Synthesis and biological evaluation of ortho-carborane containing benzoxazole as an inhibitor of hypoxia inducible factor (HIF)-1 transcriptional activity

Article abstract of DOI:10.1016/j.jorganchem.2013.04.007

ortho-Carborane and adamantane containing benzoxazoles were synthesized by intramolecular dehydration of the corresponding phenoxyacetanilides. Among the compounds synthesized, ortho-carborane containing benzoxazole 2b which has a carboxylic group on the benzoxazole ring, exhibited significant inhibition of hypoxia-induced HIF-1 transcriptional activity with the IC₅₀ value of 14.4 μM toward HeLa cell-based reporter gene assay.

Full text of DOI:10.1016/j.jorganchem.2013.04.007

Journal of Organometallic Chemistry xxx (2013) 1e6
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and biological evaluation of ortho-carborane containing benzoxazole
as an inhibitor of hypoxia inducible factor (HIF)-1 transcriptional activity
*
Hiroyuki Nakamura , Yuka Yasui, Hyun Seung Ban
Department of Chemistry, Faculty of Science, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171-8588, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
ortho-Carborane and adamantane containing benzoxazoles were synthesized by intramolecular dehy-
dration of the corresponding phenoxyacetanilides. Among the compounds synthesized, ortho-carborane
containing benzoxazole 2b which has a carboxylic group on the benzoxazole ring, exhibited significant
Received 15 March 2013
Received in revised form
31 March 2013
inhibition of hypoxia-induced HIF-1 transcriptional activity with the IC50 value of 14.4
cell-based reporter gene assay.
mM toward HeLa
Accepted 2 April 2013
Ó 2013 Elsevier B.V. All rights reserved.
Dedicated to Professor Vladimir Bregadze
on the occasion of his 75th anniversary.
Keywords:
ortho-Carborane
Hypoxia inducible factor
Transcription
Inhibition
Benzoxaziles
1. Introduction
HIF-1 transcriptional activity by inducing HIF-1
through proteasome pathway under hypoxic conditions [8]. Further
detailed study revealed that LW6 induces HIF-1 degradation by
activating the von HippeleLindau (VHL) tumor suppressor protein
[9], which recognizes specific prolyl residues of HIF-1 hydroxyl-
ated by prolyl hydroxylase (PHD) [10,11], leading to rapid degra-
dation by the ubiquitineproteasome system [12e15]. We focused
on the adamantyl moiety of LW6 and replaced it by an ortho-car-
borane moiety [16]. It is known that carboranes (dicarba-closo-
dodecaboranes: C2B10H12) exhibit remarkable thermal stability and
their icosahedral geometry and exceptional hydrophobicity may
allow their use as hydrophobic pharmacophores similar to an
adamantyl group in biologically active compounds [17,18]. We
recently found that ortho-carborane containing phenox-
a degradation
Hypoxia inducible factor (HIF)-1 is a transcription factor that
regulates angiogenesis, invasion, metastasis, glucose uptake, and
cell survival during cancer development [1,2]. HIF-1 is a basic helix-
loop-helix heterodimeric complex composed of a hypoxia inducible
a
a
subunit HIF-1
a and a constitutively expressed subunit HIF-1b. HIF-
1
a
is readily degraded through an oxygen-dependent degradation
pathway by proteasome under aerobic conditions. However, under
hypoxia, HIF-1 is stabilized and translocated into the nucleus to
dimerize with HIF-1b. This a/heterodimeric complex binds to
specific nucleotide sequences (hypoxia response elements, HREs)
with co-activators to activate various hypoxia responsive genes
including vascular endothelial growth factor (VEGFR), erythropoi-
etin, insulin-like growth factor, hem oxygenase-1, and inducible
yacetanilides possess a potent inhibitory effect on HIF-1a activation
nitric oxide synthase [3,4]. Because overexpression of HIF-1
associated with poor prognosis and resistance to treatment of
a
is
under hypoxia. Indeed the phenoxyacetanilides 1a and 1b, which
have a methyl ester or a boronic acid group on the aniline ring,
displayed significant inhibition of HIF-1 transcriptional activity in
HeLa cell-based reporter gene assay with IC50 values of 31 and
cancer patients [5], HIF-1
tumor agents [6,7].
a is one of the potential targets for anti-
Lee and coworkers reported that LW6 (Fig. 1), which contains an
adamantyl group in a molecule, possesses significant inhibition of
2.2
shock protein (Hsp) 60 is the primary target of these ortho-car-
borane containing phenoxyacetanilides and affects HIF-1 accu-
mM, respectively (Fig. 1) [19]. Furthermore, we found that Heat
a
mulation directly or indirectly [20,21]. Both compounds inhibited
Hsp60 chaperone activity using the porcine heart malate dehy-
drogenase (MDH) refolding assay [21,22]. Interestingly, the
* Corresponding author. Tel.: þ81 03 3986 0221.
