In situ racemization

It was speculated that 7 obtained in the reaction with (S)-3 is actually the enantiomeric structure (ent-1), and that it derives from (7 )-2-methylbutanal that may have been produced by racemization of the 5-isomer in situ11. It is also possible, however, that 7 derives from the reaction of (5)-2-methylbutanal and (R)-3, since the reagent that was used is not enantiomer-ically pure (ca. 95% ee). In addition, in situ racemization of 3 via reaction with adventitious chloride ions (or trace amounts of hydrogen chloride) is also possible36. 

The main application of the enzymatic hydrolysis of the amide bond is the en-antioselective synthesis of amino acids [4,97]. Acylases (EC 3.5.1.n) catalyze the hydrolysis of the N-acyl groups of a broad range of amino acid derivatives. They accept several acyl groups (acetyl, chloroacetyl, formyl, and carbamoyl) but they require a free a-carboxyl group. In general, acylases are selective for i-amino acids, but d-selective acylase have been reported. The kinetic resolution of amino acids by acylase-catalyzed hydrolysis is a well-established process [4]. The in situ racemization of the substrate in the presence of a racemase converts the process into a DKR. Alternatively, the remaining enantiomer of the N-acyl amino acid can be isolated and racemized via the formation of an oxazolone, as shown in Figure 6.34. 

For successful DKR two reactions an in situ racemization (krac) and kinetic resolution [k(R) k(S)] must be carefully chosen. The detailed description of all parameters can be found in the literature [26], but in all cases, the racemization reaction must be much faster than the kinetic resolution. It is also important to note that both reactions must proceed under identical conditions. This methodology is highly attractive because the enantiomeric excess of the product is often higher than in the original kinetic resolution. Moreover, the work-up of the reaction is simpler since in an ideal case only the desired enantiomeric product is present in the reaction mixture. This concept is used for preparation of many important classes of organic compounds like natural and nonnatural a-amino acids, a-substituted nitriles and esters, cyanohydrins, 5-alkyl hydantoins, and thiazoUn-5-ones. 

The method is not restricted to secondary aryl alcohols and very good results were also obtained for secondary diols [39], a- and S-hydroxyalkylphosphonates [40], 2-hydroxyalkyl sulfones [41], allylic alcohols [42], S-halo alcohols [43], aromatic chlorohydrins [44], functionalized y-hydroxy amides [45], 1,2-diarylethanols [46], and primary amines [47]. Recently, the synthetic potential of this method was expanded by application of an air-stable and recyclable racemization catalyst that is applicable to alcohol DKR at room temperature [48]. The catalyst type is not limited to organometallic ruthenium compounds. Recent report indicates that the in situ racemization of amines with thiyl radicals can also be combined with enzymatic acylation of amines [49]. It is clear that, in the future, other types of catalytic racemization processes will be used together with enzymatic processes. 

Kanegafuchi Chemical Industries produce D-p-hydroxyphenyl glycine, which is a key raw material for the semisynthetic penicillins ampicillin and amoxycillin. Here, an enantioselective hydantoinase is applied to convert the hydantoin to the D-p-hydroxyphenyl glycine. The quantitative conversion of the amide hydrolysis is achieved because of the in situ racemization of the unreacted hydantoins. Under the conditions of enzymatic hydrolysis, the starting material readily racemizes. Therefore, this process enables the stereospecific preparation of various amino acids at a conversion of 100% [38]. 

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. 

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. 

Gastaldi et al. discovered that in situ racemization of a chiral amine 59 was mediated by the addihon of thiyl radicals (Scheme 2.29). Combinahon with CALB... 

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. 

An elegant method to suppress the undesired spontaneous hydrolysis of a 5(47/)-oxazolone in aqueous media uses a lipase-catalyzed alcoholysis reaction. Of particular importance is the synthesis of /crt-leucine, a non-proteinogenic a-amino acid that has found widespread use both as a chiral auxiliary and as a component of potentially therapeutic pseudopeptides. Racemic 4-/ert-butyl-2-phenyl-5(47/)-oxazolone 238 was submitted to Mucor miehei catalyzed alcoholysis using butanol as a nucleophile. Addition of a catalytic amount of triethylamine promoted in situ racemization. In this way, the enantiomericaUy pure butyl ester of (5)-A-benzoyl-/ert-leucine 239 was obtained in excellent yield (Scheme 7.75). 

