Enolic compounds

Suggestions as to the methods for identifying the above classes of compounds will be found under Class Reactiona in Section XI,7. Some fimther remarks upon enolic compounds (see Table IV,1I4A) may be made here. Enols may be divided into (a) p-keto esters and (b) 1 3-diketones. With 5 per cent, sodium hydroxide solution, a p-keto ester yields the salt of the corresponding acid, which when heated with dilute hydrochloric acid is decarboxylated to a ketone ... 

Separate any phenolic or enolic compounds which may be present by extracting the sodium bicarbonate solution with two 20 ml. portions of ether remove the ether from the extract and examine any residue for phenols (or enols). 

Bravo et al. studied the reaction of various ylides with monooximes of biacetyl and benzil. Dimethylsulfonium methylide and triphenylarsonium methylide gave 2-isoxazolin-5-ol and isoxazoles, with the former being the major product. Triphenylphosphonium methylide and dimethyloxosulfonium methylide gave open-chain products (Scheme 135) (70TL3223, 72G395). The cycloaddition of benzonitrile oxide to enolic compounds produced 5-ethers which could be cleaved or dehydrated (Scheme 136) (70CJC467, 72NKK1452). 

FeCls phenols, enolic compounds 1 % aqueous FeCl3 various coloured spots... 

A very extensive and detailed study of the cationic rhodium(i)-catalyzed isomerization of allylic alcohols demonstrated that mono- and disubstituted allylic alcohols can be efficiently isomerized to the corresponding carbonyl compounds through the corresponding enol compounds (Scheme 20).45 The isomerization using cationic rhodium(l)... 

Pfizer s tenidap (CP-66,248) (157), another enolic compound, was also more potent (500-fold) toward CO over 5-LO inhibition in human ISN (0.032 /iM and 18 /iM, respectively) [379-381]. Efficacy in rheumatoid arthritis clinical trials has been reported [380,382] in patients, serum levels of acute phase proteins and synovial fluid levels of IL-1 were reduced by tenidap, in contrast to the lack of this effect with NSAIDs. Besides CO/5-LO inhibition, a variety of in vitro activities have been reported, including a number of effects on monocyte functions and differentiation [379], inhibition of neutrophil degranulation [382], inhibition of the activation of neutrophil collagenase [383], inhibition of leukocyte-endothelial cell adhesion [384], and inhibition of LTB4-induced neutrophil chemotaxis [385]. Al-... 

There are also reactions in which electrophilic radicals react with relatively nucleophilic alkenes. These reactions are represented by a group of procedures in which a radical intermediate is formed by oxidation of the enol of a readily enolized compound. This reaction was initially developed for /i-kctoacids.227 The method has been extended to /1-diketones, malonic acids, and cyanoacetic acid.228... 

The first dehydration products formed by general, acid-base catalysis are represented by the enolic forms (7, 9, and 10) of the deoxydicarbonyl sugars 7a, 9a, and 10a. The enolic compounds are formed from enediols by the removal of a molecule of water through -elimination of a hydroxyl group. For example, from the 1,2-enediol (6) derived from D-glucose or D-fructose, the enolic form (7)of3-deoxy-D-eryf/iro-hexosulose (7a) is produced, whereas from the 2,3-enediol... 

The acyclic, enolic compounds 7 and 9 may exist in either cis or trans forms. Methyl ethers of 7 have been isolated in the cis form,8 but it is not known whether the trans forms, which must be acyclic, exist. The relative proportion of isomers is controlled by the geometry of the parent sugar enediol. Although the acyclic forms are readily interconvertible tautomers, it appears that, in acidic medium, further reaction occurs much more rapidly than any equilibrating reactions. Compound 7 undergoes rapid elimination of a second hydroxyl group to give 11. This acyclic product, also, may exist as either a cis or a trans isomer, both forms of which have been isolated.8 The loss of a third molecule of water per molecule occurs after, or simultaneously with, the cyclization of 11 (see Section II, 3 p. 171), and results in formation of 5-(hydroxymethyl)-2-furaldehyde (5). 

