Isomerization pyridones

The results in Table I agree with the postulated reaction mechanism. In most of cases two, isomeric pyridones 4 and 5 are formed. The structure of the N-substituent also contributes to the ratio of products, such as in the case of a series of 3-tcrt-butylpyridinium salts where the percentage of 2-pyridone decreases from 14% (N-methyl) to 0% if the N-methyl group is replaced by sterically larger and more lipophilic substituents R . Substituents like CO2H, COMe, COPh, and NOj result in the pyridone function being specifically introduced into the 6-position, so that only 5 can be obtained after Decker oxidation. Only one case (R = Me, R = CN) has been reported (7IJCS(B)131) in which traces of a 4-pyridone 6 were formed. 

In a series of papers, van der Plas and co-workers elaborated the influence of N-substituents in 230 upon isomeric pyridone forination during enzymatic... 

The photochemical and thermal transformations of 5-methyldihydro-thiazolo[3,2-a]pyridinium-8-olate (515) have been investigated. Irradiation using a Hanovia mercury lamp and Pyrex filter gives the isomeric pyridone 516 (4%). When a Rayonet reactor was used, a product which appears to be the valence tautomer 517 (6%) was isolated. Further irradiation of 517 gave... 

Crotonate intermediates (56 R2 = H) have been heated in glacial acetic acid61 or ethylene glycol49 or its monomethyl ether61 to afford products of direct cyclization (55). Isomeric pyridones (59 R1 = Me, Ph), however, were obtained using hot Dowtherm.49 61... 

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). 

Hydroxy-imidazoles, -oxazoles and -thiazoles (484 Z = NR, O, S) can isomerize to 2-azolinones (485a). These compounds all exist predominantly in the azolinone form and show many reactions similar to those of the pyridones. They are mesomeric with zwitterionic and carbonyl canonical forms e.g. 485a 485b Z = NR, O, S). 

Pyridone also catalyzes epimerization of the anomeric position of the tetramethyl ether of glucose. The mechanism involves two double proton transfers. The first leads to a ring-opened intermediate, and the second results in ring closure to the isomerized product ... 

A different type of tautomeric relationship exists between compounds of types 323 and 324. Both types of structure can be isolated, pyridones (324, Z = N—Me) and pyrones (324, Z = 0) being formed when 323 (Z = N—Me or 0) is heated with palladium on charcoal in ethylene glycol. Similar isomerizations in the quinol-4-one series have been reported."... 

The mesoionic 1,2,3-triazolium-4-olates 361a,b and l,2,3-triazolium-4-thiolate 361c did not react with benzocyclopropene (113), and the attempted reaction between l,3-diazolium-4-olate 362 and benzocyclobutadiene failed (114). In contrast to these negative results, Ichinari et al. (249) found that the isomeric l,3-diazolium-4-olates (l,3-imidazolium-4-olates) 363 and 364 react with DMAD to form pyridones and pyrroles, respectively. 

Hydroxy-2-pyridones react with diethyl iV,A -dimethylaminomethylenemalonate to give a mixture of isomeric fused pyranones <1996JHC1041>. The three-component domino reaction of /3-keto esters with acrolein and primary amines in presence of 4A molecular sieves gives either [l,6]hydronaphthyridines or aminoazabicyclo[3.3.1]nonanes, depending on the nature of the primary amine, in a one-pot sequence (Scheme 34) <2003SL2301>. 

Anomalous isomerizations have been noted during the photolytic and thermal rearrangements of 3-acyl-2-methoxy-3//-azepines (2 R -acyl, R2 = OMe) and 3-acyl-3H- azepin-2-ones (69T5217). Irradiation in methanol solution produces mixtures of 3-azabicyclo[4.1.0]hepta-2,4-dienes (28 R1==acyl and H, R2 = OMe, R3 = H) (or -4-ene-2-ones) and 3-phenacylpyridines (or pyridones), albeit in poor yields. Detailed, but tentative, arguments involving azanorcaradiene and/or diradical intermediates are presented to explain the formation of these unusual products. 

The powerful 7V,7V-diethylcarbamate DMG serves admirably for the synthesis of substituted oxygenated pyridines. Thus, metalation of all isomeric pyridyl O-carbamates 317 with sec-BuLi/TMEDA followed by quench with numerous electrophiles affords diversely substituted products 318 in good to excellent yields (Scheme 96) (85JOC5436). Base-induced hydrolysis provides access to pyridones and hydroxypyridines. 

Hydrogenation of isoxazole 189 over Raney nickel afforded the 7-amine 191a, probably via the intermediate 190. In addition, reduction of the isomeric isoxazole 192 effected formation of the pyridone 191b.159... 

The photo-isomerization of certain pyridines to Dewar pyridines is described in Section 3.2.1.2.2, and the formation of bridged ring 6H-1,2-dihydro-3-pyridones in Section 3.2.1.10.4. 

The photoisomer of 4-methyl-2-pyridone has been used to synthesize functionalized /3-lactams (81CC880). Ozonolysis followed by reduction of (698) yielded a 1 1 mixture of (699) and (700). Monotosylation and cyclization of these products gave rise to an isomeric mixture of bicyclics (701 Scheme 164). 

Hydroxylation of 111 gave mostly 3-hydroxypyridine (120) and lesser amounts of the isomeric tautomers 2- (121) and 4-pyridone (122).175 Because the oxidation was done under strongly acidic conditions, this appears to be an... 

A sequence involving electrocyclic ring opening, E-Z isomerization, and ring closure (see Scheme 65) has been invoked319 to account for the thermal rearrangement of 2-oxopyran-5-carbaldehyde derivatives of the type (289) to 2-pyridones (290). 

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