Chiral initiator-induces self-disproportionation of enantiomers via achiral chromatography: Application to enantiomer separation of racemate

Article abstract of DOI:10.1016/j.tetlet.2013.07.061

We report here the theoretical design and proof of principle of the first example of a conceptually new approach for the preparation of enantiomerically pure compounds from the racemates by chiral initiator-induced Self-Disproportionation of Enantiomers (SDE) via achiral chromatography.

Full text of DOI:10.1016/j.tetlet.2013.07.061

Tetrahedron Letters 54 (2013) 5220–5223
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Chiral initiator-induces self-disproportionation of enantiomers via
achiral chromatography: application to enantiomer separation
of racemate
Kaori Tateishi a, Shiori Tsukagoshi a, Tsuyoshi Nakamura a, Shotaro Watanabe b, Vadim A. Soloshonok c,d
,
Osamu Kitagawa a,
⇑
a Department of Applied Chemistry, Shibaura Institute of Technology, 3-7-5 Toyosu, Kohto-ku, Tokyo 135-8548, Japan
b TAIHO Pharmaceutical Co., Ltd., 200-22, Motohara, Kamikawa-machi, Kodama-gun, Saitama 367-0241, Japan
c Department of Organic Chemistry I, University of the Basque Country UPV/EHU, 20018 San Sebastián, Spain
d IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
We report here the theoretical design and proof of principle of the first example of a conceptually new
approach for the preparation of enantiomerically pure compounds from the racemates by chiral initia-
tor-induced Self-Disproportionation of Enantiomers (SDE) via achiral chromatography.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 17 June 2013
Revised 9 July 2013
Accepted 11 July 2013
Available online 17 July 2013
Keywords:
Enantiomers
Disproportionation
Optical resolution
Chromatography
Association
One of the promising recent developments in the area of non-
conventional enantiomeric purification methods is the SDE (Self-
Disproportionation of Enantiomers) resulting in the separation of
the racemate from the excess enantiomer under totally achiral
conditions of physicochemical phase transitions or achiral
chromatography.1,2
was based on the assumption of a preferential formation of heter-
ochiral high-order species having a higher retention time as com-
pared with monomers of the excess enantiomer.4–6
While this approach holds quite tremendous practical potential,
the SDE makes use of the internal chirality of enantiomerically en-
riched compounds and cannot be applied for resolution of
racemates.
Recently, we reported an efficient method for the preparation of
a
series of enantiomerically pure N-Ac derivatives of
a
-
We hypothesized that if an optically pure compound 1 strongly
interacts with one or both enantiomers of a racemic compound 2,
then mixed homo-/heterochiral high-order species might be
formed in a solution, having different chromatographic behavior
(Scheme 1). Consequently, the original racemate will be
(phenyl)ethylamine type compounds via SDE of the corresponding
enantiomerically enriched samples under the conditions of achiral
MPLC.3 In particular, as shown in Figure 1, MPLC of non-racemic N-
acetyl-1-phenylethylamine 1a (71% ee) gave the elution profile
with a clear boundary between two fractions. The less polar frac-
tion contained enantiomerically pure 1a (>99% ee), while the more
polar fraction was considerably enantiomerically depleted (28%
ee). The magnitude of the SDE was found to strongly depend on
a substituent on the amino group.
Thus, while similar to 1a level of the SDE was observed also in
the case of N-formyl derivative, the magnitude of the SDE of N-ben-
zoyl, N-trifluoroacetyl, N-tosyl, and N-methoxycarbony derivatives
was noticeably reduced. Mechanistic rationale for these results
more polar
fraction
less polar
fraction
>99%ee
(24 mg)
28%ee
MPLC using
achiral SiO2
column
(21 mg)
NHCOCH3
Me
Ph
1a (51 mg, 71%ee)
(min)
60
70
⇑
Corresponding author. Tel.: +81 3 5859 8161; fax: +81 3 5859 8101.
E-mail address: kitagawa@shibaura-it.ac.jp (O. Kitagawa).
Figure 1. SDE via achiral chromatography of non-racemic N-acethyl-1-
phenylethylamine.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.07.061

