Enzymatic Resolutions

A chiral reagent can also be used as the source of stereoinduction in the s)mthesis of an enantioenriched compound. They are used in stoichiometric quantities and some typical examples are chiral lithium amide bases to asymmetrically deprotonate a ketone, chiral reducing reagents, such as BINAL-H, to asymmetrically reduce a 

Pseudomonas cepacia-medizted transesterification with vinyl acetate of racemic 63 gives the (Al)-monoacetate and (Al)-diacetate, leaving the enantiomer (P)-63 in 98% cc M)-63 is regenerated by hydrolysis of the (A4)-acetate and (A4)-diacetate in 77% and 94% cc, respectively. When C. antarctica lipase is used, the reaction gives (A4)-63 in 92% cc. 

Enzymatic resolution appears to be efficient but certainly needs more investigation if the preparation of large amounts of opticaUy pure thiaheh-cenes is to be achieved. 

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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... 

High yields of the enantiomerically pure alcohol and enantiomerically pure ester are reg ularly achieved The growing interest m chiral drugs (see the boxed essay on this topic p 296) has stimulated the development of large scale enzymatic resolution as a com mercial process... 

Enzymatic resolution (Section 7 13) Resolution of a mixture of enantiomers based on the selective reaction of one of them under conditions of enzyme catalysis... 

Enzymatic fat splitting Enzymatic hydrolysis Enzymatic methods Enzymatic oxidation Enzymatic resolutions... 

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). 

Fig. 5. Enzymatic resolution of amino acids by ring-opening reaction.

The biologically important anti-/8-phenylalanine, /8-2-thienylalanine (39), has been resolved into antipodes through enzymatic resolution with carboxypeptidase/ The pharmacologically important 3-piperidino-l,i-bis-2-thienyl-l-butene (40), which crystallizes as. a conglomerate, has been resolved by hand picking. ... 

Fig. 7-19. Enzymatic resolution and separation of ibuprofen sulphonmethyl ester.

Several L-amino acids are produced on a large scale by enzymatic resolution of N-acetyl-D,L-amino adds (Figure A8.4). Acylase immobilised on DEAE-Sephadex is for example employed in a continuous process while Degussa uses the free acylase retained in a membrane reactor. In the latter process the advantage of reuse of the enzyme and homogeneous catalysis are combined. 

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. 

Figure A8.10 Enzymatic resolution of D,L-amlno acid esters.

Figure A8.11 Enzymatic resolution of N-acetyl-D,L-amino add esters.

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. 

The protected E-ring moiety of (S)-camptothecin has been prepared in enantio-merically enriched form by the enzymatic resolution of a triester with PLE in a pH 7 phosphate buffer-acetonitrile (5 1) solution (Figure 6.7). The alkaloid camptothecin is an inhibitor of the enzyme topoisomerase and some of its derivatives are anticancer drugs [52]. 

The enzymatic resolution of esters via aminolysis or ammonolysis processes represents an efficient alternative to the resolution of substrates by transesterification... 

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... 

Scheme 7.25 Enzymatic resolution of pipecolic methyl esters.

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