Acetates, with lipases

Such resolution could be readily optimized by use of an appropriate acyl group which reacts efficiently with the enzyme employed [29]. For example, the acetate prepared from monofluorinated a-phenetyl alcohol was hydrolyzed with lipase MY at 34% conversion to afford the product only with 26% . Enhancement of optical purity to 73% was observed when the corresponding isobutyrate was hydrolyzed. The best results were obtained for hydrolysis of the isobutyrate by lipase PS, which afforded the product in 82% at 47% hydrolysis. Experience has shown (see Table 3) that one of the best combinations was hydrolysis of acetate with lipase MY or isobutyrate with lipase PS [30]. 

For the hydrolysis of cyano acetate with lipase, a method to improve the enantioselectivity was developed. The addition of a thiacrown ether largely improved the enantioselectivity to E = >700 from E = 53 without any additives (Figure 14(a)). 

Hydrolytic enzymes can also catalyzed the esterification of alcohols or acids with hetero atoms.1617 Some examples for the reactions of phosphorous and sulfur compounds by lipases are shown in Figure 16. By the repeated enantioselective acylation and hydrolysis of a hydroxyl phosphonate and its acetate with lipase AH, phosphonic acid analogue of carnitine (essential cofactor of fatty acid metabolism), (A)-phosphocarniiine, and its enantiomer were synthesized as shown in Figure 16 (b). 

Of course, the influence of organic solvents on enzyme enantioselectivity is not limited to proteases but it is a general phenomenon. Quite soon, different research groups described the results obtained with lipases [28]. For instance, the resolution of the mucolytic drug ( )-trans-sobrerol (11) was achieved by transesteriflcation with vinyl acetate catalyzed by the lipase from Pseudomonas cepacia adsorbed on celite in various solvents. As depicted in Scheme 1.3 and Table 1.5, it was found that t-amyl alcohol was the solvent of choice in this medium, the selectivity was so high ( >500) that the reaction stopped spontaneously at 50% conversion giving both +)4rans-sobrerol and (—)-trans-sobrerol monoacetate in 100% optical purity [29]. 

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. 

DKR reactions were performed with lipase and Pd(PPh3)4 in the presence of dppf and 2-propanol in THF. 2-Propanol was used as an acyl acceptor. Various acyclic allyhc acetates were transformed to their corresponding allylic alcohols at room temperature in good yields and excellent optical purities (Table 16). 

S)-3,7-Dimethyl-2-oxo-6-octene-l,3-diol (39) was recently identified as the aggregation pheromone of the Colorado potato beetle (Leptinotarsa decem-lineata), and synthesized by Oliver et al., starting from (S)-linalool [86]. An improved synthesis of (S)-39 by Mori is shown in Scheme 57 [87]. Enzymatic acetylation of ( )-2,3-epoxynerol (A) with vinyl acetate and lipase PS gave B together with C. The acetate B was converted to a multi-gram quantity of (S)-39 according to Oliver [86]. 

With a good route to the key meso diol 128 in hand, the authors turned their attention to desymmetrization, using the known asymmetric hydrolysis of meso diacetates by Lipase AK (Scheme 23). The meso diol 128 was first converted to diacetate 140, and then hydrolyzed with Lipase AK to cleave selectively one of the two acetates, producing chiral hydroxyester 141. Oxidation, cleavage of the acetate, and lactonization yielded the (3S,4.R) lactone 129. The corresponding lactol (3S,4 )-130 was found to be the enantiomer of the compound produced in the HLADH synthesis. 

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. 

In a similar investigation, transesterification reactions of vinyl acetate with alcohols in [BMIM]BF4 and [BMIM]PF6 in the presence of immobilized lipases CALB and PS-C were found to proceed with higher enantioselectivities than in THF or toluene, with the best result again being observed with [BMIM]PF6 (280). 

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... 

