Development of novel guanidine-bisurea bifunctional organocatalysts and their application to asymmetric α-hydroxylation of tetralone-derived β-keto esters

Article abstract of DOI:10.1071/CH14144

A series of guanidine-bisurea bifunctional organocatalysts 4, with chiral centres located outside the urea groups, were synthesized. The novel catalyst 4 is conformationally more flexible than the original catalyst 1. In α-hydroxylation of tetralone- derived β-keto esters, 4 afforded the corresponding alcohols in high yields with moderate enantioselectivity. CSIRO 2014.

Full text of DOI:10.1071/CH14144

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2014, 67, 1017–1020
Full Paper
http://dx.doi.org/10.1071/CH14144
Development of Novel Guanidine–Bisurea Bifunctional
Organocatalysts and their Application to Asymmetric
a-Hydroxylation of Tetralone-derived b-Keto Esters
A
A
A
A
Minami Odagi, Kan Takayama, Kota Furukori, Tatsuya Watanabe,
A B
,
and Kazuo Nagasawa
A
Department of Biotechnology and Life Science, Tokyo University of Agriculture
and Technology, 2-24-16, Naka-cho, Koganei City, 184-8588 Tokyo, Japan.
Corresponding author. Email: knaga@cc.tuat.ac.jp
B
A series of guanidine–bisurea bifunctional organocatalysts 4, with chiral centres located outside the urea groups, were
synthesized. The novel catalyst 4 is conformationally more flexible than the original catalyst 1. In a-hydroxylation of
tetralone- derived b-keto esters, 4 afforded the corresponding alcohols in high yields with moderate enantioselectivity.
Manuscript received: 13 March 2014.
Manuscript accepted: 16 April 2014.
Published online: 28 May 2014.
Introduction
disulfide gave thiourea 6, which was coupled with stearylamine
in the presence of mercury chloride and triethylamine (TEA) to
give guanidine 7. The tert-butoxycarbonyl (Boc) groups in 7
were deprotected with trifluoroacetic acid (TFA), and the
resulting amine was reacted with optically active isocyanates
8a–e derived from the corresponding amines in the presence of
triethylamine to give 4a–e in 56–93 % yield (2 steps).
Initially, we examined a-hydroxylation of b-keto tert-butyl
ester 2a with 4a in the presence of cumene hydroperoxide
(CHP, 1.2 equiv.) and potassium carbonate (1 equiv.) in toluene
at 08C; these conditions were previously developed for catalyst
1a (Table 1). The a-hydroxylation product 3a was obtained
in 80 % yield with 37 % ee (entry 1). (Note, the absolute
a-Hydroxy-b-dicarbonyl moieties are widely found in natural
products, as well as pharmaceuticals,[1] and many synthetic
approaches have been explored, especially from the viewpoint of
enantioselectivity. A straightforward strategy used for the con-
struction of a-hydroxy-b-dicarbonyl compounds is the enantio-
selective direct oxidation at the a-position of b-keto esters using
metal catalysts in combination with chiral ligands and/or chiral
organocatalysts.[2–4] In regards to organocatalytic approaches,
cinchona alkaloid derivatives[4a,d–f,i] and chiral phosphoric
acids[4b] have been reported as efficient catalysts for asymmetric
a-hydroxylation of b-keto esters. In addition, lappaconitine[4c]
and S-timolol analogues[4,g] are known to catalyze the reaction
withmoderate enantioselectivity. Recently, Quetal. reportedthat
TADDOL-derived guanidine effectively catalyzed asymmetric
a-hydroxylation of b-keto esters derived from indanone.[4h]
We have recently reported asymmetric a-hydroxylation of
tetralone-derived b-keto esters mediated by guanidine–bisurea
bifunctional organocatalyst 1a in the presence of cumene
hydroperoxide (CHP), which is a safe and commercially avail-
able oxidant (Scheme 1a).[5o] In this reaction, a-hydroxylation
products were obtained in high yields with high enantioselec-
tivity.[5o,p] During structure–activity relationship studies of the
catalyst 1 that focused on the acidity of the urea moiety, we
found that catalyst 1b, substituted with benzyl groups on the
urea nitrogen atoms, was also effective, and a-hydroxylated 3a
was obtained in good yield with excellent enantioselectivity
(96 % ee, Scheme 1a). This gave us the idea of shifting the chiral
centre in catalyst 1 from the linker position to outside the urea
groups, e.g. catalyst 4 (Scheme 1b). Catalyst 4 is conformation-
ally more flexible than 1, and control of the transition state of the
reaction by 4 is therefore an interesting area.
