Lipase-catalyzed amidation

Scheme 12 Lipase-catalyzed amidation resolution of CDS-4A, CDS-4B conjugated with cyanide addition reaction...

Fig. 7 1H-NMR spectra of dynamic aminonitrile system, (a) Initial inline signals before CDS-4A. (b) Twelve inline signals in CDS-4A. (c) a-Protons of 24 chiral aminonitriles in CDS-4B. (d) Methyl proton signals of three amide products from lipase-catalyzed amidation resolution of double dynamic covalent system. Adapted with permission from [50]. Copyright 2009 American Chemical Society...

The resulting dynamic aminonitrile systems were first subjected to lipase mediated resolution processes at room temperature. A-Methy] acetamide was observed as a major product from the lipase amidation resolution. In this case, free methylamine A was generated during the dynamic transimination process and transformed by the lipase. To avoid this by-reaction, the enzymatic reaction was performed at 0 °C, and the formation of this amide was thus detected at less than 5% conversion. To circumvent potential coordination, and inhibition of the enzyme by free Zn(II) in solution [54], solid-state zinc bromide was employed as a heterogeneous catalyst for the double dynamic system at 0 °C. The lipase-catalyzed amidation resolution could thus be used successfully to evaluate /V-substituted a-aminonitrile substrates from double dynamic systems in one-pot reactions as shown in Fig. 7d. Proposedly, the heterogeneous catalyst interfered considerably less or not at all in the chemo-enzymatic reaction because the two processes are separated from each other. Moreover, the rate of the by-reaction was reduced due to strong chelation between the amine and zinc bromide in the heterogeneous system. 

To obtain the best lipase-catalyzed amidation resolution, the lipase operational conditions, i.e., additives, lipase preparations, acyl donors, and solvents, were further evaluated. When molecular sieve 4 A was added to control the water content in the enzymatic resolution, decompositions of aminonitrile intermediates were observed. Among a range of lipases, the resolution process by lipase PS-C I provided the highest conversion of amide products. Phenyl acetate 37 was chosen as acyl donor because its reaction led to marginal by-reactions. Thus, the lipase-catalyzed amidation resolution of the dynamic aminonitrile systems in the presence of zinc bromide as heterogeneous catalyst was performed by lipase PS-C I and phenyl acetate as acyl donor in dry toluene at 0 °C. 

Litjens MJJ, Straathof AJJ, Jongejan JA, Heijnen JJ (1999) Synthesis of primary amides by lipase-catalyzed amidation of carboxylic acids with ammonium salts in an organic solvent. Chem Commun 1255 -1256... 

Figure 13 Lipase-catalyzed amidation of benzyl esters.

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]. 

One of the most important characteristics of IL is its wide temperature range for the liquid phase with no vapor pressure, so next we tested the lipase-catalyzed reaction under reduced pressure. It is known that usual methyl esters are not suitable for lipase-catalyzed transesterification as acyl donors because reverse reaction with produced methanol takes place. However, we can avoid such difficulty when the reaction is carried out under reduced pressure even if methyl esters are used as the acyl donor, because the produced methanol is removed immediately from the reaction mixture and thus the reaction equilibrium goes through to produce the desired product. To realize this idea, proper choice of the acyl donor ester was very important. The desired reaction was accomplished using methyl phenylth-ioacetate as acyl donor. Various methyl esters can also be used as acyl donor for these reactions methyl nonanoate was also recommended and efficient optical resolution was accomplished. Using our system, we demonstrated the completely recyclable use of lipase. The transesterification took place smoothly under reduced pressure at 10 Torr at 40°C when 0.5 equivalent of methyl phenylthioacetate was used as acyl donor, and we were able to obtain this compound in optically pure form. Five repetitions of this process showed no drop in the reaction rate (Fig. 4). Recently Kato reported nice additional examples of lipase-catalyzed reaction based on the same idea that CAL-B-catalyzed esterification or amidation of carboxylic acid was accomplished under reduced pressure conditions. ... 

