Selectivity fatty-ester synthesis

Among the aldonolactone-based surfactants are aldonolactone-linked fatty esters which have been prepared by selective acylation of unprotected aldono-1,4-lac-tones or aldono-1,5-lactones, One of the first reported examples of this type of surfactant was applied to the enzymatic synthesis of 6-0-aUcanoylgluconolactones [35], Thus, 6-0-decanoyl- and 6-0-dodecanoyl- derivatives (21a and 21b, respectively, Scheme 8) were obtained in 26-27% yield by esterification of glucono-1,5-lactone (1) at C-6 with the corresponding 2,2,2-trichloroethyl carboxylate in the presence of porcine pancreatic lipase (PPL) as catalyst. Compounds 21a,b are soluble in water at 90-96°C but precipitate when cooled to 30-37°C, NMR and GC-MS analysis after dissolution and precipitation indicated the presence in the mixture of compound 21b, the glucono-1,4-lactone-derived ester 22, and the... 

Ring opening of epoxidized fatty esters with alcohols has been studied to explore the synthesis of vicinal hydroxy ethers, which are potential lubricants725 [Eq. (5.274)]. In a comparative study of ring-opening with methanol, Nafion SAC-13 showed superior activity (TOF=lmin 1 versus 0.04 min-1 after 0.5 h at 60°C) when compared to Amberlist 15 (TOF = 0.04 min-1) both exhibiting equally high selectivity (>98%). 

Lipase-catalysed interesterification has found many applications in production of edible and specialty lipids due to mild reaction conditions, high catalytic efficiency, the inherent selectivity of natural catalysts and production of much purer products as compared to chemical methods (Sonnet, 1988). Lipases (hydrolases) are used for hydrolysis and ester synthesis. They are classified as non-specific or random, positional specific or 1,3-specific and acyl group- or structure-specific, depending on their activity towards fatty acids... 

Typically, lipases are employed in esterification however, proteases have also been utilized in organic medium although their normal function is peptidic bond hydrolysis. Their action in ester bond formation is rationalized in terms of the similarity between the active center that is mechanistically essential for transesterification. Several lipases originating from various sources have been utilized in sugar fatty acid synthesis. The activity of these enzymes is highly dependent on the reaction conditions employed and whether they are used in the free state or immobilized on a support. Immobilization confers better homogeneity of the enzyme for reaction and promotes greater thermostability of the protein [30]. The prochiral selectivity and substrate preference of lipases and protease also depend on the solvent system [34,35]. 

In the lubricant sector, oleochemically-based fatty acid esters have proved to be powerful alternatives to conventional mineral oil products. For home and personal care applications a wide range of products, such as surfactants, emulsifiers, emollients and waxes, based on vegetable oil derivatives have proved to provide extraordinary performance benefits to the end-customer. Selected products, such as the anionic surfactant fatty alcohol sulfate, have been investigated thoroughly with regard to their environmental impact compared with petrochemical based products by life-cycle-analysis. Other product examples include carbohydrate-based surfactants as well as oleochemical based emulsifiers, waxes and emollients. The catalysts used in the synthesis of these molecules need further development. 

The synthesis of long-chain fatty acid esters of carbohydrates is inherently more demanding. It was found that glucose did not react with vinyl laurate in a pure ionic liquid medium, but in biphasic tert-butyl alcohol/[BMIm][PF6], glucose could be acylated by the vinyl esters of O, 2-Cu, fatty acids. The best results were obtained with CaLB, which was twice as active as TIL, and the selectivity for acylation at C-6 was high [114]. The esterification of glucose with palmitic acid, which is, in an industrial context, to be preferred over transesterification, has recently been demonstrated in tert-butyl alcohol/[BMIm][PF6] medium [115]. 

Once bearing some substituents, the decrease of polarity of the sucrose derivatives makes them soluble in less-polar solvents, such as acetone or tert-butanol, in which some lipases are able to catalyze esterifications. Unlike proteases, which necessitate most often the use of an activated acyl donor (such as vinyl or trifluoroethyl esters), lipases are active with simple esters and even the parent carboxylic acids in the presence of a water scavenger. The selectivity of the lipase-catalyzed second esterification is specific for OH-6 allowing the synthesis of mixed T,6 -diesters.123,124 For some lipases, a chain-length dependence on the regiochemistry was observed.125 Selectively substituted monoesters were thus prepared and studied for their solution and thermotropic behavior.126,127 Combinations of enzyme-mediated and purely chemical esterifications led to a series of specifically substituted sucrose fatty acid diesters with variations in the chain length, the level of saturation, and the position on the sugar backbone. This allowed the impact of structural variations on thermotropic properties to be demonstrated (compare Section III.l).128... 

Activity and Selectivity of Proteases in Synthesis of Carbohydrate Fatty Acid Esters... 

In ERD in situ separation is used to improve the yield of reaction whereas an entrainer feed is added to make the separation feasible by selectively increasing the relative volatility of one of the products, ERD promises to be advantageous for the synthesis of fatty acid esters. The entrainer increases the relative volatility of water (by-product) compared to the alcohol (reactant), such that during the reaction the water can be continuously removed by distillation. In this way the chemical equilibrium is shifted such that higher conversions can be obtained. In Figure 1 the flowsheet of the desired process is given, in which RS stands for Reactive Section and DS for Distillation Section. 

Synthesis of a structured TAG with PUFA confined to either the l-(3-) or 2-position requires 1,3-selective lipases to interesterify a TAG using FFA or fatty acid esters (reaction 1), or to transesterify two different populations of TAG molecules (Fig. 3). Alternatively, a two-step approach can be employed, where a TAG is first subjected to 1,3-selective lipolysis, resulting in 2-MAG that is carefully isolated, then crystallized, to prevent acyl migration. The second step is 1,3-selective esterification of the 2-MAG.f " Such an approach results in a 72-85% yield and >95%... 

On the basis of the above reported results, Barrault et al. (24] studied the selective synthesis of monoglycerides 51 starling from glycerol 49 and fatty methyl esters 50 promoted by both PTBD with 2.5 mmol/g loading and homogeneous M fBI) (Scheme 13). 

Westerman, P.W., and Ghrayeb, N. (1982) Synthesis of Esters of Tetradecanoic Acid Deuterated at the Penultimate Carbon Some General Procedures for the Synthesis of Selectively Deuterated Fatty Acids, Chem. Phys. Lipids 30, 381-387. 

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