2,6-dimethyl-4-pyrone

Analogously, the reactions of ketene aminals with ketene or dimethyl ketene gave y-amino-a-pyrones and the linear acylation products, respectively 422). 

On the other hand y-pyrones or 1,3-diketones could be obtained from the reactions of ketone derived enamines with diketene 423-426). The addition of dimethyl ketene dimer to aldehyde or ketone derived enamines produced cyclohexanediones 425,426). 

The puzzling discovery of Collie and Tickle in 1899 that 2,6-dimethyl-4-pyrone (8) affords crystalline salts (9) with acids, which were the first monocyclic pj rylinm salts to be isolated, was interpreted by a formula (21) with tetracovalent oxygen.An active period of research followed. The methosulfate or methiodide of 2,6-dimethylpyrone was converted by ammonia into 4-methoxy-2,6-lutidine, therefore, the exocyclic oxygen of the pyrone must be involved in the salt formation. Thus, formula 21 was disproved and formula 22 was demonstrated for these salts. 

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 ... 

Thiopyrones and selenopyrones can be alkylated more readily than pyrones. Thus 2,6-dimethyl-4/f-pyran-4-thionc (4,6-dimethyl-4-thiopyrone) (23, Y = S) reacts rapidly with methyl iodide yielding a 4-methylmercaptopyrylium iodide (24, Y = S, R = Me, X = I). Many alkylating agents were investigated by King et al. The kinetics of the reaction between 2,6-dimethyl-4-thiopyrone and substituted phenacyl bromides was found to be described by the Hammett... 

By 0-acylation with 2-methyl-1,3-dioxolenium fluoroborate, which reacts as 0-acetyl ethylene oxide, 2,6-dimethyl-4-pyroiie is converted into 4-acetoxy-2,0-dimethylpyrylium fluoroborate (24, Y = 0, R = Ac, X=BT 4). The alleged compound with this structure which has been obtained from 8 and acetyl fluoroborate is, in fact, the BF3-complex of the pyrone. 

Hiphenylketene has been reported to react with 2,6-dimethyl-pyrone affording the diphenylmethylenepyran (28, R = R = Ph) after splitting off carbon dioxide. 

A related reaction is the acylation of yA disuhstituted benzyl ketones to isobenzopyrylium salts the unsubstituted compounds yield 4-pyrones as shown in Section II,D,3,a. Another related acylation converts unsaturated esters into 2-p3U ones, e.g., ethyl, jS-dimethyl-acrylate into 4,6-dimethyl-2-pyrone. ... 

The straightforward construction of substituted pyrone 4 proceeded as follows (see Scheme 6c). Alkylation of the monoanion of 2,4-pentanedione (8) with methyl iodide furnishes 3-methyl-2,4-pentanedione. Conversion of this substance into the corresponding dianion with sodium amide followed by selective carboxylation of the more basic site provides intermediate 7. Pyrone 4 is obtained after cyclization with l,l -carbonyldiimidazole and methylation of the resulting enol with dimethyl sulfate. 

For 2,6-dimethyl-4-pyrone and 4-thiapyrone, the kinetics were complicated by exchange in the methyl groups. Since the rates for both were relatively independent of acidity, reaction on the free base species was indicated. 

Alkoxy(carbene)iron(0) and amino(carbene)iron(0) complexes usually react with alkynes to give rj4-pyrone iron complexes and furans, respectively [54]. Nevertheless the chemoselective formation of naphthols was reported for alkoxy(carbene)iron(0) complexes with the electron-poor alkyne dimethyl... 

Enol-lactone der 2H-Pyron-Reihe werden unter Hydrogenolyse der Enol-Bindung zu ungesattigten Carbonsauren reduziert. So erhalt man z. B. aus 4,6-Dimethyl-2H-pyron mit Lithiumalanat in 47%iger Ausbeute 3 -Melhyl-hexadien- (2,4) -satire11 ... 

Dimethyl-4-pyrone forms the cage dimer (77) upon irradiation in concentrated solutions in ethanol, benzene, or acetic acid<89) ... 

Cyclic dienes also react readily with ADC compounds although in many cases the initial adducts are not isolable. 1-Substituted pyrid-2-ones give Diels-Alder adducts with ADC compounds, although 2-pyridone itself gives only the substitution product (e.g., 113).174 2-Pyrone gives a 1 2 adduct with PTAD, since the initial adduct (114) rapidly loses C02 to generate a diene which then reacts with more PTAD.175 The initial adducts of ADC compounds with cyclopentadienones,176 and 3,4-dimethyl-1-phenylphos-phole 1-sulfide (115)177 also regenerate a diene by loss of CO and PhP=S, respectively. 

A combination of an intramolecular [5+21-cycloaddition of the pyrone 110 followed by an intermolecular [4 + 2]-cycloaddition with dimethyl-butadiene leads to the fused 6,7,5-tricyclocarboxylic system 112 via 111 as described by Rodriguez et al (scheme 22).1621 The prepared skeleton is found in nature in several terpenes. The starting material for this domino reaction is obtained from maltol 109... 