Catalytic tests were performed in a glass vessel equipped with a stirrer motor. Two monoliths (diameter 4.3 cm, length 4 cm) were mounted in plane on the stirrer axis. The total reaction volume was 2.5 1. Lipase was assayed in the acylation of vinyl acetate with butanol in toluene. Initial reaction rate was followed by GC analysis. Immobilized trypsin was used in the hydrolysis of N-benzoyl-l-arginine ethyl ester (BAEE) in a 0.01 M phosphate buffer pH 8 at 308 K. The reaction was followed by UV-VIS at 253 nm, and reaction rate was calculated in the mass transfer limited situation. 

Large-scale synthesis of (Z)-3-hexenyl acetate in hexane with lipase, (Z)-3-hexenol and acetic acid was described by several authors [40-42]. (Z)-3-Hex-enyl acetate has a fruity odour and shows a significant green note flavour. It can be produced using lipase from Candida antarctica immobilised on an acrylic resin [40, 41] or using immobilised lipase from Mucor miehei [42]. The conversion was reported to be about 90%. 

For example, the acetate prepared from l,l,l-trifluoro-2-octanol was transformed into (.R)-l,l,l-trifluoro-2-octanol in 96% when hydrolyzed with lipase MY at 40% conversion. Other, trifluoromethylated chiral secondary alcohols shown in Table 2 were prepared by the same procedure. The corresponding alcohols were converted to their acetate, followed by asymmetric hydrolysis to attain the higher enantiomeric excess [28]. 

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-... 

Table 13.1 Jacobs DKR of amines with lipase, isopropyl acetate, and Pd/BaSO4.

Enzymatic hydrolysis is a nondestructive alternative to saponification for removing triglycerides in vitamin K determinations. For the simultaneous determination of vitamins A, D, E, and K in milk- and soy-based infant formulas and dairy products fortified with these vitamins (81), an amount of sample containing approximately 3.5-4.0 g of fat was digested for 1 h with lipase at 37°C and at pH 7.7. This treatment effectively hydrolyzed the glycerides, but only partially converted retinyl palmitate and a-tocopheryl acetate to their alcohol forms vitamin D and phyllo-quinone were unaffected. The hydrolysate was made alkaline in order to precipitate the fatty acids as soaps and then diluted with ethanol and extracted with pentane. A final water wash yielded an organic phase containing primarily the fat-soluble vitamins and cholesterol. 

Thus it is very easy to acetylate a racemic alcohol and treat the racemic mixture of acetates with a lipase. One enantiomer is hydrolyzed to the alcohol and the other remains as die ester. These are separated chromatographically and each component is obtained widi high optical purity. This technique is becoming more important and could be die most general technique for resolution in the future. 

Chiral carbon framework of the monoterpenoid secologanin, leading to powerful chiral synthons, from readily available meso-forms. Moderate results were obtained with lipases such as porcine liver esterase (PLE), delivering the (15,2/ )-mono-acetate at a yield of 78% with 96% ee (Fig. 10), though preparation of the enzyme seemed to be crucial for the appropriate result. In contrast, pig pancreatic lipase (PPL) was significantly more efficient in forming the (—)-(l/ ,25) enantiomer at a yield of 96% and 98% ee [86, 87]. 

The meso-diol was enzymatically acetylated, which afforded the mono-acetate with 25,6/ - configuration. Hydrolysis by lipase reaction (PLE) gave an intermediate through which four reaction steps resulted in the N-protected (Cbz-group) ethyl ester, which could be finally transformed to the (—)-enantiomer of the target alkaloid (—) -gephyrotoxin. 

Joly et al71 reported the use of Candida cylindracea lipase in the kinetic resolution of aryl-substituted /J-hydroxy ketones 44 using either the transeterification of the free /j-hydroxy ketone with vinyl acetate or the hydrolysis of its acetate. The transesterification mode afforded the alcohol with 30-70 % ee and the acetate with > 96% ee. That of hydrolysis afforded the alcohol with 64-93 % ee and the acetate with > 96% ee. 

