Carboxylic acid, enzymatic esterification

Since the imidazolide method proceeds almost quantitatively, it has been used for the synthesis of isotopically labeled esters (see also Section 3.2), and it is always useful for the esterification of sensitive carboxylic acids, alcohols, and phenols under mild conditions. This advantage has been utilized in biochemistry for the study of transacylating enzymes. A number of enzymatic transacylations (e.g., those catalyzed by oc-chymo-trypsin) have been shown to proceed in two steps an acyl group is first transferred from the substrate to the enzyme to form an acyl enzyme, which is then deacylated in a second step. In this context it has been shown[21] that oc-chymotrypsin is rapidly and quantitatively acylated by Af-fraw.s-cinnamoylimidazole to give /ra/w-cinnamoyl-a-chymotrypsin, which can be isolated in preparative quantities and retains its enzymatic activity (see also Chapter 6). 

It therefore appeared that a general mechanism for enzymatic esterification of phenolic acids with glucose was operative, whereas the reaction with other alcoholic moieties proceeded via carboxyl-activated acyl derivatives. [In this context it should be emphasized that glucose esters must not be confused with glucosides different enzymes are involved in the biosynthesis of these two types of phenolic glucose derivatives (36)]. 

Fig. 18 Enzymatic esterification of glucoside with a telecheUc carboxylic acid-terminated polysiloxane...

A relatively novel class of derivatives is obtained by the covalent incorporation of organometallic moieties into cellulose. For example, cellulose ferro-cenyl derivatives have been prepared by esterification of cellulose with an intermediate derived from ferrocene carboxylic acid and triphenyl phosphite in the presence of pyridine [84]. An enzymatically cleavable cellulose ester has been developed [85], and prodrugs have been coupled to the hydroxyl or carboxyl functions of C-terminal aromatic amino acids of cellulose peptide derivatives for controlled release applications [86]. 

Under almost anhydrous conditions in organic medium, lipases can be used in the reverse mode for direct ester synthesis from carboxylic acids and alcohols, as well as transesterifications (acyl transfer reactions) which can be divided into alcoholysis (ester and alcohol), acidolysis (ester and acid), and interesterification (ester-ester interchange). The direct esterification and alcoholysis in particular have been most frequently used in asymmetric transformations involving lipases. The parameters that influence enzymatic catalysis in organic solvents have been intensively studied and discussed. ... 

Enzymatic polymerization can be divided with regard to the polymerization mechanism into polycondensation and ROP reactions. A prominent example of polycondensation reactions is the esterification reaction (Fig. 3.8a). An activation of the carboxylic acid-containing monomer can be achieved by esterification with an alcohol. The resulting polymerization reaction is then called a transesterification (Fig. 3.37b). Since these are reversible reactions, the equilibrium needs to be shifted to the product side that requires the removal of the formed water (esterification) or alcohol (transesterification). In polycondensation reactions, the product molecular weight and the end group... 

During heating and long storage of foods, esters may also be formed in small quantities as secondary odour-active compounds. For example, esters are formed during aging of wines and spirits by esterihcation of carboxylic acids with alcohols (ethanol or fusel oil alcohols, but the non-enzymatic esterification is a very slow reaction), acidolysis (reaction of acids with esters), alcoholysis (reaction of alcohols with esters) or ester exchange (reaction of esters with other esters). 

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