Acidic enolic compounds (pyrazolidine-3,5-diones, oxicams)... 

The compounds formed are always 1 3-di-ketones. The way in which the condensing agent brings about the condensation is described under the actual preparations (see Preparation 39). The compounds themselves are di-ketones, but their sodium salts are derived from the corresponding enol compounds (B., 25, 3074). Of the three condensing agents, sodamide is the most and sodium ethylate the least effective. 

The amount of enolic compound can be estimated by adding a standard solution of bromine in alcohol, until the yellow colour just persists, but the method has the disadvantage that such a solution of bromine is unstable. 

In the following method a slight excess of an alcoholic solution of bromine is added to an alcoholic solution of the tautomeric mixture the excess of bromine is immediately removed by the addition of a few drops of alcoholic /3-naphthoI solution potassium iodide solution is next added, and the hydrogen iodide formed by interaction with the hydrogen bromide present reduces the bromo-ketone with liberation of free iodine, which is estimated by titration with standard thiosulphate (in absence of starch). One molecule of iodine = one molecule of enolic compound. For criticism of this method, see Ann. Rep., 1930, 100. 

Ascorbic acids reduce Fehling solution in the cold, and react with ferric chloride to produce the violet color typical of enolic compounds. They are readily oxidized reversibly to their primary oxidation products, 2,3-glycodiulosono-1,4-lactones (commonly known as dehydroascorbic acids 148, Scheme 16), by such mild oxidizing agents as aqueous iodine. Titration with such oxidants constitutes a quantitative method for distinguishing ascorbic acids from 2-glyculosonic acids. 

Some General Observations on the Syntheses of Metal /3-keto-enolate Compounds... 

EXAMPLE 10.8 Enols, compounds with an OH group directly bonded to a vinyl carbon, are normally less stable than the corresponding carbonyl ( keto ) isomer. [A few enols (e.g., phenols) exist predominantly or exclusively in the end form.] Though the uncatalyzed equilibration of enols with their keto isomers is usually slow, the rate of isomerization increases dramatically in the presence of a small amount of acid (HA) or base (B) ... 

Birch, A.J. Biosynthetic relations of some natural phenolic and enolic compounds. Fortschr. Chem. org. Naturstoffe 14, 186 (1957)... 

Table HI. Influence of aging wine in barrel on its content in enolic compounds with a toasty aroma (results after 6 months aging)...

Figure 16. Development of enolic compounds during barrel toasting...

Henderson and coworkers studied the reaction of MesAl with a series of aromatic ketones (25, 27, 29) to yield the precipitation of either dimethylaluminum enolates or alkoxides (see equations 9-11). In situ H NMR spectroscopic studies of the reaction between MesAl and acetophenone (29) revealed a complex mixture of products, whereas under the same conditions 2,4,6-trimethylacetophenone (25) reacts cleanly to give the corresponding enolate. The enolate compounds 26 and 28 were isolated and 26 as well as the representative alkoxide 30 were characterized by X-ray crystallography. Both 26 and 30 form dimers with a central AI2O2 core. Ab initio calculations at the HF/6-31G level indicated that both 26 and 30 are the thermodynamic products of the reactions. Equation 12 shows the alkylation and enolization reactions for the ketones 25 and 29 and... 

Most TL studies are centered on enolate compounds, in particular, rare-earth / -dike-tonates. Present evidence points to morpholinium tetrakis(dibenzoyhnethanato)europate (in) and triethylammonium tetrakis(dibenzoyhnethanato)europate(in) as the first and second most efficient triboluminescent compounds, respectively. In these cases the coexistence of disorder in the chelating ring and the non-centrosymmetric crystal structures may contribute to major charge separation upon cleavage and, consequently, to higher luminescence intensity. 

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