Dynamic kinetic resolution enables the limit of 50 % theoretical yield of kinetic resolution to be overcome. The application of lipase-catalyzed enzymatic resolution with in situ thiyl radical-mediated racemization enables the dynamic kinetic resolution of non-benzylic amines to be obtained. This protocol leads to (/f)-amides with high enantioselectivities. It can be applied either to the conversion of racemic mixtures or to the inversion of (5)-enantiomers. 

The third group of target molecules comprises chiral carboxylic acid and their derivatives esters, amides and nitriles. Enantiomerically pure esters are prepared in an analogous manner to the enantiomerically pure alcohols discussed earlier [i.e. by esterase- or lipase-catalyzed hydrolysis or (trans)esterification]. However, these reactions are not very interesting in the present context of cascade reactions. Amides can be produced by enantioselective ammoniolysis of esters or even the... 

Feng Y, Klee D, Keul H, Hdcker H (2000) Lipase-catalyzed ring-opening polymerization of morpholine-2,5-dione derivatives a novel route to the synthesis of poly(ester amide)s. Macromol Chem Phys 201 2670-2675... 

The natural substrates of lipases are triglycerides and, in an aqueous environment, lipases catalyze their hydrolysis into fatty acids and glycerol. In anhydrous media, lipases can be active in the reverse reaction [19]. In fact, in the acylation step, acids, lactones, (cyclic) carbonates [20, 21], cyclic amides [22, 23], (cyclic) thioesters [24, 25], and cyclic phosphates [26] have been found to act as suitable acyl donors. In the deacylation step, apart from water, lipases also accept alcohols [27], amines [28, 29], and thiols [30] as nucleophiles although the specificity of lipases is lower for amines and thiols than for water and alcohols [31]. 

Other authors have described the lipase-catalyzed chemoselective acylation of alcohols in the presence of phenolic moities [14], the protease-catalyzed acylation of the 17-amino moiety of an estradiol derivative [15], the chemoselectivity in the aminolysis reaction of methyl acrylate (amide formation vs the favored Michael addition) catalyzed by Candida antarctica lipase (Novozym 435) [16], and the lipase preference for the O-esterification in the presence of thiol moieties, as, for instance, in 2-mercaptoethanol and dithiotreitol [17]. This last finding was recently exploited for the synthesis of thiol end-functionalized polyesters by enzymatic polymerization of e-caprolactone initiated by 2-mercaptoethanol (Figure 6.2)... 

Scheme 7.9 Resolution of racemic amines by lipase-catalyzed enantioselective amide formation (BASF).

Proteases have been much less studied than lipases in ionic liquid media and generally require the presence of water for activity. We note that the thermolysin-catalyzed amide coupling of benzoxycarbonyl-L-aspartate and L-phenylalanine methyl ester into Z-aspartame in [BMIm][PF6] was an early example of an enzymatic reaction in an ionic liquid medium [8]. 

Another important difference between (dynamic) kinetic resolution of alcohols and amines is the ease with which the acylated product, an ester and an amide, respectively, is hydrolyzed. This is necessary in order to recover the substrate enantiomer which has undergone acylation. Ester hydrolysis is generally a facile process but amide hydrolysis, in contrast, is often not trivial. For example, in the BASF process [28] for amine resolution by lipase-catalyzed acylation the amide product is hydrolyzed using NaOH in aq. ethylene glycol at 150 °C (Fig. 9.18). In the case of phenethylamine this does not present a problem but it will obviously lead to problems with a variety of amines containing other functional groups. 

By HPLC analysis, the lipase enantiomeric discrimination could be observed from the double dynamic system. The highest enantioselectivity was around 5-10% ee for amides 33A and 36A, respectively. However, when fe/t-butyl methyl ether (TBME), commonly used in lipase-catalyzed reactions, was used as solvent, the lipase enantiomeric discrimination of the dynamic systems was much improved. High enantioselectivities were observed for all three final products 90% ee (33A), 93% ee (36A), and 73% ee (30A). Thus, only three chiral amide products were resolved from dynamic system, containing 24 chiral intermediates, through lipase-... 

Enzymes such as proteases (122), subtilisin (123), acylases, peptidases, amidases, and lipases (124) are reported to catalyze amide bond formation with, in some cases, enantiospeciflcity of over 99%. Despite limited enzyme-substrate compatibility, specific conditions have been developed to reverse their natural reactivity, which is in favor of the hydrolysis. For example, Kyotorphin (Tyr-Arg) (125), a potent analgesic, was produced on an industrial scale using a-chymotrypsin, a peptidase isolated from bovine pancreas. 

