Transesterification reactions with

Vitamin A palmitate [79-81-2] (3), a commercially important form of the vitamin, is produced from vitamin A acetate (2) via a transesterification reaction with methyl palmitate. En2ymatic preparation of the palmitate from the acetate has also been described (22). 

Steric factors are important ia transesterification reactions. With a given alcohol, primary alkyl borates react at a rate too fast to measure, secondary alkyl borates react at measurable rates, and tert-huty borate reacts very slowly. 

Metal-catalyzed allylic substitution reactions have been a mainstay of synthetic chemistry because of their ability to proceed irreversibly and with high selectivity [42]. It is also feasible, however, to produce analogous systems that are completely reversible and nonselective, or ideally situated for use in DCC. These are essentially metal-catalyzed transesterification reactions, with the added feature of potentially providing stereochemical scrambling (and selection) as well as constitutional variation. An early example of this was provided in 2000 by Kaiser and Sanders [43]. In the absence of a template, reaction of diallyl diacetate 22 with a dicarboxylic acid in the presence of catalytic Pd(0) produced a negligible amount of the cycfized compound 23 (Fig. 1.9). However, when templated with 1,3-bis(4-pyridyl) benzene, yield of the cyclic structure increased to roughly 10%, independent of the dicarboxylic acid used. 

Most industrial reactors and high pressure laboratory equipment are built using metal alloys. Some of these same metals have been shown to be effective catalysts for a variety of organic reactions. In an effort to establish the influence of metal surfaces on the transesterification reactions of TGs, Suppes et collected data on the catalytic activity of two metals (nickel, palladium) and two alloys (cast iron and stainless steel) for the transesterification of soybean oil with methanol. These authors found that the nature of the reactor s surface does play a role in reaction performance. Even though all metallic materials were tested without pretreatment, they showed substantial activity at conditions normally used to study transesterification reactions with solid catalysts. Nickel and palladium were particularly reactive, with nickel showing the highest activity. The authors concluded that academic studies on transesterification reactions must be conducted with reactor vessels where there is no metallic surface exposed. Otherwise, results about catalyst reactivity could be misleading. 

In a similar approach, Kasture and coworkers describe the use of neat substrate (ethyl acetate both as alcohol donor and as the reaction medium) in the preparation of chirally pure S-(-)-l,4-benzodioxan-2-carboxylate, an important drug intermediate used in the synthesis of doxazosin mesylate, from racemic l,4-benzodioxan-2-carboxylic acid [138]. Again, CaLB catalyzed the transesterification reaction with good enanhoselectivity (E = 160) and acceptable enantiomeric excess (>95%) and chemical yield (50%). 

This method was employed for transesterification reactions with both a-chymotrypsin and subtilisin Carlsberg with a variety of H+/Na+ buffers [53]. With both enzymes (which differ widely in secondary and tertiary structures) and two polar solvents, acetonitrile and THF, the activating effect of the solid-state buffer was clearly evident (Table 3.3). The observation that a variety of buffer pairs show success in activating two dissimilar enzymes in synthetically useful solvents makes this method for activation promising and novel. 

The gas-chromatographic separation of plasticizers can be effected directly or after conversion to low boiling point compounds. This is achieved by a transesterification reaction with methanol or diazomethane. After separation of the plasticizers mixtures with liquid chromatography, identification by spectroscopic methods is possible. 

Figure 18.5 Transesterification reaction with entropy trap (Schultz, 1993).

Fig. 40 Intramolecular (a) and intermolecular (b) transesterification reactions with phe-noxyl ester end groups of poly(e-caprolactone)...

While diketene remains a very important synthetic precursor, there has been increasing interest in the chemistry of a-methylene-/3-lactones, 3-methylene-2-oxetanones. However, unlike diketene, which can be readily synthesized by the dimerization of aldehydic ketenes, there are few methods for the synthesis of a-methylene-/3-lactones in the literature. Recent strategies for the preparation of the compounds are discussed in Section 2.05.9.2. The kinetic resolution of racemates of alkyl-substituted a-methylene-/3-lactones has been carried out via a lipase-catalyzed transesterification reaction with benzyl alcohol (Equation 21) <1997TA833>. The most efficient lipase tested for this reaction was CAL-B (from Candida antarctica), which selectively transesterifies the (A)-lactone. At 51% conversion, the (R)-f3-lactone, (R)-74, and (A)-/3-hydroxy ester, (S)-75, were formed in very high enantio-selectivities (up to 99% ee). 

