Transesterification carboxylic acids

The process of transesterification is an important way to prepare a large number of esters from more complex or more simple esters without passing through the carboxylic acid. Transesterification can be used to convert one type of ester to another type that can then be removed under a different set of conditions. This section describes many of the methods that have been found effective for ester metathesis. ... 

ROH or RO Esterification of carboxylic acids transesterification giving polyethyl-eneterephthalate (Dacron ) Lactonization of hydroxy acids (Section 6.4.2)... 

Without additional reagents /3-Diethylaminoethyl esters of thiazole carboxylic acids Transesterification... 

Chirazymes. These are commercially available enzymes e.g. lipases, esterases, that can be used for the preparation of a variety of optically active carboxylic acids, alcohols and amines. They can cause regio and stereospecific hydrolysis and do not require cofactors. Some can be used also for esterification or transesterification in neat organic solvents. The proteases, amidases and oxidases are obtained from bacteria or fungi, whereas esterases are from pig liver and thermophilic bacteria. For preparative work the enzymes are covalently bound to a carrier and do not therefore contaminate the reaction products. Chirazymes are available form Roche Molecular Biochemicals and are used without further purification. 

The cinnamyl ester can be prepared from an activated carboxylic acid derivative and cinnamyl alcohol or by transesterification with cinnamyl alcohol in the presence of the H-Beta Zeolite (toluene, reflux, 8 h, 59-96% yield). It is cleaved under nearly neutral conditions [Hg(OAc)2, MeOH, 23°, 2-A h KSCN, H2O, 23°, 12-16 h, 90% yield]or by treatment with Sulfated-Sn02, toluene, anisole, reflux. The latter conditions also cleave crotyl and prenyl esters. 

Subsequently it was found140 that ethyl 2-alkyl-1//-azepine-1-carboxylates can be isolated from a mixture of isomeric 1//-azepines by stirring the mixture with potassium hydroxide in ethanol at room temperature. Apparently, this method, which is limited to 2-alkylated azepines, depends on the slower rate of hydrolysis (and subsequent decomposition of the resulting 1H-azepine-l-carboxylic acid) of the sterically hindered 1-(ethoxycarbonyl) group. Although the yields of l//-azepines are poor (4-7%, vide supra), the method provides access to otherwise difficult to obtain, isomerically pure 2-alkyl-1//-azepines. Under the basic hydrolysis conditions aryl 2-alkyl-l//-azepine-1-carboxylates undergo transesterification to the l-(ethoxycarbonyl) derivatives. 

Transesterification and Hydrolysis of Carboxylic Acid Derivatives, Alcohols, and Epoxides... 

Conversion of Free or Silylated Carboxylic Acids into Esters, Thioesters, Lactones, or Ketenes. Transesterification of Esters with Alcohols... 

One of the most important characteristics of IL is its wide temperature range for the liquid phase with no vapor pressure, so next we tested the lipase-catalyzed reaction under reduced pressure. It is known that usual methyl esters are not suitable for lipase-catalyzed transesterification as acyl donors because reverse reaction with produced methanol takes place. However, we can avoid such difficulty when the reaction is carried out under reduced pressure even if methyl esters are used as the acyl donor, because the produced methanol is removed immediately from the reaction mixture and thus the reaction equilibrium goes through to produce the desired product. To realize this idea, proper choice of the acyl donor ester was very important. The desired reaction was accomplished using methyl phenylth-ioacetate as acyl donor. Various methyl esters can also be used as acyl donor for these reactions methyl nonanoate was also recommended and efficient optical resolution was accomplished. Using our system, we demonstrated the completely recyclable use of lipase. The transesterification took place smoothly under reduced pressure at 10 Torr at 40°C when 0.5 equivalent of methyl phenylthioacetate was used as acyl donor, and we were able to obtain this compound in optically pure form. Five repetitions of this process showed no drop in the reaction rate (Fig. 4). Recently Kato reported nice additional examples of lipase-catalyzed reaction based on the same idea that CAL-B-catalyzed esterification or amidation of carboxylic acid was accomplished under reduced pressure conditions. ... 

Esterification and transesterification using TiIV compounds are useful methods for functionalization of ester moieties under mild conditions. In the transformation of carboxylic acids to esters, a catalytic amount of TiCl(OTf)3 is effective (Scheme 30).110 Titanium alkoxides, such as Ti(OEt)4 or Ti(0 Pr)4, easily promote transesterification of alkoxy groups to other ones—even to more hindered groups.111 Anomerization of glycosides to Q-isomers using a Tilv-bascd Lewis acid is an important method for controlling the product structure.112... 