Besides cyclic esters and carbonates, six-membered cyclic depsipeptides and a five-membered cyclic phosphate were subjected to lipase-catalyzed ring-opening polymerizations, yielding poly (ester amide)s190 and polyphosphate,191 respectively. High temperatures (100—130 °C) were required for the polymerization of the former monomers. 

Some groups have reported on their search for less reactive acylating agents, to suppress noncatalyzed chemical acylation and increase product enantiomeric excess. Irimescu and Kato carried out an enantioselective lipase catalyzed acylation of 1 phenylethylamine and 2 phenyl 1 propylamine by reacting the amines with carbox ylic acids in a nonsolvent system or in ionic liquids (Figure 14.9). The reaction equilibrium was shifted toward amide synthesis by the continuous removal of the... 

Various methods for the preparation of enantiomerically pure P4-P3 mimetics had been published before this work. Among these were the classical co-crystallization with chiral amines [7, 8], the derivatization as diastereoisomeric amides [6, 9, 10], the synthesis of chiral intermediates following Evans methodology [10-12], or the use of lipase-catalyzed reactions [13], and the asymmetric hydrogenation of unsaturated derivatives [8, 14],... 

Lipases are also known to catalyze amide formation. Thus in 1987 Margolin and Klibanov [40] showed the use of porcine pancreatic lipase (PPL) for dipeptide synthesis in organic solvents. West and Wong [41] investigated several lipases and esterases for dipeptide and tripeptide synthesis, with variable yields observed. Later, Kawashiro et al. [42] also studied the dimerization of phenylalanine with PPL. 

The synthesis of chiral poly(depsipeptides), polymers with alternating amide and ester bonds, by lipase-catalyzed ring opening of 3-isopropyl morpholino-2,5-dione (19) was shown by Hocker and coworkers (Scheme 11.5) [26], Various lipases were tested for the bulk polymerization of these heterocyclic monomers at temperatures of 100 °C or above. PPL and lipase type III from a pseudomonas species were shown to be effective catalysts. The isolated polymers showed Mn values of 3.5-17.5 kgmol-1. The influence of reaction temperature, the amount of enzyme and the presence of water in the reaction medium were shown to be important factors on the high molecular weight fraction and were investigated in detail [26b]. Comparison of optical rotation values for polymers prepared by... 

Enzymes have been used for biocatalysis in organic solvents since the early 1980s, and in ILs since 2000 [17]. As biological catalysts, enzymes accelerate reactions but do not affect the equilibrium distribution. Hence, hydrolytic enzymes that, as an example, under normal circumstances in aqueous solutions hydrolyze esters and amides, will, when placed in water-free conditions, also catalyze the reverse reaction, the condensation of an acid with an alcohol or an amine, to give esters and amides, respectively. Further, under water-free conditions hydrolytic enzymes can accept alternative nucleophiles to catalyze reactions such as transesterifications. An important industrial example of this is the lipase-catalyzed transesterification of triglycerides to obtain fats with a desired melting point [18]. 

Recently, many research groups have focused their efforts oti the development of stereoselective routes leading to optically pure aminophosphinic acids. With this aim, Yamagishi and co-workers recently devised a practical methodology for the preparation of optically pure A-protected 1,1-diethoxyethyl(aminomethyl) phosphinates (12) [39] and their participation in diastereoselective alkylation reactions [40] which were first studied several years ago by McCleery and Tuck [41] (Scheme 4). In particular, they managed to obtain on a gram-scale and 99 % enantiomeric excess (ee) compound 11, after addition of paraformaldehyde to l,l-diethoxyethyl-//-phosphinate (10) and subsequent lipase-catalyzed resolution of the resulting racemic alcohol. Conversion of 11 to substrate 12 in four steps afforded a valuable substrate suitable for lithium bis(trimethylsilyl)amide (LHMDS)-promoted alkylation performed in a diastereoselective fashion (dr = 10 1) (Scheme 4). 

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