1a
1b
–
ϭ
ϭ
ϭ
ϭ
(a)
Ϫ
ϩ
n
n
-
1
Њ
3a
2a
1a
1b
(b)
Ϫ
ϩ
4
Scheme 1. a–Hydroxylation of tetralone-derived b-keto ester 2a in the
presence of guanidine–bisurea bifunctional organocatalysts 1a and 1b
(previous work[5o]), and catalysts 4 bearing chiral centres outside the urea
groups (current work).
Results and Discussion
The synthesis of the novel guanidine–bisurea compounds 4 is
illustrated in Scheme 2. Reaction between amine 5 and carbon
Journal compilation Ó CSIRO 2014
www.publish.csiro.au/journals/ajc

1018
M. Odagi et al.
Њ
Њ
5
6
Ϫ
ϩ
Ϫ
ϩ
8a–e
room
Њ
4a–e
7
temperature
8a
ϭ
ϭ
ϭ
ϭ
ϭ
8b
8c
8d
8e
8a–e
Scheme 2. Synthesis of guanidine–bisurea compounds 4a–e. Boc, tert-butoxycarbonyl; TEA, triethylamine; DMF,
N,N–dimethylformamide; TFA, trifluoroacetic acid; THF, tetrahydrofuran.
Table 1. Optimization of reaction conditions with catalyst 4aA
Table 2. Optimization of structure of catalyst 4A
Ϫ
ϩ
Ϫ
ϩ
-
4a
ϭ
-
4
S
Њ
2a
3a
Њ
2a
Catalyst 4
3a
Entry
Base
Solvent
Product 3a
Entry
Product 3a
Yield [%]B
Yield [%]B
ee [%]C
Ar
ee [%]C
1
2
3
4
5
6
7
K2CO3
DBU
Toluene
Toluene
Toluene
Toluene
CH2Cl2
MeCN
80
79
7
37
25
10
27
31
0
1
2
3
4
4b
4c
4d
4e
4-MeO-C6H4
4-NO2-C6H4
1-Naphthyl
2-Naphthyl
81
82
99
80
33
22
50
47
TEA
2 N NaOH(aq)
K2CO3
K2CO3
K2CO3
82
29
16
16
AReaction conditions: 2a (0.1 mmol), CHP (0.12 mmol), K2CO3 (0.1 mmol),
and catalyst 4 (0.005 mmol) in toluene (2 mL) at 08C for 24 h.
BDetermined by 1H NMR spectroscopy.
Ethyl acetate
13
AReaction conditions: 2a (0.1 mmol), CHP (0.12 mmol), base (0.1 mmol),
and 4a (0.005 mmol) in solvent (2 mL) at 08C for 24 h.
BDetermined by 1H NMR spectroscopy.
CDetermined by chiral HPLC.
However, no improvement in the selectivity was observed.
Moreover, the yield of 3a declined drastically (Table 1,
entries 5–7).
CDetermined by chiral HPLC.
stereochemistry of 3a was assigned by comparison with previ-
ous reported data.[3b]) Although the enantioselectivity was only
37 % ee as compared with 95 % ee in the case of 1a, it is
noteworthy that the outer chiral centres in 4a were at least
partially effective in controlling the transition state of the
reaction. Thus, the reaction conditions were further investigated
to improve the selectivity for 3a in the reaction mediated by 4a.
First, the effects of base on the reaction were investigated
(Table 1, entries 2–4). In the case of 1,8-diazabicyclo[5.4.0]
undec-7-ene (DBU), 3a was obtained in 79 % yield, but the
enantioselectivity decreased to 25 % ee (entry 2). When the base
was switched to triethylamine, the yield and enantioselectivity
decreased to 7 % and 10 % ee, respectively (entry 3). Biphasic
conditions were then examined with 2 N NaOH in toluene,
but the selectivity was still poor (27 % ee, entry 4). The solvent
was then changed to dichloromethane, acetonitrile, and ethyl
acetate in the presence of catalyst 4a and potassium carbonate.
