Volume 2 Number 4 August 2021 Pages 947–1302 RSC Chemical Biology rsc.li/rsc-chembio ISSN 2633-0679 REVIEW ARTICLE Mélanie Hall Enzymatic strategies for asymmetric synthesis RSC Chemical Biology View Article Online REVIEW View Journal | View Issue Enzymatic strategies for asymmetric synthesis ab Cite this: RSC Chem. Biol., 2021, Me´lanie Hall 2, 958 Enzymes, at the turn of the 21st century, are gaining a momentum. Especially in the field of synthetic organic chemistry, a broad variety of biocatalysts are being applied in an increasing number of processes running at up to industrial scale. In addition to the advantages of employing enzymes under environmentally friendly reaction conditions, synthetic chemists are recognizing the value of enzymes connected to the exquisite selectivity of these natural (or engineered) catalysts. The use of hydrolases in enantioselective protocols paved the way to the application of enzymes in asymmetric synthesis, in particular in the context of biocatalytic (dynamic) kinetic resolutions. After two decades of impressive development, the field is now mature to propose a panel of catalytically diverse enzymes for Received 9th April 2021, (i) stereoselective reactions with prochiral compounds, such as double bond reduction and bond Accepted 28th May 2021 forming reactions, (ii) formal enantioselective replacement of one of two enantiotopic groups of DOI: 10.1039/d1cb00080b prochiral substrates, as well as (iii) atroposelective reactions with noncentrally chiral compounds. In this Creative Commons Attribution 3.0 Unported Licence. review, the major enzymatic strategies broadly applicable in the asymmetric synthesis of optically pure rsc.li/rsc-chembio chiral compounds are presented, with a focus on the reactions developed within the past decade. Introduction brewery, bakery and wine making was not conscious, advances mostly in the 20th century in neighboring disciplines such as Biocatalysis, which can be broadly defined as the use of biochemistry, enzymology, molecular biology, and more recently enzymes to catalyze chemical reactions, is found in diverse fields in protein crystallography, genetic engineering and computational This article is licensed under a of application. While its historical use by ancient civilizations in tools, contributed to a global and molecular understanding of enzymes and their modes of action. At the same time, scientists could finally produce, manipulate and modify enzymes on a Institute of Chemistry, University of Graz, Heinrichstrasse 28, 8010 Graz, Austria. Open Access Article. Published on 01 June 2021. Downloaded 9/23/2021 4:50:48 PM. demand, which participated in the establishment of modern E-mail: [email protected] 1,2 b Field of Excellence BioHealth – University of Graz, Austria biocatalysis. Theuseofenzymesinchemistryhasalong tradition, such as in the food, textile and detergent industry,3,4 however biocatalysis started to dramatically impact the fine Me´lanie Hall studied chemistry at chemical and pharmaceutical industry only recently at the turn the E´cole Nationale Supe´rieure de of the 21st century. Besides the strong motivation to develop Chimie de Rennes (ENSCR), environmentally more acceptable chemical processes, the faster France, and received her PhD in access to proteins and the ability to modify them largely contrib- chemistry from the University of uted to a mindset shift in industry.5,6 Enzymes can compete with Graz, Austria, under Prof. Kurt traditional chemical (catalytic) methods for a broad range of Faber (2007). After a postdoctoral chemical transformations, including those long considered non- research stay with Prof. Andreas natural. Exemplary is the identification of the catalytic activity of Bommarius at the Georgia aldoxime dehydratases in the Kemp elimination reaction7 –a Institute of Technology in Atlanta, base-catalyzed deprotonation of a benzisoxazole ring – for USA, she returned to the University which, besides chemical bases, only de novo designed proteins of Graz, where she obtained her were known to be active catalysts.8 Me´lanie Hall habilitation in organic chemistry Baeyer–Villiger monooxygenases (BMVOs) represent another (venia docendi) in 2016. She is beautiful example of how nature designs efficient enzymatic currently assistant professor for sustainable bioorganic synthetic machineries to catalyze difficult chemical reactions. Chemists chemistry at the Institute of Chemistry. Her research is dedicated to employ peracids or peroxides as oxidant in the eponymic the field of biocatalysis, with particular focus on asymmetric synthesis reaction for the insertion of oxygen between two carbon atoms. and enzymatic synthetic technologies. After attack of the carbonyl group of the substrate by the oxidant, 958 | RSC