Pyridones, oxidation

Nitro-3-picoline (MI-368) is hydrolyzed on standing to l-(3 -methyl-4 -pyridyl)-3-methyl-4-pyridone and 3-methyl-4-pyridone. 4-Nitropyridine and benzyl bromide give iV-benzyl-3,5-dibromo-4-pyridone (MI-369) in 33% yield. The quaternary salt first formed is hydrolyzed to the 4-pyridone. Oxidation of hydrogen bromide by the nitrous acid product provides the bromine and water. The yield is increased to 71%by the addition of bromine. 3,5-Dibromo-4-chloro-pyridine and benzyl bromide in base also give XII-369. Similarly, 3-methyl-4-nitropyridine and benzyl bromide give W-benzyl-3-bromo-5-methyl-4-... 

It is found in practice that for a number of compounds reacting ma the predominant species an almost horizontal plot is obtained. For compounds presumed to be nitrated via the free bases, such as 2,6-lutidine i-oxide and 3- and 5-methyl-2-pyridone, slopes of approximately unity are obtained. Since this type of plot allows for the incomplete ionisation of nitric acid, it can be used at higher acidities than plots using — ( H + logio Hjo) which break down when the condition is no longer true. 

Electron-deficient alkenes add stereospecifically to 4-hydroxy-THISs with formation of endo-cycloadducts. Only with methylvinyl-ketone considerable amounts of the exo isomer are produced (Scheme 8) (16). The adducts (6) may extrude hydrogen sulfide on heating with methoxide producing 2-pyridones. The base is unnecessary with fumaronitrile adducts. The alternative elimination of isocyanate Or sulfur may be controlled using 7 as the dipolarenOphile. The cycloaddition produces two products, 8a (R = H, R = COOMe) and 8b (R = COOMe, R =H) (Scheme 9) (17). Pyrolysis of 8b leads to extrusion of furan and isocyanate to give a thiophene. The alternative S-elimi-nation can be effected by oxidation of the adduct and subsequent pyrolysis. 

Methylpyridinium quaternary salts, such as (12), undergo oxidation in alkaline solution in the presence of potassium ferricyanide to give 2-pyridones, eg, A/-methyl-2-pyridone [694-85-9] (16). Frequendy nucleophilic attack at position 2 by excess hydroxide leads to ring opening this and synthetically useful recycli2ations have been reviewed (17). 

Treatment of pyridine N-oxide (13) with acetic anhydride leads chiedy to 2-pyridone (16) formation (eq. 2) (11). 

The N-oxide function has proved useful for the activation of the pyridine ring, directed toward both nucleophilic and electrophilic attack (see Amine oxides). However, pyridine N-oxides have not been used widely ia iadustrial practice, because reactions involving them almost iavariably produce at least some isomeric by-products, a dding to the cost of purification of the desired isomer. Frequently, attack takes place first at the O-substituent, with subsequent rearrangement iato the ring. For example, 3-picoline N-oxide [1003-73-2] (40) reacts with acetic anhydride to give a mixture of pyridone products ia equal amounts, 5-methyl-2-pyridone [1003-68-5] and 3-methyl-2-pyridone [1003-56-1] (11). 

The most demanding test of cesium carbonate as base was with 2,3-dihydroxypyridine (3-hydroxypyridone). The cesium salt was found to be fairly unstable, apparently oxidizing quite rapidly. Model reactions suggested that alkylation would occur 1,3 (N, 0) to give the substituted pyridone. Nevertheless, on the basis of UV and H-nmr analysis, the product of reaction between 2,3-dihydroxypyridine and tetraethylene glycol dibromide was assigned as the pyridocrown (23% yield, mp 77—78.5°) as shown in Eq. (3.60). 

In 1931 Ing pointed out that formula (II) and (III) do not contain methyl or potential methyl groups in j ositions 6 and 8 which they occupy in cytisoline. Further, a partially reduced quinoline ought to oxidise easily to a benzenecarboxylic acid and so far the only simple oxidation, products recorded from cytisine were ammonia, oxalic acid and isovaleric acid. Distillation of cytisine with zinc dust or soda-lime yields pyrrole and pyridine, but no quinoline. On these grounds Ing suggested that cytisine should be formulated without a quinoline nucleus, and that the reactions which indicate the presence of an aromatic nucleus in the alkaloid can be accounted for by an a-pyridone ring. This a-pyridone nucleus can... 

Katada, working in the labs of Ochiai, first described the reaction of N-oxide 3 with acetic anhydride. The resultant rearrangement produced a-pyridone 4. Shortly... 

The Boekelheide reaction has been applied to the synthesis of non-natural products with the preparation of quaterpyridines serving as an example. The sequence began with the 2,4-linked bipyridyl-N-oxide 25. Execution under the typical reaction conditions produced the expected bis-pyridone 26. Treatment with POCI3 afforded the corresponding dichloride that was submitted to a palladium-catalyzed coupling with 2-stannyl pyridine to produce the desired quaterpyridine 27. 

