Chiral compounds screening

C. Chiral Compound Screening Recommendations for Purposeful Degradation Samples... 

New modifiers have traditionally been discovered by the trial-and-error method. Many naturally occurring chiral compounds (the chiral pool38) have been screened as possible modifiers. Thus, the hydrogenation product of the synthetic drug vinpocetine was discovered to be a moderately effective modifier of Pt and Pd for the enantioselective hydrogenation of ethyl pyruvate and isophorone.39 Likewise, ephedrine, emetine, strychnine, brucine, sparteine, various amino acids and hydroxy acids, have been identified as chiral modifiers of heterogeneous catalysts.38... 

Cyclic amines (including local anesthetic drugs) and amides were among the first classes of chiral compounds investigated in the early stages of the application of macrocyclic antibiotics as chiral selectors therefore, they were screened on vancomycin [7], teicoplanin [30], and ristocetin A [33] CSPs, under RPmode systems. Cyclic imides (including barbiturates, piperidine-2,6-diones, and mephenytoin) have been separated on a vancomycin CSP [157], under NP and RP mobile phase conditions. 

The approach to method development is similar to the one described for HPLC and can be characterized as a rapid stationary phase screen using column and solvent switching with gradient elution followed by development of an isocratic preparative method. SFC has been successfully applied to the analytical and preparative separation of achiral and chiral compounds. 

Reetz and coworkers developed a highly efficient method for screening of enantioselectivity of asymmetrically catalyzed reactions of chiral or prochiral substrates using ESI-MS [60]. This method is based on the use of isotopically labeled substrates in the form of pseudo-enantiomers or pseudo-prochiral compounds. Pseudo-enantiomers are chiral compounds which are characterized by different absolute configurations and one of them is isotopically labeled. With these labeled compounds two different stereochemical processes are possible. The first is a kinetic separation of a racemic mixture, the second the asymmetric conversion of prochiral substrates with enantiotopic groups. The conversion can be monitored by measuring the relative amounts of substrates or products by electrospray mass spectrometry. Since only small amounts of sample are required for this method, reactions are easily carried out in microtiter plates. The combination of MS and the use of pseudo-enantiomers can be used for the investigation of different kinds of asymmetric conversion as shown in Fig. 3 [60]. 

Screening of an impressive series of polymers derived from different bulky methacrylate esters, e.g., 42 (Chart 8), and using a variety of chiral ligands has revealed the scope of the process of forming helical poly(methacrylate ester)s and their applicability in, for example, the separation of chiral compounds.151 These polymers were prepared not only by anionic polymerization, but also by cationic, free-radical, and Ziegler—Natta techniques. Recently, Nakano and Okamoto reported the use of a co-balt(II)—salophen complex (43) in the polymerization of methacrylate ester 41.155 The free-radical polymerization in the presence of this optically active metal complex resulted in the formation of an almost completely isotactic polymer with an excess of one helical sense. 

Fructose 1,6 bisphosphate (FBP) is an allosteric activator of the thermostable l-2-hydroxyacid dehydrogenase from B. stearothermophilus, which might be useful for the asymmetric synthesis of chiral compounds. Since FBP is quite expensive, Allen and Holbrook wished to create an FBP-independent variant1208. Three rounds of shuffling and screening produced a mutant L-2-hydroxyacid dehydrogenase with three amino acid substitutions that is almost as active in the absence of FBP as the wild-type is in its presence. 

Each glycopeptide CSP has unique selectivity as well as complementary characteristics, and a considerable number of racemates have been resolved on all three of them. Interestingly, most of the resolved enantiomers have the same retention order on these macrocyclic CSPs. When they are mixed or coupled with each other, the selectivity on one CSP will not be canceled by another. Even if some compounds may not have the same retention order, the complementary effects will result in an identifiable selectivity. Therefore, the coupled chiral columns can be used as a screening tool and save chromatographers substantial time in method development. 

The screening of libraries of compounds for the desired property constitutes an essential part of the combinatorial process. The easier and the faster the screening, the higher the throughput and the more compounds can be screened in a unit of time. This paradigm has led Still s group to develop a combinatorial approach to chiral selectors that involves a visual screening step by optical microscopy that enables the manual selection of the best candidates [81]. 

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