Kinetic enzymatic resolution

Jonathan M.J. Williams, Rebecca J. Parker, and Claudia Neri 

Conventional kinetic resolution procedures often provide an effective route for the preparation of enantiomerically enriched compounds. However, a resolution of two enantiomers will only provide a maximum of 50% yield of the enantiomerically pure material. This limitation can be overcome in a number of ways, including inversion of the stereochemistry of the unwanted enantiomer, racemization and recycling of the unwanted enantiomer or dynamic kinetic resolution. 

A dynamic kinetic resolution reaction involves the interconversion of the enantiomers of a starting material under conditions where one enantiomer is converted selectively into product. This principle is shown in Fig. 9-1, where a conventional kinetic resolution reaction and a dynamic kinetic resolution reaction are compared. In both cases enantiomer A reacts to form product B more quickly than enantiomer A. However, in the conventional kinetic resolution, enantiomer A is simply left behind as unreacted starting material. In the dynamic kinetic resolution, A and A are in equilibrium, which allows for the possibility that all of the starting material will be converted into product B. The reaction conditions must be chosen that whilst the starting material enantiomers (A/A ) undergo rapid equilibration (racemization), the product B must be inert to racemization. 

Dynamic kinetic resolution reactions are not limited to enzyme-catalyzed processes, and there are reviews available that consider all aspects of such reactions[1 31. 

Conventional Kinetic Resolution Dynamic Kinetic Resolution 

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On that basis, crystallization is often used in combination with other enantiose-lective techniques, such as enantioselective synthesis, enzymatic kinetic resolution or simulated moving bed (SMB) chromatography [10, 11]. In general, when referring to crystallization techniques, the aim is to obtain an enantiomeric enrichment in the crystallized solid. However, the possibility of producing an enrichment in the mother liquors [12, 13], even if this is not a general phenomenon [14], must be taken into account. 

Enantiomers, preferential crystallization of 59 Endo selectivity 798 Ene reactions 808, 809 Enones, synthesis of 732 Enthalpies of formation 102, 103 Enynes, synthesis of 956 Enzymatic kinetic resolution 829 Epimerization 399 Episulphides, oxidation of 237 Episulphones 650, 775 Episulphoxides, photolysis of 742 a,/J-Epoxysulphones reactions of 811, 812 rearrangement of 685 synthesis of 612 / ,y-Epoxysulphones 781 y,<5-Epoxysulphones 627, 628 Epoxysulphoxides reactions of 613 rearrangement of 744 synthesis of 327, 612 Erythronolides 831... 

Table 4.20 Asymmetric Diels-Alder reactions via enzymatic kinetic resolution...

Despite its widespread application [31,32], the kinetic resolution has two major drawbacks (i) the maximum theoretical yield is 50% owing to the consumption of only one enantiomer, (ii) the separation of the product and the remaining starting material may be laborious. The separation is usually carried out by chromatography, which is inefficient on a large scale, and several alternative methods have been developed (Figure 6.2). For example, when a cyclic anhydride is the acyl donor in an esterification reaction, the water-soluble monoester monoacid is separable by extraction with an aqueous alkaline solution [33,34]. Also, fiuorous phase separation techniques have been combined with enzymatic kinetic resolutions [35]. To overcome the 50% yield limitation, one of the enantiomers may, in some cases, be racemized and resubmitted to the resolution procedure. 

Recently, an interesting example of the enzymatic kinetic resolution of a-acetoxyamide 8 was demonstrated using native wheat germ lipase and immobilized lipase PS (AMANO) (Scheme 5.6). 

Scheme 5.6 Influence of a cosolvent on enzymatic kinetic resolution.

For most chemical transformations, especially for industrial applications, the yield of 50% cannot be accepted. Since each enantiomer constitutes only 50% of the racemic mixture, the best way to increase the yield of the desired enantiomer is racemization of the unwanted one (Scheme 5.7). This reaction mustproceed simultaneously with the enzymatic kinetic resolution. In order to indicate the dynamic character of such processes, the term dynamic kinetic resolution has been introduced. 