Marazano and co-workers have also applied the reactions of tryptamine with various Zincke salts, including 115 (Scheme 8.4.39), in the synthesis of pyridinium salts such as 116. This type of product is useful for further conversion to dihydropyridine or 2-pyridone derivatives. For example, in a different study, Zincke-derived chiral pyridinium salts could be oxidized site-selectively with potassium ferricyanide under basic conditions as a means of chiral 2-pyridone synthesis (117 —> 118, Scheme 8.4.40). 

Fluoro and 3- or 5-nitro-2-chloropyridine A-oxides may be converted to the corresponding l-benzoyloxy-2-pyridones by reaction with benzoic acid alone. 

Cyclic hydroxamic acids and V-hydroxyimides are sufficiently acidic to be (9-methylated with diazomethane, although caution is necessary because complex secondary reactions may occur. N-Hydroxyisatin (105) reacted with diazomethane in acetone to give the products of ring expansion and further methylation (131, R = H or CH3). The benzalphthalimidine system (132) could not be methylated satisfactorily with diazomethane, but the V-methoxy compound was readil3 obtained by alkylation with methyl iodide and potassium carbonate in acetone. In the pyridine series, 1-benzyl-oxy and l-allyloxy-2-pyridones were formed by thermal isomeriza-tion of the corresponding 2-alkyloxypyridine V-oxides at 100°. 

Phenylethylamme and its substituted derivatives with methoxybutenone in glacial acetic acid afford the salts of the type 236, which, when oxidized with potassium permanganate, decompose to a-pyridone, whereas in an aqueous medium the compound 237 is formed (80MI1 62AG161). 

Methoxy-2,6-dimethylpyridine 4-Hydroxy-2,6-diraethylpyridine-l-oxide anion l-Hydroxy-2,6-dimethyl-4-pyridone 4-Methoxy-2,6-dimethylpyridine-l-oxide... 

Copper, hexakis(pyridine 1-oxide)-diperchlorate structure, 1, 53 Copper, hexakis(2-pyridone)-diperchlorate structure, 1,53 Copper, tetraammine-history, 1, 2... 

The site of dihydroxylation in heterocycles depends on the nature of the heteroaromatic system (Scheme 9.31) usually, electron-rich heterocycles like thiophene are readily biooxidized but give conformationally labile products, vhich may undergo concomitant sulfoxidation [241]. Electron deficient systems are not accepted only pyridone derivatives give corresponding cis-diols [242]. Such a differentiated behavior is also observed for benzo-fused compounds biotransformation of benzo[b] thiophene gives dihydroxylation at the heterocyclic core as major product, while quinoline and other electron-poor systems are oxidized at the homoaromatic core, predominantly [243,244]. 

The cycloadducts formed from the Diels-Alder reaction of 3-amino-5-chloro-2(17/)-pyrazinones with methyl acrylate in toluene are subject to two alternative modes of ring transformation yielding either methyl 6-cyano-l,2-dihydro-2-oxo-4-pyridinecarboxylates or the corresponding 3-amino-6-cyano-l,2,5,6-tetrahydro-2-oxo-4-pyridinecarboxylates. From the latter compounds, 3-amino-2-pyridones can be generated through subsequent loss of HCN <96 JOC(61)304>. Synthesis of 3-spirocyclopropane-4-pyridone and furo[2,3-c]pyridine derivatives can be achieved by the thermal rearrangement of nitrone and nitrile oxide cycloadducts of bicyclopropylidene <96JCX (61)1665>. 

The oxidation of silyl enol ethers 111 with palladium(n) acetate is a convenient nnethod for the preparation of synthetically useful 2,6-disubstituted 2,3-dihydro-4-pyridones 112 <95TL(36)9449>. 

Analogously, the mesoionic jV-methyl thiazol-5-ones and l,3-dithiol-4-ones afforded A-methyl-4-pyridones and thiapyran-4-ones when reacting with diphenyl cyclopropenone and its thione261. Benzonitrile oxide apparently gives a 1,3-dipolar cycloaddition to the C=0 group of diphenyl cyclopropenone rationalizing the formation of triphenyl-l,3-oxazin-6-one 41626i ... 

From this perspective one realizes that most previous literature data were greatly impacted by the Knackmuss reinvestigation, the results of which are summarized in a review [73AG(E) 139]. The only correct structure of an azaquinone previously reported appears to be that of compound 4c, which could not undergo dimerization due to its substitution pattern. A more recent report claims that oxidation of 2-pyridone with dibenzoyl peroxide in chloroform gave 50% of 3-benzyloxy-2-pyridone 13 and 40% of azaquinone 1 (Scheme 3) [85IJC(B)972]. 

The reaction of l-(2-pyridyl)-3,5-dinitro-2-pyridone 14 with ethyl sodio acetoacetate or diethyl sodio acetone-dicarboxylate gave a mixture of N-(2-pyridyl)nitroacetamide 15, phenol derivatives 16, and a low yield of 2-oxo-2,5-dihydropyrido[1,2-b [ 1,2,4]triazine 4-oxide 17 (79TL1393). The mechanism of the reaction is shown in Scheme 5. 

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