E-mail address: hiroyuki.nakamura@gakushuin.ac.jp (H. Nakamura).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.04.007
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j.jorganchem.2013.04.007

2
H. Nakamura et al. / Journal of Organometallic Chemistry xxx (2013) 1e6
R2
R1
O
O
O
O
H
N
H
N
H
OH
OH
LW6 (R1 = CO2Me)
GN26192 (R1 = B(OH)2)
1a: R2 = CO2Me
b: R2 = B(OH)2 (GN26361)
O
R3
O
O
This work
H
N
= BH
= C
2a: R3 = OMe
b: R3 = OH
Fig. 1. Structures of ortho-carborane containing phenoxyacetanilides 1 and benzoxazoles 2.
adamantyl derivatives such as LW6 and GN26192, having a methyl
ester and a boronic acid group at the R1 position showed no inhi-
bition of Hsp60 chaperone activity, indicating that the carborane
framework is essential for the inhibition of Hsp60 chaperone ac-
tivity [19,23]. In this paper, we developed ortho-carborane con-
taining benzoxazoles 2 as alternative candidates for inhibitors of
hypoxia-induced HIF-1 transcriptional activity.
TsOH in aqueous toluene was found to be effective for this ben-
zoxazole ring formation to give 12 in 39% yield (entry 2). Further-
more, addition of molecular sieves 3A dramatically enhanced the
reaction yield and 12 was obtained in 85% yield (entry 3).
An ethynyl moiety was introduced into the phenoxy ring of the
benzoxazole 12 using the Sonogashira coupling reaction. The
benzoxazole 12 was reacted with ethynyltrimethylsilane in the
presence of PdCl2(PPh3)2 catalyst in DMF under microwave
2. Results and discussions
In general, ortho-carboranes can be readily synthesized from the
corresponding acetylenes and decaborane in the presence of Lewis
bases. Therefore, we first tried to prepare the acetylene derivative 8
as shown in Scheme 1. 4-Iodophenol 3 was chosen as a starting
material and treated with ethyl chloroacetate under basic condi-
tions at room temperature to give phenoxyacetyl ester 4 in 99%
yield. Hydrolysis of ethyl ester 4 was carried out using LiOH in
aqueous THF at room temperature to afford the corresponding acid
5 in 98% yield. The amide formation of the 4-iodophenoxyacetic
acid 5 with aniline 11, which was prepared from 4-hydroxy-3-
nitrobenzoic acid 9 in two steps as shown in Scheme 2 [24], was
carried out using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
(EDCI) and hydroxybenzotriazole (HOBt) in the presence of diiso-
propylethylamine (DIPEA) in DMF to give corresponding 4-
iodophenoxyacetanilide 6 in 73% yield. The Sonogashira coupling
reaction was employed for the introduction of an ethynyl moiety
into the phenoxy ring. The reaction of 4-iodophenoxyacetanilide 6
with ethynyltrimethylsilane proceeded in the presence of
PdCl2(PPh3)2 catalyst in DMF under microwave irradiation condi-
tion and the resulting coupling products were treated with K2CO3
in THFeMeOH solution to remove the trimethylsilyl group from the
ethynyl function. The corresponding terminal acetylene 7 was ob-
tained in 60% in two steps. Intermolecular dehydration of 7 was
attempted under acidic conditions, such as tosylic acid (TsOH)
monohydroxide in toluene and trifluoroacetic acid (TFA) in acetic
acid (AcOH), however the desired benzoxazole 8 was not obtained.
We considered that the terminal acetylene moiety would be labile
under such harsh conditions. Therefore, we optimized the reaction
conditions of the benzoxazole ring formation from the iodide 6 as
shown in Scheme 3. Although the benzoxazole 12 was obtained in
9% yield in TFA and AcOH under microwave conditions (entry 1),
O
O
OH
a
b
OEt
I
I
3
4
O
OMe
O
O
c
O
O
OH
N
H
OH
I
I
5
6
O
OMe
O
d
O
N
H
OH
7
O
OMe
O
e
O
N
8
Scheme 1. Reagents and conditions: (a) ClCH2CO2Et, K2CO3, DMF, rt, overnight, 99%;
(b) LiOHeH2O, THF/H2O, rt, 1 h, 98%; (c) 11, EDCI, HOBt, DIPEA, DMF, rt, overnight, 73%;
(d) i. ethynyltrimethylsilane, PdCl2(PPh3)2, CuI, PPh3, DEA, DMF, m.w. 120 ꢀC, 5 min; ii.
