Esters acyloins

This modification has become the standard procedure for the acyloin ester condensation. By doing so, the formation of products from the otherwise competitive Dieckmann condensation (Claisen ester condensation) can be avoided. A product formed by ring closure through a Dieckmann condensation consists of a ring that is smaller by one carbon atom than the corresponding cyclic acyloin. 

A spectacular application of the acyloin ester condensation was the preparation of catenaries like 11. These were prepared by a statistical synthesis which means that an acyloin reaction of the diester 10 has been carried out in the presence of an excess of a large ring compound such as 9, with the hope that some diester molecules would be threaded through a ring, and would then undergo ring closure to give the catena compound ... 

The Corey-Winter reaction provides a useful method for the preparation of olefins that are not accessible by other routes. For instance it may be used for the synthesis of sterically crowded targets, since the initial attack of phosphorus at the sulfur takes place quite distantly from sterically demanding groups that might be present in the substrate molecule. Moreover the required vicinal diols are easily accessible, e.g. by the carbon-carbon bond forming acyloin ester condensation followed by a reductive step. By such a route the twistene 10 has been synthesized ... 

In general, the dissolving metal or electrochemical reductions of acyl halides or acyclic anhydrides are not useful for the preparation of primary alcohols. Such reductions invariably provide acyloin esters, ene-diolate diesters or related species. Cyclic anhydrides may be reduced to give lactones. For example, the reduction of phthalic anhydride at a mercury cathode has been used in the synthesis of phthalide (90%). In general, however, such reduction are not widely employed in synthesis. 

There are currently two proposed mechanisms for the acyloin ester condensation reaction. In mechanism A the sodium reacts with the ester in a single electron transfer (SET) process to give a radical anion species, which can dimerize to a dialkoxy dianion. Elimination of two alkoxide anions gives a diketone. Further reduction (electron transfer from the sodium metal to the diketone) leads to a new dianion, which upon acidic work-up yields an enediol that tautomerizes to an acyloin. In mechanism B an epoxide intermediate is proposed. ... 

Conventionally the acyl derivatives of acyloins and benzoins may readily be obtained by reaction with acid anhydrides. The acyloin esters can more conveniently be prepared by allowing the corresponding a-bromo ketones and the sodium or potassium salts of the appropriate carboxylic acid to react, either in ethanol or in the acid to be esterified. The esters may not necessarily be isolated, and the oxazoles are obtained directly by adding the ammonium salt to, or passing ammonia through, the mixture, which is then boiled.84 The reaction then becomes useful for the preparation of oxazoles (53) from a-bromo ketones and carboxylic acids.84 121... 

The acyloin ester can be prepared from treatment of the corresponding o-bromo ketone 10 with the sodium or potassium salt of the desired carboxylie acid 11. This reaction can be carried out neat in the carbocylic acid used or in ethanol. Using this method, the ester may not necessarily need to be isolated and the addition of ammonium acetate to... 

---