Racemic enzymatic resolution

One approach called enzymatic resolution, involves treating a racemic mixture with an enzyme that catalyzes the reaction of only one of the enantiomers Some of the most commonly used ones are lipases and esterases enzymes that catalyze the hydrol ysis of esters In a typical procedure one enantiomer of the acetate ester of a racemic alcohol undergoes hydrolysis and the other is left unchanged when hydrolyzed m the presence of an esterase from hog liver... 

The primary disadvantage of the conjugate addition approach is the necessity of performing two chiral operations (resolution or asymmetric synthesis) ia order to obtain exclusively the stereochemicaHy desired end product. However, the advent of enzymatic resolutions and stereoselective reduciag agents has resulted ia new methods to efficiently produce chiral enones and CO-chain synthons, respectively (see Enzymes, industrial Enzymes in ORGANIC synthesis). Eor example, treatment of the racemic hydroxy enone (70) with commercially available porciae pancreatic Hpase (PPL) ia vinyl acetate gave a separable mixture of (5)-hydroxyenone (71) and (R)-acetate (72) with enantiomeric excess (ee) of 90% or better (204). 

However, the products are separated using ion-exchange columns and the starting material is a derivative rattier than a precursor of the racemic amino add, thus making the total process drcuitous since it involves several chemical steps in addition to die enzymatic resolution step. Furthermore, racemisation of the unwanted isomer is not easily accomplished. 

Another example of an enzymatic one-pot multiple Diels-Alder reaction is illustrated in Table 4.20 [83]. Racemic furfuryl alcohols 130 in the presence of ethoxy vinyl methyl fumarate 131 and enzyme TOYOBO-LIP undergo enzymatic acylation followed by kinetic enzymatic resolution to give the acyl derivatives 132 which then affords the adducts 133 and 134 by intramolecular Diels-Alder reaction 3-methyl-furfuryl alcohol 130 (R = Me) in acetone gives the best results. 

The enantioselectivity of biocatalytic reactions is normally expressed as the enantiomeric ratio or the E value [la], a biochemical constant intrinsic to each enzyme that, contrary to enantiomeric excess, is independent of the extent of conversion. In an enzymatic resolution of a racemic substrate, the E value can be considered equal to the ratio of the rates of reaction for the two enantiomers, when the conversion is close to zero. More precisely, the value is defined as the ratio between the specificity constants (k st/Ku) for tho two enantiomers and can be obtained by determination of the k<-at and Km of a given enzyme for the two individual enantiomers. 

However, considering practical limitations, that is, the availability of optically pure enantiomers, E values are more commonly determined on racemates by evaluating the enantiomeric excess values as a function of the extent of conversion in batch reactions. For irreversible reactions, the E value can be calculated from Equation 1 (when the enantiomeric excess ofthe product is known) or from Equation 2 (when the enantiomeric excess ofthe substrate is knovm) [la]. For reversible reactions, which may be the case in enzymatic resolution carried out in organic solvents (especially at extents of conversion higher than 40%), Equations 3 or 4, in which the reaction equilibrium constant has been introduced, should be used [lb]. 

Racemization of amines is difficult to achieve and usually requires harsh reaction conditions. Reetz et al. developed the first example of DKR of amines using palladium on carbon for the racemization and CALB for the enzymatic resolution [35]. This combination required long reaction times (8 days) to obtain 64% yield in the DKR of 1-phenylethylamine. More recently, Backvall et al. synthesized a novel Shvo-type ruthenium complex (S) that in combination with CALB made it possible to perform DKR of a variety of primary amines with excellent yields and enantioselectivities (Figure 4.13) [36]. 

Very recently Page and coworkers have reported the D KR of sec-amines using a low catalyst loading of an Ir complex for the racemization, and Candida rugosa lipase for the enzymatic resolution [38]. 

Another example of enzyme- and acid-catalyzed DKR has been reported by Bornscheuer [45]. Acyloins were racemized by using an acidic resin through the formation of enol intermediates. The enzymatic resolution was catalyzed by CALB. Since deactivation of this enzyme occurred in the presence of the acidic resin, they designed a simple reactor setup with two glass vials cormected via a pump to achieve a spatial separation between the acidic resin and the enzyme (Figure 4.20). 

In this section we will discuss some DKRs in which racemization occurs spontaneously during the enzymatic resolution, and without further addition of any reagent. 

Here, we have selected a few representative examples of the enzymatic resolution of esters by aminolysis or ammonolysis reactions. On the other hand, the enzymatic acylation of racemic amines is also of great utility for the preparation of optically pure... 

Catalytic transformation based on combined enzyme and metal catalysis is described as a new class of methodology for the synthesis of enantiopure compounds. This approach is particularly useful for dynamic kinetic resolution in which enzymatic resolution is coupled with metal-catalyzed racemization for the conversion of a racemic substrate to a single enantiomeric product. 

Enzymatic resolutions of racemic amines or aminoalcohols via enantioselective acylation... 

