Chiral derivatization approach

Perez-Fuertes et al.6 8 have suggested the possibility of using the three-component chiral derivatizing approach for prediction of the absolute configuration of amines. Comparison of aH NMR spectra has shown significantly more deshielded signals of (S,S)-imino-boronate esters, derivative of a-amino esters than (S,R) diastereomers. 

The large number and variety of applications of the chiral derivatization approach attest to the success, viability, and importance of the technique. It is expected that despite the predictable advances to be realized in the near future in the development of direct chromatographic— mainly chiral-stationary-phase-based—separations of enantiomers, the indirect approach will continue to be widely used to solve stereochemical problems in the pharmaceutical, pharmacological, and toxicological arenas. 

A study was conducted to measure the concentration of D-fenfluramine HCl (desired product) and L-fenfluramine HCl (enantiomeric impurity) in the final pharmaceutical product, in the possible presence of its isomeric variants (57). Sensitivity, stabiUty, and specificity were enhanced by derivatizing the analyte with 3,5-dinitrophenylisocyanate using a Pirkle chiral recognition approach. Analysis of the caUbration curve data and quaUty assurance samples showed an overall assay precision of 1.78 and 2.52%, for D-fenfluramine HCl and L-fenfluramine, with an overall intra-assay precision of 4.75 and 3.67%, respectively. The minimum quantitation limit was 50 ng/mL, having a minimum signal-to-noise ratio of 10, with relative standard deviations of 2.39 and 3.62% for D-fenfluramine and L-fenfluramine. 

Indirect approaches such as chiral derivatization with chiral derivatizing reagents (CDR) offers a variety of advantages. For instance, CDRs are cheaper than chiral columns. Separation of the product diastereomers is generally more flexible than the corresponding enantiomeric separation because achiral columns can be used in conjunction with various mobile-phase... 

Using the indirect approach to achieve a chiral separation diminishes the need for a CS in the running buffer. Prior to analysis with CE, the enantiomers are derivatized with an optically pure agent to form diastereomers. An achiral environment, in which a pseudo-stationary phase is present, is sufficient to separate these diastereomers because they possess different physicochemical properties. The indirect separation is an efficient and versatile approach mainly because of the availability of numerous chiral derivatization reagents. These derivatizing reagents can contain chromo-phore, fluorophore, or electrochemical groups, which improves the detection of hardly detectable compounds. 

Schulte, M., Chiral derivatization chromatography, in Chiral Separation Techniques A Practical Approach, Subramanian, G, Lid., 2d rev. ed., WUey-VCH, Weinheim, 2001, Chap. 7. 

Most of the reactions applied to amines can also be transferred to alcohols (Eig. 7-5). One large group of chiral alcohols are the (i-adrenoreceptor blockers, for which a variety of derivatization agents was developed. One highly versatile reagent for the separation of (i-blockers is A-[(2-isothiocyanato)cyclohexyl]3,5-dinitrobenzoyl-amide (DDITC) [11]. Alternatively, unichiral drugs such as (3-blockers or (S)-naproxen [12] may be used in a reciprocal approach to derivatize racemic amine compounds. 

An example for this approach is the immobilization of (5 )-(-)-a-A-(2-naph-thyl)leucine, a 7t-donating group on silica. This chiral selector exhibits excellent recognition for 3,5-dinitrobenzoyl (DNB)- and 3,5-dintroanilido (DNAn)-deriva-tives. Amines and alcohols can be derivatized with DNB- or DNAn-chloride to the esters or carbamates and separated on the CSP, as shown by Pirkle for a wide variety of compounds [27]. 

The second approach for the synthesis of 2-amino-3-hydroxycarboxylic acids starts with a chiral isothiocyanate which is added, via the tin enolate, to aldehydes. The initially formed adducts are immediately derivatized to the heterocycles, from which. yj 7-2-amino-3-hy-droxycarboxylic acids result after a three-step procedure. The diastereomeric ratios of the intermediate bis-heterocyclic products range from 93 7 to 99 1 (desired isomer/sum of all others)104. 

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