K. Tateishi et al. / Tetrahedron Letters 54 (2013) 5220–5223
5221
NHCHO
Me
NHCHO
MPLC using SiO2 column
eluent: hexane/AcOEt = 1
Me
+
rac-2b
(S)-1b
60 mg
OMe
A)
(
S
)-1b (50 mg) + rac
-
2b (60 mg)
Scheme 1. A working hypothesis for chromatographicenantiomeric separation
based on the formation of mixed homo-/heterochiral high-order species.
more polar fraction
-2b (3%ee)
less polar fraction
-2b (>99%ee, 1.2 mg)
(R)
(S)
transformed into the (S)- and (R)-enantiomerically enriched
portions which will further undergo SDE resulting, in turn, in enan-
tiomerically pure fractions.
(S)-1b
In this Letter we present a conceptually new approach for the
preparation of enantiomerically pure compounds from the race-
mates by chiral initiator-induced Self-Disproportionation of Enan-
tiomers (SDE) via achiral chromatography. The present resolution
is based, presumably, on a preferential formation of heterochiral
high-order associations between (R)-enantiomer of a racemic sub-
strate and (S)-enantiomer of an optically pure inducing reagent,
leading to the formation of species with a different chromato-
graphic mobility, which in turn results in a transformation of the
initial racemate into enantiomerically enriched fractions and sub-
sequent SDE.
[min]
40
60
retention time
B) (S)-1b (151 mg) + rac-2b (60 mg)
less polar fraction
-2b (98%ee, 6.3 mg)
(S)
more polar fraction
(
R
)
-2b (13%ee)
(S)-1b
Building on our previous results of quite impressive SDE in a
series of chiral (phenyl)ethyl amine derivatives,3 we decided to
use a similar type of compounds as model substrates. Besides their
known SDE profile, these compounds are structurally very simple,
represent classical targets in asymmetric synthesis, and widely
used in pharmaceutical industry.
[min]
60
40
retention time
Knowing that the nature of a substituent on the amino function
has a significant effect on the magnitude of SDE, we started our
study with a detailed screening of various N-acyl derivatives of chi-
ral inducing reagent 1 and racemic compound 2 by mixing them in
a ratio of 1:1 and performing achiral MPLC experiments (Fig. 2).
Among several possible combinations in the (S)-1-phenylethyl-
amine derivatives (S)-1a–d and 1-(3-methoxyphenyl)ethylamine
derivatives rac-2a,b, the pair of N-formyl derivatives (S)-1b and
rac-2b gave the best result.
C) (S)-1b (276 mg) + rac-2b (60 mg)
less polar fraction
)-2b (>99%ee, 7.9 mg)
(
S
(S)-1b
more polar fraction
)-2b (16%ee, 48.9 mg)
(
R
In a typical experiment (S)-1b and rac-2b were mixed, and sub-
sequently subjected to MPLC on an achiral column (packed 10 lm
of silica gel, 20 Â 250 mm) using an achiral eluent (hexane–
AcOEt = 1). The obtained chromatographic profile is presented in
Figure 3. The graph (A) in Figure 3 shows MPLC chart of mixtures
of (S)-1b (50 mg) and rac-2b (60 mg) with a mole ratio of 1:1.
First of all, it should be noted that, as it was designed, the chiral
inducing reagent was completely separated, as the first eluted
[min]
60
40
20
retention time
Figure 3. MPLC chart of mixtures of (S)-1b and rac-2b.
fraction, from the components of compound 2b. The latter ap-
peared as two overlapped fractions with a noticeable boundary be-
tween them. Analysis of enantiomeric composition of these
fractions, using chiral HPLC, showed that, quite remarkably, the
less polar fraction contained enantiomerically pure 2b (1.2 mg,
>99% ee), while the more polar fraction was of 3% ee, accounting
for the complete mass and a 50:50 initial enantiomeric composi-
tion of the racemate 2b. We further envisioned, that efficiency of
these results might be improved with increasing the amount of
chiral inducing reagent (S)-1b by the virtue of increasing the num-
ber of quality interactions between (S)-1b and enantiomers of rac-
2b. Figure 3, charts B and C present the results of MPLC experi-
ments using mixtures of (S)-1b and rac-2b (60 mg) taken in mole
ratios of 3:1 and 5.5:1, respectively. As one can see, in the case of
3:1 ratio, a more clear boundary between the two fractions was ob-
served, and almost optically pure 2b [6.3 mg (98% ee)] was isolated
as the less polar fraction. This result was further improved using
the 5.5:1 ratio allowing to isolate 7.9 mg of enantiomerically pure
NHCOR
(
S
)-1 (100%ee)
NHCOR
Me
Me
(
S
)-1a (R = Me)
(
S
)-1b (R = H)
2 (racemic)
Ph
rac-2a (R = Me)
rac-2b (R = H)
(
S
)-1c (R = Et)
OCH3
(
S
)-1d (R = COOMe)
MPLC using SiO2 column
eluent: hexane- AcOEt
optical
resolution?
(
S
)-1
+
rac-2
(60 mg)
mol ratio of (
and rac-2 = 1 : 1
S
)-1
Column: 10 µm of SiO2 (20 X 250 mm)
(
S
)-1a + rac-2a
(
(
S
S
)-1a + rac-2b
)-1c + rac-2b
(
S
)-1b + rac-2b
(S)-1b + rac-2a
(
S)-1d + rac-2b
Figure 2. MPLC experiment of mixtures of (S)-1 and rac-2.