K2CO3, MeOH, THF, rt, 12 h, 60% in two steps; (e) TsOH, H2O, toluene, reflux or TFA,
AcOH, m.w. 200 ꢀC.
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H. Nakamura et al. / Journal of Organometallic Chemistry xxx (2013) 1e6
3
O
O
OH
O
OMe
O
OMe
O
a
b
OMe
O
OMe
O
a
O
N
O
N
O2N
O2N
H2N
OH
OH
10
OH
11
I
8
12
9
O
Scheme 2. Reagents and conditions: (a) H2SO4, MeOH, reflux, 7 h, 98%; (b) H2, Pd/C,
R2
EtOH, rt, overnight, 99%.
O
O
H
N
b
irradiation condition and the resulting coupling products were
treated with K2CO3 in THFeMeOH solution in a similar manner to
give the corresponding benzoxazole 8 in 92% yield.
2a: R2 = OMe
b: R2 = OH
c
Finally, the benzoxazole 8 having a terminal alkyne group was
reacted with decaborane (14) in the presence of acetonitrile in
toluene at 80 ꢀC for 2 d to give the corresponding ortho-carborane
containing benzoxazole 2a in 14% yield. The methyl ester of 2a was
converted to the corresponding carboxylic acid 2b by treating with
aluminum tribromide and dimethylsulfide in CH2Cl2 at room
temperature followed by quenching with 1 N hydrochloric acid and
2b was obtained in 91% yield (Scheme 4).
We also synthesized the corresponding adamantly derivatives
14 from adamantyl phenol 13 as shown in Scheme 5 [25]. Ada-
mantyl phenol 11 was treated with ethyl chloroacetate in DMF
using K2CO3 as a base at room temperature and the resulting ester
obtained in 86% yield was hydrolyzed with lithium hydroxide in
aqueous THF at room temperature for 1 h and the resulting to give
4-adamantylphonoxyacetic acid 14 in 94% yield. The amide for-
mation of the 4-adamantylphonoxyacetic acid 14 with aniline 11
was carried out using EDCI and HOBt in the presence of DIPEA in
DMF to give corresponding 4-adamantylphonoxyacetanilide 15 in
80% yield. Intermolecular dehydration of 15 proceeded in the
presence of TsOH monohydroxide in toluene and the desired ben-
zoxazole 16a was obtained in 68% yield. Hydrolysis of 16a using
aluminum tribromide and dimethylsulfide in CH2Cl2 at room
temperature afforded the corresponding carboxylic acid 16b in 91%
yield.
Scheme 4. Reagents and conditions: (a) i. ethynyltrimethylsilane, PdCl2(PPh3)2, CuI,
PPh3, DEA, DMF, m.w. 120 ꢀC, 5 min; ii. TBAF, THF, rt, 12 h, 92% in two steps; (b)
decaborane (14), MeCN, toluene 80 ꢀC, 2 d, 14%; (c) i. AlBr3, Me2S, CH2Cl2, rt, 2.5 h, then
1 N HCl, rt, 1.5 h, 91%.
not inhibit hypoxia-induced HIF-1 transcriptional activity signifi-
cantly at 100
derivative 2b showed a potent inhibition and its IC50 value was
14.4 M, which is slightly more potent than that of LW6. The similar
mM. Interestingly, the corresponding carboxylic acid
m
tendency was observed in the adamantyl benzoxazole derivatives
16. The methyl ester 16a did not show a significant inhibition of
hypoxia-induced HIF-1 transcriptional activity, whereas its car-
boxylic acid derivative 16b inhibited significantly with the IC50 of
22.3
mM. These results indicate that a carboxylic acid moiety is
essential for a significant inhibition of hypoxia-induced HIF-1
transcriptional activity in the case of benzoxazole derivatives.
