Lipase ammonolysis

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). 

This methodology avoids the inherent 50% yield limit of KR and the difficult separations often encountered in the resolution of racemates. The potential of enzymes, especially lipases, to catalyze the aminolysis and ammonolysis of prochiral... 

Asymmetrization of a prochiral dicarboxylic acid diester catalyzed by lipases, where the stereo center of the product is located on the acyl side, becomes a single-step process because the polar carboxylic acid and/or amide formed are not well accepted as substrates by the Upase. One example is the enantioselective hydrolysis or ammonolysis of diethyl 3-hydroxyglutarate, as shown in Scheme 7.4, a reaction which leads to the formation of a precursor for the important chiral side chain of atorvastatin, lipitor [40, 41]. The S-enantiomer was formed with high e.e. (98%), but unfortunately this is the undesired enantiomer for the production of the pharmaceutically important product. Only a-chymotrypsin gave a predominance of the... 

Ammonolysis. A preparation of primary amides catalyzed by lipase proceeds in ionic liquids at least as well as in organic media. 

The synthesis of DPP-IV inhibitor Saxagliptin 5 also required (55)-5-amino-carbonyl-4,5-dihydro-lH-pyrrole-l-carboxylic acid, l-(l,l-dimethylethyl)ester 10 (Figure 16.3C). Direct chemical ammonolyses were hindered by the requirement for aggressive reaction conditions, which resulted in unacceptable levels of amide race-mization and side-product formation, while milder two-step hydrolysis-condensation protocols using coupling agents such as 4-(4,6-dimethoxy-l,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM) [41] were compromised by reduced overall yields. To address this issue, a biocatalytic procedure was developed based on the Candida antartica lipase B (CALB)-mediated ammonolysis of (55)-4,5-dihydro-lH-pyrrole-l,5-dicarboxylic acid, l-(l,l-dimethylethyl)-5-ethyl ester 9 with ammonium carbamate to furnish 10 without racemization and with low levels of side-product formation. 

Screening experiments utilized process stream ester feed, which consisted of about 22% w/v (0.91M) of the ester in toluene. Since toluene precluded the use of free ammonia due to its low solubility in toluene, solid ammonium carbamate was employed. Reactions were performed using a mixture of neat process feed, ammonium carbamate (71 g/L, 2 mol eq of ammonia), and biocatalyst (25 g/L), shaken at 400 rpm, 50°C. Under these conditions, CALB and its immobilized forms Novozym 435 and Chirazyme L2 provided racemization-free amide with yields of 69%, 43%, and 40%, together with 21%, 18%, and 22% of side products (by HPLC), respectively, while all other biocatalysts (lipases) furnished less than 5% of the desired product [42]. The ammonolysis reaction with free CALB was then optimized with regard to the temperature and the CALB and ammonium carbamate loads to increase yield from 56% to 71%, with side products varying from 7% to 19%. 

Biocatalysts can also be used in the complete absence of water. Candida antartica lipase B (CaLB), used either as the free enzyme or in an immobilized form, catalyzes transesterification, ammonolysis andperhydrolysis. For example, octanoic acid was quantitatively transformed to amide 107 in 4 days (Scheme 46) - a significant rate... 

Successful examples were demonstrated for transesterilication, perhydrolysis and ammonolysis (using lipases, esterases, and proteases), amide/peptide synthesis (using proteases), epoxide hydrolysis (using epoxide hydrolases), and glycoside synthesis (using glycosidases). Even redox-transformations, such as carbonyl reduction and sulfoxidation were possible [113-119]. 

Scheme 3.50 Lipase-catalysed ammonolysis of a phenylglycine ester.

Later, Sheldon and coworkers [45] were able to transfer this technology to the ammonolysis of phenylglycine catalyzed by a commercial lipase (Scheme 8.6). They also found that other aldehydes, such as salicylaldehyde and pyridoxal, could carry out effectively the racemization of the substrate, without damaging the optical purity of the resolved product. 

Wegman, M., Hacking, M., Rops, )., Pereira, P., Van Rantwijk, F., and Sheldon, R. (1999) Dynamic kinetic resolution of phenylglycine esters via lipase-catalysed ammonolysis. Tetrahedron Asymmetry, 10 (9), 1739-1750. 

C. antarctica lipase catalyzes alcoholysis, ammonolysis (shown in Figure 3.23) [4a], and hydrolysis reactions in the ionic liquids l-butyl-3-methylimidazolium tetrafluoroborate or hexafluorophosphate. Reaction rates were generally same as or better than those observed in organic media. For example, the direct reaction of octanoic add and ammonia was catalyzed by Novozym 435 by bubbling ammonia through a suspension in an ionic liquid. A quantitative conversion was obtained after 4 days. 

Another approach for the enzymatic preparation of 5-ibuprofen has been demonstrated by de Zoete et al. [229]. The enantioselective ammonolysis of ibuprofen 2-chlo-roethyl ester by Candida antarctica lipase (lipase SP435) gave the remaining ester 5-(+) enantiomer in 44% yield and 96% e.e. The enantioselective enzymatic esterification of racemic ibuprofan has also been demonstrated using lipase from Candida cylindraceae [230]. The reaction was carried out in a water-in-oil microemulsion [bis(2-ethyl-hexyl)sulfosuccinate (AOT)/isooctane). The lipase showed high preference for the S-(+) enantiomers of ibuprofen which was esterified and R-(—) enantiomer remained unreacted. The reaction yield of 35% was obtained using n-propanol in the reaction mixture as nucleophile. 

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