Preparative Enantioselective HPLC

The factors influencing preparative enantiomer separations are quite similar to those of analytical chromatography. Number of plates, retention factors, and selectivity all influence resolution (see Section 1.1.3). 

In addition, particle size and column length influence the efficiency and speed of enantiomer separations. For preparative applications, it is recommended that one uses particle sizes of 16-20 pm. Spherical stationary phases of these particle 

Where possible, preparative enantiomer separations are performed under overload of the column, i.e. no baseline separation is attempted [28], In practice, one often observes that the second eluting enantiomer is obtained with a lower ee than the first eluting enantiomer. Therefore, it is advantageous if a separation system can be found in which the target enantiomer elutes first. 

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E. Francotte, History and future of preparative enantioselective HPLC, Lecture presented at the 15 International Symposium on Chirality ISCD 15, 2003, Shizuoka (Japan). 

Adsorption isotherms and Henry constants are only really of significance in preparative enantioselective HPLC and simulated moving bed (SMB) applications. 

The applicability of cinchonan carbamate CSPs for bioanalytical investigations using HPLC-ESI-MS/MS has been demonstrated by Fakt et al. [120]. The goal was the stereoselective bioanalysis of (R)-3-amino-2-fluoropropylphosphinic acid, a y-aminobutyric acid (GABA) receptor agonist, in blood plasma in order to determine whether this active enantiomer is in vivo converted to the 5-enantiomer. In this enantioselective HPLC-MS/MS bioassay, sample preparation consisted of... 

We then attempted purification of impure (.S )-144 by enantioselective HPLC. Fortunately, TBS derivative G was found to be separable by preparative HPLC on Chiralcel OD to give pure (S)-G. Deprotection of the TBS group of (S )-G under conventional conditions with TBAF caused partial racemization of (S)-144. However, treatment of (S )-G with dilute ethanolic hydrochloric acid at room temperature caused no appreciable racemization to give (.S )-144 (98.4% ee), [ah24 = -90.7 (acetone), in 40% yield. Similarly, (R)-144 was also synthesized by employing AD-mix-a instead of AD-mix-f) . (ft)-Cytosporone E (144 ), [a]o25 = +91.3 (acetone) could be obtained pure (>99% ee). As shown in the present case, use of preparative HPLC is becoming more and more important in the preparation of pure enantiomers. 

Enantioselective HPLC has evolved in recent years to a routine method carried out on analytical, preparative, and industrial scale. The optimization of separations... 

We further synthesized unsymmetrical MiniPHOS derivatives 13b (Scheme 13) [30]. Thus, enantioselective deprotonation of l-adamantyl(dimethyl)phos-phine-borane (74, R = 1 -Ad), followed by treatment with ferf-butyldichlorophos-phine or 1-adamantyldichlorophosphine, methylmagnesium bromide and bo-rane-THF complex afforded the optically active diphosphine-boranes 82b as a mixture with the corresponding raeso-diastereomer. Enantiomerically pure unsymmetrical MiniPHOS-boranes 82b were obtained by column chromatography on silica gel or separation by recycling preparative HPLC. 

Alcaro, S. et al., Enantioselective semi-preparative HPLC of two 2-arylpropionic acids on glycopeptides containing chiral stationary phases, Tetrahedron Asymmetry, 13, 69, 2002. 

Preparation of various enantiomerically pure sulfoxides by oxidation of sulfides seems feasible in the cases where asymmetric synthesis occurs with ee s in the range of 90% giving crystalline products which can usually be recrystallized up to 100% ee. Aryl methyl sulfides usually give excellent enantioselectivity during oxidation and are good candidates for the present procedure. For example, we have shown on a 10-mmol scale that optically pure (S)-(-)-methyl phenyl sulfoxide [a]p -146 (acetone, o 1) could be obtained in 76% yield after oxidation with cumene hydroperoxide followed by flash chromatographic purification on silica gel and recrystallizations at low temperature in a mixed solvent (ether-pentane). Similarly (S)-(-)-methyl o-methoxyphenyl sulfoxide, [a]p -339 (acetone, o 1.5 100% ee measured by HPLC), was obtained in 80% yield by recrystallizations from hexane. 

In order to provide a better estimate of the enantioselectivity of the catalyst, we prepared an authentic sample of (+)-chorismate by kinetic resolution of the racemate with 1F7 (37). Circular dichroism spectroscopy confirmed the identity and high optical purity of the recovered, HPLC-purified compound. Initial rate measurements with the individual isomers show that (-)-chorismate is favored over (+)-chorismate by the antibody by a factor of at least 90 to 1 at low substrate concentrations. The slight rate enhancements above background observed for the (+)-isomer may be due to general medium effects rather than interaction with a specific locus on the antibody surface. To test this possibility we are currently examining the ability of the transition state analog 3 to inhibit rearrangement of this optical isomer. 

The double dynamic system comprised 24 chiral aminonitrile intermediates, carrying different substituents on the aromatic moiety and the /V-position, and was subsequently challenged by the lipase-mediated amidation resolution process. Not only H-XMR spectroscopy was used to follow the reactions HPLC, through the integration of preparative Zorbax and chiral OD-H columns, was used to confirm the product formations and to analyze the enantioselectivities. According to both analyses, the results indicated that only /V-methyl aminonitriles, produced from methylamine A, were transformed by the lipase resolution process. The major product was found to be amide 33A, derived from its corresponding intermediate 32A. The other products were amides 36A and 30A. 

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