Next, the structure of catalyst 4 was explored, focusing on
the Ar group under the conditions listed in entry 1 of Table 1
(Table 2, entries 1–4). The phenyl group in 4a was changed to
4-methoxyphenyl 4b, 4-nitrophenyl 4c, 1-naphthyl 4d, and
2-naphthyl 4e. Among these compounds, 4d was the most
effective for the reaction, affording 3a in 99 % yield with
50 % ee. This was the best result achieved so far with 4.
Based on the conditions developed in Table 2 (entry 3), we
investigated the performance of organocatalyst 4d towards the
a-hydroxylation of b-keto esters 2b–g (Table 3). Compounds 2b
and 2c, which have a benzyloxy group at the 6-position and a
methoxy group at the 7-position on the aromatic ring, respec-
tively, afforded the corresponding a-hydroxylation products 3b
and 3c in high yields (93 % and 98 %, entries 1 and 2) with
moderate enantioselectivity (48 % ee and 44 % ee, respectively).
Bis-methoxy b-keto ester 2d afforded a low yield (11 %, entry 3)
and low enantioselectivity (20 % ee). Halogenated substrates

Guanidine–Bisurea Bifunctional Organocatalysts
1019
Table 3. Substrate performance in a-hydroxylation of 2A
[D4]methanol (MeOH) were reported on the scale relative to
[D]chloroform (d 7.26 ppm) and [D4]MeOH (d 3.30 ppm),
respectively. For 13C NMR spectroscopy, chemical shifts were
reported on the scale relative to [D]chloroform (d 77.0 ppm),
[D4]MeOH (d 49.0 ppm), and as internal references. Mass
spectra were recorded on a JMS–T100 LC (JEOL) spectrometer.
Ϫ
ϩ
-
4d
Catalyst Synthesis (4a–e)
Compound 6
Њ
2
3
To a solution of N-(tert-butoxycarbonyl)-1,2-ethylenedia-
mine (5) (3.75 g, 23.25 mmol) in ethanol (EtOH) (150 mL) was
added CS2 (780 mL, 12.79 mmol) at room temperature, and the
resulting mixture was heated at 708C for 12 h. The reaction
mixture was concentrated under reduced pressure and the
residue was purified by flash column chromatography to give
6 (2.47 g, 29 %). The spectroscopic data correspond to previ-
ously reported data.[6]
Entry
Substrate 2
Product 3
Yield [%]B
X
ee [%]C
1
2
3
4
5
6
2b
2c
2d
2e
2f
6-OBn
7-OMe
3b
3c
3d
3e
3f
93
98
11
73
70
90
48
44
20
45
42
51
5,8-OMe
6-Cl
6-Br
Compound 7
2g
7-F
3g
To a mixture of 6 (807 mg, 2.23 mmol), stearylamine
(900 mg, 3.34 mmol), and triethylamine (940 mL, 6.68 mmol)
in DMF (11 mL) was added HgCl2 (906 mg, 3.34 mmol) at room
temperature, and the resulting mixture was stirred for 12 h at
808C. The resulting mixture was diluted with ethyl acetate, and
filtered though a pad of Celite. The filtrates were washed
with saturated NH4Cl(aq), and then dried over MgSO4. After
removing the solvent under reduced pressure, the residue was
purified by flash column chromatography on silica gel to give
7 (1.19 g, 85 %). dH (CD3OD, 400 MHz) 3.40–3.17 (10H, m),
1.75–1.60 (2H, m), 1.48 (18H, s), 1.43–1.27 (30H, m), 0.92 (3H,
t, J 6.9 Hz). dC (CD3OD, 75 MHz) 160.0, 156.9, 81.35, 44.0,
43.8, 41.0, 33.9, 31.7, 31.5, 31.3, 31.3, 30.7, 29.7, 28.8, 24.6,
15.4, 15.3. m/z 598.5296. HRMS (ESI, M–Cl) Anal. Calc. for
C33H68N5O4 598.5271.
AReaction conditions: 2 (0.1 mmol), CHP (0.12 mmol), K2CO3 (0.1 mmol),
and 4d (0.005 mmol) in toluene (2 mL) at 08C for 24 h.
BDetermined by 1H NMR spectroscopy.
