HPLC enantioseparation with cellulose tris(3,5-dichlorophenylcarbamate) in aqueous methanol as a mobile phase

Article abstract of DOI:10.1246/cl.2000.352

The appropriate design of mobile and stationary phase combinations allowed the use of cellulose tris(3,5-dichlorophenylcarbamate) (CDCPC) as the chiral stationary phase (CSP) in high-performance liquid chromatography (HPLC). Together with previous data obtained in n-hexane/2-propanol as a mobile phase the present study indicates very high chiral resolving ability of CDCPC.

Full text of DOI:10.1246/cl.2000.352

3
52
Chemistry Letters 2000
HPLC Enantioseparation with Cellulose Tris(3,5-dichlorophenylcarbamate)
in Aqueous Methanol as a Mobile Phase
†
Bezhan Chankvetadze, Chiyo Yamamoto, and Yoshio Okamoto*
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603
†
Molecular Recognition and Separation Science Laboratory, School of Chemistry, Tbilisi State University, Chavchavadze Ave 1,
80028 Tbilisi, Georgia
3
(Received December 14, 1999; CL-991063)
The appropriate design of mobile and stationary phase
coated on the silica surface as a CSP.
combinations allowed the use of cellulose tris(3,5-
dichlorophenylcarbamate) (CDCPC) as the chiral stationary
phase (CSP) in high-performance liquid chromatography
The enantioseparation ability of polysaccharide-type CSPs
1
,2,5
in aqueous organic solvents is known.
The application of
these CSPs in combination with aqueous-organic mobile phases
are becoming increasingly important because this mode is bet-
ter suited for biomedical samples. The most attractive advan-
tage of separations using aqueous-organic mobile phases is a
possibility of a direct sample injection without any pretreat-
ment. Additional advantage is that achiral-chiral column-cou-
pling is more powerful in the case of reversed-phase separa-
tions. In addition, recent studies indicated that polysaccharide-
type CSPs may be successfully used even with pure alcohols as
(HPLC). Together with previous data obtained in n-hexane/2-
propanol as a mobile phase the present study indicates very
high chiral resolving ability of CDCPC.
The high chiral recognition ability of cellulose tris(3,5-
dichlorophenylcarbamate) (CDCPC) has been known for a
decade.1 However, this material is very soluble in the common
normal-phase HPLC eluents which are suitable for other poly-
6
the mobile phases. In some cases, even the separations not
1
,2
saccharide-type CSPs.
Due to this reason, it was impossible
capable of being common normal- and reversed-phase eluents
6e
to use intact CDCPC for HPLC enantioseparations. On the
are possible in pure alcohols.
For this reason, we re-exam-
ined the solubility of CDCPC in several alcohols and their
aqueous mixtures. Although CDCPC is insoluble in pure
methanol and n-hexane, it is very soluble in a binary (not even
miscible) mixture of these two solvents in almost any ratio.
Fortunately, the material is absolutely insoluble in aqueous
methanol solutions for any ratio of the components. This
opened the possibility to study CDCPC as a CSP for HPLC.
The packing material was prepared by dissolving the
CDCPC in THF and coating it onto wide pore aminopropylsi-
other hand, the corresponding dimethyl-substituted derivatives
of cellulose and amylose are insoluble in n-hexane/2-propanol
solution and belong to the most powerful CSPs for the HPLC
enantioseparations.1 Spectroscopic studies of CDCPC indicat-
ed that it possesses some unique properties from the viewpoint
of the amount of free N-H and C=O groups and their acidity
and basicity, respectively. In particular, the CDCPC contains
higher amounts of free carbamate fragments compared with the
corresponding dimethyl derivative of cellulose.3e These groups
are the most likely interaction sites with chiral analytes.
Therefore, several attempts were made in order to prepare some
hybrid-type CSPs, containing both halogen and alkyl sub-
stituents.3 The idea was that the former substituent will main-
tain a more universal chiral recognition ability and the latter
one provides material stability by creating a higher number of
intramolecular hydrogen bonds. The CSPs obtained in this way
were stable in n-hexane/2-propanol solutions and exhibited
1
lanized silica (Daisogel-1000, 7 µm) as previously described.
,2
Results of a preliminary study presented below confirmed
the previous expectations about the very high chiral recognition
