Chiral selector with multiple hydrogen-bonding sites in a macrocyclic cavity

Article abstract of DOI:10.1021/ol800940j

(Chemical Equation Presented) Chiral macrocycles with the hydrogen bond donor/acceptor sites in the cavity were synthesized and covalently bonded to silica gel to give chiral stationary phases (CSPs), which showed excellent abilities to resolve various chiral compounds, such as benzoin and Co(acac) ₃, in HPLC. Various organic solvents could be used as the mobile phase to optimize the resolution efficiency of CSPs, and in some cases, even MeCN, MeOH, and CO₂ could be used for the complete resolution of enantiomers.

Full text of DOI:10.1021/ol800940j

ORGANIC
LETTERS
2008
Vol. 10, No. 12
2365-2368
Chiral Selector with Multiple
Hydrogen-Bonding Sites in a
Macrocyclic Cavity
Tadashi Ema,*,† Daisuke Tanida,† Kyoko Sugita,† Takashi Sakai,*,†
Ken-ichiro Miyazawa,‡ and Atsushi Ohnishi*,‡
DiVision of Chemistry and Biochemistry, Graduate School of Natural Science and
Technology, Okayama UniVersity, Tsushima, Okayama 700-8530, Japan, CPI
Company, Daicel Chemical Industries, Ltd., Shinko-cho, Myoko,
Niigata 944-8550, Japan, and CPI Company, Daicel Chemical Industries, Ltd., Aboshi,
Himeji, Hyogo 671-1283, Japan
ema@cc.okayama-u.ac.jp
Received January 29, 2008
ABSTRACT
Chiral macrocycles with the hydrogen bond donor/acceptor sites in the cavity were synthesized and covalently bonded to silica gel to give
chiral stationary phases (CSPs), which showed excellent abilities to resolve various chiral compounds, such as benzoin and Co(acac)3, in
HPLC. Various organic solvents could be used as the mobile phase to optimize the resolution efficiency of CSPs, and in some cases, even
MeCN, MeOH, and CO2 could be used for the complete resolution of enantiomers.
There is an increasing demand for the accurate determination
of the enantiomeric purity with an increase in the number
of chiral drugs. Among various methods for the determination
of the enantiomeric purity, the HPLC method with a chiral
stationary phase (CSP) column has gained a dominant
position because of high sensitivity and reliability.1 In
addition to the analytical utility, chiral HPLC can also give
optically active compounds by the preparative resolution of
enantiomers. Recently, various types of CSPs have been
developed, where a chiral selector is covalently or nonco-
valently bound to silica gel.1–7 The advantage of the former,
covalently immobilized CSPs, is that the combination of
mobile phases (solvents) can be tuned to maximize the
(2) (a) Yamamoto, C.; Yashima, E.; Okamoto, Y. Bull. Chem. Soc. Jpn.
1999, 72, 1815–1825. (b) Ikai, T.; Yamamoto, C.; Kamigaito, M.; Okamoto,
Y. J. Chromatogr. A 2007, 1157, 151–158
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† Okayama University.
(3) (a) Armstrong, D. W.; Tang, Y.; Chen, S.; Zhou, Y.; Bagwill, C.;
Chen, J.-R. Anal. Chem. 1994, 66, 1473–1484. (b) Ekborg-Ott, K. H.; Liu,
Y.; Armstrong, D. W. Chirality 1998, 10, 434–483. (c) Ekborg-Ott, K. H.;
Kullman, J. P.; Wang, X.; Gahm, K.; He, L.; Armstrong, D. W. Chirality
‡ Daicel Chemical Industries, Ltd.
(1) Reviews: (a) Pirkle, W. H.; Pochapsky, T. C. Chem. ReV. 1989, 89,
347–362. (b) Okamoto, Y.; Yashima, E. Angew. Chem., Int. Ed. 1998, 37,
1020–1043. (c) Tachibana, K.; Ohnishi, A. J. Chromatogr. A 2001, 906,
127–154. Book: (d) Chiral Separations: Methods and Protocols; Gu¨bitz,
G., Schmid, M. G., Eds.; Humana: Totowa, NJ, 2004.
1998, 10, 627–660
(4) Sudo, Y.; Yamaguchi, T.; Shinbo, T. J. Chromatogr. A 1996, 736,
39–49.
.
10.1021/ol800940j CCC: $40.75
Published on Web 05/21/2008
2008 American Chemical Society