3. Conclusions
We succeeded in synthesis of ortho-carborane and adamantane
containing benzoxazoles 2 and 16. Addition of molecular sieves 4A
The synthesized ortho-carborane and adamantane containing
benzoxazoles 2 and 16 were evaluated for their ability to inhibit
hypoxia-induced HIF-1 transcriptional activity using a cell-based
reporter gene assay in HeLa cells expressing HRE-dependent
firefly luciferase reporter construct (HRE-Luc) and constitutively
expressing CMV-driven Renilla luciferase reporter [26]. The com-
pound concentrations required to inhibit the relative light units by
50% (IC50) were summarized in Table 1. The phenoxyacetanilide 1a,
which was synthesized previously, showed a moderate inhibitory
O
O
OH
OH
a,b
14
13
O
OMe
O
O
c
activity (IC50 ¼ 70.2
mM), however its benzoxazole derivative 2a did
O
N
H
OH
O
O
OMe
15
OMe
O
a
O
O
O
N
O
N
H
R3
d
OH
I
I
12
6
O
N
entry
condition
yield, %
16a: R3 = OMe
b: R3 = OH
e
TFA, AcOH, MW 200oC
TsOH, H2O, toluene, reflux
TsOH, H2O, MS 3A, toluene, reflux
1
2
3
9
39
85
Scheme 5. Reagents and conditions: (a) ClCH2CO2Et, K2CO3, DMF, rt, overnight, 86%;
(b) LiOHeH2O, THF/H2O, rt, 1 h, 94%; (c) 11, EDCI, HOBt, DIPEA, DMF, rt, overnight, 80%;
(d) TsOHeH2O, toluene, reflux, overnight, 68%; (e) i. AlBr3, Me2S, CH2Cl2, rt, 2.5 h, then
1 N HCl, rt, 1.5 h, 91%.
Scheme 3. Optimization of benzoxazole ring formation.
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4
H. Nakamura et al. / Journal of Organometallic Chemistry xxx (2013) 1e6
Table 1
(12 mL) was stirred at room temperature for 5 min. The reaction
was quenched by HCl solution (1 N) and the organic solvent was
removed under reduced pressure. The crude products were
extracted with ethyl acetate, washed with saturated aqueous NaCl
solution, dried over anhydrous MgSO4, and concentrated to give
product 5 as a spectroscopically pure form (1.36 g, 4.9 mmol, 98%)
Inhibition of HIF-1 transcriptional activity on HeLa cell-based HRE and CMV dual
luciferase assay.a
Compound
IC50
/
m
M
ꢂSD
1a
2a
2b
16a
16b
LW6
70.2
5.19
>100
14.4
>100
22.3
23.2
as a white solid: 1H NMR (400 MHz; CD3OD):
d
7.48 (d, J ¼ 8.6 Hz,
0.63
2H), 6.66 (d, J ¼ 8.6 Hz, 2H), 4.54 (s, 2H); 13C NMR (100 MHz;
0.28
4.25
CD3OD)
d 172.3, 159.4, 139.4, 118.1, 84.1, 65.8; MS (ESI) m/z 276
[M ꢁ H]ꢁ.
a
The drug concentration required to inhibit the relative light units by 50% (IC50
)
was determined from semi-logarithmic doseeresponse plots and the results
represent means ꢂ standard deviation (SD) of triplicate samples.
4.1.3. Methyl-4-hydroxy-3-[2-(4-iodophenoxy)acetamido]
benzoate (6)
A mixture of 5 (846 mg, 3.0 mmol), 11 (869 mg, 5.2 mmol), EDCI
(997 mg, 5.2 mmol), HOBt (796 mg, 5.2 mmol), DIPEA (0.89 mL,
5.2 mmol) was dissolved DMF (5 mL) and stirred at room temper-
ature overnight. The reaction was quenched by 1 N of HCl solution
and the reaction mixture was extracted with ethyl acetate,
neutralized with saturated aqueous NaHCO3 solution, washed with
saturated aqueous NaCl solution, dried over anhydrous Na2SO4, and
concentrated. The resulting solid was washed with cold methanol
to afford 6 (939 g, 2.2 mmol, 73%) as a white solid. This solid
compound was pure enough to perform the next step: 1H NMR
is essential for the formation of benzoxazole ring through intra-
molecular dehydration of 6. The decaborane coupling of the acet-
ylene derivative 8 with decaborane (14) proceeded successfully for
introduction of an ortho-carborane moiety in the molecules,
although the yield was not satisfactory. The effects of synthesized
compounds on hypoxia-induced HIF-1 transcriptional activity were
examined using a cell-based dual luciferase reporter gene assay in
HeLa cells. Both acid derivatives of ortho-carborane and ada-
mantane containing benzoxazoles, 2b and 16b, respectively, have
significant inhibitory potency similar to LW6, indicating that both
compounds are potential candidates of HIF-1 signal targeting
anticancer agents.
(400 MHz; CDCl3):
d
9.50 (s, 1H), 8.53 (s, 1H), 7.85 (d, J ¼ 8.2 Hz, 1H),
7.78 (s, 1H), 7.67 (d, J ¼ 8.4 Hz, 2H), 7.07 (d, J ¼ 8.2 Hz, 1H), 6.80 (d,
J ¼ 8.4 Hz, 2H), 4.67 (s, 2H), 3.90 (s, 3H); 13C NMR (100 MHz; DMSO)
d
166.5, 166.2, 157.7, 152.0, 138.3, 126.8, 125.7, 122.3, 120.5, 117.7,
4. Experimental section
114.9, 84.4, 67.2, 52.0; MS (ESI) m/z 428 [M þ H]þ, 450 [M þ Na]þ,
466 [M þ K]þ.
