Racemates 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... 

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). 

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... 

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. 

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. 

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.

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). 

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. 

The structure of a natural product is shown without any specification of stereochem-istiy. It is a pure substance which gives no indication of being a mixture of stereoisomers and has zero optical rotation. It is not a racemic mixture because it does not yield separate peaks on a chiral HPLC column. When the material is completely hydrolyzed, it gives a racemic sample of the product shown. Deduce the complete stereochemical structure of the natural product fiom this information. 

Complexes of unsymmetrically substituted conjugated dienes are chiral. Racemic planar chiral complexes are separated into their enantiomers 84 and 85 by chiral HPLC on commercially available /f-cyclodextrin columns and used for enantioseletive synthesis [25]. Kinetic resolution was observed during the reaction of the meso-type complex 86 with the optically pure allylboronate 87 [26], The (2R) isomer reacted much faster with 87 to give the diastereomer 88 with 98% ee. The complex 88 was converted to 89 by the reaction of meldrum acid. Stereoselective Michael addition of vinylmagnesium bromide to 89 from the opposite side of the coordinated Fe afforded 90, which was converted to 91 by acetylation of the 8-OH group and displacement with EtjAl. Finally, asymmetric synthesis of the partial structure 92 of ikarugamycin was achieved [27],... 

K. M. Kirkland, K. L. Neilson, D. A. Me Combs, and J. J. DeStef-ano, Optimized HPLC separations of racemic drugs using ovomucoid protein-based chiral column, LC-GC, 10 322 (1992). 

It has already been reported (see Section IV.A.3.b) that the two enantiomers of a racemic C-silyl aminoacetic ester have been easily separated by chiral HPLC.123 Various chiral SMAs have been prepared using classical methods. SMAs where the chiral center is not part of their framework are not considered here. 

The use of chiral auxiliaries to induce enantioselectivity was unsuccessful, as was attempted racemic separation via chiral HPLC stationary phase. It should be noted, however, that this is an excellent example of the utility of the convergent process coupled with controlled deprotection of a core unit. 

In a typical reaction, a solution of the antibody in phosphate buffered saline (PBS) is added to a solution of the racemic substrate (ca. 50-100 mM) in either toluene or chlorobenzene. The mixture is shaken, while the substrate ee is monitored by chiral HPLC. When the desired ee is reached, the reaction mixture is cooled (—20°C), allowing easy separation of the organic layer from the frozen aqueous antibody solution. The aldol product is purified by column chromatography, and the antibody solution is thawed for reuse. 

In order to install a benzophenone at the bicyclic scaffold we relied on the previously used oxazole linkage. To this end, the known amino-hydroxybenzophenone 37 (Aichaoui et al. 1990) was coupled to the free acid rac-38, which is available from Kemp s triacid in five synthetic steps. Remarkably, an 0-aeylation instead of the expected /V-acylalion was observed resulting in ester rac-39. As a consequence, oxazole formation was less straightforward but could eventually be achieved under more forceful conditions. The reaction sequence led to the racemic benzophenone rac-40, i.e. to a 1/1 mixture of the enantiomers (+)-40 and (-)-40, which was separated by chiral HPLC (Daicel Chiralpak AD). It is important to mention that a separation of enantiomers at an earlier stage is not sensible. While carboxylic acid 38 can be obtained in enantiomerically pure form, racemisation occurs upon activation, presumably due to a bridged symmetrical intermediate (Kirby et al. 1998) (Scheme 16). 

Finally, libraries aimed to chiral resolution of racemates will be covered here in particular, the use of chiral stationary phases (CSPs) has recently been reported for the identification of materials to be used for chiral separation of racemates by HPLC. The group of Frechet reported the selection of two macroporous poly methacrylate-supported 4-aryl-1,4-dihydropyrimidines (DHPs) as CSPs for the separation of amino acid, anti-inflammatory drugs, and DHP racemates from an 140-member discrete DHP library (214,215) as well as a deconvolutive approach for the identification of the best selector phase from a 36-member pool library of macroporous polymethacrylate-grafted amino acid anilides (216,217). Welch and co-workers (218,219) reported the selection of the best CSP for the separation of a racemic amino acid amide from a 50-member discrete dipeptide iV-3,5-dinitrobenzoyl amide hbrary and the follow-up, focused 71-member library (220). Wang and Li (221) reported the synthesis and the Circular Dichroism- (CD) based screening of a 16-member library of CSPs for the HPLC resolution of a leucine ester. Welch et al. recentiy reviewed the field of combinatorial libraries for the discovery of novel CSPs (222). Dyer et al. (223) reported an automated synthetic and screening procedure based on Differential Scanning Calorimetry (DSC) for the selection of chiral diastereomeric salts to resolve racemic mixtures by crystallization. Clark Still rejxrrted another example which is discussed in detail in Section 9.5.4. 

The utilization of an optically active triazolinedione for asymmetric transfer has also led to the preparation of enantiomerically pure polycyclic hydrocarbons. The method provides a straightforward means for introducing optical activity into chiral propellanes that possess a conjugated diene unit. Racemic propellane reacts with (-)-enethyl acetate at —78 °C to give, after HPLC separation, the two optically pure urazoles. Subsequent reduction with lithium aluminum hydride affords the two propellanes in enantiomerically pure form (eq 3). 

---