Chiral method development modifiers

Another application of rapid chiral method development in SFC was presented by Villeneuve and Anderegg [50]. The same columns as in Reference 49 are used, combined with four organic modifiers, including methanol, methanol with 0.1% TEA, ethanol, and isopropanol. Using a six-way column switcher, the four columns can remain constantly inside the device. For some separations, 0.1% DEA or TEA was added. The separations were generated at a flow rate of 2 mL/min, 205 atm of pressure, and a temperature of 40°C. This approach seems applicable, based on the baseline separation that was obtained for the four analyzed compounds, but lacks detailed sequential steps. Therefore, no real strategy can be derived. 

Table 1 Initial Conditions for Chiral Method Development Using Modified Carbon Dioxide as the Mobile Phase...

Table 1 Initial conditions for chiral method development using modified carhon dioxide as the mobile phase.

The basis of chiral method development screening strategies is largely dictated by whether the method is to be applied to analytical or preparative applications. Preparative chromatography tends to use volatile modifiers and mobile phases for easy removal after sample collection. Sample isolation capabilities have led to the advancement of chiral SFC since CO2 is easily driven off. Chiral separations in preparative chromatography only need to be optimized to a point where pure samples can be collected. Conversely, while analytical chromatography can use a greater variety of modifiers and mobile phases, the separation needs to be optimized to a point where suitable resolution (R > 1.5) is achieved between the enantiomers of interest. 

TABLE 7 Commonly Used Modifiers in Chromatographic Chiral Method Development... 

Method development remains the most challenging aspect of chiral chromatographic analysis, and the need for rapid method development is particularly acute in the pharmaceutical industry. To complicate matters, even structurally similar compounds may not be resolved under the same chromatographic conditions, or even on the same CSP. Rapid column equilibration in SFC speeds the column screening process, and automated systems accommodating multiple CSPs and modifiers now permit unattended method optimization in SFC [36]. Because more compounds are likely to be resolved with a single set of parameters in SFC than in LC, the analyst stands a greater chance of success on the first try in SFC [37]. The increased resolution obtained in SFC may also reduce the number of columns that must be evaluated to achieve the desired separation. 

Comparisons of LC and SFC have also been performed on naphthylethylcar-bamoylated-(3-cyclodextrin CSPs. These multimodal CSPs can be used in conjunction with normal phase, reversed phase, and polar organic eluents. Discrete sets of chiral compounds tend to be resolved in each of the three mobile phase modes in LC. As demonstrated by Williams et al., separations obtained in each of the different mobile phase modes in LC could be replicated with a simple CO,-methanol eluent in SFC [54]. Separation of tropicamide enantiomers on a Cyclobond I SN CSP with a modified CO, eluent is illustrated in Fig. 12-4. An aqueous-organic mobile phase was required for enantioresolution of the same compound on the Cyclobond I SN CSP in LC. In this case, SFC offered a means of simplifying method development for the derivatized cyclodextrin CSPs. Higher resolution was also achieved in SFC. 

Oxidation in the presence of chiral titanium tartrate (modified Sharpless method). Inspired by the Sharpless asymmetric epoxidation48 of allylic alcohols with hydroperoxides in the presence of chiral titanium complex [diethyl tartrate (DET) and Ti(0-i-Pr)4], Kagan and co-workers46 and Modena and co-workers47 developed almost at the same time two variations of this reaction leading to o.p. sulfoxides with high enantiomeric purity. 

The first chiral separation using pSFC was published by Caude and co-workers in 1985 [3]. pSFC resembles HPLC. Selectivity in a chromatographic system stems from different interactions of the components of a mixture with the mobile phase and the stationary phase. Characteristics and choice of the stationary phase are described in the method development section. In pSFC, the composition of the mobile phase, especially for chiral separations, is almost always more important than its density for controlling retention and selectivity. Chiral separations are often carried out at T < T-using liquid-modified carbon dioxide. However, a high linear velocity and a low pressure drop typically associated with supercritical fluids are retained with near-critical liquids. Adjusting pressure and temperature can control the density of the subcritical/supercritical mobile phase. Binary or ternary mobile phases are commonly used. Modifiers, such as alcohols, and additives, such as adds and bases, extend the polarity range available to the practitioner. 