Thus, racemic acid 12 (R = H) was obtained by [3+2] cycloaddition in 90-95% yield (Scheme 5.9) [28]. Its resolution into enantiomers could be achieved either by chiral preparative HPLC, or by fractional crystallization of its cinchonidine salts. Better results were obtained upon enzymatic kinetic resolution of its iso-butyl ester 12 (R = i-Bu) [29]. However, further work showed that racemic thiolester 13, which... 

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... 

A combination of an enzymatic kinetic resolution and an intramolecular Diels-Alder has recently been described by Kita and coworkers [23]. In the first step of this domino process, the racemic alcohols ( )-8-55 are esterified in the presence of a Candida antarctica lipase (CALB) by using the functionalized alkenyl ester 8-56 to give (R)-8-57, which in the subsequent Diels-Alder reaction led to 8-58 in high enantioselectivity of 95 and 91 % ee, respectively and 81 % yield (Scheme 8.15). In-... 

Additional studies featuring reactions of thiophene derivatives detail biohydrolysis of (S)-3-(thiophen-2-ylthio)butanenitrile <06TL8119>, lipase catalyzed resolution of thiotetronic acids <06TL7163>, enzymatic kinetic resolution of l,l-dioxo-2,3-dihydrothiophen-3-ol <06TL5273>, and efficient synthesis of 6-methyl-2,3-dihydrothieno[2,3-c]furan 55, a coffee... 

Ketorolac 132, a nonstereoidal anti-inflammatory drug with cyclooxygenase (COX) inhibitory activity, was marketed as a racemic mixture. It is now well established that (V)-ketorolac is the active enantiomer <1999MI382>. Therefore, efforts were devoted to the selective synthesis of this active stereomer, either by enzymatic kinetic resolution <2001TA1865> or by enantioselective synthesis <2005AGE609>. 

Scheme 1.39 Enzymatic kinetic resolution approaches to abacavir...

Nechah, M., Azzi, N., Vanthuyne, N., Bertrand, M., Gastaldi, S. and Gil, G., Highly selective enzymatic kinetic resolution of primary amines at 80°C a comparative study of carboxylic acids and their ethyl esters as acyl donors. J. Org. Chem., 2007, 72, 6918-6923. 

Procedure 3 Enzymatic Kinetic Resolution of (/ ,S)-2-Ethoxyethyl Ibuprofen Ester with Immobilized Lipase... 

The procedure shows that it is feasible to combine racemization with the kinetic resolution process (hence the DKR) of R,S)- ethoxyethyl ibuprofen ester. The chemical synthesis of the ester can be applied to any esters, as it is a common procedure. The immobilized lipase preparation procedure can also be used with any enzymes or support of choice. However, the enzyme loading will need to be optimized first. The procedures for the enzymatic kinetic resolution and DKR will need to be adjusted accordingly with different esters. Through this method, the enantiopurity of (5)-ibuprofen was found to be 99.4 % and the conversion was 85 %. It was demonstrated through our work that the synthesis of (5)-ibuprofen via DKR is highly dependent on the suitability of the reaction medium between enzymatic kinetic resolution and the racemization process. This is because the compatibility between both processes is crucial for the success of the DKR. The choice of base catalyst will vary from one reaction to another, but the basic procedures used in this work can be applied. DKRs of other profens have been reported by Lin and Tsai and Chen et al. ... 

Rodriguez, C., de Gonzalo, G., Eraaije, M.W. and Gotor, V., Enzymatic kinetic resolution of racemic ketones catalyzed by Baeyer-Villiger monooxygenases. Tetrahedron Asymm., 2007, 18, 1338. 

In a succeeding publication, the same authors reported on an enantiose-lective approach to diquinane enones 6 and ent-6 by combining the above-described synthesis with an enzymatic kinetic resolution (Scheme 4) [12]. After lipase-catalyzed enantioselective transesterification of diol rac-12. 

An efficient method to prepare enantiomerically enriched hydroperoxides is the enzymatic kinetic resolution of racemic hydroperoxides using different kinds of enzymes (mainly lipases, chloroperoxidase, horseradish peroxidase). However, the scope of these reactions may be limit by the narrow substrate specificity of the enzyme. 

A variety of further methods for the enzymatic kinetic resolution of racemic hydroperoxides (equation 14) has been published. For comparison they are summarized in Table 5. 

