Carboxylic acids cesium salts, alkylation

Especially for large-scale work, esters, may be more safely and efficiently prepared by reaction of carboxylate salts with alkyl halides or tosylates. Carboxylate anions are not very reactive nucleophiles so the best results are obtained in polar aprotic solvents54 or with crown ether catalysts.55 The reactivity for the salts is Na+ < K+ < Rb+ < Cs+. Cesium carboxylates are especially useful in polar aprotic solvents. The enhanced reactivity of the cesium salts is due both to high solubility and to the absence of ion pairing with the anion.56 Acetone has been found to be a good solvent for reaction of carboxylate anions with alkyl iodides.57 Cesium fluoride in DMF is another useful combination.58 Carboxylate alkylation procedures have been particularly advantageous for preparation of hindered esters that can be relatively difficult to prepare by the acid-catalyzed esterification method (Fischer esterification) which will be discussed in Section... 

Saponification with alkali generally used for the removal of methyl and ethyl esters is avoided if instead of simple alkyl esters rather benzyl esters or tert.butyl esters are applied for the protection of carboxyl groups. In addition to acid catalyzed esterification benzyl esters can also be secured by the reaction of cesium salts of (blocked) amino acids with benzyl chloride ... 

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