Hydrolysis reactions of esters

Enzymes are classified into six categories depending on the kind of reaction they catalyze, as shown in Table 26.2. Oxidoreductases catalyze oxidations and reductions hansferases catalyze the transfer of a group from one substrate to another hydrolases catalyze hydrolysis reactions of esters, amides, and related substrates lyases catalyze the elimination or addition of a small molecule such as H2O from or to a substrate isomerases catalyze isomerizalions and ligases catalyze the bonding together of two molecules, often coupled with the hydrolysis... 

Enzymatic Hydrolysis Reactions of Esters. Xenobiotic compounds containing esters or other acid derivatives in their structures (e.g., amides, carbamates, ureas, etc., see Table 17.3) are often readily hydrolyzed by microorganisms. To understand how enzymatic steps can be used to transform these substances, it is instructive to consider the hydrolases (i.e., enzymes that catalyze hydrolysis reactions) used by organisms to split naturally occurring analogs (e.g., fatty acid esters in lipids or amides in proteins). The same chemical processes, and possibly even some of the same enzymes themselves, are involved in the hydrolysis of xenobiotic substrates. 

Thus it is possible to study the hydrolysis reactions of esters under conditions where the substrate is completely protonated. The properties of the protonated ester, however, are more conveniently examined using more strongly acidic media, in the absence of water, where bimolecular reactions are reduced to insignificance. At sufficiently low temperatures under these conditions the rates of exchange of the added protons are slow, and the detailed structures of protonated carboxylic acids and esters can be investigated, particularly by proton nmr techniques. 

Despite the presence of water throughout the body, hydrolysis reactions of esters and amides require enzymes to proceed at an appreciable rate. Numerous enzymes throughout the body carry out hydrolysis reactions. The enzymes, both esterases and amidases, are found in the digestive system, individual cells, and plasma. The exact site of hydrolysis of a specific drug depends on the drug s structure and functionality. The ester in aspirin... 

Avery similar strategy has been applied to generate catalytic MIPs TSAs have been used as a template to generate specific binding sites in polymers using covalent and noncovalent imprinting techniques. As for catalytic antibodies, hydrolysis reactions of esters have been the focus of interest. One of the most successful systems described so far will be our first example [14]. 

Homogeneous non-catalytic reactions in supercritical solvents Most non-catalytic, supercritical, homogeneous reactions are carried out in supercritical water. Hydration/dehydration, hydrolysis, decomposition and oxidation reactions are main reaction categories. No homogeneous catalyst is added in hydrolysis reactions of esters, nitriles and butyronilriles, however the reactions are autocatalytic. Biodiesel production... 

Reactions of esters and lactones (Section 21.6) (a) Hydrolysis to yield acids... 

A chemist is studying the mechanism of the following hydrolysis reaction of the organic ester methyl acetate CH.COOCI I, + H,0 - CH,COOH + CH,OH. The question arises whether the O atom in the product methanol comes from the methyl acetate or from the water. Propose an experiment using isotopes that would allow the chemist to determine the origin of the oxygen atom. 

The monomeric metaphosphate ion itself commands a fair amount of attention in discussions of metaphosphates. It is postulated as an intermediate of numerous hydrolysis reactions of phosphoric esters 52 S4,S5) and also of phosphorylation reactions S6> kinetic and mechanistic studies demonstrate the plausibility of such an assumption. In addition, the transient formation of ester derivatives of meta-phosphoric acid — in which the double-bonded oxygen can also be replaced by thio and imino — has also been observed they were detected mainly on the basis of the electrophilic nature of the phosphorus. 

There are a few reported cases of esterases that catalyze not only hydrolysis but also the reverse reaction of ester formation, in analogy with the global reaction described for serine peptidases (Fig. 3.4). Thus, cholesterol esterase can catalyze the esterification of oleic acid with cholesterol and, more importantly in our context, that of fatty acids with haloethanols [54], Esterification and transesterification reactions are also mediated by carboxyleste-rases, as discussed in greater detail in Sect. 7.4. 