This problem was solved by Adam and coworkers in 1994-1998. They presented a high-yielding and diastereoselective method for the preparation of epoxydiols starting from enantiomerically pure allyhc alcohols 39 (Scheme 69). Photooxygenation of the latter produces unsaturated a-hydroxyhydroperoxides 146 via Schenck ene reaction. In this reaction the (Z)-allylic alcohols afford the (5, 5 )-hydroperoxy alcohols 146 as the main diastereomer in a high threo selectivity (dr >92 8) as racemic mixmre. The ( )-allylic alcohols react totally unselectively threolerythro 1/1). Subsequent enzymatic kinetic resolution of rac-146 threolerythro mixture) with horseradish peroxidase (HRP) led to optically active hydroperoxy alcohols S,S) and (//,5 )-146 ee >99%) and the... 

Following route A (Fig. 1), Yan Xiao et al. reported the chemoenzymatic synthesis of poly(8-caprolactone) (PCL) and chiral poly(4-methyl-8-caprolactone) (PMCL) microparticles [5]. The telechelic polymer diol precursors were obtained by enzymatic polymerization of the corresponding monomers in the presence of hexanediol. Enzymatic kinetic resolution polymerization directly yielded the (R)-and (S )-enriched chiral polymers. After acrylation using acryloylchloride, the chiral and nonchiral particles were obtained by crosslinking in an oil-in-water emulsion photopolymerization. Preliminary degradation experiments showed that the stereoselectivity of CALB is retained in the degradation of the chiral microparticles (Fig. 2). 

In order to extend the comparison of enzymatic kinetic resolution and asymmetric catalysis of chapter 2 and identify the potential improvements of these processes, the derivation of the equations that determine the enantiomeric excess and yield are given in detail. 

Rakels, J.L.L., Paffen, H.T., Straathof, A.J.J. and Heijnen, J.J. (1994) Comparison of enzymatic kinetic resolution in a batch reactor and a CSTR. Enzyme Microbial Technology, 16,791-794. 

I.4.2. Enzymatic Kinetic Resolution of Alcohols and Carboxylic Acids... 

Alternative synthetic approaches include enantioselective addition of the organometallic reagent to quinoline in the first step of the synthesis [16], the resolution of the racemic amines resulting from simple protonation of anions 1 (Scheme 2.1.5.1, Method C) by diastereomeric salts formation [17] or by enzymatic kinetic resolution [18], and the iridium-catalyzed enantioselective hydrogenation of 2-substituted quinolines [19]. All these methodologies would avoid the need for diastereomer separation later on, and give direct access to enantio-enriched QUINAPHOS derivatives bearing achiral or tropoisomeric diols. Current work in our laboratories is directed to the evaluation of these methods. 

Finally, Sanfilippo and coworkers describe the enzymatic kinetic resolution of atropisomeric ( )-3,3 -bis(hydroxymethyl)-2,2 -bipyridine N,N-dioxide by enantioselective esterifcation in an unusual medium of 2-propanol/vinyl acetate (20 80) [139]. Lipase from Mucor miehei (immobilized lipase preparation, Lipozyme ) was found to give good enantioselectivity with an (aS)-enantiopreference in the axial recognition and allowed efficient preparation of both enantioforms with > 98%. Despite the fact that the propanol reacts with the acyl donor, this did not diminish its positive effects on the solubility of the bipyridyl substrate. 

Enzymatic kinetic resolution is a key step in the synthesis of the platelet aggregation inhibitor Lotrafiban (Figure 10.11). A disclosed process involves CaLB in tert-butyl alcohol/water (88 12) at 50 °C the substrate concentration was only r> g I 1 owing to its low solubility in this medium [122]. By exploiting the higher solubility in 88% [BMIm][PF6] and the better thermal stability of the biocatalyst in this medium, a higher rate was observed, the reaction was performed at 40 54 1. 1 at 75 °C, and the biocatalyst (Novozym 435) could be recycled 10 times. 

Enzymatic kinetic resolution of amines Faber K (2000) Biotransformations in organic chemistry, 4th edn. Springer, Berlin Heidelberg New York, Sect. 3.1.3... 

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