Modern polysaccharide columns are based on cellulose or amylose derivatives coated onto silica. Chiral discrimination and applications have been extensively documented, but the mechanism of chiral discrimination is not yet fully understood. Whereas numerous phases are available within this subset, orthogonality can generally be obtained from a set of three or four columns as a first approach to method development. A typical choice of columns would be to try a set of different amylase (Chiralpak AD and AS) and cellulose (Chiralcel OD or OJ) columns and defer more extensive method development to the subset of samples not separated by these columns. The columns specified are run in the normal-phase mode and, accordingly, mobile phases are typically mixtures of hexane with small amounts of isopropanol or ethanol to control retention. However, selectivity is changed by different polar modifiers. Tailing may be minimized by the addition of 10-50 mM trifluoroacetic acid (TFA) or triethylamine (TEA). Analogue of the columns specified (AD-R, AS-R, OD-R, and OD-J) are available for reversed-phase separation. 

Methods development for chiral analyses has been one of the most challenging separation problems for the analytical chemist in the pharmaceutical industry. Racemic drug substances have a variety of chemical structures and several chiral selectors are available for the analyst to choose in order to obtain the enan-tioselectivity needed for chiral resolution. To alleviate this problem, a fast capillary electrophoresis procedure for the enantiomeric separation of acidic and basic compounds using native and modified cyclodextrins has been described (200). The technique is called cyclodextrin array chiral analysis. A generalized optimi-... 

This reaction has been modified to occur under mild conditions such as those of Masamune and Roush, Still, and Ando. In addition, chiral phosphonates (or phospho-namides) have been used for this reaction. In particular, the method developed by Still and Gennari using [bis(trifiuoroethyl)phosphono] esters, generally known as Gennari-Still phosphonates, is very useful in the preparation of Z-olefins. Illustrated is the HWE reaction using Gennari-Still phosphonates to give c/ -olefins, and its mechanism. 

Generally, better chromatographic performance is found with chiral separations in the normal phase for most column manufacturers. It is also likely that the easier solvent removal after collecting the isolated enantiomer, is what drove the industry to normal phase chromatography for chiral applications. It is advantageous to the chiral chromatographer that the majority of the commercially available normal phase LC CSPs and modifiers can be used on both LC and SFC instrumentation. This flexibility allows methods developed using one mode to be transferred to the other... 

Separation of enantiomers by physical or chemical methods requires the use of a chiral material, reagent, or catalyst. Both natural materials, such as polysaccharides and proteins, and solids that have been synthetically modified to incorporate chiral structures have been developed for use in separation of enantiomers by HPLC. The use of a chiral stationary phase makes the interactions between the two enantiomers with the adsorbent nonidentical and thus establishes a different rate of elution through the column. The interactions typically include hydrogen bonding, dipolar interactions, and n-n interactions. These attractive interactions may be disturbed by steric repulsions, and frequently the basis of enantioselectivity is a better steric fit for one of the two enantiomers. ... 

Enders and coworkers <96AG(E)1725> have developed an interesting general one-pot method for the asymmetric epoxidation of enones with oxygen in the presence of diethylzinc and (l ,/ )-N-methylpseudoephedrine (30), which provides a, P - epoxyketones in very high yield and high enantiomeric excess (e.g., 33 —> 34). The actual reactive species is believed to be the chirally modified alkoxy(ethylperoxy)zinc 31, which attacks the si face of the s-cis conformation of the (E) enones (cf. 32). 

Introduction Since we had already developed the novel asymmetric addition of lithium acetylide to ketimine 5, we did not spend any time on investigating any chiral resolution methods for Efavirenz . Our previous method was applied to 41. In the presence of the lithium alkoxide of cinchona alkaloids, the reaction proceeded to afford the desired alcohol 45, as expected, but the enantiomeric excess of 45 was only in the range 50-60%. After screening various readily accessible chiral amino alcohols, it was found that a derivative of ephedrine, (1J ,2S) l-phenyl-2-(l-pyrrolidinyl)propan-l-ol (46), provided the best enantiomeric excess of 45 (as high as 98%) with an excellent yield (vide infra). Prior to the development of asymmetric addition in detail, we had to prepare two additional reagents, the chiral modifier 46 and cyclopropylacetylene (37). 

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