5222
K. Tateishi et al. / Tetrahedron Letters 54 (2013) 5220–5223
2b (>99% ee) in the less polar fraction. In terms of practicality and
isolated yield, these data constitute a meaningful 21% and 26%,
respectively, of the enantiomerically pure product, obtained from
the starting racemic mixture. Determination of the absolute config-
uration of the enantiomerically pure product 2b, by comparison
with the authentic sample, has revealed its (S)-configuration.7
As one can see from chart C, the area of 2b has several shoulders
prompting us to investigate their composition in more detail. To
this end we isolated six fractions, as shown in Figure 4, and studied
their enantiomeric composition as well as the absolute configura-
tion of the excess enantiomer. The first fraction was found to be
noticeably enriched (39% ee) in (S)-2b enantiomer, while rest of
the fractions (from the second to the sixth) contained excess of
(R)-2b with gradually decreasing enantiomeric purity from 23 to
10% ee. One may assume that a similar enantiomeric composition
might be also observed in the experiments conducted with 1 and 3
equiv of reagent (S)-1b, charts (A) and (B), respectively. However,
in these cases the separation of the enantiomeric species is less
efficient and the corresponding shoulders are not pronounced.
Considering these preliminary results we may conclude that our
design was quite successful as the inducing reagent (S)-1b led to
a separation of (S)-2b enantiomer transforming the initial race-
mate 2b into an enantiomerically enriched state which further
underwent the SDE previously described for this type of
compounds.
Finally, we decided to briefly assess the generality of this meth-
od for resolution of other racemic N-formyl (phenyl)ethylamine
derivatives. For example, racemate 3b [N-formyl 1-(4-methoxy-
phenyl)ethylamine] (60 mg) was mixed with (S)-1b (275 mg), in
a mole ratio of 1:5.5 and subjected to MPLC under the standard
conditions (Fig. 5). Similar to the results obtained for resolution
of 2b, the chiral reagent 1b was eluted first followed by the area
of compound 3b, clearly showing several shoulders. The first, less
polar fraction, contained enantiomerically pure (99% ee) (S)-3b,7
which was obtained in 14% recovery yield (4.3 mg). The next frac-
tion was enantiomerically enriched (31% ee) in the same (S)-enan-
tiomer, followed by the fractions containing (R)-enantiomer in
excess of various enantiomeric purity.
NHCHO
NHCHO
Me
+
Me
rac-3b
60 mg
(S)-1b
MeO
:
eluent:
275 mg
hexane/AcOEt = 1
mol ratio 5.5
1
(S)-3b
99%ee
(S)-1b
(4.3 mg)
(S)-3b 31%ee
(R)-3b 5%ee
(R)-3b 8%ee
(R)-3b 5%ee
[min]
60
20
40
retention time
Figure 5. MPLC chart of the mixtures of (S)-1b and rac-3b with a mole ratio of
5.5:1.
NHCHO
Me
NHCHO
Me
+
rac-4b
eluent:
hexane/AcOEt = 2
(S)-1b
Me
:
60 mg
164.5 mg
mol ratio
3
1
first fraction
(S)-4b (>99%ee, 8.5 mg)
second fraction
(S)-4b (15%ee, 15.6 mg)
third fraction
(S)-1b and 4b [(R)-rich]
In another experiment, using racemate 4b [N-formyl 1-(4-
methylphenyl)ethylamine] (60 mg), mixed with the inducing re-
agent (S)-1b (164.5 mg) in a mole ratio of 1:3, we obtained a bit
different results as compared with the derivatives 2b and 3b. In
this case (Fig. 6) the first collected fraction contained enantiomeri-
cally pure (S)-4b (>99% ee, 8.5 mg, recovery yield 28%) followed by
another (S)-enantiomerically enriched fraction (15% ee, 15.6 mg).
The last, third collected fraction, contained the inducing reagent
(S)-1b and (R)-enantiomerically enriched 4b. Thus, the elution or-
der of enantiomers of 4b and the reagent (S)-1b were different
form the 2b and 3b cases, but the stereochemical outcome, result-
ing in the separation of the (S)-enantiomer 4b,7 was virtually the
same. Accordingly, one may agree that this new method might
have some degree of generality, at least in a series of structurally
[min]
90
70
100
80
retention time
Figure 6. MPLC chart of the mixtures of (S)-1b and rac-4b with a mole ratio of 3:1.
similar compounds, using the same chiral inducing reagent (S)-
1b. Obviously, different classes of compounds may require another
inducing reagent or modified conditions, but clearly, the concept
discovered in this study constitutes a new, non-conventional ap-
proach for racemate resolution.
One may agree, that elucidation of the detailed mechanism of
the observed in this study chromatographic profiles will require a
separate investigation. However, what seems to be obvious is
that the chiral inducing reagent interacts selectively with one
of the enantiomers of the starting racemate producing high-order
species having different retention times. As we have suggested in
the previous work,3 compounds of this class, tend to form syndio-
tactic hetero-chiral, hydrogen bonded associations which are
preferred in a solution over isotactic homo-chiral associations.5
Thus, we may assume that (S)-chiral inducing reagent, selectively
forms high-order species with (R)-enantiomer of the racemate
(Scheme 1, m > n), which have longer retention times as
compared to monomeric species of the (S)-enantiomer, being
eluted first.
(R)-2b (23%ee)
(S)-2b (39%ee)
(R)-2b (22%ee)
(R)-2b (20%ee)
(R)-2b (16%ee)
(S)-2b
(>99%ee)
less polar
fraction
(R)-2b (10%ee)
[min]
50
60
retention time
Figure 4. Detailed ee change of 2b in chart C (Fig. 3).