resolution efficiency. Polysaccharides,2 antibiotics,3
binaphthyls,4,5a macrocycles such as cyclodextrins,1d crown
ethers,5 and cyclophanes,6 and others1,7 have been used as
chiral selectors. Among them, the binaphthyl structure is one
of the most attractive chiral units because of its moderate
rigidity and flexibility as well as its synthetic accessibility.
In addition to the chiral unit, a binding unit is also a key
factor in the design of a chiral selector. Selector molecules
covalently attached to the silica surface in a highly accumulated
state may aggregate via the intermolecular interactions, which
would result in the compensation and masking of the binding
sites. A highly organized structure such as a helix, which has
been well-demonstrated by Okamoto and Yashima,1b,2a would
be needed to avoid the aggregation of a linear binding motif.
We envisioned that such a binding unit where the binding
site is located inside a macrocyclic cavity could retain a chiral
binding environment independently even in a highly ac-
cumulated state, which would lead to the highly efficient
resolution. Although chiral selectors bearing crown ether
motifs have been developed, the scope is rather limited
because they have only the hydrogen bond acceptor (ether)
functionality. On the other hand, an NMR chiral solvating
agent called Chirabite-AR that we have recently developed
can bind a wide range of compounds by using both hydrogen
bond donor and acceptor sites inside the macrocyclic cavity.8
In view of the unique characteristics of the receptor, we
decided to investigate whether it could be used as a chiral
selector. Here we report the synthesis of CSP-1 and the
chromatographic resolution of chiral compounds.
Scheme 1, where mild synthetic methods are employed to avoid
the decomposition of the four amide groups forming the
macrocyclic structure. Diamines 2 and 7 were prepared from
5-benzyloxyisophthalic acid and 5-tert-butyloxycarbonylisoph-
thalic acid, respectively, according to the reported procedure.9,10
Scheme 1. Preparation of CSP-1
The coupling of 2 and 7 with acid chloride 3 afforded
macrocycles 4 and 8, respectively. The deprotection of the
benzyl group in 4 followed by the attachment of the tert-
butyl ester moiety to 5 gave 6. The tert-butyl ester group in
6 or 8 was cleaved by treatment with trifluoroacetic acid.
The condensation of the resulting acids with 3-aminopropyl
silica gel afforded CSP-1a or CSP-1b, which were packed
in a stainless steel column (φ 0.46 × 25 cm). In both cases,
the coverage of the macrocycle on silica was adjusted to
0.18 mmol/g as determined by elemental analysis.
Initially, we attempted to convert the NO2 group in Chirabite-
AR to an amino group by the Pd-catalyzed hydrogenation;
however, the corresponding amine obtained was found to be
unstable for an unknown reason. We therefore searched for a
stable precursor bearing a functional group connectable to silica
or a silane coupling agent, and found that the hydroxy and
carboxyl groups can be good substitutes for the amino group.
The synthetic routes to CSP-1a and CSP-1b are shown in
The ability of CSP-1 to resolve chiral analytes A1-A6
was evaluated by HPLC. The results are summarized in Table
(7) (a) Feibush, B.; Figueroa, A.; Charles, R.; Onan, K. D.; Feibush,
P.; Karger, B. L. J. Am. Chem. Soc. 1986, 108, 3310–3318. (b) Pirkle, W. H.;
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Nakamura, T.; Nishioka, R.; Ueshige, T.; Tobe, Y. Chirality 2005, 17, 142–
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.
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Org. Lett., Vol. 10, No. 12, 2008

