Carboxylic Esters and Lactones

Carboxylic esters undergo the conversion C=0— C=CHR (R = primary or secondary alkyl) when treated with RCHBr2, Zn, and TiCl4 in the presence of A,A,A, iV -tetramethylethylenediamine. Metal carbene complexes R2C=ML (L = ligand), where M is a transition metal such as Zr, W, or Ta, have also been used to convert the C=0 of carboxylic esters and lactones to CR2. It is likely that the complex Cp2Ti=CH2 is an intermediate in the reaction with Tebbe s reagent. 

Thiolate ions are also useful for the demethylation of certain ethers, esters, amines, and quaternary ammonium salts. Aryl methyl ethers can be cleaved by heating with EtS in the dipolar aprotic solvent DMF ROAr + EtS ArO +EtSR. Carboxylic esters and lactones are cleaved (the lactones give co-alkylthio carboxylic acids) with a thiol and AICI3 or AlBrs. Esters and lactones... 

Shiina I (2004) An effective method for the synthesis of carboxylic esters and lactones using substituted benzoic anhydrides with Lewis acid catalysts. Tetrahedron 60 1587-1599... 

Thionation of Carboxylate Esters and Lactones. The reaction of esters and lactones with LR represents a well-established method for the synthesis of the corresponding thiocarbony 1 deriva-tives. - In fact, LR is the much superior reagent for the thionation of esters as compared with phosphorus pentasullide. Open-chain thionoesters have been conveniently obtained. Interestingly, natural triglycerides also smoothly react with LR to yield pure tristhionotriglycerides. Even better results are obtained with carboxylate esters if the improved solubilized LR or microwave irradiation are applied. The method has particularly been used for the preparation of cyclic thionoesters. Obviously there are very few structural or other restrictions that inhibit the formation of y-,S-, or macrocyclic thionolactones. Once more, microwave irradiation significantly improves the re-... 

Characteristic Spectroscopic Data of Carboxylic Esters and Lactones... 

Mukaiyama developed l-alkyl-2-halopyridinium salts, which are useful reagents for the preparation of carboxylic esters and lactones in the presence of tertiary amines [22]. After generation of activated onium salts from the corresponding co-hydroxycarboxylic acids, spontaneous lactonization smoothly proceeds to produce various macrolactones. Bartlett first used this lactonization method for the... 

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