CHIRALITY 21:449-467 (2009) Review Article Chemoenzymatic and Microbial Dynamic Kinetic Resolutions ND HA DUL HA School时Chemical E D ABSTRACT This review tracks a decade of dynamic kinetic resolution develop y means of urdene ents in novel reac and products of d namic kinetic solution eded t 2009.2008 Wiley-Liss.Inc. KEY WORDS:dynamic kinetic resotion;biocatalysts;chemocatalysts;chiral resoluion INTRODUCTION A cursory glance at the literatures on the aspects of amic kineti esolution (DKR)has gained im DKR and agrochemical prodt ical yield of 100%.It is an efficie ne that enantiomerically pure form of the intended solution with the e in situ enzym he,metal or bas naceuticals.The esearchers in their early-phase of sy factors that interact with chemistry causing of th oprouons ered.He key fact solution step is combined with an in situ racemisation of tosuceedinide tanding of the phys sical tion with sef the number es present,an d base cata t the o thenzyme M hi s com ondence to:Azhn ne Ma or re solution,or the enzyme 2008 Wiley-Liss.Inc
Review Article Chemoenzymatic and Microbial Dynamic Kinetic Resolutions AZLINA HARUN KAMARUDDIN,1* MOHAMAD HEKARL UZIR,1 HASSAN Y. ABOUL-ENEIN,2* AND HAIRUL NAZIRAH ABDUL HALIM3 1 School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Pulau Pinang, Malaysia 2 Pharmaceutical and Medicinal Chemistry Department, National Research Centre, Dokki, Cairo, Egypt 3 School of Bioprocess Engineering, Universiti Malaysia Perlis (UniMAP), Arau, Perlis, Malaysia ABSTRACT This review tracks a decade of dynamic kinetic resolution developments with a biocatalytic inclination using enzymatic/microbial means for the resolution part followed by the racemization reactions either by means of enzymatic or chemocatalyst. These fast developments are due to the ability of the biocatalysts to significantly reduce the number of synthetic steps which are common for conventional synthesis. Future developments in novel reactions and products of dynamic kinetic resolutions should consider factors that are needed to be extracted at the early synthetic stage to avoid inhibition at scale-up stage have been highlighted. Chirality 21:449–467, 2009. VC 2008 Wiley-Liss, Inc. KEY WORDS: dynamic kinetic resolution; biocatalysts; chemocatalysts; chiral resoluion INTRODUCTION Dynamic kinetic resolution (DKR) has gained importance for more than 2 decades since the term was first coined in 1983 for its ability to allow for a maximal theoretical yield of 100%. It is an efficient technique that produces enantiomerically pure form of the intended products. It combines enzyme, chemocatalyst or microbial kinetic resolution with the in situ enzyme, metal or base-catalysed racemisation. The classical schematic route that represents DKR is given in Figure 1. With the current method, the process depends profoundly on the ability of the enzyme/cells to select between the two enantiomers as expressed by the enantiomeric ratio, E; a relative term of the rate of reaction of the two enantiomers.1,2 The process is a well-travelled route to optically active compounds that has overcome the limited yield of the required enantiomer if kinetic resolution alone is being used. With such a combination, one can in principle obtain a quantitative yield of one of the enantiomers where the resolution step is combined with an in situ racemisation of the unreacted enantiomer. The combined route of enzymatic resolution with metal and base catalyzed racemisation in DKR is not exactly straightforward. Martı´n-Matute and Ba¨ckvall3 highlighted possible problems of incompatibility of the processes combined together in one pot. For an efficient DKR, one of the important requirements is that the compatibility of the two catalysts must be achieved but most often this is the major problem. The interference of metal with enzyme in the resolution reaction may give poor resolution, or the enzyme could also act as an inhibitor in the racemisation process. However, some groups have shown otherwise.3–7 A cursory glance at the literatures on the aspects of DKR shows that a large number of new