Porcine pancreatic lipase, resolution

Figure 11, Double kinetic resolution using enzymes with opposite enantioselectivity. PPL, porcine pancreatic lipase PLE, pig liver esterase.

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>. 

Kinetic optical resolution of racemic alcohols and carboxylic acids by enzymatic acyl transfer reactions has received enormous attention in recent years56. The enzymes generally employed are commercially available lipases and esterases, preferentially porcine liver esterase (PLE) or porcine pancreatic lipase (PPL). Lipases from microorganisms, such as Candida cylindracea, Rhizopus arrhizus or Chromobacterium viscosum, are also fairly common. A list of suitable enzymes is found in reference 57. Standard procedures are described in reference 58. Some examples of the resolution of racemic alcohols are given39. 

Scheme 2.1 Kinetic resolution of glycidyl butyrate catalyzed by porcine pancreatic lipase, PPL The preferential conversion of (S)-glycidyl butyrate into (R)-glycidol and butyric acid results from the stereochemical nomenclature rules under the Cahn, Ingold, Prelog convention, the configuration around the chiral centre is not affected.

The kinetic results for the lipase-catalysed enantioselective hydrolysis of the esters (236)-(240) can be interpreted in terms of frontier orbital localization.213 The porcine pancreatic lipase (PPL)-mediated optical resolution of 18 racemic esters can be explained by a mechanistic model involving a W-shaped active conformation of the substrate lying in a diastereo-discriminating plane.214... 

Cotterill, I. C. Sutherland, A. G. Robert, S. M. Grobbauer, R. Spreitz, J. Faber, K. Enzymatic resolution of sterically demanding bicyclo[3.2.0]heptanes evidence for a novel hydrolase in crude porcine pancreatic lipase and the advantages of using organic media for some of the biotransformations. J. Chem. Soc. Perkin Trans 1991, 1, 1365. 

Brackenridge, I. McCague, R. M. Roberts, S. M. Turner, N. J. Enzymic resolution of oxalate of a tertiary alcohol using porcine pancreatic lipase. J. Chem. Soc. Perkin Transactions 1 1993, 10, 1093-4. 

Kinetic resolution of racemic dienal iron tricarbonyl complexes by reduction using baker s yeast has been developed. Porcine pancreatic lipase-catalyzed transesterification of hydroxymethyl-substituted complexes have also been used to kinetically resolve diene complexes (Scheme 127). 

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. 

Because free-enzyme porcine pancreatic lipase had much lower activity in SCCO2 than in organic solvents, the authors decided instead to test immobilized lipases. Macroporous resin-supported lipase from M. miehei had fair activity but poor enantioselectivity. Porcine pancreatic lipase immobilized on supports had the greatest activity if the support was highly hydrophilic Sephadex G-25 and Bio-gel P6 were selected. Enantioselectivity was 83% for the (5)-gycidyl butyrate at 25-30% conversion, comparable to results in organic solvents. Several chiral alcohols were studied by Cernia et al. (80) as substrates for kinetic resolution by esterification catalyzed by silica-supported lipase from Pseudomonas sp. [Eq. (5)] ... 

In kinetic resolutions (Scheme 3.2-3.5) it is often the case that one of the products is required, while the other is not and must be discarded or recycled (e.g. racemised). Such operations can be wasteful or expensive. On the other hand, the biotransformation of wcso-compounds or prochiral compounds allows for the possibility of preparing an optically pure compound in quantitative yield. In Scheme 3.7, two examples of the use of meso-compounds are described. The diester (11) is made up of a complex dicarboxylic acid unit derivatised as the dimethyl ester. Pig liver esterase catalyses the hydrolysis of one of the ester groups to give the acid (12) (95% e.e.) in 96% yield. This compound is an excellent precursor of the natural product neplanocin. Note that the acid (12) is not a substrate for pie, and thus the reaction stops at the half-way stage. The compound (13), like (11), possesses a plane of symmetry. Hydrolysis catalysed by porcine pancreatic lipase (ppl) affords the alcohol (14) (>98% e.e.) in quantitative yield. The latter compound has been used to make fluorocarbocyclic adenosine (C -adenosine), a stable analogue of the naturally occurring nucleoside adenosine. 

The enzyme porcine pancreatic lipase (ppl) displays low catalytic activity in such esterification reactions. However, this enzyme was found to be active and extremely stereoselective for trans-esterification reactions in anhydrous organic systems. A practical use for such a trans-esterificaton process involves the resolution of the pheromone sulcatol (28) (Scheme 3.17). 

Scheme 2.55 Resolution of epoxy esters by porcine pancreatic lipase...

The resolution of racemic alcohols in which the hydroxyl residue is not at the stereogenic center is of particular interest and can be done through so-called remote resolutions [73]. The best results have been found when the stereocenter is in p-posi-tion with respect to the alcohol fxmction, while when moving the stereocenter to a further position the processes result in a loss of chiral recognition. In this context, Backvall and Deska have reported the KR of axially chiral primary allenic alcohols, which occurred with high enantioselectivities by using a crude preparation of porcine pancreatic lipase (PPL) in combination with five equivalents of vinyl butanoate using DIPE at room temperature (Table 9.2) [74]. In some cases a quaternary stereocenter contains a hydroxymethyl residue, and the remote resolution is achieved... 

A sequential approach with a combined alcoholysis-esterification reaction has been used for the porcine pancreatic lipase-catalyzed resolution of 2-phenyl-l-propanol [148] (Scheme 20). The alcoholysis reaction (A) was run to 48% conversion followed by the addition of 3 equivalents of vinyl acetate. By this, 2-phenyl-l-propanyl acetate with 92% e.e. and in 20% yield was isolated. 

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