Stereoselective separations of chiral phosphinic acid pseudodipeptides by CEC using silica monoliths modified with an anion‐exchange‐type chiral selector

Abstract

A nonaqueous CEC method for the simultaneous separation of the four stereoisomers of the N-benzyloxycarbonyl phosphinic pseudodipeptide methyl ester benzyloxycarbonyl-homophenylalanine Z-hPhepsi(PO2HCH2)Phe-OCH3 as well as of the corresponding N-2,4-dinitrophenyl (DNP)-derivative with free C-terminal carboxylic group DNP-hPhepsi(PO2HCH2)Phe-OH was developed. For this purpose, a monolithic silica capillary column modified with a cinchona alkaloid-derived anion-exchange-type chiral selector, namely O-9-(tert-butylcarbamoyl)quinidine (tBuCQD) was prepared. The mobile phase composition (ACN/methanol ratio, counterion type) was thoroughly optimized to end up with baseline resolution of all four stereoisomers with critical resolution of as high as about 2. The CEC method proved to be superior over the corresponding HPLC separations primarily due to significantly enhanced plate numbers (between 200,000 and 600,000 m(-1) in CEC). Diastereoselectivity contributions arising from electrophoretic mobility differences of the diastereomers facilitated the separation of the later eluted diastereomeric peak pair (peaks III and IV), but had a negative influence on the selectivity of the earlier eluted diastereomeric peak pair (peaks I and II). The stereoselective CEC assay allowed the assessment of the stereoisomeric purity of the individual isomers which were obtained by preparative HPLC on a CHIRALPAK QD-AX column that is based on the same tBuCQD selector. The present study demonstrates that there exist problems which are hard to solve by HPLC, yet can be conveniently solved by CEC. Moreover, it was intended to prove by this practical application that CEC with monolithic columns is robust enough to be used for solving real-life problems.

Stereoselective Synthesis of α-Amino-C-phosphinic Acids and Derivatives

“…Semi-preparative scale chromatography supplied single enantiomers in 100 mg quantities allowing the biological activity assays. In a related work, Lämmerhofer et al [61] developed a capillary electrochromatography (CEC) method for the separation of the stereoisomers of N-Cbz phosphinic pseudodipeptide methyl ester 37a and 38a and its N-DNP derivative with free C-terminal carboxylic group 37b and 38b, using a monolithic silica capillary column modified with a cinchona alkaloid-derived anion-exchange-type chiral selector 39 (Scheme 22). This method proved superiority compared to the HPLC separation, which was attributed to significantly enhanced plate numbers.…”

Recent accomplishments in the field of enantiomer separation by CEC

“…19a) as well as N-2,4-dinitrophenyl-(DNP-) derivatives (Fig. 19d) [87]. Due to the combination of chromatographic enantioselectivity (Fig.…”

“…On-column modification of a silica monolith prepared by a sol-gel process in a 100 mm id capillary (non-commercial Chromolith Si CapRod from Merck) by activation with 3-mercaptopropyl trimethoxysilane (3% v/v solution in toluene) in flow-through mode (3 h with 2 mL/min) and subsequent covalent bonding of vinyl group-containing selectors through a radical addition reaction was pursued as strategy by Preinerstorfer et al [86] to obtain functionalized cation-exchange and anion-exchange type [87] enantioselective monolithic capillary columns. The cation-exchange monolith column resulted from radical addition linkage of (S)-N-(4-allyloxy-3,5-dichlorobenzoyl)-2-amino-3,3-dimethylbutane phosphonic acid as SO onto the active sulfydryl surface [86].…”

Open-tubular Electrochromatographic Chiral Separation of Amino Acids Using an Organic Nanocrystals Immobilized Capillary

“…Preinerstorfer et al have reported the preparation of a monolithic silica capillary column modified with cinchona alkaloid derivatives to separate dipeptides in non-aqueous CEC. 12 In spite of high separation efficiencies, the monolithic capillary required labor-intensive and time-consuming synthetic processes for derivatization of the native cinchona alkaloid to immobilize onto the monolithic structure surfaces. So far, only this report has appeared to utilize the cinchona alkaloids as the chiral selector in CEC.…”