processes have been developed. These include reactions which produced pharmaceutical and agrochemical products.8,9 However, not many of these processes can be scaled up. From the chemical engineering point of view, Graviilidis’s and coworkers10 have given an excellent comprehensive review of factors that inhibit scalability of fine chemicals and pharmaceuticals. The researchers in their early-phase of synthetic work must already start bearing in mind that critical data for scaling up should be able to be extracted from their synthetic work to support the later-phase of the developmental work. Graviilidis and coworkers identified the most common factors that interact with chemistry causing a fall in the performance and they suggested ways to analyze these issues in order to generate suitable solutions. Wells11 has also mentioned a similar problem for scale up purposes, where a number of factors need to be considered. He concluded that the key factor to succeed in identifying and scaling up reactions with biocatalysts is an understanding of the physical nature of the biocatalyst itself, the number and action of the enzymes present, and how best to present the enzyme to the reaction. *Correspondence to: Azlina H. Kamaruddin, School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia. E-mail: chazlina@eng. usm.my or Hassan Y. Aboul-Enein, Pharmaceutical and Medicinal Department, National Research Centre, Dokki, Cairo 12311, Egypt. E-mail: enein@gawab.com. Received for publication 27 January 2008; Accepted 20 May 2008 DOI: 10.1002/chir.20619 Published online 24 July 2008 in Wiley InterScience (www.interscience.wiley.com). CHIRALITY 21:449–467 (2009) VC 2008 Wiley-Liss, Inc
KAMARUDDIN ET AL 2-carbethoxycycloheptanone (1)with higher cyclic ring,a PR Iso considere snaa& ENZYMATIC HYDROLYSIS The yroocrn KR) )such s.5)-naproxen5ibupro fen.etc.and pharmaceutical active inter ions in reported the DKR of (rae) for the ed the of academia icroorganisms which has 包。 on DKR the mo popular results that he fed-batch mode ext one ysis and d able in a whe res where tor is co and rea (7)from race nantiose e Chirality DOI 10.1002/chir
Several configurations of enzymatic reactors have been reported involving reaction of chiral centers, for instance, a packed bed column with immobilized lipase on resolution of naproxen and the use of enzymatic membrane reactors (EMR) in which high enantioselectivity for the (S)- enantiomer of the racemate was achievable.12–15 This shows that EMR has a great potential towards the development of chirotechnology. As the result of the role played by membrane in the separation of two immiscible fluids, EMRs are normally employed in biphasic reactions in which separation and reaction occur simultaneously. Although most of the reactions considered in the EMR are isolated to resolution reactions, but the possibility of combining both resolution and racemisation under a single operating EMR system has been described by Kamaruddin and coworkers16 in the production of (S)-ketoprofen. As mentioned earlier, microbial kinetic resolution as well as DKR has also been the focused of academia in developing novel reactions. There is a wide range of microorganisms which has been screened for microbial transformations especially for use in reactions involving dynamic kinetic resolution. However, baker’s yeast (Saccharomyces cerevisiae) has been one of the most popular organisms to perform DKR. Much have been written especially on its ability to catalyse reduction reaction on a and b-ketoesters.17–19 Other types of organisms with similar ability as that of baker’s yeast are tabulated in Table 1. The use of microbial cells as parts of the catalyst provides a stable environment as well as a complete cycle for a redox reaction in particular. As described by Uzir, in 2005 (unpublished data) in a reduction reaction, for a reaction to complete a cycle, oxidised cofactor NADP1 or NAD1 is required to release an electron to become NADPH or NADH, respectively. Such a compound is only available in a whole-cell where the cofactor is continuously produced during the