Chiral HPLC separation

The type of CSPs used have to fulfil the same requirements (resistance, loadabil-ity) as do classical chiral HPLC separations at preparative level [99], although different particle size silica supports are sometimes needed [10]. Again, to date the polysaccharide-derived CSPs have been the most studied in SMB systems, and a large number of racemic compounds have been successfully resolved in this way [95-98, 100-108]. Nevertheless, some applications can also be found with CSPs derived from polyacrylamides [11], Pirkle-type chiral selectors [10] and cyclodextrin derivatives [109]. A system to evaporate the collected fractions and to recover and recycle solvent is sometimes coupled to the SMB. In this context the application of the technique to gas can be advantageous in some cases because this part of the process can be omitted [109]. 

The above sequence was demonstrated on racemic onti-mercaptol alcohol 14 as well on a small amount of optically pure 14 (separated by chiral HPLC separation) and the chiral centers of 14 were completely retained, as expected (Scheme 5.6) [8]. With proof of concept for the ring formation strategy, some efforts were put into developing a chiral synthesis of 14, as shown in Scheme 5.7. 

With this encouraging result from the model system, a gram quantity of the racemic sulfoxide 40 was prepared by oxidation of benzoxathiin 16 with mCPBA and a small amount of chiral sulfoxide (A)-40 with 94% ee was isolated by subsequent chiral HPLC separation (Scheme 5.12). When chiral sulfoxide (S)-40 was treated with borane-dimethylsulfide, a clean reduction of the olefin and the sulfoxide was observed. More surprisingly, only the desired cis-diaryl dihydrobenzoxathiin 12 was observed in high yield and unchanged 94% ee. No trans-isomer or 16 was observed. With this proof of concept in hand, an efficient... 

Most importantly, the inefficiency of the chiral HPLC separation of the enantiomers in the penultimate step was detrimental to the speed and throughput required for the project. 

The initial route to taranabant relied on a late stage amide bond coupling between racemic amine rac-2 and pyridine acid 3 mediated by (benzotriazol-l-yloxy)tripyr-rolidinophosphonium hexafluorophosphate (Py-Bop), followed by chiral HPLC separation of the product to afford a single enantiomer (Scheme 9.1). 

V. PRACTICAL GUIDELINES TO CHIRAL HPLC SEPARATIONS OF PHARMACEUTICALS... 

Based on the high peak capacity of CE, the separation speed, and the availability of numerous chiral selectors and the simplicity of the systems, chiral CE is superior to chiral HPLC separations. This is as well reflected by the high number of publications on chiral CE in recent years. Chiral HPLC is suffering from low peak capacity (broad peaks), system stability (often normal phase systems), pressure sensitivity of columns (often cellulose-based column materials), and as a consequence long separation times. 

The analysis time for chiral HPLC separations will probably remain relatively long until CSPs with higher efficiency than the present ones become available. But monolithic columns, columns with a smaller particle size (i.e., UPLC ), and miniaturized systems would increase the efficiency and speed up the enantioseparation of existing types of CSPs. 

Caccamese, S., Manna, L., and Scivoli, G., Chiral HPLC separation and CD spectra of the C-2 diastereomers of naringin in grapefruit during maturation. Chirality, 15, 661, 2003. 

In summary, therefore, after on-line chiral HPLC separation, NMR spectroscopy has been used to characterise compounds in terms of the cis- and trans-isomers and to identify the racemic pairs on the basis of their identical NMR spectra. In addition, HPLC-circular dichroism (HPLC-CD) was used to identify... 

Martens-Lobenhoffer et al. [119] used chiral HPLC-atmospheric pressure photoionization tandem mass-spectrometric method for the enantio-selective quantification of omeprazole and its main metabolites in human serum. The method features solid-phase separation, normal phase chiral HPLC separation, and atmospheric pressure photoionization tandem mass spectrometry. The internal standards serve stable isotope labeled omeprazole and 5-hydroxy omeprazole. The HPLC part consists of Agilent 1100 system comprising a binary pump, an autosampler, a thermo-stated column component, and a diode array UV-VIS detector. The enantioselective chromatographic separation took place on a ReproSil Chiral-CA 5 ym 25 cm x 2 mm column, protected by a security guard system, equipped with a 4 mm x 2-mm silica filter insert. The analytes were detected by a Thermo Scientific TSQ Discovery Max triple quadrupole mass spectrometer, equipped with an APPI ion source with a... 

Bonato and Paias [136] developed two sensitive and simple assay procedures based on HPLC and capillary electrophoresis for the enantio-selective analysis of omeprazole in pharmaceutical formulations. Racemic omeprazole and (S)-omeprazole were extracted from commercially available tablets using methanol-sodium hydroxide 2.5 mol/1 (90 10). Chiral HPLC separation of omeprazole was obtained on a ChiralPak AD column using hexane-ethanol (40 60) as the mobile phase and detection at 302 nm. The resolution of omeprazole enantiomers by capillary electrophoresis was carried out using 3% sulfated /1-cyclodextrin in 20 mmol/1 phosphate buffer, pH 4 and detection at 202 nm. 

