Pyrid-4-one

Types i and ii are the most important. An essential difference between them is that in type i both tautomers can be aromatic [e.g., pyrid-2-one (16) and 2-hydroxypyridine (17) both satisfy the criteria for aromaticity and have large resonance energies], whereas in type ii at least one tautomer, that corresponding to 3, is nonaromatic [e.g., 3-hydroxyfuran (18) is aromatic, but furan-3-one (19) is not]. In... 

Molar refractivity is an additive property, and the predominant tautomer can be found by comparing the experimentally determined value with that calculated for the alternative forms. For example, Auwers used this method to demonstrate that pyrid-2-one exists as such and not as 2-hydroxypyridine. 

In 1951, Witkop et al. interpreted the infrared spectra of quinol-2-and -4-ones to favor the oxo formulation. Since then, many investigators, especially Mason, have reported that potential a- and y-hydroxy compounds show infrared absorption bands in the vN—H (3500-3360 cm ) and vC—O (1780-1550 cm ) regions of the spectrum and, hence, exist predominantly in the oxo form references to this work appear in Table I. A study of the bands which occur in the NH-stretching region of the infrared spectra of a series of substituted pyrid-2-ones and quinol-2-ones also supported an oxo formulation for these compounds. Detailed band assignments have been published for pyrid-2- and -4-one. Mason has reported that solutions of j8-hydroxy compounds in chloroform or carbon tetrachloride show... 

The foregoing conclusions are further supported by a refined X-ray analysis of pyrid-2-one, which indicated that the mobile hydrogen atom is attached to the nitrogen atom in the solid state and that individual molecules are bound into helices by N—H- -0 hydrogen bonds. An oxo structure is also indicated by the molar refractivity of pyrid-2-one. The dipole moment of 4-methoxypyridine is ca. 3.0 debyes in dioxane, whereas the values for pyrid-4-one and its 1-methyl derivative are much higher, ca. 6.0 debyes indicating the... 

Between 1951 and 1953 investigations by three English groups clearly demonstrated the preponderance of the oxo forms of pyrimidin-2- and -4-ones by comparing the ultraviolet spectra of these compounds with those of the N and 0-alkylated derivatives, The o-quinonoid form 91 (R = H) is favored by the evidence that A -methylation of the 6-methyl derivative of 89a does not cause a bathochromic shift in the ultraviolet spectrum (A -methylation of pyrid-4-one causes a bathochromic shift, but this is not observed for pyrid-2-one) The isomeric A -methyl derivatives of pyrimidin-4-ones [e.g., 91 (R = Me) and 90 (R = Me)] form similar cations (e.g., 92 and 93), and hence the equilibrium constant between... 

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. 

The possible involvement of bicyclic intermediates in the photorearrangement of hindered pyrid-4-ones to the corresponding pyrid-2-ones has been considered,163 but the mechanism for the photochemical conversion of 4,6-dimethyl-l-(l -piperidinyl)pyrid-2-one into the 3-piperidinyl isomer is less... 

Cycloadditions, although relatively rare, are not unknown in heterocyclic systems. The first documented example was the photodimerization of pyrid-2-one a reinvestigation of this reaction has established that three other isomeric [ 4 + 4] dimers are formed in low yield in addition to the originally reported trans-anti dimer (264).215 Attempts to effect analogous cycloadditions in a series of l,T-polymethylenedipyrid-2-ones were unsuccessful, [ 2 + 2] and [ 4 + 2] additions being preferred.216 Thus,... 

Pyrid-2- and -4-one are protonated on the oxygen atom, rather than at nitrogen, in view of the mesomeric stabilization this affords, e.g. (104) (76AHC(S1)84). The anion of pyrid-2-one carries the negative charge mainly on the oxygen atom <66JCS(B)996). 

Alkylation of the silver salt of pyrid-2-ones usually gives exclusive O-alkylation, whereas alkylation of the sodium or potassium salt gives predominantly TV-alkylation, e.g. Scheme 98. However, the course of such reactions is strongly dependent on conditions. Not only is the nature of the metal salt important but also the structure of the halide, the substituents on the pyridone ring and the solvent used (770PP5,70JOC2517,67JOC4040). 

Arenesulfonylation of pyrid-2-ones has been studied and both O- and A-sulfonylated products reported, e.g. Scheme 107 (72JHC215). 

Antimony pentachloride complexes of A-oxides of pyridine, quinoline and isoquinoline rearrange on heating to give the corresponding pyrid-2-one, carbostyril or isocarbostyril, e.g. Scheme 118 (81T1871). 

Diethyldiphenylcyclopentadienone reacts with 3,5-dinitrophenyl azide on heating to give a pyrid-2-one in a similar reaction with phenyl azide, also suggested to proceed via the nitrene, a bridged intermediate (684) is formed (77TL2463). A different adduct (685) was isolated from the reaction between phthalimidonitrene and tetracyclone when treated with... 

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