growth31 and subsequently consumed during the reaction. This means that in a situation where a reaction requires a cofactor to be generated in situ, the use of whole cell is one of the best alternatives. In the reduction of cyclic b-oxoesters with 5- or 6-membered ring with Saccharomyces cerevisiae, considerably good results were obtained. In addition, for a reduction of 2-carbethoxycycloheptanone (1) with higher cyclic ring, a microorganism such as Kloekera magna gives a better yield and selectivities of the product (2). This is shown in the reaction given by Scheme 1. This review focuses on the most recent development in DKR reactions with a biocatalytic inclination using enzymatic means for the resolution part and followed by the racemisation reactions either by enzymatic or chemical means. The discussion is arranged in such a way that different types of reactions such as hydrolysis, esterification, alcoholysis, and transesterification, aminolysis, and ammonolysis and acylation for the preparation of cyanohydrin esters were discussed in separate headings. In addition to this scope, the use of microorganisms in DKR is also considered. ENZYMATIC HYDROLYSIS Enzyme-catalysed hydrolysis is an attractive method for the kinetic resolution of racemic esters into carboxylic acid and alcohol.32–38 The hydrolysis process under in situ racemisation (DKR) from various racemic esters as substrate is well known and can be carried out by lipase for the production of non-steroidal anti-inflammatory drugs (NSAIDs) such as (S)-suprofen, (S)-naproxen, (S)-ibuprofen, (S)-fenoprofen, etc. and pharmaceutical active intermediates.15,32–37 Tsai and coworkers32,33 reported the DKR of (rac)- suprofen 2,2,2- trifluoroethyl thioester (3) for the production of (S)-suprofen (4) in different organic solvents; isooctane32 and cyclohexane.33 The DKR of racemic suprofen is shown in Scheme 2. Candida rugosa lipase was employed as the biocatalyst for enantioselective hydrolysis of (rac)- suprofen 2,2,2-trifluorothioester, in which trioctylamine was added as the catalyst to perform in situ racemisation of the remaining (R)-thioester (5). The conversion of the racemic suprofen for the desired (S)-suprofen completed with 95 e.e.32 In their work, they described a detailed investigation of the trioctylamine catalyst on the kinetic behaviours of the thioester in the DKR process. Their results indicated that the racemisation catalyst not only activates the lipase, but also enhances the enzyme stability. Tsai and coworkers then successfully integrated a hydrophobic hollow-fiber membrane into the DKR process, where the desired (S)-suprofen is continuously removed from cyclohexane to the aqueous phase circulating in the shell-side of the membrane, giving high yields of high optical purity (S)-suprofen. They developed a kinetic model for the whole process (operating in batch and fed-batch modes). The model was an extensive one, incorporating enzymatic hydrolysis and deactivation, lipase activation, racemisation and non-selective hydrolysis of the substrate by trioctylamine, and reactive extraction of (rac)- suprofen into the aqueous phase in the membrane. The production of (S)-naproxen (7) from racemic naproxen methyl ester by DKR has been reported by Xin et al.15 Candida rugosa lipase was selected as biocatalyst for enantioselective continuous hydrolysis process under in situ racemisation of racemic naproxen methyl ester (6) using sodium hydroxide as racemisation catalyst to catalyse Fig. 1. Schematic representation of a dynamic kinetic resolution. SS and SR represent enantiomers and PS and PR represent product enantiomers. 450 KAMARUDDIN ET AL. Chirality DOI 10.1002/chir
CHEMOENZYMATIC AND MICROBIAL DKR 451 TABLE 1.Optically active alcohols obtained from microbial reduction through dynamic kinetic resolution Microorganism Yield d.e. e.e.( Geotricum candidum 0 98 98 20 Chirality DOI10.12/chir