1, and representative chromatograms are shown in Figure 1.
We confirmed that racemic compounds A1-A6 could be
resolved successfully without derivatization. Among them,
Co(acac)3 (A6), which happened to be used as one of our
standard samples for the CSP assay, gave intriguing results.11
As shown in Figure 1b and Table 1, A6 was resolved
Table 1. Chromatographic Data for the Resolution of Racemic
Compounds on CSP-1a
separation factor Rb (peak separation)c
eluent
I
eluent
II
eluent
III
eluent
IV
CSP
analyte
CSP-1a
CSP-1a
CSP-1a
CSP-1a
CSP-1a
CSP-1a
CSP-1b
CSP-1b
CSP-1b
CSP-1b
CSP-1b
CSP-1b
A1
A2
A3
A4
A5
A6
A1
A2
A3
A4
A5
A6
1.14 (B)
1.07 (B)
1.87 (A)
1.00 (C)
1.00 (C)
1.57 (B)
1.15 (A)
1.09 (B)
1.83 (A)
1.06 (B)
1.08 (B)
1.75 (A)
1.31 (B)
1.07 (C)
2.26 (A)
1.24 (B)
1.00 (C)
1.50 (B)
1.44 (A)
1.07 (B)
2.43 (A)
1.51 (A)
1.08 (B)
1.67 (A)
1.00 (C)
1.00 (C)
2.19 (A)
1.17 (B)
1.37 (B)
1.61 (B)
1.26 (C)
1.00 (C)
2.38 (A)
1.04 (C)
1.25 (B)
1.61 (A)
1.00 (C)
1.00 (C)
1.54 (B)
1.00 (C)
1.00 (C)
1.00 (C)
1.00 (C)
1.06 (C)
1.70 (A)
1.08 (C)
1.00 (C)
1.20 (B)
Figure 1. Selected chromatograms for the resolution of (a) A3 on
CSP-1a with hexane/CHCl3 (7:3) and (b) A6 on CSP-1b with
MeCN. For details of the analytical conditions, see the footnote
for Table 1.
and that they are stronger on the surface of silica than in a
bulk solution.
Comparisons between CSP-1a and CSP-1b clearly indi-
cate the importance of the linker, the ether or amide group
(Table 1 and Supporting Information Tables S1-S6). In
many cases, the separation factors (R) are larger for CSP-
1b than for CSP-1a. One of the exceptions is the resolution
of sulfoxide A4 in MeCN, which was resolved by CSP-1a,
but not by CSP-1b. The hydrogen bond donor ability of
CSP-1b is expected to be higher than that of CSP-1a because
it is known that the presence of the NO2 group in Chirabite-
AR improves the hydrogen bond donor ability of the lower
amide groups of the macrocycle.8b The most impressive
results were obtained for benzoin (A3), which was resolved
completely by CSP-1b by using any of the solvent systems
that were examined, even MeOH (Table 1). We confirmed
that, in all the eluents I-IV, the enantiomers of A3 eluted
in the same order: (S)-A3 followed by (R)-A3. The binding
constants (Ka) of Chirabite-AR for (R)- and (S)-A3 in CDCl3
were determined by NMR titrations to be 240 and 40 M-1,
respectively. These results mean that (R)-A3, which has a
higher affinity for the (R)-host, eluted more slowly in all
cases, which in turn suggests that the mechanism of chiral
recognition is basically unchanged. We consider that, because
of the preorganized binding site in the macrocyclic structure,
hydrogen bonds that are essential for chiral recognition are
retained in all the eluents (I-IV).12
a CSP-1a or CSP-1b packed in a stainless steel column (φ 0.46 × 25
cm) was used for HPLC: flow rate 1.0 mL/min, detection 254 nm except
for A1 (225 nm), 25 °C, eluent I ) hexane/i-PrOH (9:1), eluent II ) hexane/
CHCl3 (7:3 except for A1 and A4 (4:6)), eluent III ) MeCN, eluent IV )
MeOH. b Calculated from k2′/k1′, where k1′ ) (t1 - t0)/t0 and k2′ ) (t2 -
t0)/t0. The retention times for the faster and slower moving enantiomers
are designated as t1 and t2, respectively, and that for 1,3,5-tri-tert-
butylbenzene used as a void volume marker is designated as t0. c A )
complete or almost complete separation, B ) partial separation, C ) little
or no separation.
successfully. Although the detailed mode of chiral recogni-
tion is unknown despite the computational calculations, the
outside lone-pair orbitals of the carbonyl O atoms of A6 are
likely to be accessible to the cavity of CSP-1 via hydrogen
bonding even if the carbonyl O atoms remain coordinated
to the Co atom.
Table 1 also indicates that various solvents can be tested
to optimize the resolution efficiency of the covalently
immobilized CSPs. In the case of CSP-1b, for example, the
complete resolution of A1, A3, and A6 was achieved by
using hexane/i-PrOH, while the excellent resolution of A1,
A3, A4, and A6 was realized by using hexane/CHCl3. When
a highly polar solvent, MeCN, was used, surprisingly, A3
and A6 could be resolved completely by CSP-1b. This
success was totally unexpected because it is known that the
ability of Chirabite-AR to discriminate between enantiomers
in NMR is deteriorated in CDCl3 containing d4-methanol or
in d6-acetone.8b The use of a more polar solvent, MeOH,
drastically diminished the peak separation in most cases.
These results suggest that the hydrogen bonds are formed
in the macrocyclic cavity of CSP-1 even in polar solvents
Chiral HPLC with CO2 as the mobile phase is very
important from the viewpoint of green chemistry.13 The
remarkable solvent tolerance of CSP-1 as demonstrated
above prompted us to examine the CO2-based mobile phase.
(12) The NMR titrations for determining the binding constant of
Chirabite-AR for A3 in MeCN or MeOH could not be performed because
of the insolubility of the reagent.
(11) Several CSPs have been reported to be effective for the resolution
of A6. For example, see ref 1b.
(13) Wang, C.; Armstrong, D. W.; Risley, D. S. Anal. Chem. 2007, 79,
8125–8135.
Org. Lett., Vol. 10, No. 12, 2008
2367

