Lipase chiral acyl donors

The improvement of the enantioselectivity E in kinetic resolution of a primary alcohol (10) through lipase-catalyzed transesterification was studied using a chiral acyl donor 11. The combination of the lipase, solvent and acyl donor was effective for the enantioselectivity.62... 

Scheme 4 A Lipase-catalyzed resolution of a chiral acyl donor (RCOOH and SCOOH) by esterification or hydrolysis (the reverse reaction). B The reaction goes through two diastereomeric acyl enzymes (RCOO-Enz and SCOO-Enz). The enantiomeric acyl groups R and S are shown in boldface.

The resolution of chiral acyl donors mainly involves carboxylic acids with the stereocenter at the a position. Candida rugosa lipase shows high enantioselectivity to many of these acids in contrast to C. antarctica lipase B. To compounds with an electron-withdrawing substituent at the stere(x enter, P. cepacia lipase shows a high selectivity as well. Two examples are presented in Scheme 8 [91,92]. 

Another approach for the coupled racemization step has been used for compounds having an acidic hydrogen on the stereocenter. Examples of such compounds are chiral acyl donors such as a-substituted esters which are prone to base-catalyzed racemization via an enolate intermediate. This approach has been frequently used and a few examples will be given here to illustrate the utility (Scheme 11). The first examples involve oxa-zolinones where it was found that porcine pancreatic lipase and lipase from Aspergillus sp. exhibited opposite enantiopreferences [106,107]. The remaining oxazolinone was spontaneously racemized via the enolate intermediate and both (l)- and (D)-iV-benzoyl amino acids could be produced this way in high chemical and optical yields. The p Ka values of thio esters are lower than those of oxo esters [108]. This has been used in the lipase-... 

The alcohol used as cosubstrate in lipase reactions with chiral acyl donors may act as an enantioselective inhibitor that will be detrimental to the enantiomeric excess. This has been reported for C. rugosa lipase-catalyzed kinetic resolution by esterification of 2-meth-ylalkanoic acids (Scheme 17) [134]. 

The catalytic alcohol racemization with diruthenium catalyst 1 is based on the reversible transfer hydrogenation mechanism. Meanwhile, the problem of ketone formation in the DKR of secondary alcohols with 1 was identified due to the liberation of molecular hydrogen. Then, we envisioned a novel asymmetric reductive acetylation of ketones to circumvent the problem of ketone formation (Scheme 6). A key factor of this process was the selection of hydrogen donors compatible with the DKR conditions. 2,6-Dimethyl-4-heptanol, which cannot be acylated by lipases, was chosen as a proper hydrogen donor. Asymmetric reductive acetylation of ketones was also possible under 1 atm hydrogen in ethyl acetate, which acted as acyl donor and solvent. Ethanol formation from ethyl acetate did not cause critical problem, and various ketones were successfully transformed into the corresponding chiral acetates (Table 17). However, reaction time (96 h) was unsatisfactory. 

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. 

Schering Plough demonstrated the kinetic resolution of a secondary amine (24) via enzyme-catalyzed acylation of a pendant piperidine (Scheme 7.13) [32]. The compound 27 is a selective, non-peptide, non-sulfhydryl farnesyl protein transfer inhibitor undergoing clinical trials as a antitumor agent for the treatment of solid tumors. The racemic substrate (24) does not contain a chiral center but exists as a pair of enantiomers due to atropisomerism about the exocylic double bond. The lipase Toyobo LIP-300 (lipoprotein lipase from Ps. aeruginosa) catalyzed the isobu-tylation of the (+) enantiomer (26), with MTBE as solvent and 2,2,2-trifluoroethyl isobutyrate as acyl donor [32]. The acylation of racemic 24 yielded (+) 26 at 97% and (-) 25 at 96.3% after 24h with an E >200. The undesired enantiomer (25)... 

The resolution of chiral amines via lipase-catalyzed enantioselective acylation is now a major industrial process, but interest in adopting ionic liquid reaction media has been surprisingly scant. Interestingly, acids could be used as the acyl donor (Figure 10.15) rather than the usual activated ester in a range ofionic liquids. CaLB was employed as the biocatalyst, and water was removed to shift the equilibrium toward the product [130, 131]. The highest rates were found in [BMMIm][TfO], [EMIm][TfO], and [EMIm][BF4]. 

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 same concept is applicable to allylic alcohols, ketones, or ketoximes. Enol acetates or ketones were successfully converted in multi-step reactions to chiral acetates in high yields and optical yields through catalysis by Candida antarctica lipase B (CALB, Novozyme 435) and a ruthenium complex. 2,6-Dimethylheptan-4-ol served as a hydrogen donor and 4-chlorophenyl acetate as an acyl donor for the conversion of the ketones (Jung, 2000a). 

Primary alcohols have been successfully used as substrates for lipases. Monterde et. Al60 reported the resolution of the chiral auxiliary 2-methoxy-2-phenylethanol 1 via Candida antarctica lipase B (CAL-B)-catalyzed acylation using either vinyl acetate (R=H) or isopropenyl acetate (R= CH3) as acyl donor (cf. fig. 8) and the alkoxycarbonylation using diallyl carbonate as the alkoxycarbonylation agent in THF at 30 °C (cf. fig. 9). 

Figure 15 Gas chromatographic chiral separation of (left) racemic l-(4-methoxy-phenyl)ethanol 22 and its corresponding acetate 22a (reference) and (right) lipase-catalyzed transesterification of l-(4-methoxy-phenyl)ethanol 22 (4 hrs) using isopropenyl acetate as acyl donor in toluene as organic solvent ees= 99.9 eep= 87 conv. =53.4, E=141.