In carrying out kinetic resolution, these in the standard approach are limited to 50% yield regarding the racemate. However, different approaches were developed [28] to overcome this limitation. The classical standard solution is to reracemize the unconverted enantiomer. A more advanced solution is the establishment of a dynamic kinetic resolution that has considerably expanded the synthetic scope of chemical processes. Here, the unconverted enantiomer is, in contrast to the latter method, racemized in situ. A great number of novel enzymatic methods have been developed [29]. Within this chapter, process solutions for enzymatic resolutions of racemic mixtures will be highlighted. 

Af-(ter -Butoxycarbonyl)-3-hydroxymethylpiperidine is a building block in the synthesis of a potent tryptase inhibitor. It is produced by stereospecific esterification of the racemic alcohol with succinic anhydride [30]. The enzymatic resolution is followed up by separation of... 

Enantiomerically pure japonilure [(4JR,5Z)-5-tetradecen-4-olide, 26] is commercially important to attract the Japanese beetle (Popillia japonica), because only pure (4.R,5Z)-26 is bioactive, while ( )-26 is totally inactive. Synthesis of (4 ,5Z)-26 via enzymatic resolution of racemic intermediates was first reported by Sugai [63], and then further studied by Fukusaki [64] to establish the process as summarized in Scheme 39. Lipase PS (Amano) from Pseudomonas sp. and lipase OF (Meito) from Candida cylindracea were shown to be the enzymes of choice. 

The three-step procedure described for the preparation of the illustrated crotylsilanes is initiated with the hydrosilation of rac-3-butyn-2-ol. This procedure is significantly improved with respect to the positional selectivity of the hydrosilation resulting in exclusive formation of the racemic (E)-vinylsilane, and as a result the present procedure is much more amenable to scale-up than those previously described in the literature.8 The enzymatic resolution of the racemic secondary allylic alcohol (vinylsilane) has also been reported using commercially available lipase extracts. The use of a Johnson ortho ester Claisen rearrangement affords the (E)-crotylsilanes 4 in nearly enantiomerically pure form. 

E. Guibe-Jampel, G. Rousseau, L. Blanco, Enzymatic Resolution of Racemic Bicyclic Lactones by Horse Liver Esterase , Tetrahedron Lett. 1989, 30, 67-68. 

Terao, Y., Tsuji, K., Murata, M., Achiwa, K., Nishio, T., Watanabe, N. and Seto, K., Eacile process for enzymatic resolution of racemic alcohols. Chem. Pharm. Bull, 1989,37,1653-1655. 

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 an attractive protocol for the production of enantiopure compounds from racemic mixtures [45]. The concept of DKR is illustrated in Scheme 5.13. In many cases, DKRs are accomplished by the combination of enzymatic resolution and transition-metal-catalyzed racemization based on hydrogen transfer. Thus, the use of Cp Ir complexes as catalysts for racemization in DKR can be anticipated. 

The synthesis of chiral carboxylic acids by enzymatic resolution of the corresponding racemate is a widely established method, and for this purpose a broad variety of esterases are commercially available. Consequently, this... 

Alternatively, enzymatic resolution of 61 by hydrolysis or of 62 by enzymatic esterification could be achieved with >99% ee and enantioselectivities of E>200, e.g. hydrolysis with common lipases like CAL-B or BCL (Amano PS) [86-88]. Wittig reaction and deprotection led to 64. Enzymatic resolution is also possible at the stage of C15-racemic 65 [86-88]. 

In the past few years, new approaches for the enantioselective synthesis of / -benzyl-y-butyrolactones appeared in the literature. Some of these approaches involve the asymmetric hydrogenation of 2-benzyl-2-butenediols (j [34]), the radical mediated rearrangement of chiral cyclopropanes (r [35]), the transition metal catalyzed asymmetric Bayer-Villiger oxidation of cyclobutanones n [36]), or the enzymatic resolution of racemic succinates (g [37]). 

The enzymatic resolution of racemic substrates now is a well-established approach for the synthesis of single enantiomers [1, 2]. A representative example is the kinetic resoluhon of secondary alcohols via lipase-catalyzed transesterification for the preparation of enantiomericaUy enriched alcohols and esters [3], The enzymatic resolution in general is straightforward and satisfactory in terms of optical purity, but it has an intrinsic Hmitation in that the theoretical maximum yield of a desirable enantiomer cannot exceed 50%. Accordingly, additional processes such as isolation, racemization and recycling of unwanted isomers are required to obtain the desirable isomer in a higher yield (Scheme 1.1). 

Zopiclone is a chiral cyclopyrrolone with hypnotic properties, possessing a pharmaceutical profile of high efficacy and low toxicity, similar to that of benzodiazepines. Zopiclone has been commercialized as a racemic mixture however, the (S)-enantiomer is more active and less toxic than the (R)-enantiomer [11]. Although enzymatic hydrolysis of esters or transesteriflcation processes of alcohols have been widely applied for enzymatic resolution or desymmetrization... 

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