Enzymatic synthesis stereoselectivity

Enzyme preparations from liver or microbial sources were reported to show rather high substrate specificity [76] for the natural phosphorylated acceptor d-(18) but, at much reduced reaction rates, offer a rather broad substrate tolerance for polar, short-chain aldehydes [77-79]. Simple aliphatic or aromatic aldehydes are not converted. Therefore, the aldolase from Escherichia coli has been mutated for improved acceptance of nonphosphorylated and enantiomeric substrates toward facilitated enzymatic syntheses ofboth d- and t-sugars [80,81]. High stereoselectivity of the wild-type enzyme has been utilized in the preparation of compounds (23) / (24) and in a two-step enzymatic synthesis of (22), the N-terminal amino acid portion of nikkomycin antibiotics (Figure 10.12) [82]. 

Zhu, D., Yang, Y. and Hua, L. (2006) Stereoselective enzymatic synthesis of chiral alcohols with the use of a carbonyl reductase from Candida magnoliae with anti-Prelog enantioselectivity. The Journal of Organic Chemistry, 71 (11), 4202-4205. 

The third contribution presents the combination of electrochemistry and enzymatic synthesis for the selective formation of complex molecules. This quite young field of research is developing rapidly because the application of the reagent-free electrochemical procedure combined with the regio- and stereoselectivity of enzymes offers the possibility of establishing new environmentally friendly process even on a technical scale. 

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. 

LB-ADH was used for the synthesis of vic-diols. Starting from benzaldehyde and acetaldehyde, (lS,2S)-l-phenylpropane-l,2-diol (de = 98%) and (lS,2R)-l-phenyl-propane-l,2-diol (de = 99%), respectively, could be produced in a stereoselective two-step enzymatic synthesis using benzaldehyde lyase (BAL) and accordingly benzoylformate decarboxylase (BFD) as well as LB-ADH [8] (Scheme 2.2.4.3). 

In 2004, Ley et al. [45] showed a stereoselective enzymatic synthesis of cis-pellitorine [N-isobutyldeca-(2 ,4Z)-dienamide], a taste-active alkamide naturally occurring in tarragon. The reactants were ethyl ( ,Z)-2,4-decadienoate— the pear ester described before—and isobutyl amine. The reaction is catalysed by lipase type B from Candida antarctica (commercially available), which shows a remarkable selectivity towards the 2 ,4Z ester. The yield was about 80%. 

Enzymatic synthesis in reaction mixtures with mainly undissolved substrates and/or products is a synthetic strategy in which the compounds are present mostly as pure solids [28, 29]. It retains the main advantages of conventional enzymatic synthesis such as high regio- and stereoselectivity, absence of racemization, and reduced side-chain protection. When product precipitates, the reaction yields are improved, so that the necessity to use organic solvents to shift the thermodynamic equilibrium toward synthesis is reduced and synthesis is made favorable even in water. 

Enzymatic synthesis relying on the use of aldolases offers several advantages. As opposed to chemical aldolization, aldolases usually catalyze a stereoselective aldol reaction under mild conditions there is no need for protection of functional groups and no cofactors are required. Moreover, whereas high specificity is reported for the donor substrate, broad flexibility toward the acceptor is generally observed. Finally, aldolases herein discussed do not use phosphorylated substrates, contrary to phosphoenolpyruvate-dependent aldolases involved in vivo in the biosynthetic pathway, such as KDO synthetase or DAHP synthetase [18,19]. 

Figure 17 Enzymatic synthesis of chiral synthon for BMS-188494, a squalene synthase inhibitor stereoselective acetylation of racemic (52).

The enzymatic synthesis of glycopeptides does not require protection of the functional groups of the amino acid side chains and sugar hydroxyls, because of the high stereoselectivity and regioselectivity of proteases. However, the substrate selectivity of these enzymes may limit a wider range of applications. 

Scriba GKE (1993) In-Vitro evaluation of 4-(2-glyceryl)-butyric acid for lipase-driven drug delivery. Pharm Res 10 S295 Udata C, Tirucherai G, Mitra AK (1999) Synthesis, stereoselective enzymatic hydrolysis and skin permeation of... 

In light of these developments, we felt that it was necessary to review the content of the first edition and to update or supplement the information presented in this work. The new topics examined in this edition include (1) enzymatic synthesis and resolution of enantiomerically pure compounds (Chapter 8) (2) toxicological consequences and implications of stereoselective biotransformations (Chapter 9) (3) stereoselective transport across epithelia (Chapter 10) and (4) assessment of bioavailability and bioequivalence of stereoisomeric drugs (Chapter 11). The chapter on stereoselective protein binding (Chapter 12) has been completely revmtten and new contributions are presented on the regulatory, industrial, and clinical aspects of stereoisomeric drugs (Chapters 13-16). In addition, the chapters... 

Since the chemical synthesis of oligosaccharides requires many synthetic steps including protection and deprotection procedures, the enzymatic approach has attracted much attention for the rapid s)uithesis of oligosaccharides. In addition, the perfect regio- and stereoselectivities of enzymatic methods with glycosyltransferases are quite attractive. Several transferases such as 8(l,4)-galactosyltransferase, a(l,3)-fucosyltransferase, and a-sialyltransferase have been used for polymer-supported enzymatic synthesis [70,71,72,73,74,75]. The selection of the pol)uner support is very important for the pol)uner-supported enz)unatic synthesis of oligosaccharides. 

Stereoselective chemical synthesis of DFAs is instrumental in obtaining pure standards for the validation of analytical methods for the identification and quantification of DFAs in mixtures, particularly in food products. Yet it is inadequate for mass production of DFAs or nutritional applications. Biotechnological approaches using inulin or levan fructosyltransferases, while limited to certain diastereomers, show much promise in these respects. Most of the reports on enzymatic synthesis of DFAs are related to the monospiranic difuranose DFA 111 (1) and the non-spiranic DFA rV (15), while reports on the dispiranic DFA I (10) are much less frequent. Their synthesis has become an issue of industrial interest [56]. 

Citramalic Acid. 2-Hydroxy-2-methylbutanedioic acid 2-methylmalic acid 2-hydroxy-2-methylsuccinic acid a-hydroxypyrotartaric acid trnns-methylbutanedioic add. CjHjO., mol wt 148.11. C 40.54%. H 5.44%. O 54.01%. Enzymatic synthesis Barker. Blair, Biochem. Prepns, 9, 21 (1962). Chemical synthesis Barker, ibid. 25 J. B. Wilkes, R. G. Wall. J. Org. Chem. 45, 247 (1980). Stereoselective synthesis E. G. J. Staring el al. ReC. Trav. Chim. 105, 374 (1986). 

Udata, C. Tirucherai, G. Mitra, A.K. Synthesis, stereoselective enzymatic hydrolysis and skin permeation of diastereomeric propranolol ester prodrugs. J. Pharm. Sci. 1999, 88, 544-550. 

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