Chem. Biol., 2021, 2, 958–989 © 2021 The Author(s). Published by the Royal Society of Chemistry View Article Online Review RSC Chemical Biology the reaction proceeds through formation of the so-called Criegee 1. Stereoselective reactions involving intermediate. The collapse of this intermediate leads to migration transformations of sp2 carbons of one of the alkyl substituents of the substrate carbonyl to the oxygen atom derived from the oxidant, and eventually results in The biocatalytic stereoselective conversion of prochiral molecules the formation of the ester or lactone product. BVMOs simply use into chiral products often involves substrates bearing at least one molecular oxygen as oxidant in the enzymatic version of the sp2 hybridized carbon atom embedded in a double bond, such as reaction and rely on a reduced flavin cofactor to activate the CQC-, CQN- or CQO-bond. Transformations of these double oxygen. The resulting peroxyflavin is a powerful nucleophile that bonds include reduction reactions (stereoselective C–H bond can attack carbonyls, leading to a biological Criegee intermediate. formation), transamination and hydroamination (stereoselective Therestofthereactioncanbeconsideredanalogoustothe C–N-bond formation), epoxidation (stereoselective C–O-bond for- chemical oxidation.9 A major advantage of the biological catalyst mation), and addition of nucleophilic carbon (stereoselective is the benign and nontoxic environment required for the C–C-bond formation). All these reactions proceed through selective oxygenation reaction: air, importantly seconded by high regio- substrate face recognition by the enzyme of interest and may lead to and enantioselectivity. the formation of up to two chiral centers (Scheme 1). The interest in using enzymes in asymmetric synthesis is Q Q Q motivated by the exquisite chemo, regio- and stereoselectivity of 1.1. C C-, C O-, C N-double bond reduction enzymes. Concepts underlying enzymatic strategies in asym- In biology, the reduction of double bonds is typically a formal metric reactions are similar to those found in organic synthesis: addition of [2H], which proceeds via a hydride addition/ – A prochiral compound can be transformed stereoselectively protonation sequence. The hydride added stereoselectively on into a chiral product in enantiopure form. Usually, such reactions the most electron-deficient (carbon) atom is derived from a involve nucleophilic attack onto sp2 hybridized carbon atoms molecule of nicotinamide cofactor NAD(P)H,27 and usually, the embedded in a CQC-, CQO-, or CQN-double bond, and imply final product is obtained by protonation – often catalyzed by an Creative Commons Attribution 3.0 Unported Licence. face recognition of the substrate molecular plane by the enzymes. acidic amino acid – through the reaction medium. – A prochiral sp3 hybridized atom can be converted into a 1.1.1. Alkene reduction. Several types of enzymes are active chiral center by the formal enantioselective replacement of one in the stereoselective reduction of CQC-double bonds.28 of the two enantiotopic groups, such as hydrogen atoms on Ene-reductases from the flavoprotein family of Old Yellow Enzymes carbon or electron lone pairs on sulfur. (OYEs) have become the golden standard for this reaction in – In case the chiral information is already present, a racemic biocatalysis owing to a broad substrate scope and large protein substrate can be converted enantioselectively according to the diversity.29,30 Bacterial enoate reductases from obligate anaerobes principles of kinetic resolution. are specific for a,b-unsaturated carboxylic acids (enoates). Due to – Finally, the case of atroposelective reactions deserves their dependence on an [Fe–S]-cluster, these enzymes are oxygen- This article is licensed under a attention. The chiral information is contained not in a center sensitive and therefore less suitable for practical applications in but in an axis that can be either formed or, if already existing, synthesis. Nicotinamide-dependent reductases from the NADP- resolved. dependent leukotriene B4 dehydrogenase subfamily of medium Open Access Article. Published on 01 June 2021. Downloaded 9/23/2021 4:50:48 PM. In this article, the major strategies relying on enzymes for chain dehydrogenases/reductases (MDR) are flavin-independent the key step of asymmetric synthesis are reviewed and classified enzymes found in plants and mammals, which have not yet in one of the following categories: (i) stereoselective reactions demonstrated generality through high stereoselectivity on a large involving transformations of sp2 hybridized carbon atoms, panel of substrates. Some of these alkene reductases have how- (ii) enantioselective reactions and (iii) atroposelective reactions. ever displayed sufficient