Scheme 6.108 Products ofthe interception of 1,2,4,6-cyclohepta-tetraene (5) with 1,3-diphenylisobenzofuran, anthracenes, 2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dien-l-one, methyl 2-pyrone-5-carboxylate, tropones, 8,8-dicyanoheptafulvene and 1,3,5-cycloheptatrienes.

Intermediates such as 224 resulting from the nudeophilic addition of C,H-acidic compounds to allenyl ketones such as 222 do not only yield simple addition products such as 225 by proton transfer (Scheme 7.34) [259]. If the C,H-acidic compound contains at least one carbonyl group, a ring dosure is also possible to give pyran derivatives such as 226. The reaction of a similar allenyl ketone with dimethyl mal-onate, methyl acetoacetate or methyl cyanoacetate leads to a-pyrones by an analogous route however, the yields are low (20-32%) [260], The formation of oxaphos-pholenes 229 from ketones 227 and trivalent phosphorus compounds 228 can similarly be explained by nucleophilic attack at the central carbon atom of the allene followed by a second attack of the oxygen atom of the ketone at the phosphorus atom [261, 262], Treatment of the allenic ester 230 with copper(I) chloride and tributyltin hydride in N-methylpyrrolidone (NMP) affords the cephalosporin derivative 232 [263], The authors postulated a Michael addition of copper(I) hydride to the electron-... 

Kottner and Klemm115 studied the Diels-Alder polymerization of bismaleimides with 4,4/-dimethyl-6,6/-(octamethylene)di-2-pyrone. When the maleimide units were connected... 

Die von Feist bei der Einwirkung von Alkali auf 3-Brom-4.6-dimethyl-2-pyron-5-carbonsaureathylester (IV) erhaltene Saure, welche fruher als l-Methylcyclopropen-2.3-dicarbonsaure (V) formuliert wurde, ist nach neueren Untersuchungen von Ettlinger (86) die isomere l-Methylencyclopropan-trans-2.3-dicarbonsaure (VI). 

Fig. 9. Normalized solubilities of l,2-dimethyl-3-hydroxy-4-pyridinone, dmppH, of its aluminum(III) and iron(III) complexes AKdmppla and Fe(dmpp)s, and of the 3-hydroxy-4-pyronate complexes Ga(malt)3 and In(etmalt)3 in methanol-water mixtures at 298.2 K (data from Refs. (114) and (234)).

It has been known since the turn of the century that amongst oxygen-containing compounds crystalline salts with protonic acids are most readily formed by 2,6-dimethyl-4-pyrone [190] (Collie and... 

Tickle, 1899). Which of the two oxygens was the site of proton addition became clear only when Hantzsch (1919) demonstrated the close optical analogy between the salts and methiodides of dimethyl-pyrone and pyridinium salts and argued that a benzene-like ring arises in the pyroxonium salts [191]. The discussion of the structure... 

A series of Tc complexes with the monobasic ligands 2-methyl-3-oxy-4-pyronate (maltolate, Hma (78)) or l,2-dimethyl-3-oxy-pyridinonate, (Hdpp, (79)) were prepared in nonaqueous solution by chloride substitution in [TcOCU]. Stepwise substitution is observed. The substitution of one... 

Anionic [ReOBr3(L)] and neutral [ReOBr(L)2] complexes are formed by reactions of [NBu4][ReOBr4] with 2-methyl-3-oxy-4-pyrone or l,2-dimethyl-3-oxy-4-pyridinone which act as monobasic ligands. The monoligand complex can be regarded as an intermediate in the formation of the bis-chelate complex. 

Pyrone, 3-hydroxy-2-methyl Sd (roasted) Pyrrole, 1-5-dimethyl Sd (roasted) Pyrrole, 1-ethyl Sd (roasted) ... 

A similar synthesis of mechanistic interest rather than preparative value involves the thermal reaction of dimethyl 2,5-bisdiazo-3,4-diketoadipate (89, Scheme 23) with benzofuran (91)." The presumed intermediate is the pyrone cation 90 produced from the adipate 89 by the Wolff rearrangement, cyclization, and loss of nitrogen. Electrophilic substitution then affords the benzofuran 92, which can be isolated. Ring opening and cyclization of the resultant ketene 93 then affords the dibenzofuran 94 in poor (0.4%) yield. 

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. ... 

The only four- - six-membered ring interconversions of any real synthetic significance are those involving diketene. Base-catalyzed dimerization of diketene is a long-established and efficient method for the preparation of dehydroacetic acid (equation 161), while mild treatment with water in the presence of tertiary amine bases gives 2,6-dimethyl-4-pyrone (equation 162). 1,3-Dioxins are obtained from the acid-catalyzed condensation of diketene with ketones (equation 163). 

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