Optimizing Enantioselectivity

Enantiomeric purity. In order to assess the efficiency of an enantioselective hydrolase-catalyzed reaction, it is imperative that one can accurately measure at least the conversion and the enantiomeric excesses of either the substrate or the product (see equations Equation 1, Equation 2, and Equation 3). Although optical rotation is sometimes used to assess enantiomeric excess, it is not recommended. Much better alternatives are various chromatographic methods. For volatile compounds, capillary gas chromatography on a chiral liquid phase is probably the most convenient method. Numerous commercial suppliers offer a large variety of columns with different chiral liquid phases. Hence it is often easy to find suitable conditions for enantioselective GC-separations that yield ee-values in excess of 

which can usually be determined with a high precision ( 0.05%) [33, 34]. For nonvolatile compounds, HPLC on a chiral phase is the method of choice. 

Solvent and water content. When reactions are performed in organic solvents [la], the solvent of choice is often a hydrophobic solvent with logP 1.5. However, despite a massive amount of work on this matter, no general rules can be formulated. Usually one has to scan a number of solvents to find the one that gives the best E for the individual case. The water activity of the reaction system can influence E quite substantially, but no general rules can be formulated, and usually one has to find the optimal conditions for the individual substrate. It is important to note, however, that if the water content is high in, e.g., an acylation reaction system, water can compete with the desired nucleophile and give an acid that can disturb the efficiency of the resolution. 

Genetic engineering. The X-ray structures are known for many hydrolases, allowing for modeling of the substrate in the active site as well as structurally based, random or rational protein mutation to magnify or invert enantioselectivity. An example of the latter is provided by the rational design of a mutant of Candida antarctica lipase (CALB), which, instead of the wild-type R-selectivity, displayed 

S-selectivity toward 1-phenylethanol [35]. Several other hydrolases have also been redesigned by protein engineering to invert or enhance their enantioselectivity [36-40]. 

---

Temperature can also be used to optimize enantioselectivity in SFC. The selectivity of most CSPs increases as temperature decreases. For this reason, most chiral separations in SFC are performed at ambient or subambient temperatures [50, 74]. Subambient temperatures are particularly useful for compounds having low conformational stability [75]. Stringham and Blackwell explored the concept of entropically driven separations [76]. As temperature increased, enantioselectivity decreased until the enantiomers co-eluted at the isoelution temperature. Further increases in temperature resulted in reversal of elution order of the enantiomers. The temperature limitations of the CSP should be considered before working at elevated temperatures. 

Another way to increase the host cavity is by using exchange reactions in y-CD (diameter of the cavity from 7.5 to 8.3 A) instead of j8-CD (diameter of the cavity from 6.0 to 6.5 A)." " The larger cavity size of y-CD decreases (and inverts) the enantioselectivity of valine from krJkL = 0.32 to kolk = 1.41 and that of isoleucine from ko/k = 0.26 to k /kL = 2.28. This observation indicates that the three amino acids have optimal enantioselectivity with j8-CD. Conversely, phenylalanine increases in selectivity from k /kL =1.2 (with j8-CD) to kolk =1.8 (with y-CD). This observation suggests that the larger cavity of y-CD allows each enantiomer of the larger amino acid to find more distinct interactions with the larger host. 

Therefore, it is not further surprising that the organic modifier type and content is an effective variable to adjust retention and to optimize enantioselectivities. Methanol and acetonitrile have been frequently found to be complementary in their selectivity profiles and these two organic modifiers are advised to be tested in a preliminary screening experiment. 

Aboul-Enein, H.Y. and Serignese, V., Optimized enantioselective separation of clenbuterol on macrocyclic antibiotic teicoplanin chiral stationary phase, J. Liq. Chrom. Rel. Technol, 22, 2177, 1999. 

The scope of Michael additions with catalysts containing cyclohexane-diamine scaffolds was broadened by Li and co-workers [95]. When screening for a catalyst for the addition of phenylthiol to a,p-nnsatnrated imides, the anthors fonnd that thiourea catalyst 170 provided optimal enantioselectivities when compared to Cinchon alkaloids derivatives (Scheme 41). Electrophile scope inclnded both cyclic and acyclic substrates. Li attributed the enantioselectivity to activation of the diketone electrophiles via hydrogen-bonding to the thiourea, with simultaneous deprotonation of the thiol by the tertiary amine moiety of the diamine (170a and 170b). Based on the observed selectivity, the anthors hypothesized that the snbstrate-catalyst... 

