Efficient synthesis of chiral binaphthol aldehyde with phenyl ether linkage for enantioselective extraction of amino acids

Article abstract of DOI:10.1002/bkcs.10354

A binaphthol aldehyde with phenyl ether linkage, compound 2, has been synthesized starting from binaphthol-3-carboxylic acid. The axially chiral binaphthol ring was racemized during the synthesis due to high temperatures required in O-phenylation reaction. The enantiomerically pure form of 2 was obtained from the resolution of the diastereomeric imine of 2. Optically pure compound (S)-2 was applied to the enantioselective liquid-liquid extraction of amino acid between CH₂Cl₂ and aqueous layers.The stereoselectivities, that is, D/L ratio of the amino acid extracted, ranged from 3.57 to 11.1. One carbon was absent in compound (S)-2 compared to the compound (S)-1 with benzyl ether linkage, which differentiated the conformations of their imines formed with amino acids.

Full text of DOI:10.1002/bkcs.10354

Article
DOI: 10.1002/bkcs.10354
BULLETIN OF THE
M. Choi et al.
KOREAN CHEMICAL SOCIETY
Efficient Synthesis of Chiral Binaphthol Aldehyde with Phenyl Ether
Linkage for Enantioselective Extraction of Amino Acids
†
‡
†,*
Misun Choi, Moo-Jin Jun, and Kwan Mook Kim
†Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea.
*E-mail: kkmook@ewha.ac.kr
‡
ReSEAT Program, Korea Institute of Science & Technology Information, Seoul 130-741, Korea
Received January 10, 2015, Accepted March 20, 2015, Published online June 26, 2015
A binaphthol aldehyde with phenyl ether linkage, compound 2, has been synthesized starting from binaphthol-3-
carboxylic acid. The axially chiral binaphthol ring was racemized during the synthesis due to high temperatures
required in O-phenylation reaction. The enantiomerically pure form of 2 was obtained from the resolution of the
diastereomeric imine of 2. Optically pure compound (S)-2 was applied to the enantioselective liquid–liquid
extractionofaminoacidbetweenCH Cl andaqueouslayers.Thestereoselectivities,thatis,D/L ratiooftheamino
2
2
acid extracted, ranged from 3.57 to 11.1. One carbon was absent in compound (S)-2 compared to the compound
(S)-1 with benzyl ether linkage, which differentiated the conformations of their imines formed with amino acids.
Keywords: Enantioselective extractor, Optical resolution, Chiral imine
Introduction
Experimental
Chirotechnologies for obtaining optically pure amino acids
include dynamic kinetic resolution (DKR), which combines
General. Chemicals such as sodium borohydride, boron
trifluoride diethyl etherate, 2,2-dimethoxy propane, palla-
dium 10 wt% on activated carbon, phenyl isocyanate, potas-
sium persulfate, triphosgene, 2-aminobenzimidazole, 3-
fluoronitrobenzene and 2,2,6,6-tetramethylpoperidine-1-oxyl
were purchased from Aldrich, Seoul, Korea and TCI, Tokyo,
1
enzymatic and chemical processes, and chemical syntheses
2–5
controlled by chiral catalysts. The enantioselective liquid–
liquid extraction (ELLE) is considered to be an economical
process due to low energy consumption, green production,
6
0
and the advantages in scaling-up. The chiral compounds of
Japan. The starting compound, 2,2 -binaphthol-3-carboxylic
7
8
crown ethers and metal complexes have been tested as extrac-
tants for ELLE of amino acids. They, however, usually suffer
from low operational enantioselectivities and the narrow range
acid, was provided by company Aminologics, Seoul,
Korea. All chemicals were used as received without further
purifications. NMR spectra were recorded on Bruker AM
9
of application especially to underivatized amino acids.
