Enols double

Writing the bromine addition step m this way emphasizes the increased nucleophilicity of the enol double bond and identifies the source of that increased nucleophilicity as the enolic oxygen... 

Acid-catalyzed formation of an enol is followed by deuteronalion of the enol double bond and dedeuteronation of oxygen. 

A high degree of syn selectivity can be obtained from the addition of enamines to nitroalkenes. In this case, the syn selectivity is largely independent of the geometry of the acceptor, as well as the donor, double bond. Next in terms of selectivity, are the addition of enolates. However, whether one obtains syn or anti selectivity is dependent on both the geometry of the acceptor and the enolate double bond, whereas anti selectivity of a modest and unreliable level is obtained by reaction of enol silyl ethers with nitroalkenes under Lewis acid catalysis. 

Molecules in which the enolic double bond is in conjugation with another double bond. Some of these are shown in Table 2.1. As the table shows, carboxylic esters have a much smaller enolic content than ketones. In molecules... 

In acyclic systems, the enolate conformation comes into play. p,(3-Disubstituted enolates prefer a conformation with the hydrogen eclipsed with the enolate double bond. In unfunctionalized enolates, alkylation usually takes place anti to the larger substituent, but with very modest stereoselectivity. 

A more traveled route to the absolute configuration represented by cyclohexa-1,4-diene 8 involves Birch reduction-alkylation of benzoxazepinone 9.2.5 heterocycle is best prepared by the base-induced cyclization of the amide obtained from 2-fiuorobenzoyl chloride and (5)-pyrrolidine-2-metha-nol. o The molecular shape of enolate 10 is such that the hydrogen at the stereogenic center provides some shielding of the a-face of the enolate double bond. Thus, alkylation occurs primarily at the 3-face of 10 to give 11 as the major diastereomer. The diastereoselectivity for alkylation with methyl iodide is only 85 15, but with more sterically demanding alkyl halides such as ethyl iodide, allyl bromide, 4-bromobut-1-ene etc., diastereoselectivities are greater than 98 2. 

A second example concerns the instability of prostacyclin in physiological mediam, which is connected to the presence of the enol ether function (ti/2 = 5-10 min at pH = 7.4 and at 37°C). Hydrolysis is so fast that its use as a vasodilator and as an inhibitor of platelet adhesion cannot be exploited. The introduction of fluorine atoms in jS of the enol double bond led to compounds with good metabolic stability while retaining the strong activity as an inhibitor of platelet adhesion (Figure 3.18) (cf. Chapter 4). Proteolysis is slowed down as the oxonium, resulting from the protonation of the enol ether, is destabilized by the CF2 group. " ... 

The enolic double bond of a difluorovinyl group in the anomeric position is able to add a radical. It can thus afford a difluoro-C-glycoside, either directly or indirectly. When the alkylating radical is generated from a 6-halogenopyranoside, the reaction can lead to difluoro-C-disaccharide. These reactions are even more efficient if the radical has an electrophilic character (e.g., with an a-halogenoester instead of an alkyl radical). 

Trifluoroacetylacetonitrile (396) can also react with nitrile imines 395 via two reaction paths depending on their structure. Those with an electron donating N substituent reacted via cycloaddition to the enolic double bond to give 397 while... 

The configurational course is generally interpreted as being a consequence of a chelated dianionic structure, such as 5, in which attack of the electrophile on the less hindered enolate face yields the o f/-isomer 3. Interestingly, however, the enolate 6, in which the enolate double bond has an (E) geometry, thus opposite to the geometry in 5 and in which the dianionic moiety is not chelated as in 5, would be expected to show the same diastereoselectivity. 

Another copper-induced reaction which may be reinterpreted in view of our mechanism is the copper-induced chlorination of ketones (64). It seems reasonable to suggest that Cu(II) bound to the ketone is oxidized to Cu(III) by C11CI2, which, in the given medium, is less stable than Cu(I). Subsequently, Cu(III) oxidizes its chlorine ligand in a double electron transfer to C1+, followed by the latter s addition to the enolic double bond. 

Cyclopentanones may also be synthesized from a,/ -unsaturated ketones and diiodo-methane. The ketone is converted to the O-silyl enol, and carbene is added to the enol double bond using the Simmons-Smith reaction (see p. 74f.). Thermal rearrangement of the resulting 1-siloxy-l-vinylcydopropane and add-catalyzed hydrolysis of the silyl enol ether leads to cyclopentanones in excellent yields (C. Girard, 1974). Very high temperatures, however, are needed, and this obviously limits the generality of this rearrangement reaction. 

Silyl enol ethers with stereogenic silicon atoms bearing chiral alkoxy groups on silicon, as in 193, induce modest stereoselectivity in peracid epoxidation of the enol double bond306. Aryl n participation has been observed in the epoxidation of the bicylooctene 194307. 

Indeed in the transition state leading to ketol 19 (cf. 21), the enolate double-bond is oriented anti peri planar to the carbonyl group. This stereochemical approach must therefore be electronically favored over that leading to the isomeric ketol 20. In the last case, the carbonyl group of the side chain is gauche (synclinal) with the enolate double-bond (cf. 22). 

Least stable enol double bond not Ethyl acetoacetate... 

Most stable enol double bond conjugated with carbonyl group ester carbonyl stabilized by resonance... 

The enolate ion derived from the nitrone 15 undergoes an unusual intramolecular dipolar cycloaddition reaction involving the enol double bond to yield 16 (95T6285). 

For l[2-pyridyl]2-propanone it is quite possible that the resonance energy of the pyridine nucleus is high enough to stabilize the initiating radical (resulting from the enolic double bond or rather the peroxide or ketohydroperoxide therefrom) to such an extent that it acts as an inhibitor. 

There are three different resonance possibilities for the enolic double bond—viz., with the aromatic nucleus, with the cyano, and with the carbonyl group. Furthermore, the a-hydrogen of the keto form would be acidic owing to the presence of the cyano group. Accordingly, all three compounds gave positive reactions to the iron chloride enol test. Further proof for the enol form in the crystalline state is given by the infrared spectra taken in Nujol and Fluorolube. 

Stuart models show that nucleus resonance stabilization of the enol double bond is possible only for the thiophene ester. It is possible that resonance stabilization of the radical resulting from the enolic double bond (with the peroxide or ketohydroperoxide forms as intermediates)... 

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