Transesterification carboxylic acid esters from

Carboxylic acid esters from carboxylic acids by transesterification... 

Figure 5.21. Reaction schemes for the most common types of step-growth polymerization. Shown are (a/c) polyester formation, (b/d) polyamide formation, (e) polyamide formation through reaction of an acid chloride with a diamine, (f) transesterification involving a carboxylic acid ester and an alcohol, (g) polybenzimidazole formation through condensation of a dicarboxyhc add and aromatic tetramines, and (h) polyimide formation from the reaction of dianhydrides and diamines.

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. ... 

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. 

The development of monoalkyl phosphate as a low skin irritating anionic surfactant is accented in a review with 30 references on monoalkyl phosphate salts, including surface-active properties, cutaneous effects, and applications to paste and liquid-type skin cleansers, and also phosphorylation reactions from the viewpoint of industrial production [26]. Amine salts of acrylate ester polymers, which are physiologically acceptable and useful as surfactants, are prepared by transesterification of alkyl acrylate polymers with 4-morpholinethanol or the alkanolamines and fatty alcohols or alkoxylated alkylphenols, and neutralizing with carboxylic or phosphoric acid. The polymer salt was used as an emulsifying agent for oils and waxes [70]. Preparation of pharmaceutical liposomes with surfactants derived from phosphoric acid is described in [279]. Lipid bilayer vesicles comprise an anionic or zwitterionic surfactant which when dispersed in H20 at a temperature above the phase transition temperature is in a micellar phase and a second lipid which is a single-chain fatty acid, fatty acid ester, or fatty alcohol which is in an emulsion phase, and cholesterol or a derivative. 

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. ... 

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). 

Esters are produced by acid-catalysed reaction of carboxylic acids with alcohols, known as Fischer esterification. They are also obtained from acid chlorides, acid anhydrides and other esters. The preparation of esters from other esters in the presence of an acid or a base catalyst is called transesterification. All these conversions involve nucleophilic acyl suhstitu-tions (see Section 5.5.5). 

Esters are less reactive than acid chlorides and acid anhydrides. They are converted to carboxylic acid hy acid or base hydrolysis, and to another ester by acid or base alcoholysis (transesterification). The 1°, 2° or 3° amides are obtained from esters by treatment with ammonia or 1° or 2° amines, respectively. 

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. ... 

Enantiomerically pure carboxylic acids are routinely obtained from N-acylsultams by Hydrogen Peroxide assisted saponification with Lithium Hydroxide in aqueous THF. 4 Alternatively, transesterification can be effected under neutral conditions in allyl alcohol containing Titanium Tetraisopropoxide, giving the corresponding allyl esters which can be isomerized/hydrolyzed with Wilkinson s catalyst (Chlorotris(triphenylphosphine)rhodium(I)) in Et0H-H20. This provides a convenient route to carboxylic acids containing base-sensitive functionality. Primary alcohols are obtained by treatment with L-Selectride (Lithium Tri-s-butylborohydride) in THF at ambient temperature. ... 

In a related approach, pentafluorophenyl esters of Fmoc amino acids 61 (see Table 14) were prepared from a mixture of acid 59 and pentafluorophenyl trifluoroacetate in dimeth-ylformamide in the presence of pyridine,which is a transesterification method developed by Sakakibara (Scheme The reaction involves formation of mixed carboxylic acid... 

The preparation of esters can be classified into two main categories (1) carboxylate activation with a good leaving group and (2) nucleophilic displacement of a carboxylate on an alkyl halide or sulfonate. For simple esters, acid-catalyzed esterification with azeotropic removal of water is also very effective, but limited to simple systems for the most part. The nucleophilic approach is generally not suitable for the preparation of esters if the halide or tosylate is sterically hindered, but there has been some success with simple secondary halides and tosylates (ROTs, DMF, K2CO3, 69-93% yield). The section on transesterification should also be consulted, since this technology can be quite useful for the preparation of esters from other esters. 

The method published by Pfizer5 relies on the formation of an ester 21 of an intermediate carboxylic acid 19 with the alcohol 20 derived from available mandelic acid and the separation of the diastereoisomers by chromatography rather than crystallisation. We can assume that the classical crystallisation of diastereoisomeric salts was not successful. Removal of the ester was simplified as a transesterification. CDI is carbonyl-di-imidazole. 

The synthetic compounds are obviously esters of a cyclopropane carboxylic acid and a cyanohydrin 202. Enantioselective transesterification of butanol and the acetate 201 of the cyanohydrin using immobilised lipases gives the required (S)-alcohol 202 and the unreacted enantiomer of the acetate (R) -201 easily racemised with Et3N. Reports using different lipases appeared at the same time CHIRAZYME L-6, the lipase from Pseudomonas immobilised on sephadex DEAE was used in i-Pr20 and the racemisation carried out with Et3N under reflux in the same solvent.55... 

Carboxylic esters cannot be brominated by elemental bromine. However, mixtures of esters and acid chlorides in molar proportions 1 1 to 5 1 can be brominated, yields being the same as from 2-6 moles of acid chloride 562 here the 2-bromo fatty acid chloride, formed as intermediate, reacts with the fatty acid ester in a transesterification that leads to the oc-bromo ester and the un-brominated chloride. 

Sensitive carboxylic acids can be prepared from their esters by acid-catalysed transesterification e.g., acrylic acid can be obtained from its methyl ester by means of formic acid containing sulfuric acid.75... 

Transesterification. Ethyl esters from carboxylic acids and acetic esters of benzylic alcohols are obtained in the catalyzed transesterification processes, using ethyl acetate as the donor of ethoxy or acetyl group. 

Various methods have been used to cleave protected peptides from the resin, including hydrazinolysis, ammonolysis, or aminolysis using a suitable amino acid ester [48-52]. Probably the most useful procedure, however, is transesterification of the peptide-resin with hydroxypiperidine. This initially forms the hydroxypiperidine ester of the protected peptide, which, after treatment with zinc in HOAc, furnishes the corresponding free carboxylic acid. This method does not lead to epimerization at the C-terminal amino acid or to loss of acid-sensitive protecting groups. Cleavage yields are usually high. 

In contrast to the carboxylic esters, no transesterification or desulfonation (cleavage of the C-S bond) is observed, in the alkyl nitration of alkyl sulfonate esters. Ethyl butanesulfonate is converted to ethyl a-nitrobutanesulfonate (54%) and the potassium salt of o-nitrobutanesulfonic acid (11%), about 33% of the ester being recovered (eq 15). The salt of a -nitrobutanesulfonic acid arises from a B-elimination with the loss of ethylene (13). 

The resin of E.foliosissima contained three carboxylic acids as the major metabolites which could be isolated more conveniently as their methyl ester derivatives. The major component was the unstable acetoxy ester (111) which displayed an allylic tertiary alcohol. Transesterification of the acetate with NaOMe, followed by heating the derived hydroxy compound in a solution of CDCI3, led to the formation of the tetrahydropyran (112). This compound proved identical to the second major metabolite isolated from the methylated acidic fraction. This interrelationship was fortunate... 

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