Carboxylic acids acyclic enolates

For acyclic systems, the anti diastereoselectivity of the (i )-enolates is lower than the syn diastereoselectivity of comparable (Z)-enolates. For example, carboxylic acid esters, which form predominantly ( )-enolates, react with aldehydes with high anti selectivity only in those cases where bulky aromatic substituents are in the alcoholic part of the ester22 25. 

The Claisen-type condensation reaction of cyclic vinylogous carboxylic acid triflates with lithium enolates and their analogues has provided acyclic alkynes bearing a 1,3-diketone-type moiety.19 The reaction mechanism has been proposed to proceed via a 1,2-addition of the enolate to the vinylogous acyl triflate, followed by fragmentation of the aldolate intermediate (Scheme 2). 

Diastereoselective Alkylations of Acyclic Enolates of Carboxylic Acid Derivatives... 

Alkylations of acyclic enolates containing a collection of chiral auxiliary groups have been used successfully for the asymmetric synthesis of carboxylic acids. The chiral, nonracemic substrates that have been used include amides, imides, esters, imine derivatives of glycinates and acyl derivatives of chiral transition metals. In these systems either extraannular or chelate-enforced intraannular chirality transfer may control the sense of the alkylation step. 

The intramolecular thioacylation of an ester enolate was used for the synthesis of 2-alkylthiopenem carboxylic acid derivatives. Sequential acylations have led to the synthesis of zwitterionic pyrazole-5-thiones from acyclic precursors, whereas 2-ethoxyoxazolidines react with the reagent to afford the products of AAacylation. ... 

Although this methodology is not successful with acyclic carboxylic acid derivatives (e.g. Weinreb amides, A-acyloxazolidinones, carboxylic esters or thioesters), it is possible to reach this kind of valuable enantiomerically enriched products by a simple oxidation with commercial bleach from the corresponding ketone (Scheme 29, top). If the ketone possesses a substituent prone to oxidation, an alternative procedure, involving the formation of a silyl enol ether (which does not need isolation/purification), can be followed instead (Scheme 29, bottom). 

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