4.1. General
4.1.4. Methyl-3-[2-(4-ethynylphenoxy)acetamido]-4-hydroxybenz-
oate (7)
1H NMR and 13C NMR spectra were measured on a JEOL JNM-AL
300 (300 MHz) or VARIAN UNITY-INOVA 400 (400 MHz) spec-
trometers. Chemical shifts of 1H NMR were expressed in parts per
To a mixture of 6 (427 mg, 1.0 mmol), PdCl2(PPh3)2, (39 mg,
0.056 mmol), CuI (11 mg, 0.056 mmol), and triphenylphosphine
(52 mg, 0.2 mmol) in DMF (1 mL) were added DEA (1.6 mL,
15 mmol) and ethynyltrimethylsilane (0.7 mL, 5 mmol) and the
mixture was stirred at 120 ꢀC under microwave conditions. The
reaction completed in 5 min. After cooling, the mixture was diluted
with ether, washed with 1 N of HCl solution, neutralized with
saturated aqueous NaHCO3 solution, washed with saturated
aqueous NaCl solution, dried over anhydrous Na2SO4, and
concentrated. The residue was dissolved in a 1:1 mixture of
methanol and THF (10 mL) and K2CO3 was added. The mixture was
stirred at room temperature for 12 h and then methanol was
removed in vacuo. The mixture was extracted with ethyl acetate,
washed with saturated aqueous ammonium chloride solution and
saturated aqueous NaCl solution, dried over anhydrous MgSO4, and
concentrated. Purification by silica gel column chromatography
with ethyl acetate gave 7 (194 mg, 0.6 mmol, 60% in 2 steps) as a
million downfield from CDCl3 as an internal standard (
CDCl3. Chemical shifts of 13C NMR were expressed in parts per
million downfield from CDCl3 as an internal standard (
¼ 77.0) in
d
¼ 7.24) in
d
CDCl3. Analytical thin layer chromatography (TLC) was performed
on a glass plates (Merck Kieselgel 60 F254, layer thickness 0.2 mm or
RP-18 F254s, layer thickness 0.2 mm). Visualization was accompa-
nied by UV light (254 nm), I2 and KMnO4. Column chromatography
was performed on silica gel (Merck Kieselgel 70e230 mesh). All
reactions were carried out under argon atmosphere using standard
Schlenk techniques. Most chemicals and solvents were analytical
grade and used without further purification.
4.1.1. Ethyl-2-(4-iodophenoxy)acetate (4)
To a mixture of 4-iodo-phenol 3 (880 mg, 4.0 mmol) and K2CO3
(1.66 g, 12 mmol) in DMF (5 mL) was added ethyl chloroacetate
(0.51 mL, 4.8 mmol) at room temperature and the mixture was
stirred overnight. The reaction was quenched by water and the
mixture was extracted with ethyl acetate, washed with saturated
aqueous NaCl solution, dried over anhydrous MgSO4, and concen-
trated. Purification by silica gel column chromatography with
hexane/ethyl acetate (20:1) gave 4 (1.21 g, 4.0 mmol, 99%) as a
white solid: 1H NMR (400 MHz; CDCl3):
d 9.52 (s, 1H), 8.54 (s, 1H),
7.85 (d, J ¼ 8.4 Hz, 1H), 7.78 (s, 1H), 7.52 (d, J ¼ 6.9 Hz, 2H), 7.07 (d,
J ¼ 8.4 Hz, 1H), 6.97 (d, J ¼ 6.9 Hz, 2H), 4.71 (s, 2H), 3.90 (s, 3H), 3.06
(s, 1H); MS (ESI) m/z 326 [M þ H]þ.
