Hydrolases, dynamic kinetic resolution

Dynamic Kinetic Resolution (DKR) of Azlactones Thioureas Can Act as Oxyanion Holes Comparable to Serine Hydrolases... 

There are basically two approaches to the synthesis of enantiomerically pure alcohols (i) kinetic resolution of the racemic alcohol using a hydrolase (lipase, esterase or protease) or (ii) reduction mediated by a ketoreductase (KRED). Both of these processes can be performed as a cascade process. The first approach can be performed as a dynamic kinetic resolution (DKR) by conducting an enzymatic transesterification in the presence of a redox metal [e.g. a Ru(ll) complex] to catalyze in situ racemization of the unreacted alcohol isomer [11] (Scheme 6.1). We shall not discuss this type of process in any detail here since it forms the subject of Chapter 1. 

Limitations of kinetic resolutions. If one is interested in preparing a single enantiomer, the major drawback of hydrolase-catalyzed kinetic resolutions is that only 50% yield of the desired enantiomer is possible from a racemic starting material. The introduction of dynamic kinetic resolution (DKR, see Chapter 6) provides one solution to this problem. Another one is to invert the configuration of the product or the remaining substrate after the hydrolase-mediated resolution, and examples of this approach are discussed below (Section 4.2.1.2). 

It is worth noting here that with two enzymes displaying opposite enantioselec-tivity it is possible to produce both enantiomers of the ester products. If the remaining alcohols can be continuously and rapidly racemized during the much slower acylation reaction, either the R- or S-esters can be obtained in high yields (>>5096) from reactions catalyzed by two hydrolases that display opposite enantio-preference. The combined process of racemization and simultaneous resolution, dynamic kinetic resolution (DKR), is described in Chapter 6. 

Hydrolase-catalyzed domino reactions incorporating a resolution and a subsequent cycloaddition reaction have been described [95-97]. This constitutes an attractive approach to complex synthetic intermediates. For example, the l-(3-methyl-2-furyl)]propanol roc-93 reacts with ethoxyvinyl methyl fumarate (94) catalyzed by Lipase LIP (from Pseudomonas aeruginosa) to furnish a dienophilic fumarate ester, which spontaneously undergoes an intramolecular Diels-Alder reaction with the furan moiety furnishing exclusively the syn-adduct, the oxabicy-clohexene 95 in excellent along with the remaining alcohol S-96 (Scheme 4.31) [95]. A similar approach has been used for a procedure that includes a series of domino reactions that includes dynamic kinetic resolution of the 3-vinylcyclohex-... 

A method that has been used to approach 100% theoretical yield in asymmetric syntheses is dynamic kinetic resolution, or DKR. Although this method has been practiced based on strictly chemical reactions, only those chemoenzymatic DKR reactions will be discussed here. Most often, the enzyme used by this method is a hydrolase (lipase, esterase, protease), but other enzymes such as hydantoinases, /V-acylamino acid racemases, and dehydrogenases have also been exploited to effectively carry out DKR reactions.196 For additional details the reader is directed to the many review articles written on DKR.197 206... 

Figure 18.22. Dynamic kinetic resolution of 2,2-disubstituted epoxides 63 by epoxide hydrolase.

An elegant way to avoid the low yields and the need for recycling half of the material in the case of kinetic resolutions is a dynamic kinetic resolution (DKR). The dynamic stands for the dynamic equilibrium between the two enantiomers that are kinetically resolved (Scheme 6.6A). This fast racemisation ensures that the enzyme is constantly confronted with an (almost) racemic substrate. At the end of the reaction an enantiopure compound is obtained in 100% yield from racemic starting material. Mathematical models describing this type of reaction have been published and applied to improve this important reaction [32, 33]. There are several examples, in which the reaction was performed in water (see below). In most cases the reaction is performed in organic solvents and the hydrolase-catalysed reaction is the irreversible formation of an ester (for example see Figs. 9.3, 9.4, 9.6, 9.12) or amide (for example see Figs. 9.13, 9.14, 9.16). 

