Esters enol, reaction with

The Mannich reaction of ketene 0,5-acetals can give rise to 3-lactams. Reaction with enol esters proceeds reasonably well, and that involving ArNH2, ethyl glyoxylate and vinyl acetate gives quinoline-2-carboxylic 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. 

In its present form, intermediate 12 is not a viable substrate for the crucial Dieckmann condensation it must undergo prior epimerization at C-16. When intermediate 12 is treated with sodium methoxide in hot methanol, enolization at C-16 occurs and an equilibrium is established between 12 and a diastereomeric substance, intermediate 11. Once formed, 11 can either revert back to 12 through the planar enolate form, or it can participate in a productive cyclization reaction to give a new six-membered ring. Under these conditions, the desired transformations take place with exceptional facility to give, after acidification of the reaction medium, enol ester 10. 

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... 

In contrast to the reactions of acyclic ADC compounds, good evidence exists for dipolar intermediates in the reaction of PTAD with enol esters. Vinyl acetate (54, R = Me) and PTAD react in dichloromethane at 60°C to give the triazole 55 via the dipolar intermediate 56.96 When the bulkier... 

Heterocycles.—The phosphonium salt (59) is an effective three-carbon synthon, as demonstrated by its reaction with enolates of /9-keto-esters (Scheme 20) to give cyclopentenyl sulphides via an intramolecular Wittig reaction.63 Ylides are also intermediates in the synthesis of dihydrofurans (60) from the cyclopropylphos-phonium salt (61) and sodium carboxylates (Scheme 21).64 Cumulated ylides are very useful for the synthesis of heterocyclic compounds, e.g. (62), from molecules which contain both an acidic Y—H bond and a carbonyl or nitroso-function, as shown in Scheme 22.65... 

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... 

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