250 spectrometer, Bruker, in CDCl and dimethylsulfoxide
3
Compound 1 has been developed as a chiral conversion rea-
(DMSO)-d6 solutions containing tetramethylsilane as an
internal standard. Melting points were measured with an
Electrothermal IA 9000 digital melting point apparatus,
Bibby Scientific. Optical rotations were measured on a DIP
360 polarimeter, Jasco Legacy. For column chromatography,
silica gel of 230–400 mesh from Aldrich was used.
gent and is also known to work as a chiral extractant in ELLE of
10,11
amino acids when it is used together with Aliquat 336.
In
this work, we have designed and synthesized compound 2 with
a phenyl ether linkage, which has a reduced distance from the
naphthol ring to the uryl group compared to that of compound
0
1
. This change would result in the difference of the stereoselec-
3-Hydroxymethyl-2,2 -binaphthol (4). Boron trifluoride
tivities of compound 2 in the imine formation with amino acids
from those of compound 1. Here we report the synthesis, enan-
tiomeric resolution, and the stereoselectivities of 2 as a chiral
extractant for ELLE of underivatized amino acids.
diethyl etherate (8 mL, 60 mmol) was added to sodium boro-
hydride (2.84 g, 75 mmol) in tetrahydrofuran (THF, 30 mL),
to which a solution of 2,2 -binaphthol-3-carboxylic acid (3)
0
(10.0 g, 30.2 mmol) in THF was transferred in oxygen- and
moisture-free condition, and the resulting mixture was stirred
ꢀ
for 20 h at 65 C. The progress of the reaction was monitored
by TLC (Thin Layered Chromatography), and THF/HCl/H O
2
O
O
HN
H HN
(1:1:1) solution (45 mL) was added to quench the reaction.
H
O
Compound 4 was isolated by column chromatography of
the crude mixture, which was obtained by the evaporation
of the oganic layer after the extraction of the reaction mixture
with CH Cl and water (9.0 g, yield 94 %). Melting point =
OH
O
H
H
OH
O
N
N
O
2
2
ꢀ
1
6
5
9 C, H NMR (CDCl , 250 MHz): δ 7.99–7.87 (m, 11H),
.91 (s, 1H, OH), 5.03 (s, 2H).
3
Compound (S)-1
Compound (S)-2
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BULLETIN OF THE
Article Efficient Synthesis of Chiral Binaphthol Aldehyde with Phenyl Ether Linkage
KOREAN CHEMICAL SOCIETY
Compound 5. Compound 5 was prepared by the procedure
that was reported by Tang et al.
130.70, 130.63, 130.20, 129.23, 128.79, 127.57, 127.44,
125.41, 125.29, 124.91, 124.69, 122.98, 122.37, 121.42,
120.18, 118.69, 117.07, 113.12, 112.13, 108.63.
1
2
Compound 6. Potassium carbonate (5.82 g, 42 mmol) was
added to a solution of compound 5 (10 g, 28 mmol) in anhy-
drous dimethylformamide (DMF), and the solution was stirred
Enantiomeric Resolution of 2. (S)-(+)-2-Phenylglycinol
(0.784 g, 5.71 mmol) was added to a solution of 2 (3.0 g,
5.71 mmol) and TPPC (2.1 g, 5.60 mmol) in CH Cl . The
ꢀ
at 80 C. After 1 h, 3-fluoronitrobenzene (5 mL, 42 mmol)
2
2
ꢀ
was added, and stirring was continued at 130 C for 70 h.
