Lipase-catalyzed

Lipase-catalyzed kinetic resolutions are often practical for the preparation of optically active pharmaceuticals (61). For example, suprofen [40828-46-4] (45), which is a nonsteroidal antiinflamatory dmg, can be resolved by Candida glindracea]i 2Lse in >95% ee at 49% conversion (61). Moreover, hpase-based processes for the resolution of naproxen [22204-53-1] and ibuprofen [15687-27-1] (61) have also been developed. 

Lipase-catalyzed enantioselective transesterification of 0-substituted-l,2-diols is another practical route for the synthesis of P-blockers. Lipase PS suspended in toluene catalyzes the transesterification of (63) with vinyl acetate to give the (5)-ester in 43% yield and >98% ee (78). The desired product, optically pure (R)-ttitylglycidol, is then easily obtained by treating the ester with alcohoHc alkaU. Moreover, Pseudomonas Hpase catalyzes the acylation of oxazohdinone (64) with acetic anhydride in very good yield and selectivity (74). PPL-catalyzed transesterification of a number of /n j -norbomene derivatives proceeds in about 30% yield and 92% ee (79,80). 

Lipase-catalyzed intermolecular condensation of diacids with diols results in a mixture of macrocycUc lactones and liuear oligomers. Interestingly, the reaction temperature has a strong effect on the product distribution. The condensation of a,(D-diacids with a,(D-dialcohols catalyzed by Candida glindracea or Pseudomonas sp. Upases leads to macrocycUc lactones at temperatures between 55 and 75°C (91), but at lower temperatures (<45°C) the formation of oligomeric esters predorninates. Optically active trimers and pentamers can be produced at room temperature by PPL or Chromobacterium viscosum Upase-catalyzed condensation of bis (2,2,2-trichloroethyl) (+)-3-meth5ladipate and 1,6-hexanediol (92). 

A benzyl carbonate was prepared in 83% yield from the sodium alkoxide of glycerol and benzyl chloroformate (20°, 24 h). It was also prepared by a lipase-catalyzed ester exchange with allyl benzyl carbonate. It is cleaved by hydrogenolysis (H2/Pd-C, EtOH, 20°, 2 h, 2 atm, 76% yield) and electrolytic reduction (—2.7 V, R4N X, DMF, 70% yield). A benzyl carbonate was used to protect the hydroxyl group in lactic acid during a peptide synthesis." ... 

Polyesters have been obtained in organic medium by polyesterification of hydroxy acids,328,329 hydroxy esters,330 stoichiometric mixtures of diols and diacids,331-333 diols and diesters,334-339 and diols and cyclic anhydrides.340 Lipases have also been reported to catalyze ester-ester interchanges in solution or in die bulk at moderate temperature.341 Since lipases obviously catalyze the reverse reaction (i.e., hydrolysis or alcoholysis of polyester), lipase-catalyzed polyesterifications can be regarded as equilibrium polycondensations taking place in mild conditions (Scheme 2.35). 

Linear polyurethanes, 26 Linear step-growth polymerizations, 13 Lipase-catalyzed polyesterifications, 83 Lipases, 82, 84 catalytic site of, 84 Liquefied MDIs, 211, 226-227 Liquid carbon dioxide, 206 Liquid-castable systems, 201 Liquid crystal devices (LCDs), alignment coating for, 269-270 Liquid crystalline aromatic polyesters, 35 Liquid crystalline polyesters, 25, 26, 48-53... 

However, whatever the mechanism of action is, the effect of solvents on enzyme selectivity is sometimes really dramatic. For example, Hrrose et al. [42] reported that in the Pseudomonas species lipase-catalyzed desymmetrization of prochiral... 

Figure 2.6 Mechanism of lipase-catalyzed hydrolysis of esters [34].

A novel approach was developed very recently by Kita et al. [15]. DKR of allylic alcohols was performed by combining a lipase-catalyzed acylation with a racemization through the formation of allyl vanadate intermediates. Excellent yields and enantioselectivities were obtained. An example is shown in Figure 4.4. A limitation with this approach for the substrates shown in Figure 4.4 is that the allylic alcohol must be equally disubstituted in the allylic position (R = R ) since C—C single bond rotation is required in the tertiary alkoxy intermediate. Alternatively, R or R can be H if the two allylic alcohols formed by migration of the hydroxyl group are enantiomers (e.g. cyclic allylic acetates). 

Several reports on DKR of cyanohydrins have been developed using this methodology The unstable nature of cyanohydrins allows continuous racemization through reversible elimination/addition of HCN under basic conditions. The lipase-catalyzed KR in the presence of an acyl donor yields cyanohydrin acetates, which are not racemized under the reaction conditions. 

Sheldon et al. have reported the DKR of phenylglycine esters via lipase-catalyzed ammonolysis [53]. Racemization was carried out by an aldehyde, such as salicyalde-hyde or pyridoxal, under basic conditions. The major problem they found was the racemization by these aldehydes of the final products. However, when performing the DKR at low temperatures (—20 °C) the substrate was racemized much faster than the product, and DKR was feasible yielding the product in good yield and high enantiomeric excess (Figure 4.27). 

Faber and coworkers have reported a DKR of mandelic acid by using a lipase-catalyzed O-acylation followed by a racemization catalyzed by mandelate racemase. However, these two transformations do not take place simultaneously in the same pot. When the sequence was repeated four times, (S)-O-acetylmandelic acid was obtained in 80% isolated yield and >98% ee [57]. 

Lipase-catalyzed methanolysis of racemic N-benzyloxycarbonyl (Cbz) amino acid trifluoroethyl esters carrying aliphatic side chains afforded the L-methyl esters and the D-trifluoromethyl esters (Figure 6.16). The released alcohol (CF3CH2OH) is a weak nucleophile that cannot attack the ester product. The nucleophilidty of the leaving group is depleted by the presence of an electron-withdrawing group [63]. 