Diols of different structures such as the meso-diol 76 (Fig. 41), the C2-symmetric diol rac-79 (Fig. 42), the diol rac-82 in which the primary hydroxy group is protected (Fig. 43) and the unprotected diol rac-84 with a primary and secondary hydroxy group (Fig. 44) were used as substrates in the lipase-catalyzed transesterification using vinyl acetate as acyl donor in organic solvents with the aim to prepare chiral buildings blocks of high enantiomeric purity.86... 

The first enzymatic desymmetrizations of prochiral phosphine oxides was recently reported by Kielbasinski et al.88 Thus, the prochiral bis(methoxycarbonylmethyl)-phenylphosphine oxide 93 was subjected to the PLE-mediated hydrolysis in buffer affording the chiral monoacetate (RJ-94 in 72% ee and 92% chemical yield. In turn, the prochiral bis(hydroxymethyl)phenylphosphine oxide 95 was desymmetrized using either lipase-catalyzed acetylation of 95 with vinyl acetate as acyl donor in organic solvent or hydrolysis of 97 in phosphate buffer and solvent affording the chiral monoacetate 96 with up to 79% ee and 76% chemical yield. 

Some of these catalyze the smooth racemization of chiral secondary alcohols at room temperature. However, a major problem which needed to be solved in order to design an effective combination of ruthenium catalyst and lipase in a DKR of secondary alcohols was the incompatibility of many of the ruthenium catalysts and additives, such as inorganic bases, with the enzyme and the acyl donor. For example, the ruthenium catalyst may be susceptible to deactivation by the acetic acid generated from the acyl donor when it is vinyl acetate. Alternatively, any added base in the racemization system can catalyze a competing selective transesterification of the alcohol, resulting in a decrease in enantioselectivity. Consequently, considerable optimization of reaction protocols and conditions was necessary in order to achieve an effective DKR of secondary alcohols. 

As an example of an enantiospecific acylation in organic solvent with the irreversible acyl donor vinyl laurate (see Fig. 3), only the (R)-enantiomer is acylated, while the (S)-2-octanol is obtained directly in 69% yield, a purity >99% and with an excellent enantiomeric ratio S R>99.5 0.5 (as determined by chiral GC-analysis). The selectivity of the forward reaction, catalyzed by a lipase in MTBE, is thereby maximized, because the use of the enolester precludes the reverse reaction. Batches of about 70 kg are produced routinely and the technology can be easily transferred to larger sizes. 

Lipases have also been widely applied for the resolution of racemic chiral amines. In principle, these reactions can be carried out in both the hydrolytic mode as well as under conditions favouring acylation. As amines are more nucleophilic than alcohols, it is necessary to use less reactive acyl donors in order to minimize the background reaction of non-enzyme catalysed acylation, and in this respect it appears that simple esters such as ethyl acetate are optimal. 

In 2002, a novel aminocyclopentadienyl ruthenium chloride complex was introduced by Park s group involving a new mode of catalytic racemisation which allowed use of the more reactive isopropenyl acetate as an acyl donor and much less lipase. This catalytic system was particularly efficient for the DKR of various aliphatic or aromatic alcohols as shown in Scheme 4.9. Not only simple alcohols, but also functionalised alcohols such as allylic alcohols, alkynyl alcohols, diols, hydroxyl esters and chlorohydrins were successfully transformed into the corresponding chiral acetates. ... 

TEA or trioctylamine, and vinyl acetate as the acyl donor, led to the corresponding chiral acetate in yields above 68% and enantioselectivities of 80-97% ee. In 2006, Wolfson et al. reported the DKR of 1-phenylethanol by hydrated ruthenium chloride in an aqueous medium using Novozym 435 as the lipase. ° This novel process, involving phenyl acetate as the acyl donor, led to the formation of the corresponding chiral acetate in 82% yield and 98% ee. Besides its low price and ideal environmental impact, performing the reaction in an... 

In 2006, Heise s group reported a novel concept for the synthesis of chiral polyesters based on a lipase-catalysed DKR polymerisation of racemic diols." As shown in Scheme 4.25, a mixture of stereoisomers of a secondary diol was enzymatically polymerised with a difunctional acyl donor (dicarboxylic acid... 

The catalytic activity of Shvo s catalyst is mainly because it dissociates into two monomeric ruthenium species in solution under thermal conditions and it can be combined well with various lipases in DKRs. Minidis and colleagues recently employed a combination of this catalyst with Novozym 435 to achieve the DKR of a series of 1-heteroaryl substituted ethanols, such as oxadiazoles, isoxazoles, l//-pyrazole, or 1//-imidazole. In the presence of / -chlorophenyl acetate as the acyl donor, the corresponding acetates were produced in moderate to high yields and excellent enantioselectivities, as shown in Scheme 4.37. In order to prepare novel chiral pincer ligands based on the 6-phenyl-2-amino-methylpyridine and 2-aminomethylbenzo[/j]quinoline scaffolds, Felluga et al. 

Biotransformations are now firmly established in the synthetic chemist s armoury, especially reactions employing inexpensive hydrolytic enzymes for the resolution of racemates and for the desymmetrization of prochiral substrates. From a practical viewpoint, biocatalytic resolution is arguably the simplest method available to obtain synthetically useful quantities of chiral synthons. As an illustration of this point, many racemic secondary alcohols ROH can be resolved without prior derivatization by combining with a lipase and a volatile acyl donor (usually vinyl acetate) in an organic solvent, to effect irreversible transesterification once the desired degree of conversion has been reached, routine filtration to remove the enzyme and concentration of the filtrate affords the optically enriched products ROAcyl and ROH directly. 

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