Acetates kinetic resolution

Enzyme-mediated hydrolysis of some racemic co-arenesulfinylalkanoic methyl esters, ArSO(CH2) COOMe, using Corynebacterium equi has led to a kinetic resolution in which the unreacted sulfinyl esters are enriched in one enantiomer at the sulfoxide center49. The enantiomeric purity of unreacted sulfinyl acetates and propionate ranges from 90 to 97%. 

In another study a hyperthermophilic esterase from Aeropyrum pemix K1 (APE1547) was used as a catalyst in the hydrolytic kinetic resolution of rac-3-octanol acetate [53]. Following a single round of epPCR, a mutant displaying a 2.6-fold increase in enantioselectivity was identified having five amino acid substitutions, which were shown to be spatially distal to the catalytic center. 

Kinetic resolution of racemic allylic acetates has been accomplished via asymmetric dihydroxylation (p. 1051), and 2-oxoimidazolidine-4-carboxy-lates have been developed as new chiral auxiliaries for the kinetic resolution of amines. Reactions catalyzed by enzymes can be utilized for this kind of resolution. ... 

Stereoinversion Stereoinversion can be achieved either using a chemoenzymatic approach or a purely biocatalytic method. As an example of the former case, deracemization of secondary alcohols via enzymatic hydrolysis of their acetates may be mentioned. Thus, after the first step, kinetic resolution of a racemate, the enantiomeric alcohol resulting from hydrolysis of the fast reacting enantiomer of the substrate is chemically transformed into an activated ester, for example, by mesylation. The mixture of both esters is then subjected to basic hydrolysis. Each hydrolysis proceeds with different stereochemistry - the acetate is hydrolyzed with retention of configuration due to the attack of the hydroxy anion on the carbonyl carbon, and the mesylate - with inversion as a result of the attack of the hydroxy anion on the stereogenic carbon atom. As a result, a single enantiomer of the secondary alcohol is obtained (Scheme 5.12) [8, 50a]. 

The low-temperature method is effective not only in the kinetic resolution of alcohols but also in the enantioface-selective asymmetric protonation of enol acetate of 2-methylcyclohexanone (15) giving (f )-2-methylcyclohexanone (16). The reaction in H2O at 30°C gave 28% ee (98% conv.), which was improved up to 77% ee (82% conv.) by the reaction using hpase PS-C 11 in /-Pt20 and ethanol at 0°C. Acceleration of the reaction with lipase PS-C 11 made this reaction possible because this reaction required a long reaction time. The temperature effect is shown in Fig. 14. The regular temperature effect was not observed. The protons may be supplied from H2O, methanol, or ethanol, whose bulkiness is important. 

Searching for a method of synthesis of enantiopure lamivudine 1, the compound having a monothioacetal stereogenic centre, Rayner et al. investigated a lipase-catalysed hydrolysis of various racemic a-acetoxysulfides 2. They found out that the reaction was both chemoselective (only the acetate group was hydrolysed with no detectable hydrolysis of the other ester moieties) and stereoselective. As a result of the kinetic resolution, enantiomerically enriched unreacted starting compounds were obtained. However, the hydrolysis products 3 were lost due to decomposition." In this way, the product yields could not exceed 50% (Equation 1). The product 2 (R = CH2CH(OEt)2) was finally transformed into lamivudine 1 and its 4-epimer. ... 

When a reverse procedure was applied, i.e. enzymatic acetylation of racemic 3, formed in situ from the appropriate aldehydes and thiols, the reaction proceeded under the conditions of dynamic kinetic resolution and gave enantiomerically enriched acetates 2 with 65-90% yields and with ees up to 95% (Equation 2). It must be mentioned that the addition of silica proved crucial, as in its absence no racemization of the initially formed substrates 3 occurred and the reaction stopped at the 50% conversion. 