R)-3-Phenoxybutanoic acid and the corresponding butyl (S)-ester were obtained by Burkholderia cepacia lipase-catalyzed enantioselective esterification of the racemic acid with 1-butanol in hexane containing anhydrous sodium sulfate to remove the water produced during the reaction (Figure 6.17) [64]. 

Orthoformates have been used in the lipase-catalyzed esterification aimed at the kinetic resolution of racemic acids such as flurbiprofen, a nonsteroidal anti-inflammatory drug (Figure 6.18). Orthoformates trap the water as it is formed through hydrolysis, and therefore prevent the reverse reaction, and, at the same time, provide the alcohol for the esteriflcation [65]. 

Asymmetric alcoholyses catalyzed by lipases have been employed for the resolution of lactones with high enantioselectivity. The racemic P-lactone (oxetan-2-one) illustrated in Figure 6.21 was resolved by a lipase-catalyzed alcoholysis to give the corresponding (2S,3 S)-hydroxy benzyl ester and the remaining (3R,4R)-lactone [68]. Tropic acid lactone was resolved by a similar procedure [69]. These reactions are promoted by releasing the strain in the four-membered ring. 

Figure 6.42 Lipase-catalyzed kinetic resolution of a p-lactam.

Figure 6.48 Favored enantiomer in lipase-catalyzed acylations of racemic alcohols containing an organometallic substituent.

Combination of lipase-catalyzed transesterification with unsaturated vinyl esters as acyl donors and ring-closing metatheses (RCMs) have also been reported [146-148]. Two groups applied this strategy for the synthesis of goniothalamin from cinnamaldehyde [147,148]. The key steps were a transesterification using vinyl acrylate as acyl donor, followed by an RCM, as depicted in Figure 6.55. 

The mechanism for the lipase-catalyzed reaction of an acid derivative with a nucleophile (alcohol, amine, or thiol) is known as a serine hydrolase mechanism (Scheme 7.2). The active site of the enzyme is constituted by a catalytic triad (serine, aspartic, and histidine residues). The serine residue accepts the acyl group of the ester, leading to an acyl-enzyme activated intermediate. This acyl-enzyme intermediate reacts with the nucleophile, an amine or ammonia in this case, to yield the final amide product and leading to the free biocatalyst, which can enter again into the catalytic cycle. A histidine residue, activated by an aspartate side chain, is responsible for the proton transference necessary for the catalysis. Another important factor is that the oxyanion hole, formed by different residues, is able to stabilize the negatively charged oxygen present in both the transition state and the tetrahedral intermediate. 

Alkanolamides from fatty acids are environmentally benign surfactants useful in a wide range of applications. It was found that most lipases catalyze both amidation and the esterification of alkanolamides however, normally the predominant final products are the corresponding amides, via amidation, and also by esterification and subsequent migration [15]. Recently, an interesting example for the production of novel hydroxyl-ated fatty amides in organic solvents has been carried out by Kuo et cd. [16]. 

Other important derivatives for the preparation of (i-aminoacids are the corresponding P-aminonitriles. Lipase-catalyzed N-acylations of racemic cis-2-aminocyclopentane and cyclohexane carbonitriles with 2,2,2-trifluoroethyl butanoate have been successfully carried out in organic solvents and ionic liquids [53], PSL yielding better results than CALB (Scheme 7.29). 

Recently, a very interesting preparation of P-peptides has been carried out by Kanerva and coworkers through a two-step lipase-catalyzed reactions in which N-acetylated P-amino esters were first activated as 2,2,2-trifluoroethyl esters with CALB [55]. The activated esters were then used to react with a P-aminoester in the presence of CALA in dry diethylether or diisopropylether (Scheme 7.31). In this peptide synthesis, the aminoester was used in excess and the unreacted counterpart was easily separated and later recyded. 

Homochiral (5)- and (f )-l-(2-furyl)ethanols were prepared from 21 by lipase-catalyzed transesterification with vinyl acetate. The pure enantiomers are preciusors for the syntheas of L-and D-daunomycin <96TA907>. 

The outstanding influence of the anionic component on the activity and selectivity of enzymes was demonstrated in the Candida rugosa lipase-catalyzed kinetic resolution of ibuprofen, a nonsteroidal antiinflammatory drug with sales of USD 183 million in 2006 (Scheme 5.15) [63]. 

Pseudomonas aeruginosa lipase-catalyzed hydrolysis of racemic ester 23 proceeds with very low enantioselectivity E = 1.1). Sequential use of error-prone PCR, saturation mutagenesis at chosen spots and DNA shuffling resulted in the formation of a mutant whose enantioselectivity was over 50. 

Lipases are the enzymes for which a number of examples of a promiscuous activity have been reported. Thus, in addition to their original activity comprising hydrolysis of lipids and, generally, catalysis of the hydrolysis or formation of carboxylic esters [107], lipases have been found to catalyze not only the carbon-nitrogen bond hydrolysis/formation (in this case, acting as proteases) but also the carbon-carbon bond-forming reactions. The first example of a lipase-catalyzed Michael addition to 2-(trifluoromethyl)propenoic acid was described as early as in 1986 [108]. Michael addition of secondary amines to acrylonitrile is up to 100-fold faster in the presence of various preparations of the hpase from Candida antariica (CAL-B) than in the absence of a biocatalyst (Scheme 5.20) [109]. 

Scheme 5.20 Lipase-catalyzed Michael addition of amines to acrylonitrile.

In a similar way, lipases catalyze Michael addition of amines, thiols [110], and even 1,3-dicarbonyl derivatives [111, 112] to a,/ -unsaturated carbonyl compounds (Scheme 5.21). 

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