Enol form

M.p. 190-192 C. The enolic form of 3-oxo-L-gulofuranolactone. It can be prepared by synthesis from glucose, or extracted from plant sources such as rose hips, blackcurrants or citrus fruits. Easily oxidized. It is essential for the formation of collagen and intercellular material, bone and teeth, and for the healing of wounds. It is used in the treatment of scurvy. Man is one of the few mammals unable to manufacture ascorbic acid in his liver. Used as a photographic developing agent in alkaline solution. 

Ethyl acetoacetate is a tautomeric substance which at room temperature exists as an equilibrium mixture of the keto and enol forms, the latter form being present to the extent of about 7%. 

It is readily prepared by the action of metallic sodium on dry ethyl acetate. The reaction, which occurs only in the presence of a trace of ethanol, is complex, but may be considered (in effect) as a condensation of two molecules of ethyl acetate under the influence of sodium ethoxide, the sodium derivative of the enol form being thus obtained. Clearly, only a trace of ethanol is thus initially... 

Thus the sodio derivative (I) of the enol form of ethyl acetoacetate is obtained. This mechanism can clearly apply also to the condensation of an ester with a suitable ketone or nitrile, as in the above reactions (ii) and (iii) respectively. 

Demonstrate both the presence and the interconversion of the keto and enol forms in the ester thus ... 

Ethyl malonate, like ethyl acetoacetate, exists as a tautomeric mixture of keto and enol forms, although in the case of ethyl malonate... 

Substitution Derivatives of Ethyl Malonate, Ethyl malonate resembles ethyl acetoacetate in that it gives rise to mono- and di-substituted derivatives in precisely similar circumstances. Thus when ethanolic solutions of ethyl malonate and of sodium ethoxide are mixed, the sodium derivative (A) of the enol form is produced in solution. On boiling this solution with an alkyl halide, e.g, methyl iodide, the methyl derivative (B) of the keto form is obtained. When this is treated again in ethanolic solution with sodium ethoxide, the... 

This preparation illustrates the ready formation of the thiazole ring by the condensation of an ot-halogeno-ketone and a thioamide. Thus chloroacetone, which may conveniently be represented in the enol form (I), condenses with thiourea (II) to give 2-amino-4-methylthiazolc (III). 

The enol form is thus temporarily removed from the solution and the ferric chloride colouration produced by the enol form consequently disappears and the solution becomes colourless. Some of the unchanged keto form of the ester then passes into the enol form in order to restore the original equilibrium and the ferric chloride colouration therefore reappears. 

Reduction of anthraquinone with tin and concentrated hydrochloric acid in the presence of boiling glacial eicetic acid gives anthrone this substance (keto form) under certain conditions passes into the enol form, anthranol ... 

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) ... 

Thermodynamic Enolate- Reversible deprotonation to give the most stable enolate more highly substituted C=C of the enol form... 

The 4-kcto group in the alkyne 262 as an enol form adds to the triple bond to give the furan 263[133], Even the conjugated keto alkyne 264 was converted into the furan 266 via isomerization to the allenyl ketone 265[134],... 

Polar solvents shift the keto enol equilibrium toward the enol form (174b). Thus the NMR spectrum in DMSO of 2-phenyl-A-2-thiazoline-4-one is composed of three main signals +10.7 ppm (enolic proton). 7.7 ppm (aromatic protons), and 6.2 ppm (olefinic proton) associated with the enol form and a small signal associated with less than 10% of the keto form. In acetone, equal amounts of keto and enol forms were found (104). In general, a-methylene protons of keto forms appear at approximately 3.5 to 4.3 ppm as an AB spectra or a singlet (386, 419). A coupling constant, Jab - 15.5 Hz, has been reported for 2-[(S-carboxymethyl)thioimidyl]-A-2-thiazoline-4-one 175 (Scheme 92) (419). This high J b value could be of some help in the discussion on the structure of 178 (p. 423). 

The olefinic proton of the enol form emerges as a sharp singlet in the region 6.2 to 7.5 ppm (DMSO) (386). while the 5-methyl protons appear at approximately 2.2 ppm. 

The conditions under which this reaction occurs are comparatively mild, and for this reason it has been assumed that dehydration takes place through the enol form without rearrangement to give the 2,5-disubstituted derivatives as indicated in Scheme 103. 

Give the structure of the enol formed by hydration of 2 butyne... 

Write the structures of the enol forms of 2 butanone that react with chlorine to give 1 chloro 2 butanone and 3 chloro 2 butanone... 

Represent the reaction of chlorine with each of the enol forms... 

The keto and enol forms are constitutional isomers Using older terminology they are referred to as tautomers of each other... 

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