CDetermined by chiral HPLC.
ϩ
4d
3a
2a
Fig. 1. Plausible transition state model of a-hydroxylation of 2a catalyzed
by 4d.
Typical Procedure for Preparation of Compound 4
To a solution of 7 (100 mg, 0.16 mmol) in CH2Cl2 (1.6 mL) was
added TFA (600 mL) at 08C. The reaction mixture was stirred at
room temperature for 1 h, and the resulting mixture was con-
centrated under reduced pressure to give diamine. To a solution
of diamine and triethylamine (190 mL, 1.33 mmol) in THF
(1.6 mL) was added (S)-(ꢀ)-1-phenylethyl isocyanate (140 mL,
1.00 mmol), and the mixture was stirred at room temperature for
12 h. The reaction mixture was concentrated under reduced
pressure, and the residue was purified by column chromato-
graphy on silica gel to give bisurea. The counter anion of the
catalyst was exchanged to chloride by treatment with saturated
NH4Cl(aq) in ethyl acetate to give 4a (208 g, 86 % in 2 steps).
[a]D 258 ꢀ33.2 (c 1.0 in CHCl3). dH (CD3OD, 400 MHz) 7.34–
7.27 (8H, m), 7.27–7.19 (2H, m), 4.82 (2H, q, J 6.9), 3.37–3.06
(8H, m), 3.01 (2H, t, J 7.3), 1.59–1.48 (2H, m), 1.42 (6H, d, J
6.9), 1.38–1.23 (30H, m), 0.92 (3H, t, J 6.6). dC (CD3OD,
75 MHz) 161.8, 156.7, 147.3, 130.3, 128.7, 127.6, 51.8, 50.9,
44.9, 43.6, 40.4, 33.9, 31.7, 31.3, 31.3, 30.6, 28.7, 24.6, 15.4.
m/z 692.5560. HRMS (ESI, M–Cl) Anal. Calc. for C41H70N7O2
692.5591.
2e–g gave 3e–g in high yields (70–90 % yield) with moderate
enantioselectivity (42–51 % ee, entries 4–6).
A plausible transition model for the 4d-catalyzed a-hydrox-
ylation of 2a is shown in Fig. 1. A guanidine and a urea group in
4d interact with b-keto ester 2a and CHP, respectively, so that
the enolate 2 approaches the oxidant from the Si face to avoid the
bulky 1-naphthyl group of 4d, predominantly affording (S)-3a.
Conclusion
We have developed a series of novel guanidine–bisurea
bifunctional organocatalysts 4, bearing chiral centres outside the
urea moieties. Asymmetric a-hydroxylation of tetralone-
derived b-keto esters in the presence of cumene hydroperoxide
as oxidant generated the corresponding a-hydroxy-b-keto esters
in excellent yields (up to 99 %) with moderate enantioselectivity
(up to 51 % ee).
Experimental
Flash chromatography was performed using silica gel 60
(spherical, particle size 0.040–0.100 mm; Kanto Co., Inc.,
Japan). Optical rotations were measured on a JASCO P–2200
polarimetre. 1H and 13C NMR spectra were recorded on AL300,
ECX400 (JEOL), and AVANCE 400 (Bruker) instruments. For
1H NMR spectroscopy, chemical shifts in [D]chloroform and
Compound 4b
[a]D 258 ꢀ38.4 (c 1.0 in CHCl3). dH (CD3OD, 400 MHz)
7.18 (4H, d, J 8.7), 6.82 (4H, d, J 8.7), 4.74 (2H, q, J 6.9), 3.73
(6H, s), 3.33–3.05 (8H, m), 3.00 (2H, t, J 7.3), 1.57–1.45 (2H,
m), 1.36 (6H, d, J 6.9), 1.28–1.19 (30H, m), 0.88 (3H, t, J 6.9).

1020
M. Odagi et al.
dC (CD3OD, 75 MHz) 161.8, 160.8, 156.7, 139.2, 128.8, 115.6,
56.5, 51.2, 44.8, 43.6, 40.4, 33.9, 31.6, 31.3, 31.3, 30.7, 28.7,
24.6, 24.5, 15.4. m/z 752.5848. HRMS (ESI, M–Cl) Anal. Calc.
for C43H74N7O4 752.5802.
