Chiral Separations in Nonaqueous Media - Chiral Separations by Capillary Electrophoresis - page 292

interactions hard to evaluate. However, it is now possible to perform a selective syn-
thesis which results in only one isomer of the CD derivative [82].
The use of charged derivatives is more limited in NACE than in aqueous systems,
where one selector is often able to enantioseparate molecules regardless of their
charge (positive, negative, or neural). In NACE systems, charged CD derivatives
have been used primarily to separate solutes of opposite charge, e.g., sulfated ones have
been used for cationic analytes and ammonium substituted ones for anions [7].
It has been suggested that the ionic-interaction compensates for the weakened inclu-
sion complexation in the nonaqueous solvent. As mentioned above, the limited
solubility (especially in alcohols) of the native CD and CD derivatives is one of the limi-
tations with these selectors in NACE [83,84]. But the sulfated single CD-isomers
have proved to be more soluble than the randomly substituted sulfated ones [11].
The single CD-isomers of heptakis(2,3-diacetyl-6-sulfato)- β -cyclodextrin (HDAS-

β -CD), heptakis(2,3-methyl-6-sulfato)- β -cyclodextrin (HDMS- β -CD), octakis

(2,3-diacetyl-6-sulfato)- γ -cyclodextrin (ODAS- γ -CD), and octakis(2,3- O -dimethyl-

6- O -sulfo)- -CD) have a moderate solubility in, e.g., MeOH

and have been utilized as selectors in this solvent [17,81,84-89]. However, the above
mentioned single isomers, and the majority of the other single isomers that have been
introduced are insoluble in ACN [90]. Sanchez-Vindas and Vigh [90] found that the low
solubility of HDAS-

γ -cyclodextrin (ODMS- γ

-CD in this solvent could be improved by the replacement of the
sodium counterion by a more hydrophobic one, and applied the single isomer sulfated
β

β

-CD as its tetrabutylammonium salt. They used the chiral selector (TBA 7 HDAS-

β -CD) in an ACN-based BGE for the enantioseparation of 13 basic compounds.

The choice of solvent and electrolytes is of great importance for the enantio-
separation in the CD systems. The inl uence of the solvent has already been men-
tioned above, and is primarily addressed at their hydrophobicity, which inl uences
the host-guest complexation. Formamide often exhibits a better environment for
the enantiomeric separation with the CDs than, e.g., MeOH which indicated that the
solvophobic interaction between the CD and the solute is of major importance for the
chiral recognition. Unless the major interactions in host-guest complexation are of
hydrophobic character, the choice of electrolytes might also inl uence the separation.
Typical BGEs for chiral separations with CDs as the chiral selectors are presented
in Table 10.5. In comparison with the ion-pair selectors, these systems more often
contain some kind of buffer, even the in water-based system commonly used phos-
phate buffer is employed in some of the systems [17,84,85,87,88]. Some examples
of enantioresolution in the CD systems are included in the same table, and are in
the range from 0.9 to 29.2, where the highest value is obtained for bupropion using
ODAS- -CD as the chiral selector.

In Figure 10.9, a 1,3,4-thiadiazine derivative is enantioseparated by use of the
chiral selector hydroxypropyl-

γ

-CD) [16]. The importance of
the choice of the electrolyte can be illustrated by the exchange of ammonium acetate/
acetic acid (Figure 10.9A) for citric acid/TRIS in the BGE (Figure 10.9B). In addi-
tion, the researchers found that MeOH was superior as a solvent for separations with
HP-

β -cyclodextrin (HP- β

β -CD and with hydroxyethyl- β -cyclodextrin (HE- β -CD), but that FA was the

solvent of choice for methyl- -CD). The inl uence of sodium,

potassium, ammonium, chloride, formate, methaneformate, and camphorsulfonate

β -cyclodextrin (Me- β