Separations of optical isomers

Chiral Chromatography. Chiral chromatography is used for the analysis of enantiomers, most useful for separations of pharmaceuticals and biochemical compounds (see Biopolymers, analytical techniques). There are several types of chiral stationary phases those that use attractive interactions, metal ligands, inclusion complexes, and protein complexes. The separation of optical isomers has important ramifications, especially in biochemistry and pharmaceutical chemistry, where one form of a compound may be bioactive and the other inactive, inhibitory, or toxic. 

Particular examples for the separation of optical isomers in the (pharmaceutical) industry include prazinquatel [51], 3-blockers [52], chiral epoxide [6],thia-diazin EMD5398 [18] and hetrazipine [7]. The Belgian company UCB Pharma uses a large-scale SMB from NOVASEP to perform optical isomer separation at a scale of several tons per year. Almost all of these separations are performed on cellulose-based stationary phases using organic eluents [4]. 

Other recent applications of suitable secondary chqmical equilibria in the mobile phase include the separation of optical isomers by using opti-... 

In MEKC, mainly anionic surface-active compounds, in particular SDS, are used. SDS and all other anionic surfactants have a net negative charge over a wide range of pH values, and therefore the micelles have a corresponding electrophoretic mobility toward the anode (opposite the direction of electro-osmotic flow). Anionic species do not interact with the negatively charged surface of the capillary, which is favorable in common CZE but especially in ACE. Therefore, SDS is the best-studied tenside in MEKC. Long-chain cationic ammonium species have also been employed for mainly anionic and neutral solutes (16). Bile salts as representatives of anionic surfactants have been used for the analysis of ionic and nonionic compounds and also for the separation of optical isomers (17-19). 

The growing interest in supramolecular chirality stems not only from the intrinsic relevance of such studies for the origin of chirality in life processes, but also from the potential technological applications, such as the separation of optical isomers for the pharmaceutical or food industries. 

B. A bonded stationary phase for the separation of optical isomers has the structure... 

Commercial bonded-phase chelation columns have also been offered for sale. The interest in these separations has risen because the chelation with metals such as Ni and Zn is asymmetric and allows the selective separation of optical isomers, such as amino acids, peptides, proteins, and carbohydrates. 

Actually, coordination complexes of different metal salts of DBTA with hydroxycarboxylic acid esters, hydroxycarboxylic acids and alcohols as well as host-guest complexes of DBTA with chiral phosphine oxides and racemic alcohols can be prepared and used for separation of optical isomers. In the next subchapters theoretical and practical aspects of these recent resolution processes are summarised. 

Chemically bonded phases may also be tailored to a specific separation problem. A case in point is the synthesis of chiral stationary phases for the separation of optical isomers. Another application of polar bonded phases, which is beyond the scope of this book,... 

Owing to the different biological activity of D- and L-enantiomers of seleno-amino acids, the chiral separation of optical isomers has been undertaken in sele-nized yeast and in yeast-based commercial supplements. Both, chiral stationary phase (crown ether) and chiral derivatization prior to reversed-phase HPLC were used [16, 77, 78],... 

It seems tc, also be worth mentioning that the described procedure has been used for micro-preparative separations of mephenytoin and hexobarbital enantiomers (26) p -CD solutions were also successfully used for resolution of 1-[2-(3-hydroxyphenyl)-l-phenylethylJ-4-(3-me-thyl-2-buteny1) piperazine enantiomers in RP systems (.20). An especially interesting example of the application of -CD is the separation of optical isomers of D,L - norgestrel (27). 

Methods for the capillary gas chromatographic separation of optical isomers of chiral compounds after formation of diastereoisomeric derivatives were developed. Analytical aspects of the GC-separation of diastereoisomeric esters and urethanes derived from chiral secondary alcohols, 2-, 3-, 4- and 5-hydroxy-acid esters, and the corresponding jf- and -lactones were investigated. The methods were used to follow the formation of optically active compounds during microbiological processes, such as reduction of keto-precursors and asymmetric hydrolysis of racemic acetates on a micro-scale. The enantiomeric composition of chiral aroma constituents in tropical fruits, such as passion fruit, mango and pineapple, was determined and possible pathways for their biosynthesis were formulated. 

The first step in method development is selecting an adequate HPLC mode for the particular sample. This choice depends on the character of the sample compounds, which can be either neutral (hydrophilic or lipophilic) or ionic, low-molecular (up to 2000 Da) or macromolecular (biopolymers or synthetic polymers). Many neutral compounds can be separated either by reversed-phase or by normal-phase chromatography, but a reversed-phase system without ionic additives to the aqueous-organic mobile phase is usually the best first choice. Strongly lipophilic samples often can be separated either by non-aqueous reversed-pha.se chromatography or by normal-phase chromatography. Positional isomers are usually better separated by normal-phase than by reversed-phase chromatography and the separation of optical isomers (enantiomers) requires either special chiral columns or addition of a chiral selector to the mobile phase. 

