Keto-enol transformation

The isomerases that catalyze the simplest reactions are tautomerases that promote the oxo-enol (keto-enol) transformation. The widely distributed oxaloacetate tautomerase (Eq. 13-24) is especially active in animal tissues.97120 Oxaloacetate exists to a substantial extent in the enolic form at 38°, 6% enol, 13% oxo, and 81% covalent hydrate.120121 A mammalian phenylpyruvate tautomerase has also been investigated.122... 

Equilibrium data and rate coefficients for keto-enol transformations at 25 °C... 

It is assumed that glucose experiences a keto-enol transformation to form an anionic enol species ... 

The nature of the chemical type of influences on keto-enol equilibrium is obvious the more basic the solvent, the higher the degree of keto-enol transformation into enol form. If solvent is indifferent with enough degrees of approximation, the well-defined dependence of the constants of keto-enol equilibrium on solventpermittivity is encountered. The difference in dipole moments of tautomeric forms is the basis of these relationships. 

After succeeding in the asymmetric reductive acylation of ketones, we ventured to see if enol acetates can be used as acyl donors and precursors of ketones at the same time through deacylation and keto-enol tautomerization (Scheme 8). The overall reaction thus corresponds to the asymmetric reduction of enol acetate. For example, 1-phenylvinyl acetate was transformed to (f )-l-phenylethyl acetate by CALB and diruthenium complex 1 in the presence of 2,6-dimethyl-4-heptanol with 89% yield and 98% ee. Molecular hydrogen (1 atm) was almost equally effective for the transformation. A broad range of enol acetates were prepared from ketones and were successfully transformed into their corresponding (7 )-acetates under 1 atm H2 (Table 19). From unsymmetrical aliphatic ketones, enol acetates were obtained as the mixtures of regio- and geometrical isomers. Notably, however, the efficiency of the process was little affected by the isomeric composition of the enol acetates. 

Tautomerism occurs elsewhere in the glycolytic pathway (see Section 15.2). The transformation of glyceraldehyde 3-phosphate into dihydroxyacetone phosphate involves two such keto-enol tautomerisms, and... 

Both the aldol and reverse aldol reactions are encountered in carbohydrate metabolic pathways in biochemistry (see Chapter 15). In fact, one reversible transformation can be utilized in either carbohydrate biosynthesis or carbohydrate degradation, according to a cell s particular requirement. o-Fructose 1,6-diphosphate is produced during carbohydrate biosynthesis by an aldol reaction between dihydroxyacetone phosphate, which acts as the enolate anion nucleophile, and o-glyceraldehyde 3-phosphate, which acts as the carbonyl electrophile these two starting materials are also interconvertible through keto-enol tautomerism, as seen earlier (see Section 10.1). The biosynthetic reaction may be simplihed mechanistically as a standard mixed aldol reaction, where the nature of the substrates and their mode of coupling are dictated by the enzyme. The enzyme is actually called aldolase. 

These seemingly anomalous results suggest that the formation and fragmentation of a-amino nitrite esters could be playing a central role in the nitrosation of aminopyrine. The characterization of both fast and slow reactions, as well as the identification of two pH optima, imply that more than one kinetically significant pathway is involved in the overall transformation. The mechanism of Fig. 5a could well be the first order component the kinetic studies show to be operative under some conditions. It is noteworthy that this pathway also leads directly in its final step to the keto-enol derivative IV, which Mirvish et al. have identified as a by-product of aminopyrine nitrosation. 

Enol and keto groups transform into each other. 

In this manner, both C- and 0-metallo derivatives of keto-enol systems were shown not to be in tautomeric ketone enolate equilibrium (in benzene), thus differing from ketones and enols themselves where ketone enol equilibria exist. Metal enolates, however, are transformed by substitution to the derivatives of the ketone species, while our C-mercury derivatives of ketones or aldehydes have been shown to produce the enol compounds. A conventional scheme (in which ketones, not their metallic derivatives, are shown) thus becomes somewhat unsatisfactory ... 

The resulting enolate 36 is then transformed into 21 in 47 % yield. However, keto-enol tautomerism leads to the formation of 39 % of the corresponding ketone 20. Fortunately, 20 can be converted into 21 in 72 % yield by a second deprotonation with LDA and reaction with Comins xesigQxit (19). ... 

