Enol form of ketone

Carbon dioxide Enol form of ketone Ketone... 

Triflate 880 can be formally considered as an ester of the enol form of ketone 879. Treatment with a base causes elimination of the triflate group to afford l-(benzotriazol-l-yl)alkynes 881 (Scheme 144) <20000L3789>. 

Likewise, the enolized form of ketones is a Br0nsted acid that can be reduced at a platinum electrode ... 

It has long been realized that the enol form of ketones can react with molecular oxygen to generate the a-hydroperoxy ketone from which a-hydroxy ketones are readily obtained by reductive work-up. The oxidation was thought to arise either fiom direct attack of the enol on molecular oxygen or through a radical-mediated process (vide infra). Necessarily the oxidation is most efficient where the proportion of... 

In spite of the fact that numerous oxidation reactions are known, that lead to a-functionalization of ketones [159,160], in most cases enol radical cations are not involved in these transformations, and rigorous evidence for their formation through selective oxidation of the enol tautomer (Fig. 2, path 2) has only been obtained in a few cases. For example, it could be inferred from kinetic studies that in many cases enols are not intermediates in aqueous oxidation reactions with V(V), Co(III), Ce(IV) and Mn(III) [161-163], whereas in acetic acid Mn(III) was postulated to attack the enol form of ketones [164,165], but not by electron transfer [166]. On the other hand, oxidants as Cr(VI), Tl(III), Hg(II) and Mn(VII) [167] as well as Pb(IV) [168] definitely react with the enol form, but since with these inner-sphere oxidants electron transfer is assumed to occur in a bonded fashion, radical cation intermediates are most likely not implicated. 

Acetylation of alcohols and phenols by ketene has limited use. Unless apparatus for the preparation of ketene is readily available, less troublesome methods can usually be found. Worthy of mention, however, are the acetylations of lactic esters in 94-98% yields and of tertiary alcohols and phenols in 89-96% yields.Catalysts are necessary even to convert a high percentage of n-butyl alcohol to n-butyl acetate. Sulfuric and p-toluenesulfonic acids are commonly used. Certain aldehydes and ketones are attacked by ketene. Acetates of enol forms of ketones may be made in this way. ° Under certain conditions / -lactones are formed (cf. method 327),... 

As a further extension of the Ochiai s procedure for a-acetoxylation, the catalytic procedure for a-tosyloxylation of ketones using mCPBA as stoichiometric oxidant and iodoarenes as catalysts in the presence of p-toluenesulfonic acid has been developed. Various a-tosyloxyketones 7 can be efficiently prepared in high yields from the reaction of ketones 6 with mCPBA (1.1 equiv) and p-toluenesulfonic acid (1.1 equiv) in the presence of a catalytic amount of iodobenzene with moderate warming (Scheme 4.4) [19]. The mechanism of this reaction involves initial oxidation of Phi by mCPBA in the presence of p-toluenesulfonic acid to generate [hydroxy(tosyloxy)iodo]benzene in situ, which then reacts with the enol form of ketone to give the a-tosyloxyketone. 

Formation of this product, in the Heck coupling reaction, is the consequence of the migration of the C=C bond repeatedly observed in this catalytic reaction [21], and transitory formation of the enol form of ketone 15. Reduction of this ketone to the S-enantiomer of benzylic alcohol 16 is the key step on the path to the (R)-enantiomer of montelukast 1. The second Grignard reaction (vi), with subsequent activation of the benzylic OH group (vii) and protection of ferf-OH group (viii) afforded the first key intermediate 18. 

Electrophilic Reactions with Ketones, Reactions of the enolic forms of ketones with various electrophilic reagents are widely used in total steroid synthesis for the formation of all the centers of asymmetry of interest to us (Table 5). As an example of electrophilic reactions we must mention in the first place the ketonization-enolization reaction. In this reaction, the splitting off and addition of a proton to a carbon atom in the O -position to the keto group is controlled by the stereoelectronic factor and takes place predominantly in the axial direction [74]. 

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