The enantiopreference of the protease subtilisin in the acylalion of chiral alcohols is known to be opposite to that observed with lipases, providing for access to both enantiomers with DKR, depending on the enzyme used [137, 138, 139]. Acylation using 2,2,2-trifluoroethyl butyrate as the acyl donor was combined with in situ racemization, affording the corresponding esters in high yield and [135]. 

The continuous reaction system could be combined with solid acid-catalyzed in situ racemization of the slow-reacting alcohol enantiomer [149]. The racemiza-tion catalyst and the lipase (Novozym 435) were coated with ionic liquid and kept physically separate in the reaction vessel. Another variation on this theme, which has yet to be used in combination with biocatalysis, involves the use of scC02 as an anti-solvent in a pressure-dependent miscibility switch [150]. 

These results, obtained with chiral substrates, agree with the general sense of enantioselective hydrogenation of prochiral 3-oxo carboxylic esters. Obviously, the chirality of the BINAP ligand controls the facial selectivity at the carbonyl function, whereas cyclic constraints determine the relative reactivities of the enantiomeric substrates. Sterically restricted transition states that lead to the major stereoisomers are shown in Scheme 66. Overall, one of four possible diastereomeric transition states is selected to afford high stereoselectivity by dynamic kinetic resolution that involves in situ racemization of the substrates. 

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. 

Asymmetric synthesis with lipases and esterases can basically be performed by two different approaches - the desymmetrization of prochiral or meso compounds and the enzymatic kinetic resolution of racemic mixtures. The main bottleneck of kinetic resolutions, product yields of maximum 50%, can be overcome if an in situ racemization of the starting material is possible. In this case all starting material can theoretically be converted to the desired product [34],... 

Resolution of cheap racemic mixtures with enzymes is a common route to enantiomerically pure chemicals on an industrial scale. However, the yield with a classical resolution is limited to 50%. An in situ racemization of the undesired enantiomer, combined with the enzymatic kinetic resolution, gives rise to a dynamic kinetic resolution (DKR) that should in principle lead to a 100% yield in the desired isomer. In spite of several Ru and Pd homogeneous systems successfully combined with enzymes and successfully applied on industrial scale in DKR [71, 72], few metal-based heterogeneous catalysts active for alcohol racemization have been reported [19, 73, 74]. 

It is well-known that catalytic amounts of aldehyde can induce racemization of a-amino acids through the reversible formation of Schiff bases.61 Combination of this technology with a classic resolution leads to an elegant asymmetric transformation of L-proline to D-proline (Scheme 6.8).62 63 When L-proline is heated with one equivalent of D-tartaric acid and a catalytic amount of n-butyraldehyde in butyric acid, it first racemizes as a result of the reversible formation of the proline-butyraldehyde Schiff base. The newly generated D-proline forms an insoluble salt with D-tartaric acid and precipitates out of the solution, whereas the soluble L-proline is continuously being racemized. The net effect is the continuous transformation of the soluble L-proline to the insoluble D-proline-D-tartaric acid complex, resulting in near-complete conversion. Treatment of the D-proline-D-tartaric acid complex with concentrated ammonia in methanol liberates the D-proline (16) (99% ee, with 80-90% overall yield from L-proline). This is a typical example of a dynamic resolution where L-proline is completely converted to D-proline with simultaneous in situ racemization. As far as the process is concerned, this is an ideal case because no extra step is required for recycle and racemization of the undesired enantiomer and a 100% chemical yield is achievable. The only drawback of this process is the use of stoichiometric amount of D-tartaric acid, which is the unnatural form of tartaric acid and is relatively expensive. Fortunately, more than 90% of the D-tartaric acid is recovered at the end of the process as the diammonium salt that can be recycled after conversion to the free acid.64... 

