Power of Biocatalysis for Organic Synthesis

Abstract

Biocatalysis, using defined enzymes for organic transformations, has become a common tool in organic synthesis, which is also frequently applied in industry. The generally high activity and outstanding stereo-, regio-, and chemoselectivity observed in many biotransformations are the result of a precise control of the reaction in the active site of the biocatalyst. This control is achieved by exact positioning of the reagents relative to each other in a fine-tuned 3D environment, by specific activating interactions between reagents and the protein, and by subtle movements of the catalyst. Enzyme engineering enables one to adapt the catalyst to the desired reaction and process. A well-filled biocatalytic toolbox is ready to be used for various reactions. Providing nonnatural reagents and conditions and evolving biocatalysts enables one to play with the myriad of options for creating novel transformations and thereby opening new, short pathways to desired target molecules. Combining several biocatalysts in one pot to perform several reactions concurrently increases the efficiency of biocatalysis even further.

Scheme 1. Early Examples of Enzyme-Catalyzed Biotransformations (Using the Wild-Type Whole-Cell Organisms) as Part of a Longer Synthetic Route for Industrial Chemical Production

(a) Vitamin C, (b) (−)-ephedrine, (c) hydrocortisone, and (d) acrylamide.

Scheme 2. Biocatalytic C-Acylation of Resorcinol Derivatives Using an Acyltransferase

(a) Designed biocatalytic reaction using esters as an acyl source to be transferred onto resorcinol derivatives. (b) Natural reaction of the acyltransferase represents a disproportionation.

Figure 1

Representation of the features of an enzyme exemplified by an acyltransferase (PDB: 5MG5). The residues involved in catalysis, required for the covalent binding of the substrate, its positioning, or activation, are located in the active site. The latter can be accessed via a tunnel. The residues are positioned by the enzyme backbone consisting of β-sheets and α-helices. The enzyme interacts with the bulk environment via its surface comprising polar and apolar areas. (Enzyme graphic provided by Verena Resch, Luminous Lab.)

Scheme 3. Transesterification, Amide Formation, and Ester Hydrolysis Catalyzed by Lipases and Esterases

(a) Acyl-enzyme intermediate formed from an ester or amine can be attacked by different nucleophiles producing, for example, esters, amides or carboxylic acids. (b) Chemoenzymatic process to produce pregabalin, applying the lipase from Thermomyces lanuginosus (Lipolase) in a kinetic resolution.

Scheme 4. Biocatalysis with Alcohol Dehydrogenases

(a) Stereoselective reduction of ketones and aldehydes catalyzed by alcohol dehydrogenases. (b) Multienzyme process for the production of (R)-4-cyano-3-hydroxybutyrate via the asymmetric reduction of ethyl 4-chloro-3-oxobutanoate with an alcohol dehydrogenase in combination with a GDH (glucose/glucose dehydrogenase) recycling system, followed by a halohydrin dehalogenase-catalyzed exchange of the chlorine for a cyano group. STY = space–time yield.

Scheme 5. Biocatalytic Formation of Amines

Via (a) amino group transfer by transaminases, (b) imine reduction by imine reductases, (c) reductive amination by amine dehydrogenases/reductive aminases (both terms are used for enzymes catalyzing the same reaction; for a discussion of differentiation, refer to ref (145)), and (d) biocatalytic production of sitagliptin using a transaminase. PLP = pyridoxal 5′-phosphate.

Scheme 6. Enzyme-Catalyzed Reactions That Are New to Nature

(a) 4-Oxalocrotonate tautomerase-catalyzed conjugate addition. (b) Photobiocatalytic cyclization catalyzed by an ene-reductase. (c) Photobiocatalytic intermolecular C–C bond formation catalyzed by an ene-reductase. (d) Biocatalytic cyclopropanation catalyzed by a cytochrome P450 variant, developed via directed evolution. (e) Biocatalytic C–H amination catalyzed by a cytochrome P411 variant, developed via directed evolution. (f) Biocatalytic Suzuki reaction catalyzed by a hybrid catalyst utilizing the streptavidin-biotin method.

Scheme 7. Multienzyme Cascade Reactions

(a) General scheme of a three-enzyme, three-step cascade reaction: Starting material A is converted via intermediates B and C to product P. (b) Process for the production of islatravir in a multienzyme cascade sequence.