In the second major use of VAM, PVA is converted to poly(vinyl alcohol) (PVOH) by a transesterification reaction with methanol, giving methyl acetate as coproduct. PVOH finds its major end use in textile sizing and adhesives. Further reaction of PVOH with butyraldehyde or formaldehyde gives polyvinyl butyral (PVB) or polyvinyl formal, which together constitute the third largest consumption of VAM. PVB is used almost exclusively in the adhesive laminating inner layer in safety glass. 

In the food industry, lipases are used in lipid modification processes. In these processes the texture, digestibility, or physical properties of natural lipids are modified by lipase-catalyzed transesterification reactions with lipids other than the original fatty acids. In the baking industry, lipases are used to influence the quality of bread through modification of the wheat flour lipids. Finally lipases are used for flavor enhancement of cheese in the dairy industry. 

Figure 7.4 Upper panel E-values at different degrees of conversion in transesterification reactions with vinyl butanoate of the alcohols la, 2a, 3a, and 4a catalyzed by Novozym 435 (this batch is different from the one used in Figure 7.3).

DCP as a Chiral Controller in Oxidative Free Radical Cyclizations. As a chiral auxiliary, DCP (1) is also reported to induce modest diastereoselection (60% de) in Mn(III)-based oxidative free-radical cyclizations of p-keto esters (eq 12). Chiral p-keto ester 25 was prepared by transesterification reaction with methyl ester 23, 1, and 0.3 equiv of DMAP (catalyst) in anhydrous toluene at reflux for 3-5 d as described by Taber. Oxidative cyclization of a 0.1 M solution of 24 in AcOH with 2 equiv of Mn(OAc)3-2HzO and 1 equiv of Cu(OAc)3 HzO provided bicyclo[3.2.1]octan-2-one (25). 

Other procedures to obtain the polymer include the use of terephthalic acid esterified to its dimethyl ester, which by a transesterification reaction with ethylene glycol generates the polymer. In addition to ethyleneglycol, other diols can be used in the esterification readion, such as 1,4-butanediol, 1,4-cyclohexanedimethanol, 1,6-hexanediol, etc. 

Other possibilities to prepare chiral cyanohydrins are the enzyme catalysed kinetic resolution of racemic cyanohydrins or cyanohydrin esters [107 and references therein], the stereospecific enzymatic esterification with vinyl acetate [108-111] (Scheme 2) and transesterification reactions with long chain alcohols [107,112]. Many reports describe the use of fipases in this area. Although the action of whole microorganisms in cyanohydrin resolution has been described [110-116],better results can be obtained by the use of isolated enzymes. Lipases from Pseudomonas sp. [107,117-119], Bacillus coagulans [110, 111], Candida cylindracea [112,119,120] as well as lipase AY [120], Lipase PS [120] and the mammalian porcine pancreatic lipase [112, 120] are known to catalyse such resolution reactions. 

Transesterification reactions with phosphites and alkoxyphosphoranes allow the synthesis of... 

Solid base catalysts have gradually gained importance in the catalytic field due to their well-known advantages of non-corrosive and easier product separation[l]. However, in contrast to the extensive application of solid acid catalysts, the utilization of solid bases was limited for their rapid catalytic deactivation [2]. The deactivation problem was also foimd in the continuous synthesis of dimethyl carbonate (DMC). DMC has attracted more and more attention in recent years because of its low toxic and nicely biodegradable property [3,4]. Transesterification between methanol and propylene carbonate (PC) or ethylene carbonate (EC) is an attractive route for the synthesis of DMC. Both acid and base catalysts catalyze the reaction, and base catalyst was reported to be more effective [5]. Among bases, CaO showed unique catalytic activity for the transesterification reaction with high yield and selectivity [6]. Unfortunately, when CaO based catalyst was employed in the continuous synthesis of DMC, its activity gradually decayed with time-on-stream due to... 

Once a thioester with the appropriate number of carbon atoms is obtained, it undergoes a transesterification reaction with glycerol in order to form fats, oils, and phospholipids (Sections 26.3 and 26.4). 

Lipases are being used in several reactions of synthesis for the production of valuable compounds. Biodegradable polymers, like butyl oleate and some polyesters, have been synthesized by esterification and transesterification reactions with lipases... 

CSTR Figure 5.2f). As long as the residence time for diffusion of molecules from the input to the exit is long enough to achieve an acceptable conversion of reactants to products, a standard batch reactor can be turned into a CSTR. As an example, a 4 L flask in a MARS has been used for the preparation of biodiesel via a transesterification reaction with a flow rate of 7.2 L/min (33 s residence time). ... 

Ready replacement of the bromine atoms in MeBiBr2 and PhBiBr2 occurred on treatment with sodium ethoxide, and the resulting diethoxides served as starting materials for a series of transesterification reactions with thiols, 1,2-diols, 1,2-dithiols, etc. ... 

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