Both esters and amides undergo hydrolysis reactions. In a hydrolysis reaction, the ester or amide bond is cleaved, or split in two, to form two products. As mentioned earlier, the hydrolysis of an ester produces a carboxylic acid and an alcohol. The hydrolysis of an amide produces a carboxylic acid and an amine. There are two methods of hydrolysis acidic hydrolysis and basic hydrolysis. Both methods are shown in Figure 2.9. Hydrolysis usually requires heat. In acidic hydrolysis, the ester or amide reacts with water in the presence of an acid, such as H2SO4. In basic hydrolysis, the ester or amide reacts with the OH ion, from NaOH or water, in the presence of a base. Soap is made by the basic hydrolysis of ester bonds in vegetable oils or animal fats. 

Hubbard and Kirsch (1972) have recently proposed that histidine may act as a nucleophile in a-chymotrypsin acylation reactions of esters having a good leaving group (jO-nitrophenol). This suggestion was based on a similarity in p-value for acylation by p-substituted nitrophenyl and dinitrophenyl benzoates and nucleophilic attack on these compounds by imidazole, in contrast with less positive p-values for hydroxide ion catalysis. Hammett p-values for hydrolysis of substituted phenyl esters are given in Table 6 and show little apparent trend. The values for hydroxide ion and alcoholate ions are... 

Curtius reaction of ester 196 in benzyl alcohol leads to benzylurethane 197, which by hydrolysis and catalytic reduction gives amine 198 (72JCS(P1)878, 69JCS(C)2235). The primary amino group of compound 198 was converted into dimethylamino using the Eschweiler reaction, and then selectively quaterniz. Hofmann cleavage of this salt then leads to furo[4,3,2-/g]benzazocine 199 (Scheme 54). 

Not surprisingly, it is rather difficult to separate the different contributions of the different interactions as they occur in the micellar Stern region. In an attempt to solve this problem, the group of Engberts used a series of hydrolysis reactions of activated esters and amides to probe the reaction environment offered by micelles. The reactions initially involved the water-catalyzed pH-independent hydrolysis reactions of i-methoxy-phenyl dichloroacetate 4 and l-benzoyl-3-phenyl-l,2,4-triazole 5, as extensive information on the rate retarding effects of added cosolutes on this reaction was available. ... 

Novak and co-workers have examined the hydrolysis reactions of a series of esters, 158a-i, of hydroxylamines or hydroxamic acids that are derived... 

Asymmetric transformations of ot-amino acids promoted by optically active metal complexes have been reported by several groups 269). The control of the stereoselective hydrolysis reactions of racemic esters by chiral micellar compounds prepared from amino acids has been intensively investigated 270). 

Major hydrolysis reactions are ester and amide hydrolysis. These are catalyzed by a group of enzymes with overlapping substrate specificity and activity. Hydrazides can also undergo hydrolysis. Some of the newer drugs such as hormones, growth factors, and cytokines now being produced are peptides, and certain toxins are also peptides or proteins, so the role of peptidases may be important. 

A chemist would like to study the mechanism of the hydrolysis reaction of the organic ester methyl acetate as shown below. 

The most important reaction of esters is their conversion by a carbonyl-group substitution reaction into carboxylic acids. Both in the laboratory and in the body, esters undergo a reaction with water—a hydrolysis—that splits the ester molecule into a carboxylic acid and an alcohol. The net effect is a substitution of -OC by -OH. Although the reaction is slow in pure water, it is catalyzed by both acid and base. Base-catalyzed ester hydrolysis is often called saponification, from the Latin word sapo meaning "soap." Soap, in fact, is a mixture of sodium salts of long-chain carboxylic acids and is produced by hydrolysis with aqueous NaOH of the naturally occurring esters in animal fat. 

The hydrolysis reactions of A -phospho amino acids seen as models for protein dephosphorylation have been studied in Tris-HCl buffer (pH7.5)-DMSO. The reactions were first order and the rates were very much faster than those of simple phosphoamidates. A pentacoordinated phosphorus intermediate is proposed on the reaction pathway.265 The rates of ester exchange reactions of alcohols (nucleoside models) with the oxyphosphorane (299) have been studied and the rates of exchange are much faster for diols than for mono-alcohols.266... 

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