TABLE 1. Optically active alcohols obtained from microbial reduction through dynamic kinetic resolution Product Microorganism Yield d.e. (%) e.e. (%) Reference Geotricum candidum 80 98 98 20 Saccharomyces cerevisiae 94 94 99 21 Saccharomyces cerevisiae 94 92 99 21 Saccharomyces cerevisiae 88 100 96 22 Saccharomyces cerevisiae 72–85 99 99 23 Rhizopus arrizus 97 98 99 24 Saccharomyces sp. 95 98 98 24 Mucor recemosus 75 98 99 24 CHEMOENZYMATIC AND MICROBIAL DKR 451 Chirality DOI 10.1002/chir
452 KAMARUDDIN ET AL TABLE 1.(Continued) Microorganism Yield de. ee Reference oe味era magna 100 94 25 Yarrowia lipolytica 26 Saccharomyces cerevisiae 74 27,28 71 29,30 of ra (p-TBD)and trioctylamine35 was added as an in situ rac hiaewacnatmedamtaacaTtitceoetp tional dynam esolutio c0 DKR to the production of()n Two diff 2 of 15.7 xen e ster i ne at 45 to polystyrene cr 1.Reduction of 2-carbethoxycycloheptanone via DKR Chirality DOI 10.1002/chir
the remaining (R)-methyl ester (8). The conversion of racemic naproxen methyl ester was greater than 60% with an enantiomeric excess (e.e.) of (S)-naproxen greater than 96%. The DKR of racemic naproxen ester is shown in Scheme 3. This reaction took place in an aqueous-organic biphasic system where a tubular silicone rubber membrane was used in a stirred-tank reactor. This whole set-up allows for the separation of the chemical catalytic racemisation and enzyme resolution processes which served to avoid the key problem associated with Naproxen conventional dynamic resolution. Besides working on (S)-suprofen as a valuable product, Tsai and coworkers also extended their research on the DKR to the production of (S)-naproxen. Two different substrates were used in their study which were 2,2,2-trifluoroethyl ester34 and 2,2,2-trifluoroethyl thioester.35 Candida rugosa lipases immobilized on polypropylene powders were employed as biocatalysts for the enantioselective hydrolysis of (rac)-naproxen ester in isooctane at 458C. The organic base of 1,5,7-triazabicyclo[4,4,0] dec-5-ene bound to polystyrene cross-linked with 2% divinylbenzene (DVB) (p-TBD)34 and trioctylamine35 was added as an in situ racemisation catalysts, respectively. The enantiomeric excess (e.e.) for the (S)-naproxen was 58.1% at the racemate conversion of 95.5%.34 Using the trioctylamine as racemization catalyst, they integrated the DKR process with a hollow- fiber membrane to reactively extract the desired (S)-naproxen out of the reaction medium. The DKR of (rac)-fenoprofen thioester (9) by using Lipase MY as biocatalyst was reported by Tsai and coTABLE 1. (Continued) Product Microorganism Yield d.e. (%) e.e. (%) Reference Kloekera magna 80 100 94 25 Yarrowia lipolytica 95 – 95 26 Saccharomyces cerevisiae 74 – 98 27, 28 Saccharomyces cerevisiae 71 98 85 29, 30 Scheme 1. Reduction of 2-carbethoxycycloheptanone via DKR technique. 452 KAMARUDDIN ET AL. Chirality DOI 10.1002/chir
CHEMOENZYMATIC AND MICROBIAL DKR 45 Scheme 2.DKR of racemic supro s 36 The DKR of (rae)-fe thio ster is sh 1 s of a unde acid (17)has b the in sD) reporte he ()-o-chl d on constant with Kamaruddin and coworker eelopedsher5Ds ee of the d(acetic ac ortan acemic 1,2,3 1 arthritis C234 hydro inolin c-1carb drug ntaining the e ap .1n osa has produ ) ed(® trile (4:1)containing 1 quiv of added md0.25 04 hydroxide as he base (20 the rat can be achie d.The th ved the r bm by starting the of racemic ibuproten este or low enzvme conten .O CH.CF (8) CF,CH,SH NaOH In situ racemisation Chirality DOI 10.1002/chin
workers.36 The DKR of (rac)-fenoprofen thioester is shown in Scheme 4. The enantioselective hydrolysis process of (rac)-fenoprofen 2,2,2- trifluoroethyl thioester in isooctane was catalyzed by Lipase MY from Candida rugosa under the in situ racemisation of the remaining (R)-thioester substrate (11) with trioctylamine as racemisation catalyst. The conversion of racemic (rac)-fenoprofen thioester was 91% with an enantiomeric excess of (S)-fenoprofen (10) of 91% when 75 mM of trioctylamine was added as racemisation catalyst. They showed that the racemisation process follows a first order reversible kinetics, in which a linear relationship between the inter-conversion constant with trioctylamine concentration was found. Their work also indicated that trioctylamine also acted as a lipase