Tryptophan (and also tryptophanol) undergoes a stereoselective cyclocondensation with racemic compound 249, in a very interesting process involving a kinetic resolution with epimerization of the tryptophan stereocenter and simultaneous desymmetrization of the two diastereotopic acetate chains <2005CC1327>, affording the enantiomeri-cally pure lactam 250. A subsequent treatment of the latter compound with trifluoroacetic acid led to the indolo[2,3- ]quinolizidine 251 through an intermediate acyliminium cation (Scheme 50) <20050L2817>. 

Molybdenum catalysts that contain enantiomerically pure diolates are prime targets for asymmetric RCM (ARCM). Enantiomerically pure molybdenum catalysts have been prepared that contain a tartrate-based diolate [86], a binaph-tholate [87], or a diolate derived from a traris-1,2-disubstituted cyclopentane [89, 90], as mentioned in an earlier section. A catalyst that contains the diolate derived from a traris-1,2-disubstituted cyclopentane has been employed in an attempt to form cyclic alkenes asymmetrically via kinetic resolution (inter alia) of substrates A and B (Eqs. 45,46) where OR is acetate or a siloxide [89,90]. Reactions taken to -50% consumption yielded unreacted substrate that had an ee between 20% and 40%. When A (OR=acetate) was taken to 90% conversion, the ee of residual A was 84%. The relatively low enantioselectivity might be ascribed to the slow interconversion of syn and anti rotamers of the intermediates or to the relatively floppy nature of the diolate that forms a pseudo nine-membered ring containing the metal. 

Takaya and co-workers (256) disclosed that chiral copper alkoxide complexes catalyze the transesterification and kinetic resolution of chiral acetate esters. Selec-tivities are very poor (E values of 1.1-1.5) but it was noted that the Lewis acid BINAP CuOTf was not an effective catalyst. The observation thatp-chlorophcnyl-BINAP-CuOf-Bu complex gave faster rates than BINAP-CuOt-Bu suggests that both the Lewis acidic and Lewis basic properties of the copper alkoxide are required for optimal reactivity. 

When the allyl group is part of a cyclic structure, a k-g reaction cannot take place to epimerise one of the chiral carbon atoms after having formed the k-g intermediate, the same Jt-face comes back-on to palladium. Thus, if we start from a racemic cyclic alkenyl acetate mixture, racemisation at the complex is blocked and the product will also be a racemate at 100% conversion. Kinetic resolution is still an option to obtain chiral product and starting material. Nevertheless high ee s were obtained when cyclic alkenyl acetates were used. 

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. 

The kinetic resolution of racemic substrate with CPDMO reaction was stopped at 50 % conversion and immediately extracted with ethyl acetate. Combined extracts were washed once with brine and dried with anhydrous Na2S04. 

Catalysts lacking phosphorus ligands have also been used as catalysts for allylic substitutions. [lr(COD)Cl]2 itself, which contains a 7i-accepting diolefin ligand, catalyzes the alkylation of allylic acetates, but the formation of branched products was only favored when the substitution reaction was performed with branched allylic esters. Takemoto and coworkers later reported the etherification of branched allylic acetates and carbonates with oximes catalyzed by [lr(COD)Cl]2 without added ligand [47]. Finally, as discussed in Sect. 6, Carreira reported kinetic resolutions of branched allylic carbonates from reactions of phenol catalyzed by the combination of [lr(COE)2Cl]2 and a chiral diene ligand [48]. 

Inagaki, M. Hiratake, J. Nishioka, T Oda, J. One-pot synthesis of optically active cyanohydrin acetates from aldehydes via lipase-catalyzed kinetic resolution coupled with in situ formation and racemization of cyanohydrins. J. Org. Chem. 1992, 57, 5643-5649. 

A novel continuous-flow SCCO2 process for the kinetic resolution of 1-phenyethanol enantiomers (Figure 30) using Novozym 435 immobilized enzyme from Candida antarctica was described by Matsuda et al. [51], The lipase enzyme, selectively acetylated the R)-alcohol component. A mixture of starting material and vinyl acetate was passed through the enzyme with supercritical carbon-dioxide (Figure 31). The reaction zone was pressurized and heated, so the reaction could be performed imder supercritical conditions, synthesizing the desired (i )-acetate with 99.7% ee. and 47% yield. 

