Single isomer cyclodextrins as chiral selectors in capillary electrophoresis

Since decades, cyclodextrins are one of the most powerful selectors in chiral capillary electrophoresis for the enantioseparation of diverse organic compounds. This review concerns papers published over the last decade (from 2009 until nowadays), dealing with the capillary electrophoretic application of single isomer cyclodextrin derivatives in chiral separations. Following a brief overview of their synthetic approaches, the inventory of the neutral, negatively and positively charged (including both permanently ionic and pH-tunable ionizable substituents) and zwitterionic CD derivatives is presented, with insights to underlying structural aspects by NMR spectroscopy and molecular modeling. CE represents an ideal tool to study the weak, non-covalent supramolecular interactions. The published methods are reviewed in the light of enantioselectivity, enantiomer migration order and the fine-tuning of enantiodiscrimination by the substitution pattern of the single entity selector molecules, which is hardly possible for their randomly substituted counterparts. All the reviewed publications herein support that cyclodextrin-based chiral capillary electrophoresis seems to remain a popular choice in pharmaceutical and biomedical analysis.

Enhanced single-isomer separation and pseudoenantiomer resolution of new primary rim heterobifunctionalized α-cyclodextrin derivatives

The synthesis of batch-to-batch reproducible cyclodextrin (CD) derivatives often requires functionalization at specific positions of the CD skeleton. However, the regioselective preparation of this type of CD derivatives remains a challenge in synthetic chemistry. Thus, the present study aimed to prepare all positional regioisomers on the primary rim of homobifunctionalized diazido-α-CDs by selective substitution on the primary rim. Specifically, three positional regioisomers 6^A,6^B-, 6^A,6^C-, and 6^A,6^D-diazido-α-CDs were prepared via different intermediates (using sulfonylation with capping agents, bromination and tosylation). Furthermore, heterobifunctionalized 6^A-azido-6^X-mesitylenesulfonyl-α-CDs were also synthesized, and all regioisomers were successfully separated by HPLC. Moreover, the heterobifunctionalized α-CD regioisomers were isolated in gram-scale quantities, isomers AB and AC in the form of a pseudoenantiomeric mixture. The pseudoenantiomers AC/CA and AB/BA were resolved on an analytical scale by HPLC–MS at 10 °C. Thus, the presented synthetic and analytical methods for homo- and heterodisubstituted α-CDs are efficient and reproducible for the preparation of various pure regioisomeric CD derivatives. Accordingly, our findings indicate, (i) the versatility of selectively modified azido and mesitylene CD skeletons in preparing new types of α-CD derivatives and (ii) the potential of using resolved α-CD pseudoenantiomers in other research fields such as organocatalysis.

Methoxyethylammonium monosubstituted β-cyclodextrin as the chiral selector for enantioseparation in capillary electrophoresis

Methoxyethylamine monosubstituted β-cyclodextrin, mono-6(A)-(2-methoxyethyl-1-ammonium)-6(A)-β-cyclodextrin chloride (MEtAMCD), is synthesized and analytically characterized. Bearing a methoxy group in cyclodextrin rim, MEtAMCD exhibits outstanding enantioselectivities toward ampholytic and acidic racemates in capillary electrophoresis. Driven by inclusion complexation, electrostatic interactions and/or hydrogen bonding, the enantioseparation of MEtAMCD is found to be strongly dependent on various separation parameters including buffer pH, cyclodextrin concentration, applied voltage, separation temperature and organic solvent additives. MEtAMCD demonstrates as a versatile cationic chiral selector for the studied 26 acidic and ampholytic enantiomers.