activator. Kamaruddin and coworkers,37 developed the enzymecatalysed enantioselective hydrolysis process for (S)-ibuprofen (13) production which is one of the most important members of NSAIDs that belongs to the family of propionic acid. It is widely used to treat rheumatoid arthritis, headache, muscular strain, cephalalgia, etc.39 The process involved the hydrolysis of (rac)-ibuprofen ester, 2-ethoxy- 2-4-(isobutylphenyl) propionate (12). The kinetic resolution of (rac)-ibuprofen ester with lipase from Candida rugosa has produced (S)-ibuprofen acid and unreacted (R)- ibuprofen ester (14). Under the in situ racemisation, the unreacted (R)-ibuprofen ester was racemised with sodium hydroxide as the base catalyst. Through this method, the enantiopure (S)-ibuprofen (13) can be obtained at 99.4% and the conversion higher than 85% can be achieved. The DKR of racemic ibuprofen ester is shown in Scheme 5. A lipase-catalysed dynamic hydrolytic resolution of (rac)-2,2,2-trifluoroethyl a-chlorophenyl acetate (15) and (16) in water-saturated isooctane for the production of tri- fluoroethyl (R)-a-chlorophenyl acetic acid (17) has been reported by Tsai and coworkers.40 The (R)-a-chlorophenyl acetic acid (17) is a type of a-haloarylacetic acids which are known as important intermediates for synthesizing many drugs such as prostaglandin, prostacyclin, semi-synthetic penicillin, and thiazolium salts.41–43 The DKR of (rac)-2,2,2-trifluoroethyl a-chlorophenyl acetate is shown in Scheme 6. The authors reported that the best hydrolysis reaction was catalysed by Lipase MY(I) at 358C and the racemisation was catalyzed by trioctylamine as the racemization agent. The reaction gave 93.0% yield and 89.5% e.e. of the desired (R)-a-chlorophenyl acetic acid (17). Recently, Fulop and coworkers44 reported lipase-catalysed kinetic and DKR of racemic 1,2,3,4-tetrahydroisoquinoline-1-carboxylic acid (ethyl ester (rac)-(18) to produce enantiopure (R)-1,2,3,4-tetrahydroisoquinoline-1-carboxylic acid (19). The drugs containing the isoquinoline skeleton can be applied to a wide range of therapies.44 Candida antarctica lipase B catalyzed the enantioselective hydrolysis of the corresponding ethyl ester (rac)-1 in toluene/acetonitrile (4:1) containing 1 equiv of added water and 0.25 equiv of dipropylamine as racemisation catalyst to catalyse the unreacted S-ethyl ester (20). However, they found that during the DKR process using base-catalyst, the rate of racemisation was much slower than the rate of enzymatic hydrolysis. They then solved the problem by starting the hydrolysis reaction in the presence of low enzyme content Scheme 2. DKR of racemic suprofen. Scheme 3. DKR of racemic naproxen ester. CHEMOENZYMATIC AND MICROBIAL DKR 453 Chirality DOI 10.1002/chir
KAMARUDDIN ET AL 9 CF3CH,SH 13 (14 Scheme 5.DKR of racemic OCH CF. +CF3CH2OH OCH.CF OH (15) (16) (17 In situ racemisation Scheme 6.DKRof (acetate Tohenclacee ( H,0(1eq) (18) 02H CO.E (19 (20 Scheme 7.DKR ofethylester the e of th ction,r hesis of both opis racemic thiob ramethoxy n of sel d monolayers on surlaces re ENZYMATIC ESTERIFICATION Chirality DOI 10.1002/chir
and during the course of the reaction, more enzymes was added. This method allowed the preparation of (19) (e.e. 5 96%) with 80% isolated yield. The DKR of ethyl ester (18) is shown in Scheme 7. ENZYMATIC ESTERIFICATION The reaction of carboxylic acids with alcohols to form ester through a condensation reaction is known as esterifi- cation. Sanfilippo et al.45 reported the synthesis of both atropisomers of racemic thiobiphenyl (2,2’6,6’-tetramethoxybiphenyl-3,3’-diyl) dimethanethiol (21) by DKR process. Atropisomeric biphenyls are used in the design of selfassembled monolayers on surfaces relevant as biosensors. The esterification reaction of racemic thiobiphenyl in the presence of vinyl acetate and Pseudomonas cepacia lipase resulted in the DKR of epimerizing hemithioacetal intermediates (Scheme 8). This group synthesized a new C2- Scheme 4. DKR of (rac)-fenoprofen thioester. Scheme 5. DKR of racemic ibuprofen ester. Scheme 7. DKR of ethyl ester. Scheme 6. DKR of (rac)-2,2,2-trifluoroethyl a-chlorophenyl acetate. 454 KAMARUDDIN ET AL. Chirality DOI 10.1002/chir