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Compound 4c
[a]D 258 ꢀ45.1 (c 1.0 in CHCl3). dH (CD3OD, 400 MHz) 8.20
(4H, d, J 8.7), 7.57 (4H, d, J 8.7), 4.93 (2H, q, J 7.1), 3.38–3.14
(8H, m), 3.03–2.96 (2H, m), 1.54–1.42 (8H, m), 1.36–1.21 (30H,
m), 0.92 (3H, t, J 6.9). dC (CD3OD, 75 MHz) 161.6, 156.8,
155.5, 149.0, 128.8, 125.5, 51.7, 50.7, 44.6, 43.7, 43.6, 40.5,
33.9, 31.6, 31.3, 31.2, 30.6, 28.7, 24.6, 24.1, 15.3. m/z 782.5244.
HRMS (ESI, M–Cl) Anal. Calc. for C41H68N9O6 782.5293.
[4] (a) For catalytic asymmetric a-hydroxylation of 1,3-dicarbonyl com-
pounds using organocatalysts, see: M. R. Acocella, O. G. Mancheco,
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JO048655W
Compound 4d
[a]D 258 ꢀ33.2 (c 1.0 in CHCl3). dH (CD3OD, 400 MHz) 8.13
(2H, d, J 7.8), 7.92–7.83 (2H, m), 7.70 (2H, d, J 8.2), 7.56–7.40
(8H, m), 5.65 (2H, q, J 6.4), 3.37–2.95 (8H, m), 2.89–2.79
(2H, m), 1.55 (6H, d, J 6.4), 1.42–1.03 (32H, m), 0.91 (3H, t, J
6.9). dC (CD3OD, 75 MHz) 161.7, 156.6, 142.8, 136.1, 132.7,
130.7, 129.4, 127.9, 127.3, 124.9, 123.8, 50.7, 47.8, 44.9, 43.5,
40.4, 33.9, 31.6, 31.5, 31.3, 31.1, 30.5, 28.5, 24.6, 23.7, 15.4. m/z
792.5854. HRMS (ESI, M–Cl) Anal. Calc. for C49H74N7O2
792.5904.
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Compound 4e
[a]D 258 þ23.7(c 1.0 inCHCl3). dH (CD3OD, 400 MHz)7.34–
7.27 (8H, m), 7.47–7.19 (6H, m), 4.82 (2H, q, J 6.9), 3.37–3.06
(8H, m), 3.01 (2H, t, J 7.3), 1.59–1.48 (2H, m), 1.42 (6H, d, J 6.9),
1.38–1.23 (30H, m), 0.92 (3H, t, J 6.6). dC (CD3OD, 75MHz)
161.9, 156.7, 147.3, 130.3, 128.7, 127.6, 51.8, 50.9, 44.9, 43.6,
40.4, 33.9, 31.7, 31.3, 31.3, 30.6, 28.7, 24.6, 15.4. m/z 792.5871.
HRMS (ESI, M–Cl) Anal. Calc. for C49H74N7O2 792.5904.
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talysts, see: Y. Sohtome, A. Tanatani, Y. Hashimoto, K. Nagasawa,
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Supplementary Material
(c) Y. Sohtome, Y. Hashimoto, K. Nagasawa, Eur. J. Org. Chem. 2006,
2894. doi:10.1002/EJOC.200600307
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1H and 13C NMR spectra of catalysts 4a–e and HPLC charts of
3a–g are available on the Journal’s website.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research on Inno-
vative Areas ‘‘Advanced Molecular Transformations by Organocatalysts’’
(No. 23105013) from The Ministry of Education, Culture, Sports, Science and
Technology, Japan.
(e) K. Takada, N. Nagasawa, Adv. Synth. Catal. 2009, 351, 345.
doi:10.1002/ADSC.200800692
(f) S. Tanaka, K. Nagasawa, Synlett 2009, 667.
(g) K. Takada, S. Tanaka, K. Nagasawa, Synlett 2009, 1643.
(h) Y. Sohtome, B. Shin, N. Horitsugi, R. Takagi, K. Noguchi,
K. Nagasawa, Angew. Chem. Int. Ed. 2010, 49, 7299. doi:10.1002/
ANIE.201003172
(i) T. Sohtome, S. Tanaka, K. Takada, T. Yamaguchi, K. Nagasawa,
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