Stereoselective enzymatic transformation

During the course of all of these reactions, any type of chirality in the substrate is recognized by the enzyme, which causes a preference for one of the two possible stereochemical pathways for a reaction. The value of this discrimination is a crucial parameter since it stands for the selectivity of the reaction. The latter is governed by the reaction kinetics. It should be noted, that the following chapter is not an elaboration on enzyme kinetics, but rather a compilation of the most important conclusimis needed for obtaining optimal results from stereoselective enzymatic transformations. 

The results presented in Tables 3 and 4 deserve some comments. First, a variety of enzymes, including whole-cell preparations, proved suitable for the resolution of different hydroxyalkanephosphorus compounds, giving both unreacted substrates and the products of the enzymatic transformation in good yields and, in some cases, even with full stereoselectivity. Application of both methodologies, acylation of hydroxy substrates rac-41 and rac-43 or the reverse (hydrolysis of the acylated substrates rac-42 and rac-44), enables one to obtain each desired enantiomer of the product. This turned out to be particularly important in those cases when a chemical transformation OH OAc or reverse was difficult to perform. As an example, our work is shown in Scheme 3. In this case, chemical hydrolysis of the acetyl derivative 46 proved difficult due to some side reactions and therefore an enzymatic hydrolysis, using the same enzyme as that in the acylation reaction, was applied. Not only did this provide access to the desired hydroxy derivative 45 but it also allowed to improve its enantiomeric excess. In this way. 

Recent developments in the enzymatic synthesis of carbohydrates can be classified into four approaches 1) asymmetric C-C bond formation catalyzed by aldolases (1-10 2) enzymatic synthesis of carbohydrate synthons (loll) 3) asymmetric glycosidic formation catalyzed by glycosidases (12.-17) and glycosyl transferases (18-23.) and 4) regioselective transformations of sugars and derivatives (24-25). These enzymatic transformations are stereoselective and carried out under mild conditions with minimum protection of functional groups. They hold promise in preparative carbohydrate synthesis. In connection with this book, we focus on the first two approaches. 

Surprisingly, the introduction of the pyridine ring not only influences the velocity of the enzymatic transformations, but also induces promising stereochemical effects (Table 1). For instance, at 40% conversion (R)-phenylethanol is obtained from the pyridyl acetate 25 with 73 % ee, whereas the value for the corresponding phenylacetate is only 28%. Also, the secondary alcohol liberated from the ester 26 displays 98% ee at 40% conversion, whereas the respective phenylacetate leads to 1-phenylpropanol with 94% ee but at a conversion rate of 12% only [19,20]. These results demonstrate that the stereoselecting properties of penicillin acylase may be enhanced by appropriate engineering of the substrate. This is of particular interest since this enzyme has already been used for the kinetic resolution of various chiral alcohols [21-24], e.g. furyl alkyl carbinols [24], which are valuable precursors for the de novo synthesis, with moderate to high ee values, of carbohydrates. 

The biosynthesis of lignans is a rather unexplored area. Experimental evidence of all the postulated logical biosynthetic sequences has still not been found. Research has focused on enzymatic transformations occurring in lignan biosynthesis, with particular emphasis on the stereoselectivity and enantiospecificity of such conversions. 

Enzymes are an increasingly available and important tool in the arsenal of the synthetic chemist. Enzymatic reductions are often straightforward and highly stereoselective. There are now many enzymatic transformations that are compatible with the use of organic solvents.6l5 Other solvents can be used as well, illustrated by the enzyme alcohol dehydrogenase from Geotrichum candidum, which is active in supercritical carbon dioxide.6i6 Prelog studied the reduction of ketones with several enzymatic systems. Reduction of... 

Scheme 40 describes the second part of the Hoechst Marion Roussel process of 7-AC A (129) manufacture - the first enzymatic transformation has already been described in Scheme 6. Glutaric acid derivative 20 is now subjected to treatment with immobilized Pseudomonas sp, cu-amidodicarboxylate amido-hydrolase (recombinant in Escherichia coli). The enzyme catalyzes the chemo-and stereoselective hydrolysis of the amide and gives the free amine 129 in reasonable yield and optical purity. The whole process has also been established in several other companies, with minor modifications. Anbics, for example, is presently setting up a fermentation process with its subsidiary Bioferma Murcia in Spain for the production of 7-AC A. A typical isolated yield of 82% has been reported for 129, which can be further optimized to >85% by applying techniques such as reversed osmosis on the production scale [115].

Ogston [20,21], seemingly unaware of the Easson-Stedman model, proposed a similar three-point attachment model to rationalize the observed stereoselectivity in the enzymatic transformation of symmetrical prochiral substrates, e.g., citrate and aminomalonate (Fig. 3) [22]. Similarly, Dalgleish [23], also unaware of the Easson-Stedman model [17], rationalized his observations concerning the resolution of the enantiomers of a number of amino acids on paper chromatography by a three-point attachment. In a subsequent telephone conversation with Bentley [24], Dalgleish stated that he was terribly impressed by the Ogston hypothesis. It is therefore... 

Many stereoselective enzymatic hydrolyses of nonnatural esters do not show a perfect selectivity, but are often in the range of 50-90% e.e., which corresponds to E values which are considered as moderate to good (E = 3-20). In order to avoid tedious and material-consuming processes to enhance the optical purity of the product, e.g., by crystalhzation techniques or via repeated kinetic resolution, several methods exist to improve the selectivity of an enzymatic transformation itself [24, 277], Most of them can be applied to other types of enzymes. 

The enzymatic transformation for stereoselective direct conversion of keto ester 2 to amino ester 3 using a transaminase was attractive, but considered risky as commercial application of transaminases for API synthesis was unprecedented at the time (since then several new reports have appeared on the successful use of transaminases) [15] and few transaminases were commercially available. However, these risks were considered acceptable in view of the perceived value of the biocatalytic route, and subsequent efforts resulted in partial optimization of a transaminase [16] prior to development being halted for the project. 

Interestingly, for the transformation of both the racemic 1-hydroxyalkanephosphonates 41 and 2-hydroxyalkanephosphonates 43 into almost enantiopure acetyl derivatives 42 and 44, respectively, a dynamic kinetic resolution procedure was applied. Pamies and BackvalP used the enzymatic kinetic resolution in combination with a ruthenium-catalysed alcohol racemization and obtained the appropriate O-acetyl derivatives in high yields and with almost full stereoselectivity (Equation 25, Table 5). It should be mentioned that lowering... 

In the present review we concentrate on the induction of asymmetry for the case in which the chiral reagent (5) is represented by an amino acid or a derivative thereof. Only those papers are considered in which the formation of a new center of asymmetry is induced. This can take place with the simultaneous incorporation of the chiral amino acid (or a derivative thereof) in the target molecule or by the action of catalytic amounts of this amino acid on a prochirale molecule. Reactions in which only the asymmetric center of the amino acid is modified without the stereoselective appearance of a new chiral center, have not been considered. Enzymatically catalyzed transformations 241 of molecules are not treated here. 

The incentive for industrial utilization of microorganisms is based on several favorable factors. The most important of these is that a fermentation process is very often the only feasible way to obtain a complex product, or a specific chemical transformation. Also a chemical change conducted enzymatically (via microorganisms or their enzymes) has a far lower energy barrier than by the same chemical synthetic step. This means that microbiological processes may often be carried out at easily attainable near-ambient conditions. Under milder conditions, the enzymatic processes can be 10 to 10 times as rapid as the corresponding chemical route to the same product [1]. Another feature of importance for the production of some complex biological products is that many enzymes are totally specific and stereoselective for the product of interest, unlike many chemical reactions. 

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