Allylic ethers of cyanohydrins are easily prepared through phase transfer allylation. Deprotonation of these ethers with LDA in THF at -78 °C effects 2,3-rearrangement to transient p. y-unsaturated ketone cyanohydrins, which are transformed during work-up to the ketones (Table 16). In an extension of this work, the mixed acetal cyanohydrin ethers (187), prepared by mild acid treatment of the cyanohydrins with 2-methoxy-1,3-butadiene or 1-f-butoxyallene, rearranged to the keto enol ethers (189 equation 40). Hydrolysis of the enol ethers (189) leads to 1,4-dicarbonyl compounds, which can be cyclized to cyclopentenones. 

The tautomeric transformations of phenols can be subdivided into two groups (i) keto-enol tautomerism which is accompanied by loss of the aromatic character of the ring, and (ii) tautomeric equilibrium involving participation of substituents where the aromatic phenol nucleus is conserved. 

The tautomerism of hydroxyarenes occupies a particular position among keto-enol tautomer transformations of various organic compounds because of the aforementioned loss of aromaticity. In contrast to carbonyl compounds (e.g. the keto form 1 is more energetically favored than the enol form 2 by 42 kJmoU , equation 1), the phenols 3 are much more stable than their keto tautomers (4 or 5) because the energy gained by... 

The coordination of transition metals is known to influence the keto-enol tautomerism in the condensed phase" . The effect of coordination of bare Fe+ ions on the keto-enol equilibrium of phenol was investigated by means of generation of various cyclic [Fe,Cg, He, 0]+-isomers. These isomers were characterized by collisional activation (CA) and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry" . It was shown that the energy difference between the phenol-iron complex 65 and the keto isomer 66 is not perturbed by the presence of the iron cation in comparison with the uncom-plexed isomers 3 and 4 (equation 25). Thus, the energy difference for both the neutral and the Fe+-coordinated systems amounts to ca 30 kJ moC in favor of the phenolic tautomer. 

An interesting situation arises when a tendency of ketodiene tautomer to transform into phenol results in a disturbance of the conjugation system in the whole structure in which this tautomer is a fragment. Thus, in the series of porphyrinoids 74 containing a semiquinone moiety, the macrocycle achieves the aromatization by undergoing a keto-enol tautomerization, whereby the phenolic subunit in structure 73 is transformed in such a way that the inner three carbon atom moiety becomes part of the 18 jt-electron... 

The mushroom tyrosinase-catalyzed oxidative decarboxylation of 3,4-dihydroxyphenyl mandelic acid (111, R = H) and a-(3,4-dihydroxyphenyl) lactic acid (111, R = Me) proceeds via the quinone methide intermediate 112. The coupled dienone-phenol rearrangement and keto-enol tautomerism transforms the quinone methide 112 into 1-acyl-3,4-dihydroxyphenyl compounds 113 (equation 48) . ... 

Hydroxy-substituted triazines148,149 are normally transformed into better leaving groups before they are replaced by another nucleophile. As such compounds often exhibit keto-enol tautomerism (see also p 670), nucleophiles may add to the carbonyl group and subsequently lead to the elimination of water, e.g. as in the formation of triamine 2. 

Several mechanisms have been proposed to explain the variations of the bioluminescence color for native and mutant luciferases. According to White at al1 changes in bioluminescence spectra are the result of keto-enol tautomerization of oxyluciferin (LO). The efficiency of this process depends on a correct location of the B, and B2 bases in proximity of the thiazole ring for effective transformation of the ketone form of LO (L0=0) to the enol (LO-OH) which can interact with the B3 base to form enolate-ion (LO-O ). When the Bi base is absent or protonated (e g. at pH < 6.0) bioluminescence of the L0=0 will be observed with Xmax in the red region of the spectrum. At intermediate pH all forms L0=0, LO-OH and (LO-O ) - will be observed in the bioluminescence spectra.2... 

Isomerization reactions are of high importance to the exploitation of carbohydrates. These transformations retain the functionality parameter of the feedstock, while maintaining complete atom-efficiency, since the reaction only involves a reshuffle of atoms within the molecule. Keto-enol tautomerization between aldoses and ketoses as encountered in Fig. 13, as well as the furanose or pyranose ring formation via cychc hemi-acetalization of pentoses and hexoses, are simple examples. The epimerization... 

Besides being prepared by oxidation, aldehydes and ketones can also be prepared by reactions in which the first step includes the addition of water to the triple bond of the alkyne molecule. The first intermediate, the unsaturated alcohol (enol) is unstable and undergoes isomerization to the stable ketone. This type of reaction in which one isomer is transformed to another is called rearrangement. The older name for this molecular rearrangement is taulomerism and this special case is called the keto-enol tautomerism. 

Show that this transformation can be regarded as two enzyme-catalyzed keto-enol tautomerizations (Section 12.8). 

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