Transesterification, of esters

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

Biocatalytic hydrolysis or transesterification of esters is one of the most widely used enzyme-catalyzed reactions. In addition to the kinetic resolution of common esters or amides, attention is also directed toward the reactions of other functional groups such as nitriles, epoxides, and glycosides. It is easy to run these reactions without the need for cofactors, and the commercial availability of many enzymes makes this area quite popular in the laboratory. 

Figure 11 Probable reaction mechanism for the transesterification of esters catalyzed by a supported titanate catalyst on silica (based on Blandy et al.f ...

Catalytic transesterification of esters 7.14 or 7.15 in EtO /EtOH afforded the free hemiacetal velutinal (7.11) (38), while both velutinal esters gave the methyl acetal 7.13 (38, 40) in HPLC grade methanol. 

It has been shown that it is the vinyl group that is transferred in this reaction, not an alcoholic one as in acid- or alkali-catalyzed transesterification of esters with saturated alcoholic components. With 0-labeled acetic acid in the transvinylation of vinyl-propionate, the labeled oxygen remained completely in the acetate group [68]. 

Transesterification of esters occurs by a mechanism very much like amide synthesis, but the reaction requires a catalytic amount of base (usually the Na salt of the alcohol). The nucleophile is the alkoxide. The reaction is driven in the forward direction by the use of a large excess of the starting alcohol. 

FIGURE 14.1. Esterification of carboxylic acids and transesterification of esters with extended processing... 

Transesterification of esters occurs by a mechanism very much like amide synthesis, but the reaction requires a catalytic amount of base (usually the Na salt... 

Syntheses of tellurol esters 3 are limited in comparison to thiol and selenol esters. A conventional preparative method is the reaction of tellurolate anions with acid chlorides or acid anhydride [113]. Co2(CO)8-mediated carbonylation of dichalcogenides (described in Sect. 3.1.10) is applicable to diphenyl ditel-luride, but the yields of tellurol esters are poor. They can also be obtained by transesterification of esters with z-Bu2AlTeBu. Reactions of tellurol esters 3 have not been extensively explored yet. However, generation of acyllithiums by Li-Te exchange is characteristic. They can be employed as acyl radical precursors in tin free radical reactions. 

Transesterification of Esters and the Acyiation and Deacyiation of Protected Aicohois... 

The key structural feature of POST-1 - the presence of dangling pyridine groups in the channels - affords a unique opportunity to perform asymmetric heterogeneous catalysis. Thus, potentially, any base catalyzed reactions (e.g., esterification or hydrolysis) can be performed with POST-1. Moreover, chiral pores should induce a degree of enantioselectivity in the final product mixture. The catalytic activity of POST-1 in the transesterification reaction was examined. Although the reaction of 16 and ethanol in the presence of POST-1 in carbon tetrachloride produced ethyl acetate in 11% yield, little or no transesterification occured without POST-1 or with the iV-methylated POST-1 (Sect. 2.2). The post chemical modification of the pyridine groups in POST-1 proves the role of free pyridine moiety in transesterification reaction. Transesterification of ester 16 with bulkier alcohols such as isobutanol, neopentanol, and 3,3,3-triphenyl-l-propanol occurs at a much slower rate under otherwise identical reaction conditions. Such size selectivity suggests that catalysis mainly occurs in the channels. 

Each trinuclear zinc unit contains six pyridyl groups, three of which are coordinated to the zinc ions, and two of the remaining three protonated. These exposed pyridine groups showed catalytic activity in the transesterification of ester E (Scheme 12). When racemic l-phenyl-2-propanol was used, the ester product exhibited a modest 8% ee in favor of the 5-enantiomer. 

These exchange reactions of Fischer carbene complexes occur through tetrahedral intermediates, just like the transesterification of esters. Reaction of a Fischer carbene complex with an alcohol in the presence of base generates the same type of anionic intermediate that an ester does. This intermediate breaks down like an ester to form a new alkoxy-substituted carbene complex. Reactions of amines with Fischer carbene complexes occur similarly, but the high basicity of amines and the high electrophilicity of the carbene complexes alleviates the need for any additional base. 

Another important concern is related to the chemical stability in solution of the analyte or SIS itself. Special consideration should also be given to the dangers inherent in use of protic solvents (water, alcohols etc.), e.g., hydrolysis or transesterification of ester analytes, D/H back exchange of a D-labeled SIS. 

Since the anion-exchange resins are frequently used in the hydroxy form, certain precautions should be observed when employing them with non-aqueous solvents. If acetone is used, excessive diacetone alcohol may be formed via base-catalyzed aldol condensation. A small amount of water should be present in all organic solvents containing alcohol to repress transesterification of esters (15) and esterification of acids (16) with alcohol as solvent. Without water, up to 5% of transesterification in 15 to 20 min has been observed with DEAE-Sephadex. After an overnight contact the transesterification approached 35 %. With 1 % water in the solvent, however, less than 0.3% transesterification and 0.3% saponification occurred when 70 mg of butyl stearate was passed through a DEAE-Sephadex column (15). 

The critical feature of the NCL method is that ligation occurs at an N-terminal Cys residue, no matter how many additional internal Cys residues are present in either segment. Such fascinating chemoselectivity allows the use of unprotected functional groups present in either of the two reacting proteins. For detailed studies of the mechanism of the NCL reaction, the interested reader should consult the studies by Dawson et al. [31, 32]. Here it suffices to say that transesterification of esters into thioesters represents an energetically uphill process in view of the higher reactivity (lower stabihty) of the latter. It can be achieved if the subtle interplay between entropic and enthalpic factors favours the process, or if the product is present in a low steady-state concentration, and is consumed in the subsequent irreversible step, as for the NCL process shown in Scheme 12.2. 

The first reaction designed specifically as an N-heterocyclic carbene catalyzed process was the transesterification of esters in the presence of IMes 13. This process, reported simultaneously by Hendrick and Waymouth and by Nolan, allowed the facile transesterification of esters with an added alcohol (Scheme 14.3). A number of researchers have also developed chiral NHCs for kinetic resolutions of alcohols using these processes. ... 

Transesterification of ester linkers can be conducted by many protocols. Scheme 11 gives a short overview illustrating the most popular procedures for a transesterification into methylesters [ 113,114]. 

Scheme 16 Enantioselective transesterification of esters with 1-butanol.

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