Werner s synthesis and separation of optical isomers proved the octahedral shape conclusively, because none of the other six-coordinate geometries could have similar optical activity. 

The immohilization of the cage complexes on the surface through apical groups offers interesting application possibilities. This approach enables one to obtain ion-exchange resins especially selective for metal ions. The immobilized optically active cations allow one to obtain ion-exchange resins for the separation of optical isomers, such as racemic amino acids or optically active complexes. 

S. Fanali, Separation of optical isomers by capillary zone electrophoresis based on host-guest complexation with cyclodextrins,/. Chromatogr. 474 441 (1989). 

Szepesi et al. reported an ion-pair separation of eburnane alkaloids on a chemically bonded cyanopropyl stationary phase. As counter-ion, di-(2-ethyl hexyl)phosphoric acid or (+)-10-camphorsulfonic acid were used in a mobile phase consisting of hexane - chloroform -acetonitrile mixtures (Table 8.8, 8.9). Because of the poor solubility of the latter pairing ion, diethylamine (Table 8.9) was added to the mobile phase. Addition of diethylamine considerably reduced the k1 of the alkaloids, due to suppression of the ionization of the alkaloids. However, due to the strong acidic character of the pairing ion, ion-pairs were still formed under these conditions. The camphorsulfonic acid containing mobile phases were found to be very useful for the separation of optical isomers (Table 8.10, 8.11, Fig.8.8) 6. It was also found that the selectivity of the system could be altered by choosing different medium-polarity solvents (moderator solvents) as dioxane, chloroform or tetrahydrofuran. The polar component of the solvent system affected peak shape. Based on these observations, a method was developed to analyze the optical purity of vincamine and vinpocetine. For the ana-... 

For the ferric siderophore complexes, comparison of the CD spectra of the chromium complexes of ferrichrome and enterobactin with the CD spectra of their iron complexes [and the separation of optical isomers of even ferric(benzhydroxamate)3 complexes in nonaqueous solution 192)] have shown that the same rule applied to the CD spectra for chromium complexes can be used for iron siderophore complexes as well iron(III) complexes will have a predominant A configuration in solution if the CD band in... 

This special case illustrates the power of continuous cooling processes with massive amounts of seed to reject impurities that have the potential to crystallize at equilibrium. Batch cooling to achieve this separation of optical isomers is not a practical alternative because the resolution is not based on equilibrium solubility. The time required for batch cooling would result in the nucleation of the undesired isomer when any practical amount of product is to be harvested in each cycle. 

The successful application of enantioselective chromatography as a valuable approach to the separation of optical isomers on a preparative and even production scale has attracted the attention of most pharmaceutical companies. 

However, the real potential of enantioselective chromatography for the preparative separation of optical isomers was definitely established in 1973 by Hesse and Hagel who introduced fully acetylated cellulose (triacetylcellulose) as a new efficient chiral CSP [14]. They successfully achieved the preparative separation of the enantiomers of various chiral compounds. For many years, triacetylcellulose was practically the only chiral stationary phase available for preparative separations and it has been used for the chromatographic resolution of a broad variety of chiral molecules [1-3, 15, 16]. 

Resolution (or separation) of optical isomers was achieved by SP-Sephadex column chromatography using 0.02M Na2[Sb2(L-tart)2] For the N-C-C-P and the P-C-C-P complexes, the bands of optical isomers were separated incompletely, so that the chromatography was repeated until no further increase in Ae/e values was observed. From the eluates, the optical isomers were isolated by a method similar to that for the racemates. 

Chiral columns are used routinely for the analytical separation of optical isomers.20 Typical columns use tribenzoates, or tris(phenyl carbamates), of cellulose or amylose as well as cyclodextrins. Flash chiral chromatography has... 

Because of their kinetic inertness, Pt11 complexes of the type PtCl2L2 allow separation of optical isomers of olefins, allenes, tertiary phosphines or arsines. The complexes are optically stable at 25° and the enantiomorphic forms can be separated by suitable manipulations.26... 

Of particular interest is the use of esters of a-oxyphosphonic acid for the analysis of trace amounts of carboxylic acids using a selective phosphorus detector [353]. TMS esters are also used in chromatographic analysis [254,354-366]. Interesting methods of analysing keto-acids have been described [367-372] and the separation of optical isomers has been studied [199,373,374]. 

Armstrong, D.W. Han, S.M. Han, Y.I. Separation of optical isomers of scopolamine, cocaine, homatro-pine, and atropine. Anal.Biochem., 1987, 167, 261-264... 

---