The general drawback of all these resolution concepts is the maximum yield of 50%. In the last part of this chapter the state-of-the-art and recent developments in 100% yield concepts in crystallization-induced resolution are discussed. This so-called crystallization-induced asymmetric transformation combines classical resolution with in situ racemization. Most examples in this chapter originate from day-to-day research efforts at DSM Pharma Chemicals and at Syncom. 

Combination of (classical) resolution with in situ racemization is a powerful but highly underestimated technology. This technology is often referred to as crystallization-induced asymmetric... 

Constant reaction conditions (e.g., solvent, pH, T°C) No isolation of intermediate products In situ racemization 14... 

General theoretical [2] and practical [3, 4] aspects of catalyzed KR and of ASD [5-7] processes mediated by both enzymes [8] and also by non-enzyme-based [9, 10] catalysts have been widely reviewed. Moreover, two well-documented variants on the standard KR reaction (1) parallel KR (PKR) [11, 12], in which the selectivity of a KR process can be significantly improved by running enantiocomplemen-tary KRs in parallel in one-pot processes and (2) dynamic KR (DKR) [13, 14], in which the 50% yield limit can be circumvented by achieving in-situ racemization of the substrate during the KR process, have also been reviewed recently. Conse-... 

Indeed, when we studied various phosphoric acid catalysts for the reductive amination of hydratopicaldehyde (16) with p-anisidine (PMPNH2) in the presence of Hantzsch ester 11 to give amine 17, the observed enantioselectivities and conversions are consistent with a facile in situ racemization of the substrate and a resulting dynamic kinetic resolution (Scheme 16). TRIP (9) once again turned out to be the most effective and enantioselective catalyst for this transformation and provided the chiral amine products with different a-branched aldehydes and amines in high enantioselectivities (Hoffmann et al. 2006). 

Inagaki M, Hiratake J, Nishioka T, Oda J (1991) Lipase-catalyzed kinetic resolution with in situ racemization one-pot synthesis of optically active cyanohydrin acetates from aldehydes. J Am Chem Soc 113 9360-9361... 

This eoncept has been known for a long time in pure enzymatic synthesis, e.g. amino acid synthesis via hydantoins [1] or oxazolidinones [2]. Cyanohydrins [3] and lactols [4] are prone to in situ racemization as well and may serve as substrates in kinetic resolutions. 

The method is of general applicability in the deracemization of secondary alcohols and amines and consists of a Upase-catalyzed irreversible acylation and in situ racemization of the non-reacted enantiomer catalyzed by a ruthenium catalyst. 

The method of combined enzyme- and transition metal-catalyzed reactions widely applied to the DKR of secondary alcohols has also been applied to the DKR of a-hydroxy acid esters rac-1. The principle is based on the enantioselective acylation catalyzed by Pseudomonas species lipase (PS-C from Amano Ltd) using p-Cl-phenyl acetate as an acyl donor in cyclohexane combined with in situ racemization of the non-acylated enantiomer catalyzed by ruthenium compounds [7]. Under these conditions, various a-hydroxy esters of type 1 were deracemized in moderate to good yields and high enantioselectivity (Scheme 13.2). 

The requisites for successful DKR are that the starting material is racemizable under the reaction conditions while the product is configurationally stable under the same conditions. Due to the industrial importance of amino acids, a number of studies have been devoted to the problem of effective racemization of the unused enantiomer that remained after a standard kinetic resolution [35]. However, racemization methods based on high temperature or extreme pH values are unsuitable for in situ racemization. Thus, if enzymes are involved, the racemization reaction must be carried out under mild conditions, using the catalytic action of a second enzyme (racemase) or a base, exploiting the difference in the pKj value of the a-methine carbon due to difference in the structure between the substrate and product... 

Deracemization of a-Amino Acids via Enzyme-catalyzed DKR Coupled with In Situ Racemization... 

Scheme 13.14 4-Substituted-2-phenyloxazolin-5-one as substrates for lipases or esterases, with opposite selectivity allowing DKR with in situ racemization.

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