(+)-(21) Lipase CAL- R R 22 240 R、 R R H-(CH 3= Yield (%e.e.(%) (CH- 689 n-P ase PS-D 25 (26) 00 Scheme9 DKR flly alcoho by the combination ofipses and [VOOSPha
Scheme 8. Esterification reaction of racemic thiomethanebiphenyl. Scheme 9. DKR of allyl alcohols by the combination of lipases and [VO(OSiPh3)3]
456 KAMARUDDIN ET AL al racem S-C-R pR--R-C-R R--RS-C-s Ru/CALB R--R chiral polymer racemic monomer mixture RIS--RIS X--x= fymi kinee e oyeri KRP)m diol andioevCAl obtain the single enantiomers:first through dire component could b e 435 with the 00 lyst.After 7 ction,92%conv )was a actions are usua at high tem the rec method was as efective for the cyclic allylic alcohol mic mandelic acid (D is shown in Scheme 12on cdocepiagarcteeetcrsafsoonihp'atlcohos om the sation of un-rea cted (5)-mandelic acid (3D)in ag acemic mon DKR originat HO OH Novozyme 435 + (27) Rtegm -(CH2)- OMe (29) (28) -MeOH Scheme 11.Esterification of 1,4-benzenedimethano Chirality DO110.1002/chir
symmetric chiral thiomethanebiphenyl (621) in racemic and developed two different biocatalytic strategies to obtain the single enantiomers: first through direct esterifi- cation catalysed by lipase using vinyl acetate as the acylating agent and second via the alcoholysis of acetate derivative using the same lipase and n-butanol as nucleophile. Akai et al.46 reported the feasibility of DKR of allyl alcohols (22/23) by the combination of lipases and [VO(OSiPh3)3] to produce corresponding esters (24), (Scheme 9). Enantiopure of allyl alcohols and their derivatives are important intermediates in the synthesis of natural and non-natural compounds. It is known that the isomerisation reactions are usually performed at high temperatures, thus has limited application. Their work showed that the combination of oxovanadium compound with lipases produces a novel DKR process with excellent enantiomer resolution and chemical yields. They found too that this method was also effective for the cyclic allylic alcohol (25) to produce the corresponding esters (26). They highlighted the fact that their route provided a way to produce optically active esters of secondary alcohols from the corresponding ketones via available tertiary alcohols which is not possible by the existing DKRs with ruthenium complexes. A novel concept for the synthesis of chiral polyester by DKR polymerization of racemic monomers has been reported by Hilker et al.47 The concept of DKR polymerization (DKRP) from a racemic diol and a dicarboxylic acid derivative is shown in Scheme 10. The enantioselective esterification of rac/meso mixture of a,a0 -dimethyl-1,4-benzenedimethanol (27) (Scheme 11) shows that the diol component could be catalyzed by Novozyme 435 with the presence of dimethyl adipate (DMA) as the acyl donor (28) to produce chiral polyester (29). In situ racemisation of hydroxyl-functionalize R-configured centres into the reactive R configuration was catalysed using ruthenium catalyst. After 70 h of reaction, 92% conversion of hydroxyl group was achieved with a maximum theoretical number of average molecular weight of ca. 9300 g mol21 . It is believed that this concept offers an efficient route for a one-pot synthesis of chiral polymers from the non-natural monomers. The DKR of racemic mandelic acid by esterification was recently described by Choi et al.48 (R)-mandelic acid (30) is a key intermediate for the production of semi-synthetic cephalosporins and penicillins.49 It can also be used as a chiral resolving agent and chiral synthon for the synthesis of anti-tumor and anti-obesity agents.50 The DKR of racemic mandelic acid (30)/(31) is shown in Scheme 12. One pot enantioconvergent synthesis of (R)-mandelic acid ester (32) was achieved by enantioselective esterification of racemic mandelic acid (30)/(31) with ethanol in organic solvent (hexane) with the presence of lipase and in-situ racemisation of un-reacted (S)-mandelic acid (31) in aqueous phase by recombinant E. coli cell containing mandalate racemase. This is an example of a two enzyme catalysing the DKR process. The DKR of racemic mandelic acid (30)/(31) using CHIRAZYME L-2, originated from Scheme 10. Concept of dynamic kinetic resolution polymerization (DKRP) from a racemic diol and a dicarboxylic acid derivative. CALB 5 Candida antarctica lipase B.47 Scheme 11. Esterification of racemic a,a’-dimethyl-1,4-benzenedimethanol.47 456 KAMARUDDIN ET AL. Chirality DOI 10.1002/chir