In summary, we have demonstrated that the synthetic
macrocycles with multiple hydrogen bonding sites in the
cavity can function as excellent chiral selectors. CSP-1,
having a distinct site preorganized for chiral recognition, can
resolve a variety of chiral compounds including an unex-
pected one, Co(acac)3, without derivatization of the analytes.
Various organic solvents can be used as the mobile phase to
optimize the resolution efficiency, including even the highly
polar solvents such as MeCN and MeOH as well as the CO2-
based mobile phase. CSP-1, showing excellent performance
and durability, is very promising, and the alteration and
tuning of the macrocyclic structure will lead to a further
enhancement in the chiral HPLC performance.
Acknowledgment. This work was supported by a Grant
for Research for Promoting Technological Seeds from Japan
Society for the Promotion of Science (JSPS). We are grateful
to the SC-NMR Laboratory of Okayama University for the
measurement of NMR spectra.
Figure 2. Chromatograms for the resolution of (a) A3 and (b) A6
on CSP-1a, using the CO2-based mobile phase: flow rate 2.5 mL/
min, detection 220 nm, 25 °C, CO2/i-PrOH (9:1 for A3, 8:2 for
A6), 10 MPa.
Supporting Information Available: Synthetic procedures
for CSP-1a and CSP-1b, determination of the coverage of
1
the macrocycle on silica, copies of H and 13C NMR, and
The results are shown in Figure 2. Complete resolution of
A3 and A6 was achieved by CSP-1a, and the peak separation
was much better than we had expected. These results
convinced us of the excellence of CSP-1.
copies of chromatograms. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL800940J
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Org. Lett., Vol. 10, No. 12, 2008