4.1.5. Methyl-2-[(4-ethynylphenoxy)methyl]benzoxazole-5-carbox-
ylate (8)
white solid: 1H NMR (400 MHz; CDCl3):
d
6.80 (d, J ¼ 6.8 Hz, 2H),
6.69 (d, J ¼ 6.8 Hz, 2H), 4.59 (s, 2H), 4.27 (q, J ¼ 7.0 Hz, 2H), 1.30 (t,
To a mixture of 12 (409 mg, 1.0 mmol), PdCl2(PPh3)2, (39 mg,
0.056 mmol), CuI (11 mg, 0.056 mmol), and triphenylphosphine
(52 mg, 0.2 mmol) in DMF (1 mL) were added DEA (1.6 mL,
15 mmol) and ethynyltrimethylsilane (0.7 mL, 5 mmol) and the
mixture was stirred for 5 h under reflux. After cooling, the mixture
was diluted with ether, washed with 1 N of HCl solution, neutral-
ized with saturated aqueous NaHCO3 solution, washed with satu-
rated aqueous NaCl solution, dried over anhydrous MgSO4, and
concentrated. The residue was dissolved in THF (5 mL) and TBAF
J ¼ 7.0 Hz, 2H); 13C NMR (100 MHz; CDCl3)
d 168.4, 157.6, 138.3,
116.9, 84.0, 65.3, 61.4, 14.1; MS (ESI) m/z 307 [M þ H]þ, 329
[M þ Na]þ, 345 [M þ K]þ.
4.1.2. Methyl-3-[2-(4-ethynylphenoxy)acetamido]-4-hydroxybenz-
oate (5)
A mixture of 4 (1.53 g, 5.0 mmol), lithium hydroxide mono-
hydrate (258 mg, 6.1 mmol) in a 1:1 solution of THF and water
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H. Nakamura et al. / Journal of Organometallic Chemistry xxx (2013) 1e6
5
(4 M in THF; 0.08 mL, 0.3 mmol) was added. The mixture was
stirred at room temperature for 0.5 h and quenched by addition of
saturated aqueous ammonium chloride solution. The mixture was
extracted with ethyl acetate, washed with saturated aqueous NaCl
solution, dried over anhydrous MgSO4, and concentrated. Purifi-
cation by silica gel column chromatography with hexane/ethyl ac-
etate (5:1) gave 8 (238 mg, 0.8 mmol, 92% in 2 steps) as a white
7.45 (d, J ¼ 6.8 Hz, 2H), 7.00 (d, J ¼ 6.8 Hz, 2H), 5.34 (s, 2H), 3.96 (s,
3H), 3.87 (s, 3H); MS (ESI) m/z 426 [M ꢁ H]ꢁ, 461 [M þ Cl]ꢁ.
4.1.10. Methyl-2-[(4-carboranylphenoxy)methyl]benzoxazole-5-
carboxylic acid (2b)
To a mixture of 2a (72 mg, 0.16 mmol) and dimethylsulfide
(5 mL) in CH2Cl2 (6 mL) were added aluminum tribromide (682 mg,
2.6 mmol) dropwise at 0 ꢀC and the mixture was stirred at room
temperature. After 2 h, aqueous HCl solution (0.5 N; 24 mL) was
added and the mixture was stirred for another hour at room tem-
perature. The reaction mixture was filtered by Celite pad and
diluted with ethyl acetate. The organic layer was washed with
saturated aqueous NaCl solution, dried over anhydrous MgSO4, and
concentrated. Purification by silica gel column chromatography
with ethyl acetate gave 2b (84 mg, 0.21 mmol, 91%) as a white solid:
solid: 1H NMR (400 MHz; CDCl3):
d
8.46 (s, 1H), 8.14 (d, J ¼ 9.0 Hz,
1H), 7.60 (d, J ¼ 9.0 Hz, 1H), 7.46 (d, J ¼ 8.6 Hz, 2H), 7.01 (d,
J ¼ 8.6 Hz, 2H), 5.35 (s, 2H), 3.96 (s, 3H), 3.01 (s, 3H); 13C NMR
(100 MHz; CDCl3)
d 166.5, 162.3, 158.0, 153.8, 140.8, 133.8, 127.6,
127.3, 122.6, 115.8, 114.8, 110.8, 76.4, 62.7, 52.4, 29.7; MS (ESI) m/z
308 [M þ H]þ, 330 [M þ Na]þ.
4.1.6. Methyl-4-hydroxy-3-nitrobenzoate (10)
To a methanol solution (100 mL) of 4-hydroxy-3-nitrobenzoic
acid 9 (1.83 g, 10 mmol) was added conc. H2SO4 (2.0 mL) and the
mixture was stirred for 12 h under reflux. Methanol was removed
in vacuo and the residue was neutralized with saturated aqueous
NaHCO3 solution, extracted with ethyl acetate, washed with satu-
rated aqueous NaCl solution, dried over anhydrous MgSO4, and
concentrated. Purification by silica gel column chromatography
with hexane/ethyl acetate (1:1) gave 10 (1.94 g, 9.8 mmol, 98%) as a
1H NMR (400 MHz; CD3OD):
d
8.27 (s, 1H), 8.05 (d, J ¼ 9.0 Hz, 1H),
7.61 (d, J ¼ 9.0 Hz, 1H), 7.44 (d, J ¼ 8.2 Hz, 2H), 6.98 (d, J ¼ 8.2 Hz,
2H), 5.36 (s, 2H); MS (ESI) m/z 412 [M ꢁ H]ꢁ, 448 [M þ Cl]ꢁ.
