Pyrylium 2.6- dimethyl

Pyrylium 2,6-Dimethyl-4-methyI-thio- -iodid E7b/2, 903 [2,6-(CH3)2 - 4-thiono - 4H-pyran + H3C-I]... 

Pyrylium perchlorate, 4-chloromethyl-2,6-dimethyl-ring opening recyclization and, 3, 660... 

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 ... 

Tetramethylethylene behaves in the AlCIs-catalyzed diacetylation as 155 (Aik = iso-Pr)affording2,6-dimethyl-4-isopropylpyrylium. Although the olefin acylation had been investigated by many chemists beginning with Kondakov (cf. Nenitzescu and Balaban and Balaban and Nenitzescu ), the formation of pyrylium salts had escaped notice because they are water-soluble and had been discarded after hydrolysis of the reaction mixture. Only in the study of the ZnCla-catalyzed acetylation of diisobutene had a crystalline product been observed by Byrns and Doumani its reaction... 

Nitropyrido[l,2-A]quinolin-6-ium-l 1-olate (6%) was isolated from a reaction mixture obtained by photoirradiation of crystalline photochromic 2-(2, 4 -dinitrobenzyl)pyridine for 10 days <2004JP0865>. Reaction of l-(3-chloropropyl)-6,7-dimethoxy-3-methylbenzo[c]pyrylium perchlorate with hydrazines and hydroxylamine in refluxing MeOH afforded 9,10-dimethoxy-6-methyl-, -4,6-dimethyl-l,2,3,4-tetrahydropyridazino[6,l-tf]isoquinoli-nium, and 9,10-dimethoxy-6-methyl-l,2,3,4-tetrahydro[l,2]oxazino[3,2- ]isoquinolinium, perchlorates, respectively <2004CHE1131>. 

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. ... 

Hydroxide ion adds to 2,4,6-TriphenylpyryIium ion in the presence of sodium acetate260,261 or in aqueous solution at pH 14 to give 2//-pyran 40, which isomerizes to the corresponding dienone at lower pH.88 The same reaction was discovered for 2,6-dimethyl-4-(/j-methoxyphenyl)pyrylium ion.2614... 

In contrast to 4//-pyrans, the 2//-pyrans have been rarely reported to be aromatized to pyrylium salts. 2,6-Dimethyl-4-methoxy-2//-pyran (323) was easily converted to the corresponding perchlorate 373 with perchloric acid.318,319 A similar oxidation was reported for 2,4,6-triphenyl-2//-pyran.217 The formation of pyrylium salts 375 and 377 from 374 and hydrogen chloride261 or from 376 and perchloric acid181 are not oxidations. 

The formation of 2,6-dimethyl-4-methoxy-l-nitrobenzene (551) from intermediate 2,6-dimethyl-4-methoxy-2//-pyran (323) and nitromethide ion319 involves aromatization to pyrylium ion 373 followed by the expected process shown in Eq. (35). 

The two features of interest in the structure of 6-(2-hydroxyprop-l-enyl)-2,4-dimethyl-pyrylium cation (179) are the enolic nature of the 6-substituent and the shortness of the C(6)—C(l ) bond. The latter is considered to indicate a significant contribution from the methylenepyran structure (180) (75CC284). 

Alkylthio groups at C-4 are amongst the best leaving groups and are useful precursors for several 4-substituted pyrylium salts, for example 2,6-dimethyl-4-piperidinopyrylium iodide (125) (55JOC448). 

The preparation of highly functionalized cyclopentadiene derivatives may by achieved by using oxalenes or their pyrylium salts (90 and 92, respectively), (see Scheme 14, where X = O). In this way, azalenes 26,51 29,8Z 88 32,51 or 3651 can be obtained by heating salts (90) with primary amines in dimethyl-formamide (Eq. 6). The reaction with sodium hydrogen sulfide gives thialenes... 

For explanation of experimental results and for correlation of charge densities with NMR data, semiempirical quantum-chemical calculations of benzo[c]pyrylium cation have been employed. Interestingly, the first calculation of 1,3-dimethyl-benzo[r]pyrylium cation by the simple linear combination of atomic orbitals/molecular orbital (LCAO/MO) method (70KGS1308) revealed a preference for the resonance from a in which the value of the charge density at C was three times as much as at C3. 

In principle, alkylation and thus formation of pyrylium salts proceed easier with thiopyrones containing a thiocarbonyl group than with those containing a carbonyl group. In the last-mentioned cases O-methylation is possible, however, as in the O-methylation of 2,6-diphenylthiopyran-4-one with dimethyl sulfate.31 The product can be made to crystallize as the perchlorate. 

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