Pyrylium salt

Pyrylium Salts. - New uses for known and novel pyrylium salts continue to emerge. New routes to pyrylium salts consist of acylation of the 1,5-diene (21) in the presence of perchloric acid and the reaction of 3-chloro-2-methylprop-l-ene with aluminium chloride and perchloric acid to give the useful 4-chloromethyl-2,6-dimethylpyrylium perchlorate.  

6-trimethylpyrylium salt which is safer to store than the perchlorate is the sulphonic acid salt (22), which has been synthesized from isobutene and acetic anhydride. Pyrylium salts that carry acid or ester groups are becoming more common one of these has been synthesized in two steps, in 

The less reactive pyrylium salts added on tributylphosphine (but not tri-phenylphosphine) to give a phosphonium salt (24), which, on heating with di-isopropylethylamine, gave the bi-4/f-pyran (25) in high yield. When the tertiary amine (with or without a catalytic amount of tributylphosphine) was 

Dulenko, N. N. Alekseev, and V. M. Golyak,XWwi. Pronu-st, Sec. Reakt-Osobo Chist. Veshchestva, 1981,79 (Chem. Abstr., 1982, 96, 68 751). 

6- triphenylpyrylium salt may occur at C-2 or C-4 or at both positions. A kinetic study of this reaction, and the effect of changing the 4-substituent, has suggested that the reaction may be controlled more by distribution of electron charge than by orbital characteristics.  

Pyrylium Salts.—Acetylation of isoparaffins, such as (68), under Friedel-Crafts conditions at low temperatures produces pyrylium salts (69). The ratio of reactants and the presence or absence of chloroform as a solvent affect the yield of the mixture of isomers which is often obtained, through the intermediacy of a carbenium ion. The pyrylium salts were converted into pyridines (70) by aqueous ammonia. However, 3-methylpentane gave only 3-ethyl-2,4,6-trimethyl-pyridine. 

Pyrylium salts undergo some interesting reactions for example, addition of methoxide ion, which may occur at C-2 or C-4, according to the structure of the substrate. The rates of some of these reactions in buffers have been measured spectrophotometrically and the behaviour of the salts has been explained in electronic and structural terms. 2,6-Diphenylpyrylium perchlorate reacts with tributyl phosphite (by an Arbuzov rearrangement) to give a high yield of the phosphite (71). ° 

Applications of di- and tri-phenylpyrylium salts continue to be exploited in the synthesis of pyridines, thiocarbonates, thiocyanates, nitriles, and NN -diaryl-carbodi-imides. Conversion of pyrylium salts into 2-acyl-furans and 2-pyrones by sulphuric acid is dependent on the concentration of the acid.  

An account of domino reactions of benzopyrylium triflates with l,3-bis(silyl enol ethers) has been published 07SL1016 . 

A range of fluorescent pyrylium salts bearing 2-thienyl and 2-pyridyl substituents have been prepared using classical methods and converted into functionalised. 3-phosphinines by reaction with P(SiMe3)3 and thence into X5-phosphinines 07CEJ4548 . 

The adduct obtained from the stereo- and regiospecific [5+2] cycloaddition of the 3-oxidopyrylium ylide 27 to 3-methylenetetrahydrofuran-2-one is readily converted into the polycycle 28 which is a late intermediate in the synthesis of the polygalolides 07OL873 . 

Molecular oxygen inserts into substituted cyclopentadienes under acidic conditions producing pyrylium salts a hydroperoxide rearrangement is proposed 05JOC5768 . In a further development of the use of benzotriazole (Bt) in synthesis, P-lithiation of the vinyl ether 16 and quenching of the anion with chalcone affords an enol ether. Cyclisation with PCI5 produces 2,4,6-triarylpyrylium salts 05S245 . 

The basis of a reusable colorimetric probe for cyanide ion in water is the opening of a polymer-bound pyrylium ring by the anion 05CC2790 . Oligomers derived from the reaction of a -phenylene-bis-4,4 -(2,6-diphenylpyrylium) salt with amines are rod-like and range in length from 2-9 nm 05JOC405 . 

2/ -tetrahydropyran, the latter better known simply as tetrahydropyran (THP), are valuable laboratory chemicals 3,4-dihydro-2//-pyran is used as a reagent to protect alcohols and THP is a non-protic solvent. 

The products of nucleophilic addition are pyrans that frequently ring 

Q What sequence of steps takes place when piperidine reacts with a 2,4,6-trimethylpyrylium salt to form A -(3,5-dimelhylphenyl)-piperidine  

A Piperidine acts both as a nucleophile and as a base. First it combines with the pyrylium cation at C-2, forming an adduct which then ring opens, deprolonates and tautomerizes to an enamino ketone (Schema 4.3), This product cyclizes through an intramolecular reaction between the enamine unit and the carbonyl group, followed by dehydration to form the phenylpiperidine. 

Although the valence bond description of pyran-2-one could include a zwitterionic contribution involving the carbonyl group, thus making the ring aromatic, there is little evidence to support this. For example, the IR carbonyl frequency (1740 cm ) is typical of an unsaturated lactone, and the chemical shifts of the ring protons in the H NMR spectrum indicate that there is no ring current. 

---

The normal pattern of coupling constants for aromatic six-membered rings is found in the heterocyclic aza systems, except that the ortho coupling to a proton a to a heterocyclic nitrogen is reduced from 7-8 Hz to 4.5-6 Hz. The J2.3 of pyrylium salts is still lower... 

Finally, a novel synthetic route involves formation of the pyridine ring from a fused pyran intermediate, e.g. (139) - (140) (70CB1250, 80JOC1918, 73JCS(P1)823). If a pyrylium salt is used, a quaternary pyridopyrimidinium salt such as (141) is formed 77KGS14S4). 

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. 

Azulene, 4,b,8-trimethyl-from pyrylium salts, 3, 660 Azulenes... 

Benzofuro[3,2-c]pyrroles reduction, 4, 1079 Benzofuro[3,2-6]pyrylium salts synthesis, 4, 995 Benzofuro[2,3-c][l, 2]thiazine... 

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... 

Wolff rearrangement, 7, 193 pyrylium salt synthesis from, 3, 871... 

Naphtho[l,2-h]pyrylium salts, 5,6-dihydro-2,4-diphenyl-synthesis, 3, 869 Naphtho[2,l-h]pyrylium salts synthesis, 3, 866 Naphthoquinolines, tetrahydro-synthesis, 2, 469 N aphtho[ 1,2-7]quinolines Skraup synthesis, 2, 467... 

K-pyran synthesis from, 3, 761-762 pyridazine synthesis from, 3, 54 pyrylium salt synthesis from, 3, 872 reduced, 2, 45... 

Pyrylium salts alkyl groups reactivity, 3, 662 aromaticity, 3, 640 arylammes from, 3, 657 benzenoid compounds from, 3, 656, 658 benzisoxazol-3-yl-synthesis, 6, 124 bicyclic... 

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, 2,6-dimethoxycarbonyl-synthesis, 3, 873 Pyrylium salts, 2,6-diphenyl-synthesis, 3, 865, 866 Pyrylium salts, 3-ethoxycarbonyl-synthesis, 3, 870... 

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... 

---