ability of CDCPC. At first, almost all racemates (Figure 1) pre-
viously resolved into enantiomers using this material in n-hexa-
ne/2-propanol (CDCPC is slightly soluble in this mobile phase
1
and the column was destroyed after several runs) were also
resolved in aqueous methanol without any adjustment of the
buffer and optimization of the separation conditions (Table 1).
In addition, several chiral analytes not studied before (com-
pounds 10-20 in Figure 1) were separated into enantiomers in
the methanol-water mobile phase. In most cases, the selectivity
of the enantioseparation was slightly lower in the aqueous
methanol than in the n-hexane/2-propanol mobile phase.
However, the plate numbers were rather high for the most
resolved analytes in the aqueous methanol (Figure 2). Base-
line enantioseparation of the several racemates with low capaci-
ty factors indicates that the material exhibits a quite high chiral
recognition ability even in a polar medium. Under the
unbuffered conditions, the neutral compounds were, as expect-
ed, better resolved than the charged ones. However, among the
well resolved analytes, two contained a tertiary (compound 10)
and a primary amino (compound 20) group. CDCPC seems to
be an especially useful CSP for the separation of chiral binaph-
thyl and biphenyl derivatives (compounds 13, 14, 17, 18, and
3
quite interesting chiral recognition properties. An alternative
way for stabilizing the CSP is immobilization on a silica sur-
face. This technique for polysaccharide derivatives was first
4a
applied by Okamoto et al. in 1987 and recently used by sever-
4b-d
al groups with various modifications.
Both of above men-
tioned strategies resulted in CSPs with increased stability in
organic solvents. However, these immobilized CSPs exhibited
a somewhat lower chiral recognition ability. This may be the
result of the loss of their regular higher-ordered structure.
Therefore, the goal of this study was to use an intact CDCPC
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
353
This work was partially supported by Grants-in-Aid for
Scientific Research on Priority Areas No. 10208103 from the
Ministry of Education, Science, Sports and Culture, and the
Venture Business Laboratory Project “Advanced Nanoprocess
Technologies” at Nagoya University. Bezhan Chankvetadze
thanks the Venture Business Laboratory at Nagoya University
for financial support of his stay at the Department of Applied
Chemistry, Nagoya University.
References and Notes
1
a) Y. Okamoto, M. Kawashima, and K. Hatada, J. Chromatogr.,
3
63, 173 (1986). b) Y. Okamoto and E. Yashima, Angew. Chem.,
Int. Ed. Engl. 37, 1020 (1998).
2
3
E. Yashima, C. Yamamoto, and Y. Okamoto, Synlett, 1998, 344.
a) B. Chankvetadze, E. Yashima, and Y. Okamoto, Chem. Lett.,
1
993, 745. b) B. Chankvetadze, E. Yashima, and Y. Okamoto, J.
Chromatogr. A, 670, 39 (1994). c) B. Chankvetadze, E. Yashima,
and Y. Okamoto, J. Chromatogr. A, 694, 101 (1995). d) E.
Yashima, C. Yamamoto, and Y. Okamoto, Polym. J., 27, 856
(
1995). e) B. Chankvetadze, L. Chankvetadze, S. Sidamonidze, E.
Kasashima, E. Yashima, and Y. Okamoto, J. Chromatogr. A, 787, 67
1997).
(
4
a) Y. Okamoto, R. Aburatani, S. Miura, and K. Hatada, J. Liq.
Chromatogr., 10, 1613 (1987). b) E. Yashima, H. Fukaya, and Y.
Okamoto, J. Chromatogr. A, 677, 11 (1994). c) C. Minguillon, P.
Franco, L. Oliveros, and P. Lopez, J. Chromatogr. A, 728, 407
(1996). d) N. Enomoto, S. Furukawa, Y. Ogasawara, H. Akano, Y.
2
0) as well as for enantioseparation of succinimide derivatives
Kawamura, E. Yashima, and Y. Okamoto, Anal. Chem., 68, 2789
(1996).
(compounds 11 and 12), an imidazole-containing animal-anes-
5
6
a) K. Ikeda, T. Hamasaki, H. Kohno, T. Ogawa, T. Matsumoto, and
J. Sakai, Chem. Lett., 1989, 108. b) A. Ishikawa and T. Shibata, J.
Liq. Chromatogr., 16, 859 (1993).
thetic (compound 10) and antimicotic drugs, etc. Some inter-
esting structure-enantioseparation dependencies were observed
in the series of chiral biphenyl and binaphthyl derivatives.
Further studies on the screening of CDCPC under HPLC
conditions and on its use in capillary electrochromatography
a) J. Dingenen in: “A Practical Approach to Chiral Separations by
Liquid Chromatography,” ed by G. Subramanian, VCH, New York
(
1994), Chapt. 6, pp. 115-181. b) M. Girod, B. Chankvetadze, and G.
Blaschke, J. Chromatogr. A, accepted. c) M. Meyring, B.
Chankvetadze, and G. Blaschke, J. Chromatogr. A, accepted.
(CEC) are in progress.