mixture was stirred at room temperature for 12 h. On confirm-
ing the formation of the imine by investigating the TLC, the
solvent was removed with a rotary evaporator. The two dia-
stereomers were separated by silica column chromatography
with the eluent EtOAc/CH Cl (1:20). The purity of the dia-
The mixture was cooled to room temperature, quenched by
M HCl, and extracted with EtOAc and brine. The organic
1
layer was dried over anhydrous MgSO . The crude product
4
was chromatographed on silica gel column with EtOAc/
2
2
1
CH Cl /hexane (1:1:10) to obtain compound 6 (8.4 g, yield
stereomer was determined by H NMR. The pure form of the
diastereomer was hydrolyzed at room temperature for 12 h in
CH Cl and 1 M HCl aqueous solutions. The crude product in
2
2
ꢀ
1
=
3
2
62%). Melting point = 97.7 C,
H NMR (CDCl3,
00 MHz): δ 7.96–7.0 (m, 15H), 5.10–4.97 (dd, J = 15,
5.5 Hz, 2H), 1.35 (s, 3H), 1.23 (s, 6H).
2
2
the organic solution was column-chromatographed with
Compound 7. Palladium 10 wt% on activated carbon (0.9
g) was added to a solution of compound 6 (9.20 g, 19 mmol) in
EtOAc/CH Cl (1:15) to give pure (S)-2. (0.62 g, yield
2
2
ꢀ
41%). Melting point = 171 C, [α] = −111.85(c 0.005, Ace-
D
THF, and stirred for 20 h at room temperature while H gas in a
tone); HRMS (FAB) calcd for C H N O 525.1809; found:
2
34 25 2 4
balloon was injected. The mixture was filtered, and the evap-
525.1809.
oration of the filtrate gave compound 7 (8.46 g, yield 98%).
Enantioselective Liquid–Liquid Extraction. An aqueous
solution of the sodium salt of racemic amino acids (20 equiv
relative to (S)-2) was stirred with an equal volume of CDCl3
solution containing (S)-2 and the phase-transfer reagent tetra-
phenylphosphonium chloride (TPPC) or Aliquat 336 (1.3
equiv relative to (S)-2) at room temperature. The imine forma-
ꢀ
1
Mp = 212.2 C, H NMR (CDCl , 300 MHz): δ 7.91–6.95
3
(m, 13H), 6.28–6.18 (m, 3H), 5.10–5.09 (d, J = 1.2 Hz, 2H),
1.36 (s, 3H), 1.28 (s, 3H).
Compound 8. Phenyl isocyanate (2.70 mL, 24 mmol) was
added to a solution of compound 7 (8.56 g, 19 mmol) in
CH Cl andstirred atambient temperature. After 5 h, the solu-
1
tion in the organic layer was investigated by H-NMR
2
2
tion was evaporated by a rotary evaporator, and the residue
was chromatographed on silica gel column with EtOAc/hex-
ane (1:3) to give compound 8 (9.40 g, yield 86%). Melting
spectroscopy.
ꢀ
1
point = 229.7 C, H NMR (CDCl , 300 MHz): δ 7.78–6.40
Results and Discussion
3
(m, 22H), 5.03–4.89 (dd, J = 15.6, 26.4 Hz, 2H), 1.29 (s,
3
H), 1.20 (s, 3H).
Compound 9. HCl (35%, 4.2 mL, 48 mmol) was added to a
Optically pure compound (S)-2 was synthesized starting from
2,2n-binaphthol-3-carboxylic acid(3)followingtheprocesses
shown in Scheme 1. The compound 5 was prepared by the pro-
solution of compound 8 (9.40 g, 16 mmol) in THF/MeOH
ꢀ
12
(1:1), and the mixture was heated to 50 C for 2 h. The reaction
cedure reported by Tang et al. Phenylation of 5 with 3-
mixture was extracted with EtOAc and water, and the organic
fluoronitrobenzene in DMF solution produced compound 6.
However, this reaction required a high temperature, 130 C,
ꢀ
layer was dried over anhydrous MgSO . The crude product
4
was purified by silica column chromatography with EtOAc/
and a long reaction time, 70 h, and unfortunately resulted in
the racemization of the axially chiral binaphthol rings even
though the starting compound 3 was optically pure. Thus, a
special resolution method was to be developed to obtain opti-
cally pure target product, (S)-2. The reduction of the nitro
group to amine to obtain compound 7 was successful by using
hexane (1:2) to obtain compound 9 (8.41 g, yield 96 %). Melt-
ꢀ
1
ing point = 208.2 C, H NMR (CDCl , 300 MHz): δ
3
7
.84–6.20 (m, 23H), 4.76–4.61 (dd, J = 12.9, 59.4 Hz, 2H),
.70 (s, 1H).
