Keto-enol tautomerism preparation

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... 

Another approach for the synthesis of enantiopure amino acids or amino alcohols is the enantioselective enzyme-catalyzed hydrolysis of hydantoins. As discussed above, hydantoins are very easily racemized in weak alkaline solutions via keto enol tautomerism. Sugai et al. have reported the DKR of the hydantoin prepared from DL-phenylalanine. DKR took place smoothly by the use of D-hydantoinase at a pH of 9 employing a borate buffer (Figure 4.17) [42]. 

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. 

Dibenzoylmethane (8b) has been the subject of much interest as regards the possibility that its polymorphism is associated with keto-enol tautomerism. Chemical and spectroscopic studies showed that this is not so (33a). This compound had previously been reported to be trimorphic (33b), but one form appears, in fact, to be a eutectic mixture of the other two. The molecules in these two polymorphs are both in the same state of tautomerism they differ in the torsional angle about the (CH)-(CO) bond and in the type of hydrogen bonding in which they participate. It is noteworthy that solutions prepared from these forms at low temperature have differences in chemical and spectroscopic properties that are maintained for some time. For example, such solutions prepared and held at —35° react at different rates with FeCl3. 

The initial product has a hydroxy group attached to a carbon-carbon double bond. Compounds such as this are called enols (ene + ol) and are very labile—they cannot usually be isolated. Enols such as this spontaneously rearrange to the more stable ketone isomer. The ketone and the enol are termed tautomers. This reaction, which simply involves the movement of a proton and a double bond, is called a keto—enol tautomerization and is usually very fast. In most cases the ketone is much more stable, and the amount of enol present at equilibrium is not detectable by most methods. The mechanism for this tautomerization in acid is shown in Figure 11.6. The mercury-catalyzed hydration of alkynes is a good method for the preparation of ketones, as shown in the following example ... 

However, the lack of analytical methods to distinguish clearly diastereoisomers such as 7a and 7b casts some doubt on the above assumptions. Specifically, it cannot be ensured with ultimate certainty that the reactions have really occurred in the desired manner or whether epimerization has taken place to a certain extent. In the case of 7a/Fe+ and 7b/Fe+, for example, the experimental data clearly reveal the occurrence of diastereoselective dehydrogenation, but the intrinsic SE might even be larger if the samples were not the pure diastereoisomers 7a and 7b but only enriched samples. Epimerization at C(3) is in fact facile via keto/enol tautomerism of the carbonyl compound occurring upon work-up. Therefore, the isotopolog 7c was prepared via an independent synthetic route (N2D2 reduction of 2-methyl-but- -2-enoic acid). As the results obtained with... 

Problem 27.5 Acrolein, CH2 CHCHO, is prepared by heating glycerol with sodium hydrogen sulfate, NaHS04. (a) Outline the likely steps in this synthesis, which involves acid-catalyzed dehydration and keto-enol tautomerization. (Hinf Which OH is easier to eliminate, a primary or a secondary ) (b) How could acrolein be converted into acrylic acid ... 

Also, all the -situated hydrogen atoms of ketones are readily replaceable by deuterium in an aqueous alkaline medium, since these atoms take part one after the other in the reversible keto-enol tautomerism. This applies, for instance, to the four -positions of cyclohexanone and to the one of 2,2,6-tri-methylcyclohexanone, whereas no exchange occurs in camphor or camphor-quinone which cannot enolize owing to the particular stereochemistry of the molecules.90 Open-chain ketones and also steroidal ketones91 that are not subject to this limitation are often used for exchange reactions. The following description of the preparation of [D6]acetone illustrates the point 29... 

P-Keto acids, decarboxylation, 762—763, 768, 838, 840-841, 850 Keto-enol isomerism, 355, 705—707 Keto-enol tautomerism. See Keto-enol isomerism P-Keto esters acidity of, 831 alkylation of, 839-841, 850 Michael addition of, 846—847 nomenclature of, 832 preparation of... 

How would you prepare the soluble Phenytoin Sodium from Phenytoin Explain the mechanism of reaction involving keto-enol tautomerism. 

MCMs formed from (p-vinylphenyl)-3-phenyl-l,3-propandione (73) have been prepared from EuCU (in the presence of Py) by means of the keto-enol tautomerism of 1,3-propandiones. 

The primary product of biosynthesis is the (3R,7S)-jasmonate. The thermodynamic equilibrium lies at 7 93 in favour of the trans isomers, which results very easily from keto-enol tautomerism. It is not clear whether epimerisation takes already place in the plant or first occurs duringthe extraction process. The four diastereomers of methyl (Z)-jasmonate can be separated by preparative HPLC. 

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. 

There is no simple, commonly accepted method for the preparation of imidazoles, but rather many different approaches. One approach, somewhat related to chemistry seen in previous chapters, involves the reaction of an a-hydroxy-ketone such as 121 with formamide, 122. The -NH2 unit of forma-mide attacks the carbonyl (acyl addition), and loss of water (elimination) gives an enol that tautomerizes to the ketone. (Keto-enol tautomerism was first discussed in Chapter 10, Section 10.4.5.) A second molecule of formamide reacts with this ketone via acyl addition to give a product, which loses water. An intramolecular attack of the nitrogen atom from this product to one -CHO rmit on the carbonyl of the other CHO unit, followed by loss of water under the reaction conditions, gives imidazole, 123. 

Phentolamine (as a water-soluble methanesulfonic acid salt) is an antihypertensive that has recently been introduced into dentistry It cuts in half the time taken to recover from the numbing effect of local anesthetics. The key step in its preparation dates from 1886, the reaction shown below. What is its mechanism (Caution This is not a nucleophilic aromatic substitution. Hint Think keto-enol tautomerism). 

Whilst azo compounds prepared from diazonium salts and phenolic or keto-enol coupling components are often depicted in the hydroxyazo form (4.11), an alternative tautomeric structure can be drawn for such compounds (Scheme 4.19). This ketohydrazone tautomer (4.21) can, in cases where the azo and hydroxy groups are located on adjacent carbon atoms, exhibit hydrogen bonding between the two groups as shown. Similar pairs of structures, but without hydrogen bonding, can be drawn for p-hydroxyazo compounds. 

Hydroboration-oxidation of alkynes preparation of aldehydes and ketones Hydroboration-oxidation of terminal alkynes gives syn addition of water across the triple bond. The reaction is regioselective and follows anti-Markovnikov addition. Terminal alkynes are converted to aldehydes, and all other alkynes are converted to ketones. A sterically hindered dialkylborane must be used to prevent the addition of two borane molecules. A vinyl borane is produced with anU-Markovnikov orientation, which is oxidized by basic hydrogen peroxide to an enol. This enol tautomerizes readily to the more stable keto form. 

For several tautomeric systems ketones/enols, imines/enamin and others) a distinct reversal of the stability order is observed when going from the neutral compounds to the radical cations, the first use of which in a new preparative a-Umpolung reaction has been documented for keto/enol systems. The present review provides a critical evaluation of the chemistry of enol radical cations in solution with a special emphasis on the Umpolung reaction and the intermediates thereof. Other enol type of radical cations are discussed with respect to their potential to provide a-carbonyl radical and a-carbonyl cation intermediates. Hence, this article does not constitute a comprehensive summary on all enol type of radical cation reactions. All potentials in this review are referenced versus SCE, unless noted otherwise. Potentials measured against the ferrocene/ferrocenium couple were converted to SCE by adding 0.334 V. 

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