Lipase-catalyzed resolutions water activities

Table 1 Altered E Values at Various Water Activities in Lipase-Catalyzed Resolutions...

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 dynamic cyanohydrin system was next challenged with lipase-catalyzed transesterification resolution using different operational conditions. Thus, different lipases, organic solvents, additives, and acyl donors were evaluated. Isopropenyl acetate 26 was chosen and used as acyl donor because its reaction produces acetone as by-product, which does not interfere in the reaction and the NMR spectra. Molecular sieve 4 A was also added in the dynamic resolution process to control the water activity. The lipase preparation PS-C I was chosen in the resolution process since it expressed the highest lipase activities for both the substrate structure and the enantiomeric selectivities. Different organic solvents were also... 

For the synthesis of p-lactam antibiotics, the presence of asymmetrical carbon at the 3 and 4 positions is critical to prepare optically active -lactams [197]. Nagai et al. [198] developed enzymatic synthesis of optically active p-lactams by lipase-catalyzed kinetic resolution using the enantioselective hydrolysis of iV-acyloxymethyl p-lactams 108 in an organic solvent (isopropyl ether saturated with water) and the transesterification of N-hydroxymethyl P-lactam 109 in organic solvent (metiiylene chloride) in tiie presence of vinyl acetate as acyl donor (Fig. 37). The reaction yield of 35-50% and e.e. s of 93 to more than 99% were obtained depending on the specific substrate used in the reaction mixture, Lipase B from Pseudomonas fragi and lipase PS-30 from Pseudomonas sp. were used in the reaction mixture. 

A beneficial feature of subtilisin, and in particular subtilisin-CLECs, is their high catalytic activity in polar and non-polar organic solvents, allowing for transesterifications of alcohols in the presence of small amounts of water. Transesterifications catalyzed by subtilisin were mostly done with vinyl acetate. Apparently, the acetaldehyde formed during transesterification is not harmful to the enzyme as it is in the case of some lipases and pig liver esterase. Although resolution of such alcohols either through hydrolysis of the corresponding esters or transesterification is the domain of lipase, in some cases useful selectivities were achieved with subtilisin (1-9) (Table 11.1-26). 

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)]. 

In this two-step resolution, the enzyme catalyzes the hydrolysis of the diester to monoester, then the monoester to the diol. Each step contributes to the overall enantioselectivity. The reaction mixture is biphasic buffered water containing the cmde enzyme and an ether solution of the diester. The cmde enzyme is bovine pancreas acetone powder that contains many enz5mies, but the substrate rearts only with the cholesterol esterase so the other enzymes do not interfere, other than to make the work-up messy. The enz5mie cholesterol esterase requires a bile salt, taurocholate, for fiiU activity. This bile salt helps emulsify the two phases. Cholesterol esterase seems to behave more like a lipase than an esterase in this example since it works with the substrate in the organic phase. The dipentanoate ester is chosen to simplify the separation of starting diester and product diol (Figure 5.7). 

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