3
Compound 2. 2,2,6,6-Tetramethylpiperidine-1-oxyl
(
9
TEMPO, 66 mg, 2 wt%) was added to a slurry of compound
(3.33 g, 6.32 mmol) in acetonitrile and heated to 50 C.
H gas in the presence of Pd/C catalyst. The reaction of com-
2
ꢀ
pound 7 with phenyl isocyanate followed by the deprotection
and the oxidation of the alcohol yielded the racemic product 2.
In the oxidation step, the product was obtained as a pure form
in yellow crystalline solids by precipitation from the extracted
methylene chloride solution.
Methyltriphenylphosphonium peroxydisulfate (MTPPP,
7
6
.10 g, 9.50 mmol) was added to the mixture and refluxed at
ꢀ
0 C. ThereactionwasmonitoredbyTLC. After5 h, thereac-
tion mixture was extracted with CH Cl and water, and a yel-
2
2
lowsolidprecipitatedfromtheorganicsolution, whichwasthe
The enantiomeric separation of the racemic compound 2
was successful by the formation of diastereomeric imines of
2 with (S)-(+)-2-phenylglycinol followed by the chromato-
graphic separation of the diastereomers with an eluent of
EtOAc/CH Cl (1:20) and normal silica column. Finally, an
pure compound 2 in racemic form (2.96 g, yield 90%). Melt-
ꢀ
1
ing point = 197 C, H NMR (DMSO, 300 MHz): δ 10.36 (s,
1
8
7
1
H, OH), 10.26 (s, 1H, CHO), 8.69–8.59 (m, 3H),
1
3
.11–8.01 (m, 3H), 7.49–6.48 (m, 17H); C NMR (DMSO,
5 MHz): δ 197.51 ( CHO), 157.97, 153.49, 152.83,
52.74, 141.52, 139.95, 137.68, 137.23, 133.87, 130.79,
2
2
optically pure (S)-2 was obtained by the hydrolysis of the sin-
gle diastereomeric imine in an acidic condition.
Bull. Korean Chem. Soc. 2015, Vol. 36, 1834–1837
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BULLETIN OF THE
Article Efficient Synthesis of Chiral Binaphthol Aldehyde with Phenyl Ether Linkage
KOREAN CHEMICAL SOCIETY
Reasonable explanations for the preference of the (S)
formof1toward theformationoftheimine withD-aminoacids
over L-amino acids were provided in a previous paper.
cation of the imine. The imine formed between (S)-2 and an
anionic amino acid has a negative charge.
10
The stirring of the MC solution containing (S)-2 and TPPC
together with the aqueous layer containing excess amount of
the sodium salt of a racemic phenylalanine led to the ELLE of
the amino acid from the aqueous layer to the organic layer by a
stereoselective imine formation. Figure 2(a) shows the partial
Figure 1 shows the energy-minimized structures calculated
by using Spartan program for the imines formed between
1
3
D-alanine and (S)-1 and (S)-2.
The uryl plane of the imine of (S)-2 is more tilted toward the
one naphthalene ring and is pulled more toward to the other
naphthalene ring. Itappears that theimineof2hasamorecom-
pact structure. This difference would bring out the difference
in the stereoselectivity of 2 from 1. However, it was meaning-
less topredict bycalculations whether 1or 2 had a higher selec-
tivity because the calculations could not correctly reflect on
factors such as pH and solvent effects.
1
H NMR spectra for the free form of (S)-2 in the presence of
TPPC. The singlet at 8.20 ppm was assigned to the proton of
C-3 carbon of the naphthol ring. Figure 2(b) and (c) shows par-
1
tial H NMR spectra of the imines formed by the reactions of
(S)-2 with optically pure L- and D-phenylalanine, respectively.
