Tautomer keto-enol

The compounds 2- (16) and 4-pyridone (38) undergo chlorination with phosphoms oxychloride however, 3-pyridinol (39) is not chlotinated similarly. The product from (38) is 4-chloropyridine [626-61-9]. The 2- (16) and 4-oxo (38) isomers behave like the keto form of the keto—enol tautomers, whereas the 3-oxo (39) isomer is largely phenolic-like, and fails to be chlotinated (38). 

Nomenclature is based on the keto-enol tautomers. The trihydroxy form is variously designated cyanuric acid, j -triaziae-2,4,6-triol or 2,4,6-trihydroxy-j -triaziQe. The trioxo stmcture, or j -triaziae-2,4,6(lJT,3JT,5JT)-trione is the basis for the isocyanuric acid nomenclature. 

For 3-acetyl-5-benzylpyrrolin-2-one, the keto-enol tautomer 109 predominates according to spectral data (UV, IR, and NMR) (80HCA121) (for further work on pyrrolin-2-ones see 77JHC681 95MI1). 

Once such effects had been noted, it became necessary to interpret the observed results and to classify the solvents. The earliest attempts at this were by Stobbe, who reviewed the effects of solvents on keto-enol tautomers [4]. Since then many attempts have been used to explain solvent effects, some based on observations of chemical reactions, others on physical properties of the solvents, and yet others on spectroscopic probes. All of these have their advantages and disadvantages and no one approach can be thought of as exclusively right . This review is organized by type of measurement, and the available information is then summarized at the end. 

If one considers a TMS group to be equivalent to a proton, a recent paper describes the formation of a phosphanyl-substituted allene by a keto-enol tautomer-ization-like silyl migration [346],... 

The large temperature range is due to the distillation of a mixture of the keto-enol tautomers. 

A mechanistic study of acetophenone keto-enol tautomerism has been reported, and intramolecular and external factors determining the enol-enol equilibria in the cw-enol forms of 1,3-dicarbonyl compounds have been analysed. The effects of substituents, solvents, concentration, and temperature on the tautomerization of ethyl 3-oxobutyrate and its 2-alkyl derivatives have been studied, and the keto-enol tautomerism of mono-substituted phenylpyruvic acids has been investigated. Equilibrium constants have been measured for the keto-enol tautomers of 2-, 3- and 4-phenylacetylpyridines in aqueous solution. A procedure has been developed for the acylation of phosphoryl- and thiophosphoryl-acetonitriles under phase-transfer catalysis conditions, and the keto-enol tautomerism of the resulting phosphoryl(thiophosphoryl)-substituted acylacetonitriles has been studied. The equilibrium (388) (389) has been catalysed by acid, base and by iron(III). Whereas... 

Myrtaceae used in Australian traditional medicine), exists in a keto-enol tautomer ratio of 3 2. This compound was known previously as a synthetic product but only limited physical (melting point) and no spectroscopic data were available. The (3-hydroxychalcone constituent obtained from Millettia leucantha (26) is a further example of a retrochalcone. ... 

Hydrogen Bonding Using NMR A New Look at the 2,4-Pentanedione Keto-Enol Tautomer Experiment 54... 

Suelter90 has classified enzymes that are activated by monovalent cations into two groups. One involves the catalysis of phosphoryl-transfer reactions and the other a variety of elimination and/or hydrolytic reactions in which a keto-enol tautomer can be invoked as an intermediate. The M+ cation is then required to stabilize the enolate anion. It is still not possible to verify this hypothesis, but it seems unlikely in view of the comments above. 

X,Y=0,S,Se,Te], has been undertaken.628 The stabilities of different tautomeric forms of 4-hydroxycoumarins have been evaluated629 by MNDO calculations, and the four lowest-energy oxo-hydroxy tautomers of 5-fluorouracil have been studied630 using density functional methods. Semiempirical calculations have been carried out on the keto-enol tautomerism of triazolopyrimidines.631 A base-catalysed keto-enol tautomer-ism has been proposed632 to be responsible for the observed deuterium exchange of the hydrogens at the 3-position of diazepam when the molecule is treated with alkaline deuteriated methanol. 

An intramolecular [2+2] photocycloaddition of allyl ethers with dioxinones followed by a base-induced fragmentation leads to substituted tetrahydropyran-4-ones <1997TL5579>. A one-pot scandium triflate catalyzed diastereoselec-tive cyclization between aldehydes and (3-hydroxy dioxinones 1046 followed by alkoxide addition to the resulting bicycles 1047 leads to 3-carboxy-substituted tetrahydropyran-4-ones 1048 with high levels of diastereoselectivity as a mixture of keto/enol tautomers (Scheme 268, Table 49) <20050L1113>. 

A synthesis of 2-cyanocyclohexanone 4.45 from cyclohexanone is shown below. Formylation of cyclohexanone produces a mixture of keto/enol tautomers 4.42 and 4.43, the equilibrium lying to the side of the enol 4.42. Treatment with hydroxylamine affords isoxazole 4.44, and base-induced fragmentation of the isoxazole ring affords 4.45. Explain the regioselectivity of the isoxazole formation, and the mechanism of the fragmentation process. 

Draw keto-enol tautomers of carbonyl compounds, identify acidic hydrogens, and draw the resonance forms of enolates. 

Diisopropylamine (112.72mmol) was dissolved in 50ml of THE, cooled to —50°C, and 1.6 M butyllithium (112.72 mmol) added over 30 minutes so that the temperature remained between -55° and —45°C. The reaction mixture was warmed to —5°C, cooled to -65°C, and t-butyl acetate (112.72 mmol) added dropwise over 30 minutes. Thereafter, the product from Step 4 (28.18 mmol) dissolved in 50 ml THE was added, stirred 1 hour at -60 °C, warmed to -5°C over 45 minutes, and stirred 30 minutes at this temperature. The intermediate was hydrolyzed with 65 ml NH4CI solution and extracted twice with 250 ml EtOAc. The organic phase was washed with 10 ml IM HCl portions until the pH was acidic, then washed with brine until neutral, dried, concentrated, and an orange product isolated. The keto-enol tautomers were resolved by HPLC on a Chiracel OD column using n-hexane/ethyl alcohol, 98 2, at a flow rate of 1 ml/min, and had retention times of 21.7 minutes and 28.0 minutes, respectively. 

Despite the importance of keto/enol tautomers [57] only a small amount of work has been devoted to the study of enol radical cations in condensed phase. This is directly related to the fact that simple enols as the thermodynamically less stable tautomers [58] are usually not isolable, sinre the kinetic barrier for ketonization is rather low [59,60]. Much more is known about the chemistry of enol and keto radical cations in the gas-phase [61]. For details the reader is referred to recent comprehensive reviews [62]. The only available data on the thermodynamics of enol/keto radical cations in solution stem from a recent study [63]. Using stable dimesityl substituted enols the relative stabilities were determined by a thermochemical cycle approach. 

It is long known that -diketones, which exist as keto-enol tautomers, have a well defined electron transfer ability. In general, they exibit a first, partially chemically reversible, one-electron rednction to the corresponding enolato monoanion (minor amounts of the corresponding dimer are also formed), which in tnrn undergoes either a reduction process or an irreversible oxidation. The electrode potentials of a selection of symmetrically substituted (R = R ) S-diketones are reported in Table 7. 

Tautomers differ in the position of a double bond and a hydrogen atom. In Chapter 23 an in-depth discussion of keto-enol tautomers is presented. 

Which of the following pairs of compounds represent keto-enol tautomers ... 

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