Esters intramolecular catalysis

The catalysis of ester hydrolysis by other groups within the ester molecule (intramolecular catalysis) has been extensively studied (17,18). These reactions are important because they simulate catalysis by en2ymes. Intramolecular catalysis of esters has been used as a model in dmg discovery efforts (19). 

Each of the following molecules has been considered to be capable of some form of intramolecular catalysis of ester hydrolysis. For each reactant, indicate one or more mechanisms by which intramolecular catalysis might occur. Depict a transition-state arrangement that shows this catalysis. 

Yet another distinction is between intermolecular catalysis, in which the catalytic function and the reaction site are on different molecules, and intramolecular catalysis, in which the catalytic function and the reaction site are within the same molecule. All of the above examples constitute intermolecular catalyses. The following reaction, the hydrolysis of a monomaleate ester, is an intramolecular nucleophilic catalysis. 

Table 7-17. Relative Rates of Intramolecular Catalysis of Ester Hydrolysis by Carboxylate Groups...

Because esters 745 a, b of imidazole-acetic acid are unstable when stored for long periods, owing to intramolecular catalysis by the imidazole moiety, the esters should be converted into their salts or free acids and stored as such. Only tert-butyl imidazole-(4,5)acetates derived from tert-butyl 4-chloroacetoacetate seem to be stable [232, 233]. N-alkyl-substituted amidines give rise to a mixture of alkyl N-alkylimidazole-4- and 5-acetates [232, 233]. 

Ester hydrolysis has been a very important system for the investigation of intramolecular catalysis. The types of displacements involved have been... 

These relations both demonstrate the inhibition of formation of the tetrahedral state which can be clearly attributed to steric crowding. Such a correlation as (5) confirms the attack at the neighbouring carbonyl group and this intramolecular catalysis for all this series. The activation parameters for the alkaline hydrolysis of these esters were also measured and are shown in Table 1. The enthalpies of activation of the 2-formyl, 2-acetyl, 2-propionyl, 2-isobutyryl and 2-pivaloyl esters are exceptionally small. These are... 

The use of mesitoate esters in the elucidation of reaction mechanisms has been pioneered by Burrows and Topping (1969,1970). This system has been used to suppress the competitive intermolecular reaction by steric bulk effects and to detect participation by the identification of the products formed. Under identical conditions (pH 11.28 at 30°C in 9.5% ethanol-water), 2-acetylphenyl mesitoate [41]is hydrolysed 130 times more readily than 4-acetylphenyl mesitoate, clearly indicating intramolecular catalysis. However, the products of hydrolysis provided no clue to the mechanism of... 

The use of mesitoic acid esters has again been successfully employed by Burrows and Topping (1975) in the elucidation of intramolecular carbon acid participation. Under basic aqueous conditions, 2-acetylphenyl mesitoate [41] hydrolyses to yield mesitoic acid and 2-hydroxyacetophenone, reacting with intramolecular catalysis via the monoanion of the ketonic hydrate (see p. 192). However, in 47.5% aqueous ethanol containing potassium hydroxide, the reaction products from l-acetyl-2-naphthyl mesitoate [45] were found... 

Just as intramolecular catalysis has been observed in the hydrolysis of carboxylate esters, a variety of neighbouring groups can participate in the hydrolysis of phosphates. The alkaline hydrolysis of dimethyl phosphoacetoin [47] has found to be ca. 2 x 106 times faster in water at 25°C than that of... 

Fig. 7.2. a) The most common mechanism of base-catalyzed ester hydrolysis, namely specific base catalysis (HCT catalysis) with tetrahedral intermediate and acyl cleavage. Not shown here are an W mechanism with alkyl cleavage observed with some tertiary alkyl esters, and an 5n2 mechanism with alkyl cleavage sometimes observed with primary alkyl esters, particularly methyl esters, b) Schematic mechanism of general base catalysis in ester hydrolysis. Intermolecular catalysis (bl) and intramolecular catalysis (b2). c) The base-catalyzed hydrolysis of esters is but a particular case of nucleophilic attack. Intermolecular (cl) and intramolecular (c2). d) Spontaneous (uncatalyzed) hydrolysis. This becomes possible when the R moiety is... 