K. Tateishi et al. / Tetrahedron Letters 54 (2013) 5220–5223
5223
Balentova, E. J. Org. Chem. 2001, 66, 3940; (m) Soloshonok, V. A. Angew. Chem.,
Int. Ed. 2006, 45, 766; (n) Soloshonok, V. A.; Berbasov, D. O. J. Fluor. Chem. 2006,
127, 597; (o) Takahashi, M.; Tanabe, H.; Nakamura, T.; Kuribara, H.; Yamazaki,
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In conclusion, we have designed a new approach for racemate
resolution based on chiral initiator-induced self-disproportion-
ation of enantiomers via achiral chromatography. Direct resolution
methods of racemates via chromatography using chiral mobile8 or
stationary phases9 are known.10 However, chiral mobile phase
method requires the use of very large excess (>100 equiv) of a chi-
ral selector added to an eluent, resulting in mixtures of resolved
enantiomers with the chiral selector. Although separation of enan-
tiomers using chiral stationary phase method is the most popular
on analytical scale, the only disadvantage of this approach is that
preparative chiral stationary phase columns are still very expen-
sive. Our approach opens up a new area of research with a remark-
able practical potential for inexpensive, available in any research
laboratory technique for generation of enantiomerically pure
compounds starting from racemates. The following features of this
work should be particularly emphasized: (1) This Letter presents
the proof of principle of a new approach for racemate resolutions
via achiral chromatography; (2) Methodologically, this new ap-
proach offers an unexpected and practically advantageous benefit
of combining chiral mobile method with the SDE. Taking sepa-
rately, the chiral mobile method has a significant shortcoming
requiring to use large quantities of enantiomerically pure reagent,
and the SDE is limited to the application to already enantiomeri-
cally enriched mixtures. (3) Taking into account that the chiral mo-
bile method and the SDE are of very general nature, this new
approach is also expected to be of general application for the prep-
aration of enantiomerically pure samples from racemic mixtures.
2. For reviews on SDE via achiral chromatography: (a) Soloshonok, V. A.;
Berbasov, D. O. Chim. Oggi/Chem. Today 2006, 24, 44; (b) Soloshonok, V. A.;
Roussel, C.; Kitagawa, O.; Sorochinsky, A. E. Chem. Soc. Rev. 2012, 41, 4180; (c)
Sorochinsky, A. E.; Aceña, J. L.; Soloshonok, V. A. Synthesis 2013, 45, 141.
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Soloshonok, V. A.; Aceña, J. A.; Kitagawa, O. Tetrahedron 2012, 68, 4013.
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7. The absolute stereochemistry of the separated enantiomers (S)-2b-4b were
determined by comparison with the authentic samples that were prepared by
N-formylation of optically pure 1-(3-methoxyphenyl)ethyamine, 1-(4-
methoxyphenyl)ethyamine, and 1-(4-methylphenyl)ethyamine (commercially
available).
Supplementary data
8. Representative literatures on chiral mobile phase methods: (a) Lam, S.; Chow,
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Hatada, K.; Yuki, H. J. Am. Chem. Soc. 1979, 101, 4673; (c) Yuasa, S.; Shimada, A.;
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839.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
07.061.
References and notes
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