Lipases dynamic kinetic resolution

The resolution of racemic ethyl 2-chloropropionate with aliphatic and aromatic amines using Candida cylindracea lipase (CCL) [28] was one of the first examples that showed the possibilities of this kind of processes for the resolution of racemic esters or the preparation of chiral amides in benign conditions. Normally, in these enzymatic aminolysis reactions the enzyme is selective toward the (S)-isomer of the ester. Recently, the resolution ofthis ester has been carried out through a dynamic kinetic resolution (DKR) via aminolysis catalyzed by encapsulated CCL in the presence of triphenylphosphonium chloride immobilized on Merrifield resin (Scheme 7.13). This process has allowed the preparation of (S)-amides with high isolated yields and good enantiomeric excesses [29]. 

Biooxidative deracemization of racemic sec-alcohols to single enantiomers [47,48] is complementary to combined metal-assisted lipase-mediated strategies [49,50]. In general, deracemization can be realized by either an enantioconvergent, a dynamic kinetic resolution, or a stereoinversion process. The latter concept is particularly appealing, as only half of the substrate needs to be converted, as the remaining half already represents the product with correct stereochemistry. 

Kim and co-workers recently reported an excellent example of dynamic kinetic resolution (DKR) using lipase-ruthenium combo catalyst in an IE solvent system (Fig. 7). Applied to this protocol, the authors succeeded in preparing (R)-ester or (5 )-ester using lipase PS or subtilisin, respectively. An IE solvent system is truly appropriate for DKR because racemizafion takes place easily in a highly polar solvent. 

Another approach to the synthesis of chiral non-racemic hydroxyalkyl sulfones used enzyme-catalysed kinetic resolution of racemic substrates. In the first attempt. Porcine pancreas lipase was applied to acylate racemic (3, y and 8-hydroxyalkyl sulfones using trichloroethyl butyrate. Although both enantiomers of the products could be obtained, their enantiomeric excesses were only low to moderate. Recently, we have found that a stereoselective acetylation of racemic p-hydroxyalkyl sulfones can be successfully carried out using several lipases, among which CAL-B and lipase PS (AMANO) proved most efficient. Moreover, application of a dynamic kinetic resolution procedure, in which lipase-promoted kinetic resolution was combined with a concomitant ruthenium-catalysed racem-ization of the substrates, gave the corresponding p-acetoxyalkyl sulfones 8 in yields... 

The ability of enzymes to achieve the selective esterification of one enantiomer of an alcohol over the other has been exploited by coupling this process with the in situ metal-catalysed racemisation of the unreactive enantiomer. Marr and co-workers have used the rhodium and iridium NHC complexes 44 and 45 to racemise the unreacted enantiomer of substrate 7 [17]. In combination with a lipase enzyme (Novozyme 435), excellent enantioselectivities were obtained in the acetylation of alcohol 7 to give the ester product 43 (Scheme 11.11). A related dynamic kinetic resolution has been reported by Corberdn and Peris [18]. hi their chemistry, the aldehyde 46 is readily racemised and the iridium NHC catalyst 35 catalyses the reversible reduction of aldehyde 46 to give an alcohol which is acylated by an enzyme to give the ester 47 in reasonable enantiomeric excess. 

The use of an enzyme in a cascade using nanoencapsulation has also been demonstrated [23]. In this case, the dynamic kinetic resolution (DKR) of secondary alcohols was achieved with an acidic zeolite and an incompatible enzyme, Candida antarctica lipase B (CALB) (Scheme 5.8). 

Hoyos, P., Buthe, A., Ansorge-Schumacher, M.B. et al. (2008) Highly efficient one pot dynamic kinetic resolution of benzoins with entrapped Pseudomonas stutzeri lipase. Journal of Molecular Catalysis B, Enzymatic, 52-53,133-139. 

Dynamic Kinetic Resolution for the Synthesis of Esters, Amides and Acids Using Lipases... 

Dynamic Kinetic Resolution of 1-Phenylethanol by Immobilized Lipase Coupled with In Situ Racemization over Zeolite Beta... 