Complex 218, in the presence of chlorotrimethylsilane (TMSCl) as a promoter, was found to be the most useful and gave the highest enantioselectivity and best yield in this reaction. Dichloromethane, chloroform, and 1,2-dichloroethane proved to be the best solvents for optimizing enantioselectivity. A limited survey of substituents suggested that this catalyst was not particularly sensitive to the electron demand of the aryl system, although steric effects may be important (Table 37, entry 4). 

Deprotonation of 4-substituted cyclohexanones using Koga s chiral base 39 gave silylenol ethers with 93-94% ee (Scheme 25)58,59. Variation of the aromatic ring in such chiral bases did not display any noticeable improvement in enantioselectivity60,61. However, some drawbacks have to be noted as optimal enantioselectivity requires HMPA as co-solvent. 

These transformations provide an indication of how different salen and metal combinations have been developed to optimize enantioselectivities and efficiency diverse reactions. The modular nature of the salen ligand allows... 

Having chosen appropriate chiral auxiliary, solvent and reaction parameters, it is necessary to apply tailor-made metal/carrier catalysts in order to optimize enantioselectivity and catalytic activity. Since the carrier material significantly influences the properties of the active Pt particles. 

L-valinate, N-dodecoxycarbonylvaline), polymer surfactants (e.g. poly[sodium N-undecenyl-L-valinate]), and n-alkyl-P-glucopyranosides. An important property of the glycosidic surfactants is that they can be charged in situ through complexation with borate anions, which allows the surfactant charge density to be adjusted to optimize enantioselectivity [204]. 

Several strategies, including metabolic engineering, have been applied to improve the optical purity of LA optimizing enantioselective biosynthesis. For instance, a Lc. lactis mutant strain obtained by UV mutagenesis was able to produce d-LA from molasses and hydrolyzed sugarcane with 73% yield [308], while a recombinant strain of Lb. plantarum NCIMB 8826 produced... 

The formal [4 + 2]-Diels-Alder could also be achieved using nonequivalent a,p-unsaturated aldehydes employing L-proline as an aminocatalyst. Chiral dienes were accessible in good yields (61-82%) and modest to good enan-tioselectivities (41-63%) (Scheme 5.37). Reaction temperatures varied from 25 °C to —25 °C to produce optimal enantioselectivities. Hong et al. also described the formal [3 + 3] and [4 + 2] cycloadditions of ot,p-unsaturated aldehydes, providing access to poly-substituted aromatic products in modest to good yields (Scheme 5.38). ... 

We examined various chiral phosphines (Fig. 6) in the reaction of NBD with several acetylenes (Table 4) under the standard cycloaddition conditions as described previously. S,5-Chiraphos and / -Prophos usually give the best yields and optimize enantioselectivities for most of the tested acetylenes. BPE and Duphos ligands are less selective, and / -BINAP and (+)-DIOP gives no cycloadduct. ... 

A different fit of the two enantiomers into the asymmetric cavities—the key-lock principle—of these polymers effects separation of the antipodes. For optimal enantioselectivity the secondary structure of the chiral spatially fixed matrix is decisive. This type of separation is usually called inclusion chromatography. 

Chiral Cu(II)/bisoxazoline complexes have proven efficient in catalyzing Friedel-Crafts reactions to a number of substrates. Cu(OTf)2/t-Bu-bisoxazoline (13) promotes addition of a variety of electron-rich aromatic compounds (11) to ethyl glyoxylate (12) (Scheme 17.2) [5]. An interesting detail in this work was the importance of triflate as the counterion. When Cu(Sbp6)2 was employed as the Lewis acid, the yield and enantioselectivity was dramatically decreased. Furthermore, the choice of solvent had a significant effect on reactivity and selectivity. CH2CI2 generally led to improved chemical yield, while THF provided optimal enantioselectivities. 

Later, Melchiorre s group employed a similar catalyst system in the P-hydroxylation of (x,P-unsaturated ketones 79 (Scheme 43.17) [28], aziridination of enones (Scheme 43.18) [29], and sulfa-Michael addition of a,p-unsaturated ketones (Scheme 43.19) [30]. High chemical yields and excellent enantioselectivities for various chiral products were obtained, which proved the use of chiral co-catalysts to be necessary for optimal enantioselection. 

---