Lipase synthetic applicability

Several synthetic applications are given in Scheme 16 (40). The hydrogenation of a /3-keto ester on a 6-100-kg scale is used for the synthesis of tetrahydrolipostatin, a pancreatic lipase inhibitor developed at Hoffmann-LaRoche Company (1c). Tartaric acid is the best chiral modifier a-amino acids or a-hydroxy acids are not satisfactory. The source... 

As the use of lipase for industrial chemical synthesis becomes easier, several chemical companies have begun to increase significantly their biocatalytic process used in synthetic application. Among these companies, BASF, in which enantiomerically pure alcohols and amines are produced on industrial scale.98... 

Lipases, and many other enzymes as well, have been shown to be active in organic solvents with low water content [54] (< 1% v/v of water). From this finding some synthetic applications are derived [55], the most important are the following ... 

Having developed a technique for the rapid evaluation of immobilized biocatalysts, the authors subsequently compared a series of lipase enzymes for the model reaction depicted in Scheme 6.39, whereby CaLB (146), lipase Pseudomonas cepacia IM and Amano lipase AK were found to afford the highest throughputs of 10.2, 10.2, and 10.6 pmol 148min 1g 1, respectively. The synthetic applicability of the kinetic... 

From the hydrolases toolbox, probably lipases have been the most demanding catalysts for synthetic application. Their natural funchon involves the hydrolysis of triacylglycerol ester bonds, compoimds that are poorly soluble in water. Thus, the reaction usually occurs in an organic-aqueous interface. This phenomenon involving the conformational change of the selected lipase is called interfacial achvahon [35], and it provides an inherent affinity for hydrophobic media to the enzyme. 

Another interesting synthetic application is the lipase-catalyzed preparation of en-antiomerically pure epoxy esters as intermediates of the calcium channel blocker diltiazem [283] (Scheme 67). 

Special attention should be devoted to less conventional applications of the enzymatic transesterification methodology such as resolution of unstable substrates as racemic secondary hydroperoxides [291]. The development of new reactions in the presence of enzymes should pursued, as, for example, the simultaneous formation of a hemithioac-etal and die irreversible transesterification in the presence of a lipase [292]. Also, for synthetic applications, the combination of enzymatic and chemical asymmetrical methods could lead to interesting results, such as the one-pot lipase-catalyzed acylation and the Mitsonobu inversion of the configuration of the unreacted alcohol, which should lead to only one enantiomeric ester [293]. 

In conventional synthetic transformations, enzymes are normally used in aqueous or organic solvent at moderate temperatures to preserve the activity of enzymes. Consequently, some of these reactions require longer reaction times. In view of the newer developments wherein enzymes can be immobilized on solid supports [183], they are amenable to relatively higher temperature reaction with adequate pH control. The application of MW irradiation has been explored with two enzyme systems namely Pseudomonas lipase dispersed in Hyflo Super Cell and commercially available SP 435 Novozym (Candida antarctica lipase grafted on an acrylic resin). 

Hydroperoxides play an important role as oxidants in organic synthesis [56-58]. Although several methods are available for the preparation of racemic hydroperoxides, no convenient method of a broad scope was until recently [59] known for the synthesis of optically active hydroperoxides. Such peroxides have potential as oxidants in the asymmetric oxidation of organic substrates, currently a subject of intensive investigations in synthetic organic chemistry [60, 61]. The application of lipoxygenase [62-65] and lipases [66,67] facilitated the preparation of optically active hydroperoxides by enzymes for the first time. 

Whereas several areas of biocatalysis - in particular the use of easy-to-use hydrolases, such as proteases, esterases and lipases - are sufficiently well research to be applied in every standard laboratory, other types of enzymes are still waiting to be discovered with respect to their applicability in organic-chemistry transformations on a preparative scale. This latter point is stressed in this volume, which concentrates on the newcomer-enzymes which show great synthetic potential. 

Enzymes that belong to the class of hydrolases are by far the most frequently-applied enzymes in polymer chemistry and are discussed in Chaps. 3-6. Although hydrolases typically catalyse hydrolysis reactions, in synthetic conditions they have also been used as catalysts for the reverse reaction, i.e. the bond-forming reaction. In particular, lipases emerged as stable and versatile catalysts in water-poor media and have been applied to prepare polyesters, polyamides and polycarbonates, all polymers with great potential in a variety of biomedical applications. 

In order to enhance the potential of synthetic reactions of lipids and the transesterification in organic solvents, a fungal lipase from Phycomyces nites was chemically modified. The promotion of dispersibility in orgaiuc solvents resulted in a much higher reactivity. Chemically modified lipases showed higher reactivity than unmodified lipase when they were utilized for the transesterification of triglycerides and other lipids. The initial rate of transesterification in organic solvents by modified lipase was 40 times faster than that of unmodified lipase. Chemically modified lipase was also found applicable for the syndesis of other fatty acids esters. 

The application of lipases in synthetic biotransformations encompasses a wide range of solvolytic reactions of the carboxyl group, such as esterification, transesterification (alcoholysis), perhydrolysis, and aminolysis (amide synthesis) [103]. Transesterification and amide synthesis are preferably performed in an anhydrous medium, often in the presence of activated zeolite, to suppress unwanted hydrolytic side reactions. CaLB (which readily tolerates such conditions [104,105]), PsL, and PcL are often used as the biocatalyst [106]. 

The applications of microbial lipases in the food industry involve the hydrolytic as well as the synthetic capabilities of these enzymes and have been summarized by Godtfredsen (1993) in Table 10-7. 

A typical research example includes the synthesis of a K -channel blocker intermediate (Fig. 7.10). Here, an aminotransferase is used to replace a keto group by an amino function to yield the desired intermediate [20]. Other potential applications include the biosynthesis of flavouring esters. In nature such compoimds are often formed by a specific acyl CoA transferase. For practical purposes, however, the alternative, less complicated route, using lipases and esterases in their synthetic mode, seems more appropriate. 

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