CHEMOENZYMATIC AND MICROBIAL DKR 457 EtOH (R)-Mandelica (R)-Mandelic acidester OH 3 (S)-Mandelic acid Scheme 12.Dynamic kinetic resolution for the preparation of (R)-mandelic acid ester dcltypcid dld nst orfor enzymatic acylation of alcohols() 32 the e 15).They hav the operation OH Ruthenium catalyst 9c R R (33) (34 The DKR of various alcohols (33)at an ambient temper (34)has Yield (%e.e.(% solven p-PhC.Ha found that similar type of com (ne (37)at ambient emperature.A the pro hols (5 results in a highly effcient synthesis of enanti 27品 s pre PRCH-CH the undes (Me).CCH(CH Chirality DOI 10.1002/chin
Candida antarctica type B gave 65% isolated yield and 98% e.e. of (R)-mandelic acid ester (32) as the sole product. The whole process was performed in a hollow-fiber membrane bioreactor to facilitate the preparative scale operation. ENZYMATIC ALCOHOLYSIS AND TRANSESTERIFICATION REACTIONS The DKR of various alcohols (33) at an ambient temperature using Candida antarctica lipase B and ruthenium complex (Scheme 13) to give the corresponding esters (34) has been reported by Ba¨ckvall and coworkers.51 The enzymatic reaction via acylation occurred in the presence of isopropenyl acetate as an acyl donor in toluene solvent. The necessity for premixing ruthenium complex with KOtBu in toluene suggests that a new ruthenium complex intermediate (ruthenium alkoxide) which is crucial for initiating the racemisation under DKR conditions. Most of the alcohols tested gave >92% yield and >99% enantiomeric excess. It was also found that similar type of complexes (pentaphenylcyclopentadienyl ruthenium) could provide excellent catalysts for the racemisation of secondary alcohols (37) at ambient temperature. A combination of the process with enzymatic resolution of the (rac) secondary alcohols (35) results in a highly efficient synthesis of enantiomerically pure acetates (36) at room temperature with short reaction times for most substrates. The work of Ba¨ckvall and coworkers52 is presented in Scheme 14. The selection of a suitable acyl donor is needed in order to minimize the undesired reaction with side products. Verzijl et al.53 reported the use of simple alkyl esters such as isopropyl butyrate or methyl phenylacetate as acyl donor for enzymatic acylation of secondary alcohols (38) in toluene to produce the corresponding esters (39) (Scheme 15). They have reported that simple alkyl esters Scheme 12. Dynamic kinetic resolution for the preparation of (R)-mandelic acid ester. Scheme 13. The DKR of various alcohols at an ambient temperature ruthenium as racemisation catalyst. CHEMOENZYMATIC AND MICROBIAL DKR 457 Chirality DOI 10.1002/chir
KAMARUDDIN ET AL linase Ph 35 Ph (37) n'-PhsCpRu(CO)X.t-BuOK Scheme 14.The DKR of sody lhusing combined uthenu)and lipase catalysis can be used as acyl donors if the alcohol or ketone residuc ess resulted in faster reacti removed tone in % R (38) (39) Yield (% e.e.(%) 469 99 p-MeOC.H m-CF3C&H4 2.Eu n-Hex % /-Bu Methyl phe (40) R R Shvo's catalyst Enzyme con entration Ph Me 0.9 PhOCH 1.0 110 0.5 Scheme 15.The DKR of secndary alohols using allyl esters acyl donors. Chirality DO110.1002/chir
can be used as acyl donors if the alcohol or ketone residue formed during the enzymatic acylation was continuously removed during the reaction. The addition of ketone in the racemization process resulted in faster reaction rate and reduced the amounts of enzyme (Novozyme 435) and Shvo’s ruthenium catalyst needed in the process. The Scheme 14. The DKR of secondary alcohols using combined ruthenium (II) and lipase catalysis. Scheme 15. The DKR of secondary alcohols using alkyl esters as acyl donors. 458 KAMARUDDIN ET AL. Chirality DOI 10.1002/chir
