Enol tautomer

Write a structural formula for the enol tautomer of cytosine... 

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. 

Benzisoxazole and alicyclic ketones undergo a mercury sulfate-catalyzed cycloaddition in boiling xylene. The enol tautomers (141) are apparently involved and the yields of... 

Carbanions derived from carbonyl compoimds are often referred to as etiolates. This name is derived from the enol tautomer of carbonyl compounds. The resonance-stabilized enolate anion is the conjugate base of both the keto and enol forms of carbonyl... 

The study of the chemistry of carbonyl compounds has shown that they can act as carbon nucleophiles in the presence of acid catalysts as well as bases. The nucleophilic reactivity of carbonyl compounds in acidic solution is due to the presence of the enol tautomer. Enolization in acidic solution is catalyzed by O-protonation. Subsequent deprotonation at carbon gives the enol ... 

Spectroscopic investigation of enamines conjugated with ketone, ester and nitrile groups established the prevalence of enamine rather than imine-enol tautomers in examples of secondary amines (206-212). Similar studies have been made with enamines of acylpyridines and acetophenones (213,214). 

Aldehydes and ketones ( keto forms) normally exist in equilibrium with their enol tautomers. 

If the refractivity of the pure tautomeric constituents is known, the composition of the equilibrium mixture can be determined. This method has been used to study, for example, the keto and enol tautomers of ethyl acetoacetate. So far it has not been applied to heterocyclic compounds in this series the isolation of the pure... 

Any doubt about the existence of individual tautomers is now long past some tautomers can be crystallized separately (desmotropy), and others can be observed simultaneously in the same crystal (Section V,D,2) in summary, tautomers are not intrinsically different from isomers. Maybe it is worth mentioning that even two identical tautomers can differ. This is the case for the two intramolecular hydrogen-bonded (IMHB) enol tautomers of acetylacetone and for many NH-azoles they correspond to a doublewell profile for the proton transfer with both wells having the same energy (autotrope). 

As reported earlier (76AHCS1, p. 225), 2-acetyl-3-hydroxyfuran exists as the enol tautomer. 3-Hydroxybenzofurans with an ester group at the 2-position also exist mainly in their hydroxy form 30 with a small amount of the keto isomer 31 (48CB203 75AK72). 

In cases where R = aryl, hetaryl, R = H (and others) a-keto-y-butyrolactones seem to exist entirely as enol tautomers [81JCS(P1)1969, 81TL1591 82G1 85CC340 86TL2015 92JOC4558],... 

NMR spectra of thiophene 77 indicate a small amount (8.4% in CS2) of the second keto tautomer 78. The enol tautomer 76 eould not be detected [78JCS(P1)292]. 

Formyl- and 2-acetyl-3-hydroxyselenophenes have been formulated as enol tautomers (72JHC355). 

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). 

The formation of a stable imino-enole tautomer 358 is due to the conjugation of the C=C and C=N double bonds with the aromatic ring and hydroxyl group. The enaminoketone tautomer 357 is present in a negligible amount. 

Hydroxythianaphthene has been isolated in tw o forms,and it has been suggested that these may be the individual keto and enol tautomers. A reinvestigation of this system using modem techniques would be welcome. 

Replacement of a benzene ring by its isostere, thiophene, is one of the more venerable practices in medicinal chemistry. Application of this stratagem to the NSAID piroxicam, gives tenoxicam, 136, a drug with substantially the same activity, nie synthesis of this compound starts by a multi-step conversion of hydroxy thiophene carboxylic ester 130, to the sulfonyl chloride 133. Reaction of that with N-methylglycinc ethyl ester, gives the sulfonamide 134. Base-catalyzed Claisen type condensation serves to cyclize that intermediate to the p-keto ester 135 (shown as the enol tautomer). The final product tenoxicam (136) is obtained by heating the ester with 2-aminopyridine [22]. 

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. 

Most carbonyl compounds exist almost exclusively in the keto form at equilibrium, and it s usually difficult to isolate the pure enol. For example, cyclohexanone contains only about 0.0001% of its enol tautomer at room temperature, and acetone contains only about 0.000 000 1% enol. The percentage of enol tautomer is even less for carboxylic acids, esters, and amides. Even though enols are difficult to isolate and are present only to a small extent at equilibrium., they are nevertheless responsible for much of the chemistry of carbonyl compounds because they are so reactive. 

Q Loss of H+ from the n- position by reaction with a base A-then yields the enol tautomer and regenerates HA catalyst. 

Problem 22.1 Draw structures for the enol tautomers of the following compounds ... 

Carbonyl compounds are in a rapid equilibrium with called keto-enol tautomerism. Although enol tautomers to only a small extent at equilibrium and can t usually be they nevertheless contain a highly nucleophilic double electrophiles. For example, aldehydes and ketones are at the a position by reaction with Cl2, Br2, or I2 in Alpha bromination of carboxylic acids can be similarly... 

Only brief reference is made in this section to azepinones as there is scant evidence for the existence of their azepinol tautomers. 1 //-Azepin-3(2//)-ones, e.g. 12, exist exclusively as the nonconjugated, nonplanar 3-oxo forms in a wide range of solvent systems.43 44 All attempts to generate the enolates have failed. Likewise, l//-azepin-2(3//)-ones, for which lactim and enol tautomers are possible, prefer the amide form 13.45... 

Although the conversion of an aldehyde or a ketone to its enol tautomer is not generally a preparative procedure, the reactions do have their preparative aspects. If a full mole of base per mole of ketone is used, the enolate ion (10) is formed and can be isolated (see, e.g., 10-105). When enol ethers or esters are hydrolyzed, the enols initially formed immediately tautomerize to the aldehydes or ketones. In addition, the overall processes (forward plus reverse reactions) are often used for equilibration purposes. When an optically active compound in which the chirality is due to an asymmetric carbon a to a carbonyl group (as in 11) is treated with acid or base, racemization results. If there is another asymmetric center in the molecule. 

Katsumura, Kitaura and their coworkers [74] found and discussed the high reactivity of vinylic vs allylic hydrogen in the photosensitized reactions of twisted 1,3-dienes in terms of the interaction in the perepoxide structure. Yoshioka and coworkers [75] investigated the effects of solvent polarity on the product distribution in the reaction of singlet oxygen with enolic tautomers of 1,3-diketones and discussed the role of the perepoxide intermediate or the perepoxide-Uke transition state to explain their results. A recent review of the ene reactions of was based on the significant intervention of the perepoxide structure [76], which can be taken as a quasi-intermediate. 

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