M. I. Page, D. Render, G. Bemath, Stereochemical Studies. Part 112. Geometrical Dependence of Intramolecular Catalysis in the Hydrolysis and Aminolysis of Aryl Esters , J. Chem. Soc., Perkin Trans. 2 1986, 867-871. 

One must be careful not to generalize from the above discussion that hydrolysis of pivaloyl esters will always be slow. Indeed, a notable exception may well exist for monoesters of catechols, where intramolecular catalysis accelerates hydrolysis. This was seen for L-3-[3-hydroxy-4-(pivaloyl-oxy)phenyl] alanine (8.81 4-pivaloyl-L-dopa), a potentially valuable prodrug of L-dopa [114], When given to rats and dogs, 4-pivaloyl-L-dopa displayed markedly longer duration of action and a higher bioavailability of L-dopa than the drug itself complete conversion to L-dopa was noted in rat. The... 

Dunn and Bruice (1970, 1971) presented evidence that the mechanism of hydrolysis of methoxymethoxybenzoic acid is actually A-1. Evidence included a Bronsted coefficient for intramolecular catalysis of DO, based upon points for the unsubstituted compound and the 5-NO2 derivative, and p -values that were the same (—3-0) for both the methyl ester and the carboxyl derivatives of [72] when... 

Intramolecular catalysis has become a large subject in its own right in recent years, and is discussed adequately elsewhere. As so often in the study of the mechanisms of chemical reactions, esters have been used more often than any other type of compound in the study of intramolecular catalysis. So the reader is referred to the comprehensive survey of intramolecular catalysis by Bruice and Benkovic (ref. 3, pp. 119-211) and the more recent review by Kirby and Fersht305, which brings the topic up to date (December 1969). 

This reaction process is depicted in general terms in equation (34), and includes the category generalized in equation (35). The earliest example of this type of reaction appears to be the ready transesterification of uncomplexed ester groups shown in Scheme 39,133,134 which intramolecular participation forms a new chelate ring in the transition state. Since this discovery, numerous studies have been made on the intramolecular catalysis of ester hydrolysis by metal-complexed hydroxide... 

Intramolecular catalysis of amide bond isomerization is believed to play a key role in the folding of several proteins and this process has now been demonstrated experimentally including evidence for an H-bond between the side-chain and the prolyl Na in a cis-proline peptidomimetic.143 The amide (178) and the ester (179) have been used as substrates for these studies. Support for intramolecular nucleophilic attack... 

Work by Sigwalt, Bischoff and Cypryk22 have used this inter-and intramolecular catalysis to explain the condensation kinetics and cyclic formation processes in siloxane condensations. The kinetics show a very complex dependence on siloxane chain length, complicated by equilibria involving acid, silanol and water. They do indicate that the dominating reaction in the process is condensation and that chain disproportionation and chain scrambling are negligible The kinetics of condensation are influenced by the involvement of triflic acid in several equilibria, i.e. the formation of triflate esters, shown in equation 3, the possible involvement of triflic acid in the reaction of these esters with silanol, shown... 

Intramolecular catalysis by the carbonyl group is important in some reaction systems and can sometimes be a component of the ori/zo-effect. Bowden has recently reviewed intramolecular catalysis by carbonyl groups in ester hydrolysis115. 

Succinate esters serve as examples of derivatives that exhibit less than optimal pH-hydrolysis rate behavior owing to their increased reactivity in water as a result of intramolecular catalysis of hydrolysis by the terminal carboxylic acid functionality (Anderson and Taphouse, 1981 Anderson etal., 1984 Damen etal., 2000). Since intramolecular catalytic effects are quite sensitive to geometric factors and distances separating the interactive groups (Anderson and Conradi, 1987), intramolecular catalysis by a terminal ionizable group should be easily controlled by varying the alkyl chain length. 

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