One of the first fluorescence-based ee assays uses umbelliferone (14) as the built-in fluorophore and works for several different types of enzymatic reactions 70,86). In an initial investigation, the system was used to monitor the hydrolytic kinetic resolution of chiral acetates (e.g., rac-11) (Fig. 8). It is based on a sequence of two coupled enzymatic steps that converts a pair of enantiomeric alcohols formed by the asymmetric hydrolysis under study (e.g., R - and (5)-12) into a fluorescent product (e.g., 14). In the first step, (R)- and (5)-ll are subjected separately to hydrolysis in reactions catalyzed by a mutant enzyme (lipase or esterase). The goal of the assay is to measure the enantioselectivity of this kinetic resolution. The relative amount of R)- and ( S)-12 produced after a given reaction time is a measure of the enantioselectivity and can be ascertained rapidly, but not directly. 

A typical example that illustrates the method concerns the lipase- or esterase-catalyzed hydrolytic kinetic resolution of rac-1-phenyl ethyl acetate, derived from rac-1-phenyl ethanol (20). However, the acetate of any chiral alcohol or the acetamide of any chiral amine can be used. A 1 1 mixture of labeled and non-labeled compounds (S)- C-19 and (f )-19 is prepared, which simulates a racemate. It is used in the actual catalytic hydrolytic kinetic resolution, which affords a mixture of true enantiomers (5)-20 and (J )-20 as well as labeled and non-labeled acetic acid C-21 and 21, respectively, together with non-reacted starting esters 19. At 50% conversion (or at any other point of the kinetic resolution), the ratio of (5)- C-19 to (1 )-19 correlates with the enantiomeric purity of the non-reacted ester, and the ratio of C-21 to 21 reveals the relative amounts of (5)-20 and (J )-20 (98). 

In work concerning the directed evolution of enantioselective enzymes, there was a need for fast and efficient ways to determine the enantiomeric purity of chiral alcohols, which can be produced enzymatically either by reduction of prochiral ketones (e.g., 26) using reductases or by kinetic resolution of rac-acetates (e.g., 19) by lipases (111). In both systems, the CD approach is theoretically possible. In the former case, an LC column would have to separate the educt 26 from the product (A)/(J )-20, whereas in the latter, (5)/(J )-20 would have to be separated from (S)/(R)-19. 

The beneficial effect of the hydrophobicity of [BMIM]PFg was shown to extend to other enzymes a remarkably enhanced enantioselectivity was observed for lipases AK and Pseudomonas fluorescens for the kinetic resolution of racemic P-chiral hydroxymethanephosphinates (Scheme 31) (278). The ee values of the recovered alcohols and the acetates were about 80% when the enzymatic reactions were conducted in the hydrophobic [BMIMJPFg. In contrast, there was little enantioselectivity (<5%) observed with the enzymes in hydrophilic [BMIM]BF4. The lack of stereoselectivity in [BMIM]BF4 was attributed to the high miscibility of [BMIM]BF4 with water. The relatively hydrophilic ionic liquid is capable of stripping off the essential water from the enzyme surface, leading to insufficient hydration of the enzyme and a consequently strong influence on its performance (279). 

When the water-miscible ionic liquid [MMIM][MeS04] was used as a neat medium for the enzymatic transformations, however, poorer performance was observed. For the kinetic resolution of mc-l-phenylethanol by transesterification with vinyl acetate with a set of different lipases dispersed in the pure ionic liquid, it was found that [MMIM][MeS04] was among the poorest media for the enzymes (291). It has been recognized that some water-miscible ionic liquids in the pure form are denaturants (27), but, when they are used in the presence of excess water, their tendency to... 

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