Figure 3. Initial Velocity of Hydrolysis of Mixtures of (R)- and (S)-HMPC Acetate with Arthrobacter Lipase.

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... 

Inagaki M, Hatanaka A, Mimura M, Hiratake J, Nishioka T, Oda J (1992) One-pot synthesis of optically active cyanohydrin acetates from aldehydes via quinidine-catalyzed transhydro-cyanation coupled with lipase-catalyzed kinetic resolution in organic solvent. Bull Chem Soc Jpn 65 111-120... 

However, the reaction has long reaction times and it suffers from several experimental details which lower the reaction yields. Recently improvements in the original procedures allowed (S)-mandelonitrile acetate to be obtained from the corresponding rac-alcohol in a 97% yield and 98% e.e. while also shortening the reaction time. Best results were obtained with (lipase from C. antarctica type B) (CALB) adsorbed over celite [24] (Scheme 13.8). 

Thiel and coworkers reacted imidazole with epoxycyclohexane to form the racemic hydroxycyclohexyl imidazole. Attempts to separate the enantiomers by kinetic resolution with lipase B of Candida antarctica and isopropenyl acetate as acylating agent [11,12] failed, but gave the racemic ester in high yields (see Figure 4.1). Alkylation was then... 

The ruthenium-catalyzed racemization of a-methylbenzyl alcohol was combined with an enzyme-catalyzed transesterification with lipase. Dinuclear ruthenium complex 64 effectively catalyzes the racemization of a-methylbenzyl alcohol and the combination of 64, p-chlorophenyl acetate, and enzyme N-435 in the reaction of racemic amethylbenzyl alcohol gave enantiomerically pure (R)-a-methylbenzyl acetate in the excellent yield (Eq. 12.26) [29]. 

Regioselective acylations of polyhydroxylated compounds such as carbohydrates, glycerols, steroids, or alkaloids have been carried out with lipases, esterases, and proteases [13, 20]. One example is the Candida antartica lipase (immobilized on acrylic resin) catalyzed monoacylation of the signalling steroid ectysone (1) giving selectively the 2-C)-acetate 2 (eq. (1)). Using vinyl acetate for this transesterification the reaction was irreversibly pushed to the product side, since the liberated enol instantaneously isomerizes to acetaldehyde [21]. The sometimes unfavorable aldehyde is avoided when 1-ethoxyvinyl acetates [22], trichloro- or -fluoroethyl esters [23 a, b], oxime esters [23 c] or thioesters [23 d] are employed for the quasi-irreversible reaction courses. 

In a second investigation, racemic 2-hydroxy-4-phenylbutanoic acid (42) was reacted with lipase PS (LPS) and vinyl acetate (VA) (ethenyl ethanoate) in /-butyl methyl ether (2-methoxy-2-methylpropane) to give acetate (S)-43 in 35% yield and in 99% ee unreacted alcohol 44 was found to have 99% ee with R configuration predominant (Scheme 3.3). 

Romero, M.D. Calvo, L. Alba, C. Habulin, M. Primozic, M. Knez, Z. Enzymatic synthesis of isoamyl acetate with immobilized Candida ant-arc tica lipase in supercritical carbon dioxide. J. Supercritic. Fluids 2005, 33 (1), 77-84. 

Different groups have studied the kinetic resolution of rac-l-phenylethanol by transesterification with vinyl acetate using lipases in ionic liquids as the reaction media [6,17,46] (Fig. 7.8). Schofer et al. [17] screened a set of nine lipases and two esterases for activity in ten different ionic liquids and compared the results with the reaction performed in methyl tert-butyl eter (MTBE) as the solvent. The lipases showed good activity and, in some cases, improved enantioseleclivity in these new reaction media. For example, the enantiomeric excess with PiL and AsL in [bmim ]... 

Romero MD, Calvo L, Alba C et al (2005) Enzymatic synthesis of isoamyl acetate with immobilized Candida antarctica lipase in supercritical caibon dioxide. J Supercrit Fluids 33 77-84... 

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