The one-pot dynamic kinetic resolution (DKR) of ( )-l-phenylethanol lipase esterification in the presence of zeolite beta followed by saponification leads to (R)-l phenylethanol in 70 % isolated yield at a multi-gram scale. The DKR consists of two parallel reactions kinetic resolution by transesterification with an immobilized biocatalyst (lipase B from Candida antarctica) and in situ racemization over a zeolite beta (Si/Al = 150). With vinyl octanoate as the acyl donor, the desired ester of (R)-l-phenylethanol was obtained with a yield of 80 % and an ee of 98 %. The chiral secondary alcohol can be regenerated from the ester without loss of optical purity. The advantages of this method are that it uses a single liquid phase and both catalysts are solids which can be easily removed by filtration. This makes the method suitable for scale-up. The examples given here describe the multi-gram synthesis of (R)-l-phenylethyl octanoate and the hydrolysis of the ester to obtain pure (R)-l-phenylethanol. 

The complete transformation of a racemic mixture into a single enantiomer is one of the challenging goals in asymmetric synthesis. We have developed metal-enzyme combinations for the dynamic kinetic resolution (DKR) of racemic primary amines. This procedure employs a heterogeneous palladium catalyst, Pd/A10(0H), as the racemization catalyst, Candida antarctica lipase B immobilized on acrylic resin (CAL-B) as the resolution catalyst and ethyl acetate or methoxymethylacetate as the acyl donor. Benzylic and aliphatic primary amines and one amino acid amide have been efficiently resolved with good yields (85—99 %) and high optical purities (97—99 %). The racemization catalyst was recyclable and could be reused for the DKR without activity loss at least 10 times. 

Dynamic kinetic resolution enables the limit of 50 % theoretical yield of kinetic resolution to be overcome. The application of lipase-catalyzed enzymatic resolution with in situ thiyl radical-mediated racemization enables the dynamic kinetic resolution of non-benzylic amines to be obtained. This protocol leads to (/f)-amides with high enantioselectivities. It can be applied either to the conversion of racemic mixtures or to the inversion of (5)-enantiomers. 

Dynamic kinetic resolution (DKR) is a process in which the resolution process is coupled with in situ racemization of unreacted substrate. This has been shown to be a potential and feasible method to produce 100 % theoretical yield. We have developed a chemo-enzymatic DKR to obtain higher desired yield for (5)-ibuprofen. The combined base catalyst with lipase has resulted in high conversion and excellent ee of the product. 

Lin, C.N. and Tsai, S.W., Dynamic kinetic resolution of suprofen thioester via coupled triocty-lamine and lipase catalysis. Biotechnol. Bioeng., 2000, 69, 31-38. 

Chen, C.Y, Cheng, Y.C. and Tsai, S.W., Lipase-catalyzed dynamic kinetic resolution of (R,S)-fenoprofen thioester in isooctane. J. Chem. Technol. Biotechnol., 2002, 77, 699-705. 

Dynamic Kinetic Resolution Synthesis of a Fluorinated Amino Acid Ester Amide by a Continuous Process Lipase-mediated Ethanolysis of an Azalactone... 

Chapter 3 describes the application of lipases, proteases and sulfatases for the kinetic resolution of a range of interesting molecules. A selection of dynamic kinetic resolution (DKR) procedures is disclosed in Chapter 4. DKRs are attracting a significant amount of... 

The reversibility of hydrogen transfer reactions has been exploited for the racemi-zation of alcohols and amines. By coupling the racemization process with an enantioselective enzyme-catalyzed acylation reaction, it has been possible to achieve dynamic kinetic resolution reactions. The combination of lipases or... 

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. 

A prominent example of chemoenzymatic catalysis in bio-organic chemistry is the dynamic kinetic resolution (DKR) of secondary alcohols (Scheme 9) [94, 95] and amines [96-99], In this process, a lipase is employed as an enantioselective acylation catalyst, and a metal-based catalyst ensures continuous racemization of the unreactive enantiomer. 

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-... 

