Isomerism keto-enol tautomerism

Sequential keto-enol tautomeric isomerization (Eq. 7) could convert 118 to the tetrahydropterin derivative, the structure of which is widely accepted as (6R)-pyruvoyltetrahydropterin (124). Contrary to the importance of 124 in the biosynthesis of tetrahydrobiopterin (43) (Sect. 5.1), thus far, only a few... 

The aldehyde or ketone is called the keto form and the keto enol equilibration referred to as keto-enol isomerism or keto-enol tautomerism Tautomers are constitu tional isomers that equilibrate by migration of an atom or group and their equilibration IS called tautomerism The mechanism of keto-enol isomerism involves the sequence of proton transfers shown m Figure 9 6... 

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. 

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

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. 

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. 

There is a distinct relationship between keto-enol tautomerism and the iminium-enamine interconversion it can be seen from the above scheme that enamines are actually nitrogen analogues of enols. Their chemical properties reflect this relationship. It also leads us to another reason why enamine formation is a property of secondary amines, whereas primary amines give imines with aldehydes and ketones (see Section 7.7.1). Enamines from primary amines would undergo rapid conversion into the more stable imine tautomers (compare enol and keto tautomers) this isomerization cannot occur with enamines from secondary amines, and such enamines are, therefore, stable. 

Glucose 6-phosphate is then isomerized to fructose 6-phosphate. This conversion of an aldose sugar to a ketose sugar is easy to rationalize in terms of keto-enol tautomerism (see Box 10.1). 

This is achieved by two keto-enol tautomerism reactions and a common enol (see Box 10.1). Mechanistically, it is identical to the isomerization of glucose 6-phosphate to fructose 6-phosphate seen earlier in the sequence, so we can move on to the next step of the pathway. 

Even for a simple reaction, involving just one reactant species and one product species, such as a keto-enol tautomerism or a cis-trans isomerization, the above equation for a given solvent is complicated. StUl, in specific cases it is possible to unravel the solvent effects of cavity formation, for the solute species have different volumes, polarity/polarizability if the solute species differ in their dipole moments or polarizabilities, and solvent Lewis acidity and basicity if the solutes differ in their electron-pair and hydrogen-bond acceptance abilities. 

Isomer stabilities and activation energies have been calculated for keto-enol tautomerization of simple carbonyl compounds, MeC(R)=X (X = O R = H, Me) 129 both specific and bulk solvent effects have been analysed. Related isomerizations of acid derivatives (R = F, CN) and other related structures (R = H X = CF12, NH, S) are compared. 

Enols tend to be unstable and isomerize to the ketone form. As shown next, this isomerization involves the shift of a proton and a double bond. The (boxed) hydroxyl proton is lost, and a proton is regained at the methyl position, while the pi bond shifts from the C = C position to the C=O position. This type of rapid equilibrium is called a tautomerism. The one shown is the keto-enol tautomerism, which is covered in more detail in Chapter 22. The keto form usually predominates. 

An isomerism involving the migration of a proton and the corresponding movement of a double bond. An example is the keto-enol tautomerism of a ketone or aldehyde with its enol form. (p. 1047)... 

Step 5 is an isomerization of dihydroxyacetone phosphate to glyceraldehyde 3-phosphate that occurs by keto-enol tautomerization. 

Keto-Enol Tautomerism. It is well known that ketones such as acetone have an isomeric structure, which results from proton movement, called the enol tautomer, an unsaturated alcohol ... 

Keto-enol tautomerism cannot take place without at least a trace of acid or base, " since the acidic or basic center or both in the tautomeric substance is too weak. In this equilibrium, the heteroatom is the basic site the proton is the acidic site. For tautomerism in general (see p 98), the presence of an acid or a base is not necessary to initiate the isomerization since each tautomeric substance possesses amphiprotic properties. Keto-enol tautomerism is therefore the exception. 

Tautomerism Isomerism in which isomers, called tautomers, are readily interconvertible. Keto-enol tautomerism is an example... 

Phosphorylation and isomerization. Glucose, produced by the digestion of dietary carbohydrate., is first phosphorylated at the hydroxyl group on C6 by reaction with ATP in a process catalyzed by the enzyme hexokinase. The glucose 6-phosphate that results is isomerized by glucose 6-phosphate isomerase to fructose 6-phosphate. As the open-chain structures in Figure 29.4 show, this isomerization reaction takes place by keto-enol tautomerism (Section 22.1), since both glucose and fructose share a common enol ... 

Keto/enol tautomerism The tautomerism between the keto and enol forms R CH- C(=0)R and R2C=C(-OH)R. This type of isomerism requires at least one a-hydrogen atom. 

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