric acid to produce tris-(9-crown-3)-triphenylene 28 in 25% yield <2001CJC195>. 

Primary amines via pyrilium salts react with 2-pyrid ie N-oxides and on decomposition give aldehydes in fair yields (equation 36). ... 

The pyrilium salts are stable aromatic cations and are responsible as metal complexes for some flower colours. 

Reaction of nitromethane with pyhlium salts. Dimroth 62) in a number of papers since 1956 63] has described an interesting reaction in the formation of aromatic nitrocompounds by reacting nitromethane with pyrilium salts (e.g. perchlorate) substituted in positions 2.4,6 in basic medium (7) with a yield varying from 25 to 607o ... 

Since many furans are not particularly sensitive to light they often form as products of photochemical reactions. A high pressure mercury arc can be used to eject nitrogen from pyridazine Ai-oxides and form cyclopropenyl ketones and their furan isomers.29 High yields of 2-acetylfuran can be achieved by irradiating the pyrilium ion in Scheme 5 (i.e., the requisite pyrone dissolved in sulfuric acid) although the conditions, mainly the acidity, have to be carefully controlled.30 Surprisingly, the intermediate cyclopen-tenone diol can be isolated with 50% sulfuric acid it is converted to the furan in a separate step. 

Pyrans, pyrones, pyrilium salts, mutual transformations of 85UK1971. 2-Pyrones, 6-substituted, synthesis and reactivity of 83MI6. Tetrahydropyrans, 2.6-disubstituted, synthesis of 83CRV379. 47/-Tetrahydropyrans, 4-functionalized, chemistry of 81AKZ728. Tetrahydropyrans, synthesis via intramolecular cyclization of unsaturated... 

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