Enols keto-enol tautomerism

Space does not permit any further detailed discussion except for a brief account of two interesting subjects. The first is concerned with keto-enol tautomerism. The classical example is ethyl acetoacetate, which can exist in the keto form (I) and the enol form (II) ... 

Enols are related to an aldehyde or a ketone by a proton transfer equilibrium known as keto-enol tautomerism (Tautomensm refers to an mterconversion between two struc tures that differ by the placement of an atom or a group)... 

A single Kekule structure does not completely descnbe the actual bonding in the molecule Ketal (Section 17 8) An acetal denved from a ketone Keto-enol tautomerism (Section 18 4) Process by which an aldehyde or a ketone and its enol equilibrate... 

Hydroxyquinolines (Quinolinols). A number of methods have been employed for their preparation. A modified Chichibabia reaction of quinoline ia fused KOH—NaOH at 240°C produces 70% of 2-hydroxyquiQoline [59-31-4] (121). Alternative names based on the facile keto—enol tautomerism of two of these compounds are 2(1H) and 4(lJd)-quiQolinone none of the other quinolinols show this property. The treatment of... 

The keto-enol tautomerism of 1,2-benzisoxazoles has been examined and the existence of either form can be postulated on the basis of reactivity. IR analysis on the solid indicates the exclusive existence of the enol form, while in CHCI3 solution both appear to be present (71DIS(B)4483). 

The keto-enol tautomerization in the excited triplet state of 2-methylacetophenone involves the transfer of an H atom in the CHO fragment... 

Figure 1.11. NMR analysis of the keto-enol tautomerism of 2,4-pentanedione [CDCIa, 50% v/v, 25 °C, 60 MHz for H, 20 MHz for C]. (a) H NMR spectrum with integrais [resuit keto enoi = 13 87] (b) H broadband de-coupied C NMR spectrum (c) C NMR spectrum obtained by inverse gated H decoupiing with integrals [result keto enol = 15 85 ( 1)]...

Many nitrogen-containing compounds engage in a proton-transfer equilibrium that is analogous to keto-enol tautomerism ... 

The aromaticity of the pyrimidine and purine ring systems and the electron-rich nature of their —OH and —NHg substituents endow them with the capacity to undergo keto-enol tautomeric shifts. That is, pyrimidines and purines exist as tautomeric pairs, as shown in Figure 11.6 for uracil. The keto tautomer is called a lactam, whereas the enol form is a lactim. The lactam form vastly predominates at neutral pH. In other words, pA) values for ring nitrogen atoms 1 and 3 in uracil are greater than 8 (the pAl, value for N-3 is 9.5) (Table 11.1). 

Methylphenylhydrazine and both 1- and 2-naphthylhydrazines are also reported to react similarly. Phenols, in general, do not undergo this reaction, which is favoured by compounds exhibiting keto-enol tautomerism. ... 

Naphtho[2,l-h]furan-2-(3 -one 28 has been described as a keto tautomer (91JA2301). Naphtho[l,2-h]furan-3-(2//)-ones of type 29 (R = H, Me, Et, Pr, pentyl, heptyl) show keto-enol tautomerism with the enol form predominating (88RRC917). 

Armulated thiophenes of types 195 and 197 (A benzo, naphtho) were studied concerning keto-enol tautomerism. The ring fusion has a remarkable influence upon these equilibria. Whereas for the c-fused thiophenes 197 only keto tautomers were present, for h-fused derivatives 195 also the enol forms 196 were found (the equilibria are solvent dependent) (82JOC705). 

A carbonyl compound with a hydrogen atom on its a carbon rapidly equilibrates with its corresponding enol (Section 8.4). This rapid interconversion between two substances is a special kind of isomerism known as keto-enol tautomerism, from the Greek Canto, meaning "the same," and meros, meaning "part." The individual isomers are called tautomers. 

Keto-enol tautomerism of carbon) ] compounds is catalyzed by both acids and bases. Acid catalysis occurs by protonation of the carbonyl oxygen atom to give an intermediate cation that Joses H+ from its a carbon to yield a neutral enol (Figure 22.1). This proton loss from the cation intermediate is similar to what occurs during an El reaction when a carbocation loses H+ to form an alkene (Section 11.10). 

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... 

Figure 25.8 Fructose, a ketose, is a reducing sugar because it undergoes two base-catalyzed keto-enol tautomerizations that result in conversion to an aldose.

Glucose 6-phosphate is isomerized to fructose 6-phosphate by ring opening followed by a keto-enol tautomerization. 

Reduction of the acyl phosphate gives glyceraldehyde 3-phosphate, which Q undergoes keto-enol tautomerization to yield dihydroxyacetone phosphate. 

Following hydrolysis, keto-enol tautomerization of the carbonyl group from C2 to Cl gives glucose 6-phosphate. The isomerization is the reverse of step 2 in glycolysis. 

Keto-enol tautomerism (Sections 8.4, 22.1) The rapid equilibration between a carbonyl form and vinylic alcohol form of a molecule. 

Keto-enol tautomerism, 264, 842-844 Kiliani, Heinrich, 994 Kiliani-Fischer synthesis, 994-995 Kimbail, George, 216 Kinetic control, 491 Kinetics, 362... 

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