4.1.11. (4-Adamantan-1-yl-phenoxy)-acetic acid (14)
This compound was synthesized from 4-adamantylphonol (13)
(6.91 g, 30 mmol), ethyl chloroacetate (3.9 mL, 36 mmol), and
K2CO3 (1.26 g, 91 mmol) using the procedures described for 4 and 5
to give 14 (6.94 g, 24 mmol, 94%) as a white solid: 1H NMR
yellow solid: 1H NMR (400 MHz; CDCl3):
d 10.8 (s, 1H), 8.83 (s, 1H),
8.24 (d, J ¼ 8.8 Hz, 1H), 7.22 (d, J ¼ 8.8 Hz, 1H), 3.95 (s, 3H); 13C NMR
(400 MHz; CD3OD):
4.60 (s, 2H), 2.10 (bs, 3H), 1.94 (bs, 6H), 1.89e1.81 (m, 6H); 13C NMR
(100 MHz, CD3OD) 172.9, 157.2, 145.7, 126.8, 115.1, 65.9, 44.6, 37.9,
d
7.31 (d, J ¼ 9.0 Hz, 2H), 6.90 (d, J ¼ 9.0 Hz, 2H),
(100 MHz; CDCl3)
d 164.7, 158.1, 137.9, 133.2, 127.3, 122.7, 120.2,
52.6; MS (ESI) m/z 198 [M þ H]þ, 220 [M þ Na]þ, 236 [M þ K]þ.
d
36.7, 30.5; IR (KBr) 2874, 2577, 1705, 1589, 1489, 1435, 1285, 1231,
4.1.7. Methyl-3-amino-4-hydroxybenzoate (11)
1088, 1003, 957, 821 cmꢁ1; MS (ESI) m/z 285 [M ꢁ H]ꢁ.
A heterogeneous methanol solution (150 mL) of 10 (9.40 g,
48 mmol) and 10% Pd/C (3.42 g) was stirred overnight under
hydrogen atmosphere and the palladium catalysts were removed
by Celite pad filtration. The solvent was removed in vacuo and the
residue was purified by silica gel column chromatography with
hexane/ethyl acetate (2:1) to give 11 (6.31 g, 38 mmol, 74%) as a
white solid. 1H NMR data were in agreement with those reported in
4.1.12. 3-[2-(4-Adamantan-1-yl-phenoxy)-acetylamino]-4-hydroxy
-benzoic acid methyl ester (15)
This compound was synthesized from 14 (1.60 g, 5.6 mmol), 11
(1.67 g, 10 mmol), EDCI (1.92 g, 10 mmol), HOBt (1.53 g, 10 mmol),
and DIPEA (1.7 mL, 10 mmol) using the procedure described for 6 to
give 15 (1.95 g, 4.5 mmol, 80%) as a white solid: 1H NMR (400 MHz;
the literature [24]: 1H NMR (400 MHz; CDCl3):
d
7.46 (s, 1H), 7.42 (d,
CDCl3):
d
8.75 (s,1H), 8.61 (s,1H), 7.83 (d, J ¼ 8.4 Hz,1H), 7.75 (s,1H),
J ¼ 8.2 Hz,1H), 6.75 (d, J ¼ 8.2 Hz, 1H), 3.87 (s, 3H); MS (ESI) m/z 168
7.36 (d, J ¼ 8.8 Hz, 2H), 7.06 (d, J ¼ 8.4 Hz, 1H), 6.96 (d, J ¼ 8.8 Hz,
2H), 4.69 (s, 2H), 3.90 (s, 3H), 2.10 (bs, 3H), 1.90 (bs, 6H), 1.81e1.73
[M þ H]þ, 190 [M þ Na]þ, 206 [M þ K]þ.
(m, 6H); 13C NMR (100 MHz; CDCl3)
d 168.9, 166.2, 154.4, 153.2,
4.1.8. Methyl-2-[(4-iodophenoxy)methyl]benzoxazole-5-carboxy-
late (12)
146.2, 129.3, 126.4, 124.5, 124.3, 122.4, 120.0, 114.5, 67.2, 52.1, 43.3,
36.7, 35.7, 28.9; MS (ESI) m/z 436 [M þ H]þ.