Multiplets appearing at 4.0–4.5 ppm are due to the α-proton of
the amino acid of the imines. Figure 2(d) is the spectrum of the
imine formed by (S)-2 and excess DL-phenylalanine under
ELLE condition. The comparison of the spectra apparently
shows that the imine ofD-Phe exists in larger amounts than that
of L-Phe, i.e., the ELLE is successful. The stereoselectivity
could be determined by integration of the signals.
The stereoselectivities for several representative amino
acids of the ELLE, which were measured by the same way,
are listed as the D/L ratios in Table 1. The table also includes
the data of the ELLE experiments carried out using both Ali-
quat 336 and TPPC.
The aldehyde (S)-2 is freely soluble in CH Cl when mixed
2
2
with tetraphenylphosphonium chloride (TPPC) or trioctyl-
methylammonium chloride (Aliquat 336). Aliquat 336 is
1
4
widely used as a phase-transfer catalyst, which is considered
to assist in bringing an anionic amino acid from the aqueous
layer into the organic layer. In this study, TPPC and Aliquat
336 were used in more than a stoichiometric amount relative
to (S)-2 because they play an additional role as a counter-
The aldehyde (S)-2 showed D/L ratios in the range
3.57–11.1, which are similar but slightly lower than those
O
O
OH
OH
OH
OH
OH
OH
a
b
O
OH
3
4
5
O
O
O
O
d
O
O
c
e
NO2
NH2
6
7
O
OH
Figure 1. Model structures of the imines of (S)-1 (a) and (S)-2 (b)
O
O
H
H
N
f
OH
O
H
H
formed with D-alanine, which were calculated by the Spartan
N
N
N
13
program.
O
O
8
9
O
H
OH
O
H
H
N
O
N
H
O
g
OH
O
H
N
H
N
h
i
(S)-2
O
H
OH
O
O
2
H
N
H
N
O
(
R)-2
/BF /THF, 65 C,
0 h, 94%; (b) 2,2-Dimethoxypropane/acetone, H , rt, 20 h, 86%;
ꢀ
Scheme 1. Reagents and conditions: (a) NaBH
2
(
4
3
+
ꢀ
3
/DMF, 80 C, 1 h; (ii) 3-Fluoronitrobenzene/DMF,
c) (i) K
2
CO
ꢀ
1
30 C, 70 h, 62%; (d) Pd/C, H
2
(g)/THF, rt, 20 h, 98%; (e) Phenyliso-
ꢀ
cyanate/CH
g) MTPPP, TEMPO/CH
2
Cl
2
, rt, 5 h, 86%; (f ) HCl/THF, MeOH, 50 C, 2 h, 96%
Figure 2. (a) Free form of (S)-2. (b) Imine formed between (S)-2 and
L-Phe. (c) Imine of D-Phe. (d) Imine formed with excess amount of
DL-Phe.
ꢀ
(
3
CN, 60 C, 5 h, 90%; (h) (S)-(+)-2-phenyl-
glycinol/CH Cl
2
2 2 2
, rt, 12 h; (i) HCl (1 M)/CH Cl , rt, 12 h, 41%.
Bull. Korean Chem. Soc. 2015, Vol. 36, 1834–1837
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BULLETIN OF THE
Article Efficient Synthesis of Chiral Binaphthol Aldehyde with Phenyl Ether Linkage
KOREAN CHEMICAL SOCIETY
Table 1. Diastereomeric ratio (D-amino acid imine)/(L-amino acid
2. (a) V. A. Soloshonok, X. Tang, V. J. Hruby, L. Van Meervelt,
Org. Lett. 2001, 3, 341; (b) D. Z. Lin, J. Wang, X. Zhang,
S. B. Zhou, J. Lian, H. L. Jiang, H. Liu, Chem. Commun.
2013, 49, 2575.
imine) and enatiomeric excess (ee) values (in parentheses) at the
1
organic layer after ELLE with (S)-2, which were determined by H-
NMR studies.
