Chemical fatty-ester synthesis

Among the compounds that have been isolated from the brassins were a new group of glucosyl esters of fatty acids, the structures of which were established by spectral methods (254, 255) as well as by chemical and biochemical synthesis (256, 257, 258). Although these esters were much less active than brassinolide in the bean second internode bioassay, their presence may be essential for seed germination since the enzymatic synthesis of these esters was correlated to germination (257, 258). 

Chemical esterification methods use an alcohol and a carboxylic acid in the presence of a mineral acid as catalyst. Sulfuric acid, which is commonly used, leads to the formation of undesirable byproducts, requiring a difficult separation step (3). Moreover, in this case, the starting material is a high-value component (fatty acid). Consequently, researchers are interested in the alcoholysis reaction using a vegetable oil with low cost and largely produced in Brazil as a raw material for ester synthesis. 

In the interesterification of fats, 1,3-positional specific lipases catalyze reactions in which only the fatty acids in the a-positions of the triglycerides take part, whereas positional nonspecific lipases are able to catalyze reactions in which the fatty acids from both the a- and / -positions take part. In transesterification between two types of fat, the positional non-specific lipase is therefore able to randomize the fatty acids, resulting in the same fatty acid composition in the triglycerides as obtained in the commercially important chemical randomization process. In ester synthesis, positional non-specific lipases catalyze the reaction with both primary and secondary alcohols whereas positional specific lipases are more or less specific for primary alcohols. 

The synthesis of furan fatty esters containing a phenyl substituent at the 3- or 4-position of the furan ring was reported these involved methyl 9,10-epoxy-12-oxostearate as a key intermediate (85). The mono-, di-, and triacylglycerols of a Cj g furan fatty acid were prepared by chemical and enzymatic means (86). Furanoacety-lene phytoalexins (wyerone [19] and dihydro-wyerone [20]) were prepared in multigram quantities from furfural (87). The synthesis route of compound [19] is presented in Scheme 6. 

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... 

The fatty esters of polyethers are known to be good surfactants, and many commercial products are available in the market place. Most people use chemical methods to achieve the required synthesis. Enzymatic methods can also be used however, the reaction needs to be optimized. An example is the lipase-catalyzed synthesis of fatty acid diester of poly(ethylene glycol) EG). 

Higher molecular primary unbranched or low-branched alcohols are used not only for the synthesis of nonionic but also of anionic surfactants, like fatty alcohol sulfates or ether sulfates. These alcohols are produced by catalytic high-pressure hydrogenation of the methyl esters of fatty acids, obtained by a transesterification reaction of fats or fatty oils with methanol or by different procedures, like hydroformylation or the Alfol process, starting from petroleum chemical raw materials. 

Hydrolases, which catalyze the hydrolysis of various bonds. The best-known subcategory of hydrolases are the lipases, which hydrolyze ester bonds. In the example of human pancreatic lipase, which is the main enzyme responsible for breaking down fats in the human digestive system, a lipase acts to convert triglyceride substrates found in oils from food to monoglycerides and free fatty acids. In the chemical industry, lipases are also used, for instance, to catalyze the —C N —CONH2 reaction, for the synthesis of acrylamide from acrylonitril, or nicotinic acid from 3-pyridylnitrile. 

Esters are common components in cosmetics and skin-care products. They can be synthesized from fatty acids and alcohols using either chemical or enzymatic reactions. The chemical reactions are normally catalysed by acid catalysts. Enzymatic synthesis is carried out under milder conditions and therefore it provides products of very high purity. A range of esters such as isopropyl palmitate and isopropyl myristate are now produced industrially using enzymatic synthesis. The reactions are carried out in solvent-free systems using an immobilised lipase as catalyst. In order to get high yields in the reactions, water is removed continuously. 

We then focused on the synthesis of lipid A analogs which contain 3-hydroxy fatty acids. For this purpose, sufficient amount of (R)-3-hydroxytetradecanoic acid ( ), which is the commonest hydroxy acid in Salmonella lipid A, was first prepared by means of an asymmetric reduction of the corresponding keto ester, i. ., methyl 3-oxotetradecanoate (j ) (7). Catalytic hydrogenation of 21 in the presence of Raney Ni modified with (R, R)-tartaric acid foaBr (8) afforded the crude (R)-ester in 85% enantiomeric excess. After saponification, the resultant acid was purified through its dicyclohexylammonium salt to give the optically and chemically pure (R)-acid In a yield of 61% from... 

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... 

Sulfonic acids and sulfonate salts contain the -S03H and -SO) groups, respectively, attached to a hydrocarbon moiety. The structural formulas of benzene sulfonic acids and of sodium 1 -(p-sulfophenyl)decane, a biodegradable detergent surfactant, are shown in Figure 1.19. The common sulfonic acids are water-soluble strong acids that lose virtually all ionizable H+ in aqueous solution. They are used commercially to hydrolyze fat and oil esters to produce fatty acids and glycerol used in chemical synthesis. 

This case study investigates the possibility of applying reactive distillation to the synthesis of fatty-acid esters as a generic multiproduct process. As representative species we consider the lauric (dodecanoic) acid and some alcohols the series Q-C8, such as methanol, n-propanol and 2-ethyl-hexanol (isooctanol). The generic reversible chemical reaction is ... 

These are just a few of the many chemical reactions of the hexoses. Gluconic acid, glucono-5-lactone from this acid, sorbitol, sorbitol esters, sorbitans (tetra-hydric anhydrosorbitol), mannitol, mannitol esters, copolymers of fatty acid sorbitol esters and alkylene oxides, and a few other derivatives are currently marketed as commodity chemicals derived from the hexoses. Vitamin C (l-ascorbic acid) is perhaps one of the best examples of a commercial derivative of D-glucose that is manufactured primarily by chemical synthesis ... 

In addition to uvidins A (3.14) and B (3.36) (10), more recently several new fatty acid esters of drimenol (3.2-3.6) and uvidin A (3.16-3.20) (11), and three new sesquiterpenes (12) with the bicyclofarnesane skeleton have been isolated from L. uvidus (Table 3). The stereostructures of the three uvidins C (3.29), D (3.34), and E (3.22) have been established by speetroscopic data and chemical reactions (12). In the interesting synthesis of uvidin E (3.22) from uvidin A (3.14), the Rubottom s procedure was employed for the regiospecific and stereoselective a-hydroxylation at C-5 of the enone 3.10 (12) (Scheme 2). 

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. 

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