A toluene solution (30 mL) of 6 (854 g, 2.0 mmol) and TsOHeH2O
(942 mg, 4.8 mmol) was stirred in the presence of molecular sieves
3A overnight under reflux. The molecular sieves were filtered and
the solvent was removed in vacuo. The residue was dissolved in
ethyl acetate, neutralized with saturated aqueous NaHCO3 solution,
washed with saturated aqueous NaCl solution, dried over anhydrous
MgSO4, and concentrated. Purification by silica gel column chro-
matography with hexane/ethyl acetate (5:1) gave 12 (695 mg,
4.1.13. 2-(4-Adamantan-1-yl-phenoxymethyl)-benzoxazole-5-
carboxylic acid methyl ester (16a)
This compound was synthesized from 15 (1.31 g, 3.0 mmol),
TsOHeH2O (1.37 g, 7.2 mmol), and toluene (30 mL) using the pro-
cedure described for 12 to give 16a (851 mg, 2.0 mmol, 68%) as a
white solid. In this case, the reaction proceeded with molecular
sieves: 1H NMR (400 MHz; CDCl3):
d
8.46 (s, 1H), 8.13 (d, J ¼ 8.6 Hz,
1.7 mmol, 85%) as a white solid: 1H NMR (400 MHz; CDCl3):
1H), 8.14 (d, J ¼ 8.8 Hz,1H), 7.61 (m, 3H), 6.84 (d, J ¼ 9.2 Hz, 2H), 5.31
(s, 2H), 3.96 (s, 3H); 13C NMR (100 MHz; CDCl3)
166.5, 162.3, 157.6,
d
8.45 (s,
1H), 7.60 (d, J ¼ 8.6 Hz, 1H), 7.29 (d, J ¼ 9.4 Hz, 2H), 7.01 (d,
J ¼ 9.4 Hz, 2H), 5.33 (s, 2H), 3.96 (s, 3H), 2.08 (bs, 3H), 1.88 (bs, 6H),
d
1.80e1.71 (m, 6H); 13C NMR (100 MHz; CDCl3)
d 166.5, 163.0, 155.6,
153.7,140.8,138.5,127.6,127.3,122.5,117.2,110.8, 84.5, 62.7, 52.4; MS
153.8, 145.2, 140.9, 127.4, 127.1, 126.0, 122.5, 114.3, 110.8, 62.8, 52.4,
(ESI) m/z 410 [M þ H]þ, 432 [M þ Na]þ.
43.3, 36.7, 35.6, 29.7, 28.9; MS (ESI) m/z 418 [M þ H]þ.
4.1.9. Methyl-2-[(4-carboranylphenoxy)methyl]benzoxazole-5-
carboxylate (2a)
4.1.14. 2-(4-Adamantan-1-yl-phenoxymethyl)-benzoxazole-5-
carboxylic acid (16b)
To a mixture of 8 (307 mg, 1.0 mmol) and decaborane (134 mg,
1.1 mmol) in toluene (5 mL) was added acetonitrile (1.0 mL,
20 mmol) and the mixture was stirred for 18 h under reflux. The
solvents were removed and the residue was purified by silica gel
column chromatography with hexane/ethyl acetate (5:1) to give 11
(63 mg, 0.15 mmol, 14%) as a white solid: 1H NMR (400 MHz;
This compound was synthesized from 16a (100 mg, 0.23 mmol),
dimethylsulfide (6 mL), and aluminum tribromide (981 mg,
3.7 mmol) using the procedure described for 2b to give 16b (84 mg,
0.21 mmol, 91%) as a white solid: 1H NMR (400 MHz; CDCl3):
d 8.52
(s, 1H), 8.18 (d, J ¼ 8.4 Hz, 1H), 7.63 (d, J ¼ 8.4 Hz, 1H), 7.29 (d,
J ¼ 9.5 Hz, 2H), 7.01 (d, J ¼ 9.5 Hz, 2H), 5.34 (s, 2H), 2.08 (bs, 3H),
CDCl3):
d
8.46 (s, 1H), 8.14 (d, J ¼ 8.6 Hz, 1H), 7.59 (d, J ¼ 8.6 Hz, 1H),
1.88 (bs, 6H), 1.83e1.71 (m, 6H); 13C NMR (100 MHz; CDCl3)
d 163.3,
Please cite this article in press as: H. Nakamura, et al., Journal of Organometallic Chemistry (2013), http://dx.doi.org/10.1016/
j.jorganchem.2013.04.007

6
H. Nakamura et al. / Journal of Organometallic Chemistry xxx (2013) 1e6
155.6, 154.3, 145.2, 128.0, 126.1, 123.3, 114.3, 110.9, 76.7, 62.8, 43.3,
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Please cite this article in press as: H. Nakamura, et al., Journal of Organometallic Chemistry (2013), http://dx.doi.org/10.1016/
j.jorganchem.2013.04.007