Histidine residues are efficient nucleophiles in aqueous solution at pH 7, much more so than lysines, and this is the basis for the site-selective functionalization of lysine residues in folded polypeptides and proteins [24, 25]. p-Nitrophenyl esters react with His residues in a two-step reaction to form an acyl intermediate under the release of p-nitrophenol followed by the reaction of the intermediate with the most potent nucleophile in solution to form the reaction product. In aqueous solution the reaction product is the carboxylic acid since the hydroxide ion is the most efficient nucleophile at pH 7. If there is an alcohol present the reaction product will be an ester and the overall reaction is a transesterification reaction. 

In addition, iodine snccessfnlly catalyzed the electrophilic snbstitntion reaction of indoles with aldehydes and ketones to bis(indonyl)methanes [225], the deprotection of aromatic acetates [226], esterifications [227], transesterifications [227], the chemoselective thioacetalization of carbon functions [228], the addition of mercaptans to a,P-nnsatnrated carboxylic acids [229], the imino-Diels-Alder reaction [230], the synthesis of iV-Boc protected amines [231], the preparation of alkynyl sngars from D-glycals [232], the preparation of methyl bisnlfate [233], and the synthesis of P-acetamido ketones from aromatic aldehydes, enolizable ketones or ketoesters and acetonitrile [234],... 

Vinyl acetate was produced by the catalytic acetylation of acetylene, but this monomer is now produced by the catalytic oxidative condensation of acetic acid and ethylene (structure 17.32). Other vinyl esters can be produced by the transesterification of vinyl acetate with higher boiling carboxylic acids. 

Whereas the Markovnikov addition of carboxylic acids to propargylic alcohols produces P-ketoesters, resulting from intramolecular transesterification [30, 31], the addition to propargylic alcohols in the presence of Ru(methallyl)2(dppe) 1 at 65 °C leads to hydroxylated alk-l-en-l-yl esters via formation of a hydroxy vinylidene intermediate [32, 33]. The stereoselectivities are lo ver than those obtained from non-hydroxylated substrates. These esters, which are protected forms of aldehydes, can easily be cleaved under thermal or acidic conditions to give conjugated enals, corresponding to the formal isomerization products of the starting alcohols (Scheme 10.6). 

Fig. 11 Copolymerization of CL with aliphatic co-mercapto carboxylic acids (top) and transesterification of pCL with aliphatic co-mercapto carboxylic acids (bottom) to give random copolymers with thioester linkages [30]...

Contrary to the optical resolutions described in Sections 2.1.1.-2.1.3., which depend on the solubility or chromatographic properties ( Thermodynamic resolution ), the kinetic resolution rests on rate differences shown by the enantiomers when reacted with an optically active reagent. In the ideal case, only one enantiomer is converted into the envisaged product and the other enantiomer is unchanged. In this way, optical resolution is reduced to the more simple separation of two different reaction products. In practice, only two methods of kinetic resolution are reasonably general and reliable the Sharpless epoxidation of allylic alcohols and the enzymatic transesterification of racemic alcohols or carboxylic acids. 

Catalysis by Solid Acids. Two aspects are considered here. The first aspect is concerned with transesterification reactions catalyzed by solid acids. Unfortunately, little research dealing with this subject has been reported in the literature. The second aspect deals with esterification reactions of carboxylic acids (or FFAs). This second part addresses an important characteristic of inexpensive TG feedstocks, i.e., high FFA content. Ideally, an active solid catalyst should be able to carry out transesterification and esterification simultaneously, thus eliminating pretreatment steps. It is likely that heterogeneous catalysts that perform well in esterification should also be good candidates for transesterification since the mechanisms for both reactions are quite similar. 

Naturally occurring fatty alcohols used in the fragrance industry are produced principally by reduction of the methyl esters of the corresponding carboxylic acids, which are obtained by transesterification of natural fats and oils with methanol. Industrial reduction processes include catalytic hydrogenation in the presence of copper-chromium oxide catalysts (Adkins catalysts) and reduction with sodium (Bouveault—Blanc reduction). Unsaturated alcohols can also be prepared by the latter method. Numerous alcohols used in flavor compositions are, meantime, produced by biotechnological processes [11]. Alcohols are starting materials for aldehydes and esters. 

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