3
. (a) K. Maruoka, T. Ooi, Chem. Rev. 2003, 103, 3013; (b) T. Ooi,
K. Maruoka, Angew. Chem. Int. Ed. 2007, 46, 4222.
Diastereomeric ratio (ee%)
Amino acids
TPPC
Aliquat 336
4. (a) S. M. Kim, J. W. Yang, Org. Biomol. Chem. 2013, 11, 4737;
b) H. Yan, J. S. Oh, J.-W. Lee, C. E. Song, Nat. Commun. 2012,
, 1212.
(
3
Phenylalanine
Serine
Valine
Leucine
Alanine
7.60 (76.8)
3.76 (58.0)
11.1 (83.4)
7.69 (77.0)
3.57 (56.2)
7.14 (75.4)
3.84 (58.8)
11.1 (83.4)
8.33 (78.6)
4.00 (60.0)
5
. (a) P. Merino, E. Marqués-López, T. Tejero, R. P. Herrera, Tet-
rahedron 2009, 65, 1219; (b) J. Gawronski, N. Wascinska,
J. Gajewy, Chem. Rev. 2008, 108, 5227;(c)S. J. Connon, Angew.
Chem. Int. Ed. 2008, 47, 1176.
6
. B. Schuur, B. J. V. Verkuijl, A. J. Minnaard, J. G. de Vries,
H. Heeres, B. L. Feringa, Org. Biomol. Chem. 2011, 9, 36.
. (a) S. C. Peacock, L. A. Domeier, F. C. A. Gaeta, R. C. Helgeson,
J. M. Timko, D. J. Cram, J. Am. Chem. Soc. 1978, 100, 8190; (b)
D. S. Lingenfelter, R. C. Helgeson, D. J. Cram, J. Org. Chem.
of 1. The selectivities of (S)-2 were not significantly affected
by change of Aliquat 336 to TPPC.
7
In these ELLE experiments, the imine formation was com-
pleted in several hours of stirring, and the imine in the organic
layer could be hydrolyzed by stirring with an acidic aqueous
solution. The hydrolysis reproduced the organic layer com-
pletely as in its initial state and transferred the hydrolyzed
amino acid to the aqueous layer. The organic layer could be
recycled again and again without isolation processes such as
evaporation and precipitation. This ensured the economical
1
981, 46, 393; (c) A. Galan, D. Andreu, A. Echavarren,
P. Prados, J. de Mendoza, J. Am. Chem. Soc. 1992, 114, 1511;
(d) K. Naemura, K. Nishioka, K. Ogasahara, Y. Nishikawa,
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H. Itoh, M. Shiode, T. Iwachido, O. Yonemitsu, Inorg. Chem.
8
11
management of the ELLE, as reported in the previous paper.
1
998, 37, 1585; (c) T. B. Reeve, J.-P. Cros, C. Gennari,
U. Piarulli, J. G. de Vries, Angew. Chem. Int. Ed. 2006, 45,
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Conclusion
2
In summary, a new type of chiral binol-aldehyde with phenyl
ether linkage, (S)-2, has been synthesized. During the synthe-
sis, the axially chiral binaphthol ring was racemized as a result
of high reaction temperatures. The enantiomerically pure
product was obtained through the resolution of diastereomeric
imine of 2 formed with an aminoalcohol by normal column
chromatography. The stereoselectivities of (S)-2 toward the
imine formation with amino acids in the ELLE condition were
1
B. L. Feringa, J. Org. Chem. 2009, 74, 6526; (f )
B. J. V. Verkuijl, B. Schuur, A. J. Minnaard, J. G. de Vries,
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1
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measured by H NMR spectroscopic studies.
Acknowledgments. This work was supported by Basic Sci-
ence Research Program through the National Research Foun-
dation of Korea (NRF) funded by the Ministry of Education,
Science and Technology (NRF-2013R1A1A2008959). M.-J.
Jun is grateful to the ReSEAT Program, Korea Institute of Sci-
ence & Technology Information, for support.
1
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