Resolution Reaction

Kinetic resolution reactions on C2-symmetric substrates have important applications. Desymmetrization is just one example of such a kinetic resolution reaction. Enzymatic desymmetrization is outlined in Scheme 8-1.5,6... 

Carbonylative kinetic resolution of a racemic mixture of trans-2,3-epoxybutane was also investigated by using the enantiomerically pure cobalt complex [(J ,J )-salcy]Al(thf)2 [Co(CO)4] (4) [28]. The carbonylation of the substrate at 30 °C for 4h (49% conversion) gave the corresponding cis-/3-lactone in 44% enantiomeric excess, and the relative ratio (kre ) of the rate constants for the consumption of the two enantiomers was estimated to be 3.8, whereas at 0 °C, kte = 4.1 (Scheme 6). This successful kinetic resolution reaction supports the proposed mechanism where cationic chiral Lewis acid coordinates and activates an epoxide. 

Pellissier, H., Recent developments in dynamic kinetic resolution. Tetrahedron, 2008, 64, 1563-1601 Turner, N.J., Enzyme catalysed deracemisation and dynamic kinetic resolution reactions. Curr. Opin. Chem. Biol., 2004, 8, 114-119 Gmber, C.C., Lavandera, I., Faber, K. and Kroutil, W., From a racemate to a single enantiomer deracemisation by stereoinversion. Adv. Synth. Catal., 2006, 348, 1789-1805 Pellissier, H., Dynamic kinetic resolution. Tetrahedron, 2003, 59, 8291-8327 Pmnies, O. and Backvall, J.-E., Combination of enzymes and metal catalysts. A powerful approach in asymmetric catalysis. Chem. Rev., 2003, 103, 3247-3261. 

The reversibility of hydrogen transfer reactions has been exploited for the racemi-zation of alcohols and amines. By coupling the racemization process with an enantioselective enzyme-catalyzed acylation reaction, it has been possible to achieve dynamic kinetic resolution reactions. The combination of lipases or... 

The lipase-catalyzed resolutions usually are performed with racemic secondary alcohols in the presence of an acyl donor in hydrophobic organic solvents such as toluene and tert-butyl methyl ether (Scheme 1.3). In case the enzyme is highly enantioselective E = 200 or greater), the resolution reaction in general is stopped at nearly 50% conversion to obtain both unreacted enantiomers and acylated enantiomers in enantiomerically enriched forms. With a moderately enantioselective enzyme E = 20-50), the reaction carries to well over 50% conversion to get unreacted enantiomer of high optical purity at the cost of acylated enantiomer of lower optical purity. The enantioselectivity of lipase is largely dependent on the structure of substrate as formulated by Kazlauskas [6] most lipases show... 

The enantiomeric ratio is an intrinsic feature of enzyme-enantiomer couples. The actual realization of this property in a resolution reaction affects the enantiomeric excess value, ees (for the substrate) and eeP (for the product) (Equation 3) ... 

By what appears to be a convenient coincidence, it turns out that the barriers that contribute to ksp for a more realistic kinetic scheme, notably the bi bi ping pong scheme adopted by the majority of hydrolases that are currently employed in biocatalytic resolutions reactions, are equally simple to identify. Figure 2.4 shows the barriers that contribute. By straightforward manipulation of the kinetic equations one obtains Equation 16 ... 

From the results reviewed above, one might get the impression that the choice of an organic solvent that optimizes the enantioselectivity of the enzyme in a given resolution reaction is a matter of tedious trial and error, with little guidance from established rules or insights. In practice, however, one has to consider several mitigating circumstances. In many cases of interest, the choice will be limited to a relatively small number of solvents that are either industrially approved or readily available in the laboratory. Since most practical resolutions start from a racemic mixture obtained by chemical synthesis, batch-mode enrichment requiring relatively modest 5-values will be an attractive method. In that case, solubility and easy... 

Fermentation methods for synthesis and resolution Reaction with cyclic lactam intermediates Reaction with glycine and aldolase Fractional crystallization... 

Relatively rapid. Sharp, black, high-resolution reaction product. Sensitive. Permanent preparations. Nontoxic reagents. Can be studied using various forms of microscopy including dark ground, epipolarization, confocal laser scanning and electron microscopy. 

The heading substitution reactions has been used to describe the conversion of a stereogenic center to another. Of course, this means that the substrate stereogenic center has had to be obtained by one of the reaction types outlined earlier, from the chiral pool, or by resolution. Reactions that fall into this category include epoxide and cyclic sulfate openings and iodolactonizations (Chapter 22). Perhaps the most important reaction of this type for asymmetric synthesis is allylic substitution in the presence of a transition metal catalyst. 

Turner, N. J. 2004. Enzyme catalyzed deracemization and dynamic kinetic resolution reactions. Curr. Op. Chem. Biol., 8(2), 114-119. 

The precatalyst used in these water-based kinetic resolution reactions is the cobalt Schiff-base complex 9.40. Its structural similarity to the asymmetric epoxidation catalysts 9.38A and 9.38B is to be noted. In the actual catalytic system 9.40 is activated with small amounts of acetic acid and air to give a cobalt(III) complex where CH3C02 and H20 are additional ligands. The mechanistic details of this reaction are as yet unknown. 