Separation of optically active isomers is one of the most important areas of HPLC apphcation in the pharmaceutical industry. Since most of biological systems are predominantly homochiral, different enantiomers of the same drug could have different effect and potency, and the development of enan-tioselective analytical (and preparative) separation methods is very important. Detailed description of chiral HPLC separation is given in Chapter 22 of this book here we only briefly review the specifics of distinctive types of chiral stationary phases (CSP). 

Some examples of chiral HPLC separations of racemic drugs are the following. Typical chromatograms of the simultaneous determination of isopyramide and its active metabolite, mono-N-dealkyldisopyramide, in drug-free human plasma, human plasma spiked with dis-opyramide and mono-N-dealkyldisopyramide, and treated subject plasma are presented in Fig. 

A similar type of procedure is shown in Scheme 2 for the isolation of glucopiericidinols from a Streptomyces sp. fermentation, the main difference being that this extraction incorporates chromatography on Sephadex LH20. This purification is also notable in that the final stage involved a chiral HPLC separation of two diastereoisomers (4). 

The use of chiral HPLC (separative method) in tandem with a chiraUty detector (chirality assessment) presents a decisive advantage in the determination of absolute configuration of a series of l-(thi)oxothiazolinyl-3-(thi)oxothiazolinyl toluene atropisomers by the chemical transformation method. Such a correlation method could be performed on a mixture of a very limited quantity of compounds, without the tedious purification steps that are normally required in the classical chemical correlation method (02CHI665). 

J. Hermansson et al.. Chiral HPLC separations of vinca alkaloid analogues on al-acid glycoprotein and human serum albumin columns, J. Chromato., 609(1992)163. 

Figure 6 Chiral HPLC separation of amino acid derivatives on imprinted stationary phases packed with Z-L-Glu-OH (a), Boc-L-phe-Gly-OEt (b), Z-L-Ala-L-Ala-Ome (c) Z-L-Ala-Gly-L-Phe-OMe (d). Mobile phase chloroform - acetic acid. Column 250 x 4.6 mm. Flow rate, 1 mL/min. Detection, UV 260 nm. Reproduced from Ref. 22, with permission.

Figure 7 Chiral HPLC separation of 2-arylpropionic acid derivatives on nonimprinted (a) and (5)-naproxen-imprinted stationary phase (b). (1) Racemic ketoprofen, (2) racemic ibu-profen, (3) (R)-naproxen, (4) (5)-naproxen. Mobile phase, 20 mM phosphate buffer pH 3.2 -acetonitrile 1 + lv/v. Columns 100 x 4.6 mm. Flow rate, ImL/min. Detection, UV 254 nm. Reproduced from Ref. 45, with permission.

Figure 8 Chiral HPLC separation of a equimolar mixture of (-)-cinchonidine (1) and (+)-cinchonidine (2) on a (—)-cinchonidine-imprinted stationary phase. Mobile phase, methanol -acetic acid 7 + 3 v/v. Columns 150 x 4.6 mm. Flow rate, 0.5mL/min. Detection, UV 280 nm. Reproduced from Ref. 48, with permission.

The original discovery synthesis of the P2 domain of telaprevir utilized bicycloproline derivative 56 (Scheme 10),"° which was prepared in racemic form using a four-step, two-pot protocol starting from 2-cyclopentenone, as described by Monn and Valli." In this approach, enantiomerically pure 56 was obtained via chiral HPLC separation." Reduction of the ketone of 56 produced secondary alcohol 57, which was further reduced to 58 under Barton-McCombie deoxygenation conditions. The synthesis of P2 fragment 59 was completed by hydrogenolysis of the benzyl carbamate. 

The team carried an extensive screening process to determine the optimum conditions for a classical resolution of the racemate 29, and thus avoid the need for a chiral HPLC separation. Accordingly, the amine was treated with dibenzoyl-L-tartaric acid as the resolving chiral acid. Eventually, the desired isomer (R)-29 was obtained in a 74% yield with an ee of 96% by treating with a 4 1 mixture of isopropyl acetate/methanol (25 vol.) and aging for 16 h at room temperature. This ee proved to be sufficient to prepare the desired API at an acceptable quality (as determined by evaluating the downstream chemistry). 

Ethanesulfonic acid showed a beneficial effect on the chiral HPLC separation of basic compounds (45). Lfsing a single chiral column and a starting mobile phase, more than half of a diverse set of amines was baseline separated. Changing the alcohol content and the alcohol type increased the success rate. 

A unique example of a planar chiral catalyst displaying an imine as reactive function was reported by Kunz et al. [29] in 2007 (Scheme 8.10). Readily accessible pseudo-gem aldehyde-methyl ester Pc [30] in racemic form was condensed with fully protected galacto-pyranosylamine to give diastereoisomeric imines with respect to the Pc chirality. HPLC separation delivered the (Rp) stereoisomer in pure form, which showed a remarkable ability to perform highly enantioselective Strecker reactions (up to 99% ee). It is assumed that the imine catalyst acts as a Br0nsted base toward hydrocyanuric acid produced in situ and generates an iminium-cyanide pair that can interact with the imine substrate and direct cyanide addition on a unique face of the latter. 

Chiral HPLC separation techniques have some significant advantages over classical resolution methods ... 

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