The integration of a catalyzed kinetic enantiomer resolution and concurrent racemization is known as a dynamic kinetic resolution (DKR). This asymmetric transformation can provide a theoretical 100% yield without any requirement for enantiomer separation. Enzymes have been used most commonly as the resolving catalysts and precious metals as the racemizing catalysts. Most examples involve racemic secondary alcohols, but an increasing number of chiral amine enzyme DKRs are being reported. Reetz, in 1996, first reported the DKR of rac-2-methylbenzylamine using Candida antarctica lipase B and vinyl acetate with palladium on carbon as the racemization catalyst [20]. The reaction was carried out at 50°C over 8 days to give the (S)-amide in 99% ee and 64% yield. Rather surpris-... 

While enzymes and chiral chemical catalysts compete for best performance in a variety of situations, they have also been used jointly to afford a desired reaction result (Choi, 1999). By far the most frequent application of this concept, termed an enzyme-metal combi reaction (EMCR) , is the dynamic kinetic resolution (DFR) of a racemic mixture with a lipase and an organometallic complex to afford in-situ racemization. 

Racemic resolution of a-hydroxy esters was achieved with Pseudomonas cepacia lipase (PCL) and a ruthenium catalyst (for a list, see Figure 18.13) as well as 4-chlorophenyl acetate as an acyl donor in cyclohexane, with high yields and excellent enantiomeric excesses (Huerta, 2000) (Figure 18.14). Combining dynamic kinetic resolution with an aldol reaction yielded jS-hydroxy ester derivatives in very high enantiomeric excesses (< 99% e.e.) in a one-pot synthesis (Huerta, 2001). 

J. Y. Jung, and M. J. Kim, Dynamic kinetic resolution of acyclic allylic acetates using lipase and palladium, J. Org. Chem. 1999, 64(22), 8423-8424. 

J. H. Koh, and J. Park, Dynamic kinetic resolution of allylic alcohols mediated by ruthenium- and lipase-based catalysts,... 

Kinetic resolutions in general are regularly applied in organic synthesis. Since enzymes are highly attractive for asymmetric synthesis, various types of biocatalysts have been used in enzymatic (dynamic) kinetic resolutions, but the focus will remain on lipase- and esterase-mediated resolutions as the most common tools in early steps of natural product syntheses. 

Dynamic Kinetic Resolutions with Lipases and Esterases... 

Akai S, Tanimoto K et al (2006) A dynamic kinetic resolution of allyl alcohols by the combined use of lipases and [VO(OSiPh3)3]. Angew Chem Int Ed 45 2592-2595... 

Dynamic kinetic resolutions of secondary alcohols and amines have been achieved by the combination of biocatalysts with metal catalysts.12 For example, a metal catalyst was used to racemize the substrate, phenylethanol, and a lipase was used for the enantioselective esterification as shown in Figure 12. The yield was improved from 50% in kinetic resolution without racemization of the substrate to 100% with metal catalyzed racemization. 

The following sections discuss selected examples of the use of esterases and lipases to hydrolyze prochiral or meio-substrates, where yields of 100% can theoretically be attained, followed by a brief discussion of dynamic kinetic resolution (DKR) where reaction yields of 100% can also potentially be achieved. 

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... 

A completely different enzyme-catalyzed synthesis of cyanohydrins is the lipase-catalyzed dynamic kinetic resolution (see also Chapter 6). The normally undesired, racemic base-catalyzed cyanohydrin formation is used to establish a dynamic equilibrium. This is combined with an irreversible enantioselective kinetic resolution via acylation. For the acylation, lipases are the catalysts of choice. The overall combination of a dynamic carbon-carbon bond forming equilibrium and a kinetic resolution in one pot gives the desired cyanohydrins protected as esters with 100% yield [19-22]. 

In general the hydrolysis of amides is performed as a kinetic resolution and not as a dynamic kinetic resolution. It is applied industrially [96] but in most cases the industrial kinetic resolution of amines, as performed for instance by BASF, is an acylation of racemic amines [38], rather than the hydrolysis of racemic amides. For the acylation of amines many different acyl donors [27] and enzymes can be used, including lipases (or a review see [97]). 

Subsequently the groups of Williams [7] and Backvall [8] showed, in 1996 and 1997, respectively, that lipase-catalyzed transesterification of alcohols could be combined with transition metal-catalyzed racemization to produce an efficient dynamic kinetic resolution of chiral secondary alcohols (Fig. 9.2). 

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