It is well recognized that the medium of the resolution reaction can influence the selectivity. The medium is primarily the solvent, which in hydrolysis mainly... 

Keith, J. M. Larrow, J. R Jacobsen, E. N. Practical Considerations in Kinetic Resolution Reactions, Adv. Synth. Catal. 2001, 34, 5-26. 

Preparative Methods (i) preparation of racemic DPEN and its optical resolution Reaction of benzil and cyclohexanone in the presence of ammonium acetate and acetic acid at reflux temperature gives a cyclic bis-imine (1) (eq 1). Stereoselective reduction of the bis-imine with lithium in THF-liquid ammonia at —78 °C followed by addition of ethanol, then hydrolysis with hydrochloric acid and neutralization with sodium hydroxide produces the racemic diamine (2). Recrystallization of the l-tartaric acid salt from a 1 1 water-ethanol mixture followed by neutralization with sodium hydroxide, recrystallization from hexane results in (5,5)-DPEN (3) as colorless crystals. 

This methodology has been extended successfully to polymer-supported chiral (salen)Co complexes [88] and to intramolecular kinetic resolution of epoxy alcohols (with (R,R)-L Co OAc)) [82]. The ceiling of 50 % yield in kinetic resolution reactions can be extended if the starting material undergoes racemization under the reaction conditions. This has been shown to be possible with epichlorohydrin in reaction with TMSN3, the dynamic kinetic resolution process affording now a 76 % product yield (97 % ee) and 12 % each of the dichloro and diazido products [89]. 

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. 

Hbhne et al. reported a substrate protection strategy that enhanced both the rate and the enantioselectivity of transaminase catalyzed kinetic resolution reactions [32]. The co transaminase catalyzed resolution of the pharmaceutically important syn thons 3 amino pyrrolidine 53 and 3 aminopiperidine 54 was imp roved by the addition of protecting groups to the substrate amines. Reaction rates were improved by up to 50 fold, and product ee was improved from 86 to 99% (Figure 14.23). 

Walsh and co-workers have developed a one-pot method for the synthesis of hydroxyepoxides via an initial synthesis of an allylic alcohol followed by an asymmetric epoxidation <05JOC1262,05JA14668,05JA16416>. This reaction provides an improvement in overall yields over the typical kinetic resolution reaction. The method involves an initial asymmetric addition to the aldehyde followed by a diastereoselective epoxidation reaction. 

The enantioselective hydrolysis of racemic N-acetylated a-amino acids d,l-1 at De-gussa represents a long established large-scale process for the production of L-ami-no acids, l-2 [4]. This enzymatic resolution requires an L-aminoacylase as the biocatalyst. The starting materials for this process are readily available, since racemic N-acetyl amino acids d,l-1 can be economically synthesized by acetylation of racemic a-amino acids with acetyl chloride or acetic anhydride under alkaline conditions via the so-called Schotten-Baumann reaction [5]. The enzymatic resolution reaction of N-acetyl d,L-amino acids, d,l-1, is achieved by a stereospecific L-aminoacylase which hydrolyzes only the L-enantiomer and produces a mixture of the corresponding L-amino acid, l-2, acetate, and N-acetyl D-amino acid, d-1 (Fig. 4) [6],... 

In addition, the amino acylase process can be also applied in the production of other proteinogenic and non-proteinogenic L-amino acids such as L-valine and l-phenylalanine. It is worth noting that racemases have recently been developed by several companies which allow (in combination with the L-aminoacylases) an extension of the existing process towards a dynamic kinetic resolution reaction [10]. It should be mentioned that the same concept can be also applied for the synthesis of D-amino acids when using a D-aminoacylase as an enzyme. 

The original procedures for catalyst preparation and use have proven sufficient for large-scale production of many building block targets, but improvements were necessary to enhance the industrial and economic practicality of batch-wise production utilizing this technology. Active catalyst preparation was a general problem because it remained coupled to the resolution reaction, due to the inability to physically isolate the catalyst by means other than concentration to dryness. More specifically, the HKR of epichlorohydrin was additionally handicapped by the presence of a racemization pathway with concomitant side-product formation (see Sec-... 

We then carried out the optical resolution of (R.S-)-BNOAc (10%) on a 500 mL scale to produce (S)-BNOAc (Fig. 12), and (S)-BONAc was obtained with >98% ee in a yield of 32% in 24 hours. This resolution reaction was reproduced up to a 1 kL scale, so that this technology should be suitable for industrial-scale production. Moreover, in order to make this method more efficient, if (R)-BN with 45% ee was reacted to (R)-BNOAc after extraction of (S)-BNOAc, and treated by the reverse type of strain, Pseudomonas sp. DS-K-717, the productivity would be substantially increased. 

Fig. 13 Stereoselectivity for various esters of CHB with Enterobacter sp. DS-S-75. The resolution reaction was carried out in a 500 mL Erlenmeyer flask with 100 mL of 20 mM phosphate buffer (pH 7.2) containing 2% (v/v) of...

Fig. 17 Resolution reaction of (R,S)-CHBM with Enterobacter sp. DS-S-75 under pH control with 25% NaOH aq.

---