Vinyl enzyme kinetic resolution with

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. 

Biphasic systems consisting of ionic liquids and supercritical CO2 showed dramatic enhancement in the operational stability of both free and immobilized Candida antarctica lipase B (CALB) in the catalyzed kinetic resolution of rac- -phenylethanol with vinyl propionate at 10 MPa and temperatures between 120 and 150°C (Scheme 30) 275). Hydrophobic ionic liquids, [EMIM]Tf2N or [BMIM]Tf2N, were shown to be essential for the stability of the enzyme in the biotransformation. Notwithstanding the extreme conditions, both the free and isolated enzymes were able specifically to catalyze the synthesis of (J )-l-phenylethyl propionate. The maximum enantiomeric excess needed for satisfactory product purity (ee >99.9%) was maintained. The (S)-l-phenylethanol reactant was not esterified. The authors suggested that the ionic liquids provide protection against enzyme denaturation by CO2 and heat. When the free enzyme was used, [EMIM]Tf2N appeared to be the best ionic liquid to protect the enzyme, which... 

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... 

Kinetic resolutions by means of the selective formation or hydrolysis of an ester group in enzyme-catalyzed reactions proved to be a successful strategy in the enantioseparation of 1,3-oxazine derivatives. Hydrolysis of the racemic laurate ester 275 in the presence of lipase QL resulted in formation of the enantiomerically pure alcohol derivative 276 besides the (23, 3R)-enantiomer of the unreacted ester 275 (Equation 25) <1996TA1241 >. The porcine pancreatic lipase-catalyzed acylation of 3-(tu-hydroxyalkyl)-4-substituted-3,4-dihydro-2/7-l,3-oxazines with vinyl acetate in tetrahydrofuran (THF) took place in an enantioselective fashion, despite the considerable distance of the acylated hydroxy group and the asymmetric center of the molecule <2001PAC167, 2003IJB1958>. 

Efficient kinetic resolution of chiral unsaturated secondary alcohols by irreversible enzyme-mediated acylation (with vinyl acetate as acylating agent, a crude preparation of Pseudomonas AK, and hexane as solvent) is possible, provided one relatively large and one small substituent are attached to the carbinol carbon. However, the method can be used to resolve substrates that are not amenable to asymmetric epoxidation (see examples 23, 25, 27, 29, where the double bond is either deactivated by an electron-withdrawing substituent, or is of the propargyl alcohol type). Acylation of the / -enantiomer consistently proceeds faster than that of the 5-enantiomer. An example of an allenic alcohol was also reported248. 

Initially, kinetic resolutions of 2-(trimethylsilyl)-5-[l -(2, 2, 2 -trifluoro-l -hydroxy-ethyl)]furan with a wide variety of lipases and vinyl alkanoates were examined in 1,2-dichloroethane. On the basis of these results, the system consisting of an enzyme (Novozym 435, Candida antarctica, Novo Nordisk Co. Ltd.) and vinyl propionate was sufficient to obtain optical pure alcohol and ester with a high -value. Moderate effect on the optical purity was observed on changing the organic solvents. Obviously, Novozym 435-CH2ClCH2Cl system is the most convenient practical system for obtaining the optically pure alcohol and the ester on the basis of comparison of the reaction time and the -value. 

The integration of a catalyzed kinetic enantiomer resolution and concurrent racemization is known as a dynamic kinetic resolution (DKR). This asymmetric transformation can provide a theoretical 100% yield without any requirement for enantiomer separation. Enzymes have been used most commonly as the resolving catalysts and precious metals as the racemizing catalysts. Most examples involve racemic secondary alcohols, but an increasing number of chiral amine enzyme DKRs are being reported. Reetz, in 1996, first reported the DKR of rac-2-methylbenzylamine using Candida antarctica lipase B and vinyl acetate with palladium on carbon as the racemization catalyst [20]. The reaction was carried out at 50°C over 8 days to give the (S)-amide in 99% ee and 64% yield. Rather surpris-... 

Other possibilities to prepare chiral cyanohydrins are the enzyme catalysed kinetic resolution of racemic cyanohydrins or cyanohydrin esters [107 and references therein], the stereospecific enzymatic esterification with vinyl acetate [108-111] (Scheme 2) and transesterification reactions with long chain alcohols [107,112]. Many reports describe the use of fipases in this area. Although the action of whole microorganisms in cyanohydrin resolution has been described [110-116],better results can be obtained by the use of isolated enzymes. Lipases from Pseudomonas sp. [107,117-119], Bacillus coagulans [110, 111], Candida cylindracea [112,119,120] as well as lipase AY [120], Lipase PS [120] and the mammalian porcine pancreatic lipase [112, 120] are known to catalyse such resolution reactions. 

Enzymes other than CAL B have also been reported to operate under the biphasic conditions. CAL A and CAL B or a lipase from Mucor miehei were tested for the kinetic resolution of glycidol using vinyl acetate or vinyl butyrate. The enzymes were used either suspended in the free ILs or immobilized, when the reactions were carried out in [EMIM][NTf2] [71]. CALAwas inactive, butthe other two enzymes showed activity, albeit at 10-20% of that in the absence of COj whether they were free or immobilized. In general, the supported enzymes showed superior performance [71]. Chymotripsin, a specific protease for aromatic amino acids, was found to catalyze the hydrolysis or transesterification of the ethyl ester of N-acetyl-phenylalanine with propanol in scC02-[RMIM][PF5] (R = butyl or octyl) with or without added water [Eq. (15)]. 

Consequently, in a stereoselective reaction, one stereoisomer is formed preferentially over other possible stereoisomers, as illustrated in Fig. 2.24, or enantiomers can be separated as shown in Fig. 2.25 for acylation of 1-phenyl-ethanol with vinyl acetate using lipase as enzyme in the so-called kinetic resolution approach. 

The lipase PS from Burkholderia cepacia was supported on a Kynol ACC 507-15 active carbon cloth with and without modification by IL phase for the kinetic resolution (KR) of aromatic secondary alcohols with vinyl acetate. Scheme 2.30 [124]. For the asymmetric acylation of 1-phenylethanol, the conversion was about 50% using the ACC-supported [EMIm][NTf2] containing the enzyme. If other supports such as active carbon and alumina were used, the conversion of 1-phenylethanol was <10% even when a higher catalyst loading was employed. By applying optimized reaction conditions, the conversions of l-(furan-2-yl) ethanol and N-(2-hydroxy-2-phenylethyl) propionamide in the acylation reactions were 30-50% with 86-99% ee, respectively. 

The combination of a resin and covalently supported IL with SCCO2 was also used in the KR and dynamic kinetic resolution (DKR) of 1-phenylethanol with vinyl propionate catalyzed by Candida antarctica lipase B (CALB) [125]. The IL molecule covalently supported on Merrifield resin was realized through the reaction of 1-butyl imidazole with chloromethylated resin. Subsequently, NTf2 was introduced via ion exchange. Under improved conditions, the conversion of 1-phenylethanol was 50% with 99.9% ee to the product. In order to develop a more efficient process, the KR of 1-phenylethanol was tested on a flow system, and it remained stable for 6 days with 99% ee Moreover, by combing two fixed-bed reactors loaded with the supported enzyme (biocatalytic reactor, CALB-SILLP (SILLP, supported ionic liquid-like phase) 11, 150 mg) and an additional one with an acid zeolite (chemical racemization catalyst, 100 mg). Figure 2.40, the DKR of 1-phenylethanol... 

---