Ketones reaction with enol esters

The mixed Claisen condensation of two different esters is similar to the mixed aldol condensation of two different aldehydes or ketones (Section 23.5). Mixed Claisen reactions are successful only when one of the two ester components has no a hydrogens and thus can t form an enolate ion. For example, ethyl benzoate and ethyl formate can t form enolate ions and thus can t serve as donors. They can, however, act as the electrophilic acceptor components in reactions with other ester anions to give mixed /3-keto ester products. 

Therefore, transesterification reactions frequently fail when R is tertiary, since this type of substrate most often reacts by alkyl-oxygen cleavage. In such cases, the reaction is of the Williamson type with OCOR as the leaving group (see 10-14). With enol esters, the free alcohol is the enol of a ketone, so such esters easily... 

Now let us look at the ease of forming the enolate anion nucleophiles. Ketones are more acidic than esters (see Section 10.7). Taken together, these factors mean the more favoured product is going to be the P-diketone (acetylacetone), formed from a ketone nucleophile by a Claisen reaction with an ester. This is the reaction observed. 

With enol esters, the free alcohol is the enol of a ketone, so such esters easily undergo the reaction... 

Coupling with enol esters (7, 93). A new synthesis of an alkyl-substituted alkene involves coupling of a lithium dialkyl cuprate with an enol triflate,1 available from a ketone by reaction with triflic anhydride and 2,6-di-t-butylpyridine.2 A wide variety of organocuprates can be used and the geometry of the enolate is largely retained. Reported yields are in the range 60 100%. 

With enol esters such as 102, reaction with an alcohol gives an ester and the enol of a ketone, which readily tautomerizes to the ketone as shown. Hence, enol esters are good acylating agents for alcohols.This transformation has been... 

The different methods available for doing aldol reactions with enolates of aldehydes, ketones, and esters... 

Weedon and coworicers used the condensation of the methyl ketone (100) with polyene ester (101) to form the enolic 3-diketone portion (102) of the carotenoids mytloxanthin and trikentriorhodin (equation 28)." A similar condensation was used to form the 3-diketone (105) of all-( )-wallemia C by reaction of (103) with the methyl ketone (104 equation 29). ... 

Early extensive accounts of the 4v participation of a,/)-unsaturated carbonyl compounds in [4 + 2] cycloadditions detailed their reactions with electron-deficient dienophiles including a,/3-unsaturated nitriles, aldehydes, and ketones simple unactivated olefins including allylic alcohols and electron-rich dienophiles including enol ethers, enamines, vinyl carbamates, and vinyl ureas.23-25 31-33 Subsequent efforts have recognized the preferential participation of simple a,/3-unsaturated carbonyl compounds (a,/3-unsaturated aldehydes > ketones > esters) in inverse electron demand [4 + 2] cycloadditions and have further explored their [4 + 2]-cycloaddition reactions with enol ethers,34-48 acetylenic ethers,48 49 ke-tene acetals,36-50 enamines,4151-60-66 ynamines,61-63 ketene aminals,66 and selected simple olefins64-65 (Scheme 7-1). Additional examples may be found in Table 7-1. 

The reaction can be carried out by combining ketone (6) with the ester and a b strong enough (Table 18.2) to produce only a small concentration of the H4atc, often EtO. The reaction is therefore an equilibrium and it is driven Mr by formation of the stable delocalised enolate (7) of the product. Add nrk-up then releases TM(5). This last step applies to all reactions of this sort Mt is not usually written down. 

Diketones. The reaction of enol esters with ketones at 50-70° in the presence of manganic acetate (2 eq.) leads to 1,4-diketones in moderate yields ... 

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