Instead of starting with racemic starting material it is also possible to use symmetric substrates [25]. The hydrolase selectively catalyses the hydrolysis of just one of the two esters, amides or nitriles, generating an enantiopure product in 100% yield (Scheme 6.7). No recycling is necessary, nor need catalysts be combined, as in the dynamic kinetic resolutions, and no follow-up steps are required, as in the kinetic resolutions plus inversion sequences. Consequently this approach is popular in organic synthesis. Moreover, symmetric diols, diamines and (activated) diacids can be converted selectively into chiral mono-esters and mono-amides if the reaction is performed in dry organic solvents. This application of the reversed hydrolysis reaction expands the scope of this approach even further [22, 24, 27]. 

The dynamic kinetic resolution (DKR) of secondary alcohols and amines (Scheme 11.11) is a prominent, industrially relevant, example of chemo-enzymatic chemistry in which a racemic mixture is converted into one enantiomer in essentially 100% yield and in high ee. This is in sharp contrast to enzyme-catalyzed kinetic resolutions that afford the desired end-product in a yield of at most 50%, while 50% of the starting material remains unreacted. In DKR processes, hydrolases are typically employed as the enantioselective acylation catalyst (which can be either R or S selective) while a concurrent racemization process racemizes the remaining substrate via an optically inactive intermediate. This ensures that all starting material is converted into the desired end-product. The importance of optically pure secondary alcohols and amines for the pharmaceutical industry triggered the development of a number of approaches that enable the racemization of sec-alcohols and amines via their corresponding ketones and imines, respectively [42],... 

The synthesis of chiral building block (S)-tert-leucine ((S)-45) was accomplished by Chiroscience via a hydrolase-catalyzed dynamic kinetic resolution... 

A.R. (2012) Dynamic kinetic resolution via hydrolase-metal combo catalysis in stereoselective synthesis of bioactive compounds. Adv. Synth. Catal., 354, 2585-2611. 

Enzymes - and thus hydrolases - can realize all kinds of selectivities such as chemo-, regio-, diastereomer and diastereotopic selectivity, as well as enantiomer and enantiotopic selectivity [83]. Accordingly, lipases were apphed in all possible kinds of stereoselective biotransformations [29, 30, 79, 81, 83] such as KR [79, 84], deracemization, and dynamic kinetic resolution (DKR) [85]. In this review, we wish to concentrate on methods enabling the continuous-mode hydrolase-mediated production of compounds in high enantiomeric purity. 

The most common application of hydrolase-catalyzed reactions is the preparation of enantiopure compounds. The three possible routes are kinetic resolutions, desymmetrizations and dynamic kinetic resolutions (Figure 5.3 [21-23]). The choice of substrate determines the possible routes. If the substrate is a racemate, then the choice is either a kinetic resolution or a dynamic kinetic resolution. If the substrate is a meso or prochiral compound, then the route is a desymmetrization. Since racemates are more numerous than meso or prochiral compounds, the most common routes are resolutions. 

Three routes to enantiopure compounds using hydrolase-catalyzed reactions, (a) Kinetic resolution starts with racemic substrate and converts one enantiomer into product. This separation yields one enantiomer as the product alcohol and one as the starting acetate, both with a maximum yield of 50%. (b) Desymmetrization of a prochiral compound transforms one of prochiral groups to yield a chiral product with a maximum yield of 100%. (c) A dynamic kinetic resolution combines rapid racemization of racemic starting material with a hydrolase catalyzed acylation of one enantiomer. The maximum yield is 100%. 

Structures of suitable acyl donors in hydrolase-catalyzed dynamic kinetic resolution of racemic alcohols. 

DYNAMIC KINETIC RESOLUTION VIA HYDROLASE-METAL COMBO CATALYSIS... 

Akai, S. (2014). Dynamic kinetic resolution of racemic allylic alcohols via hydrolase-metal combo catalysis An effective method for the synthesis of optically active compounds. Chem. Lett., 43,746-754. 

Dynamic kinetic resolution (DKR) is a method that allows for conversion of the racemic mixture into the desired enantiomer with up to 100% of the theoretical yield. " DKR is a powerful approach to asymmetric synthesis and can be achieved by the application of transition-metal catalysts, Lewis acids, organocatalysts, or enzymes (e.g., hydrolases, dehydrogenases, haloalcohol dehalogenases, and transaminases). 

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