Synthesis and structure-activity relationship of p-carborane-based non-secosteroidal vitamin D analogs

Article abstract of DOI:10.1016/j.bmc.2014.01.015

1α,25-Dihydroxyvitamin D₃ [1α,25(OH) ₂D₃: 1] is a specific modulator of nuclear vitamin D receptor (VDR), and novel vitamin D analogs are therapeutic candidates for multiple clinical applications. We recently developed non

Full text of DOI:10.1016/j.bmc.2014.01.015

Bioorganic & Medicinal Chemistry 22 (2014) 1227–1235
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Synthesis and structure–activity relationship of p-carborane-based
non-secosteroidal vitamin D analogs
Shinya Fujii a,b, Atsushi Kano a, Chalermkiat Songkram c, Hiroyuki Masuno a, Yoshiyuki Taoda d,
Emiko Kawachi a, Tomoya Hirano a, Aya Tanatani e, Hiroyuki Kagechika a,
⇑
a Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062, Japan
b Institute of Molecular and Cellular Biosciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
c Department of Pharmaceutical Chemistry, Faculty of Pharmaceutical Sciences, Prince of Songkla University, Kho-Hong, Hat Yai, Songkhla 90112, Thailand
d Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
e Department of Chemistry, Faculty of Science, Ochanomizu University, 2-1-1 Otsuka, Bunkyo-ku, Tokyo 112-8610, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
1a,25-Dihydroxyvitamin D3 [1a,25(OH)2D3: 1] is a specific modulator of nuclear vitamin D receptor
Received 22 December 2013
Revised 8 January 2014
Accepted 10 January 2014
Available online 21 January 2014
(VDR), and novel vitamin D analogs are therapeutic candidates for multiple clinical applications. We
recently developed non-secosteroidal VDR agonists bearing a p-carborane cage (a carbon-containing
boron cluster) as a hydrophobic core structure. These carborane derivatives are structurally quite differ-
ent from classical secosteroidal vitamin D analogs. Here, we report systematic synthesis and activity eval-
uation of carborane-based non-secosteroidal vitamin D analogs. The structure–activity relationships of
carborane derivatives are different from those of secosteroidal vitamin D derivatives, and in particular,
the length and the substituent position of the dihydroxylated side chain are rather flexible in carborane
derivatives. The structure–activity relationships presented here should be helpful in development of non-
secosteroidal vitamin D analogs for clinical applications.
Keywords:
Vitamin D
Nuclear receptor
Carborane
Non-secosteroid
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
such as LG190178 (3),4 had been developed as potent non-secos-
teroidal VDR ligands.
Vitamin D receptor (VDR) is a ligand-inducible nuclear receptor
specific for vitamin D,1 and plays important roles in many physio-
logical processes, including phosphate and calcium homeostasis,
immune regulation, bone metabolism, cell proliferation and differ-
entiation.2 VDR is activated by binding of the endogenous VDR
We recently reported development of a potent non-secosteroi-
dal VDR ligand 4 using p-carborane (1,12-dicarba-closo-dodecabo-
rane), in place of the CD ring of the secosteroid structure of 1.5,6
Carboranes are carbon-containing boron clusters with unusual
chemicophysical characteristics, including spherical geometry
and a hydrophobic B–H surface.7,8 We have shown that carboranes
can be used as the hydrophobic core of biologically active mole-
cules, especially nuclear receptor ligands.9 Though the overall
structure of compound 4 is quite different from those of classical
vitamin D analogs, X-ray crystallographic analysis using VDR li-
gand-binding domain (LBD) complexed with the (S)-isomer of 4 re-
vealed that the binding mode of compound 4 to VDR is similar to
that of 1.5 The terminal primary hydroxyl group and the secondary
hydroxyl group of the 1,3-diol part of compound 4 correspond to
agonist 1a,25-dihydroxyvitamin D3 [1a,25(OH)2D3: 1] (Fig. 1),
and regulates expression of specific target genes. Because VDR
and its ligands have significant roles in the pathogenesis and ther-
apy of diseases such as osteoporosis, arthritis, psoriasis and can-
cers, many
1a,25(OH)2D3 analogs bearing
a
secosteroidal
skeleton have been developed as drug candidates. For example,
maxacalcitol (2) was reported to potently suppress keratinocyte
proliferation and has been approved for treatment of psoriasis.3
On the other hand, the structural complexity, synthetic inconve-
nience and chemical instability of the secosteroidal compounds
are disadvantageous for potential clinical application. Develop-
ment of novel non-secosteroidal VDR ligands is therefore impor-
tant, but until recently only one series of bisphenol derivatives,
the 1a-hydroxyl group and the 3-hydroxyl group of 1, respectively.
The tertiary alcohol of the other alkyl chain corresponds to the
25-hydroxyl group of 1. These results suggested that structural
modifications used in reported vitamin D analogs might also be
useful for the development of novel non-secosteroidal VDR ligands.
On the basis of these considerations, we planned to investigate the
structure–activity relationship of a series of carborane-containing
vitamin D analogs.
⇑
Corresponding author. Tel.: +81 3 5280 8032; fax: +81 3 5280 8127.
E-mail address: kage.chem@tmd.ac.jp (H. Kagechika).
0968-0896/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmc.2014.01.015

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S. Fujii et al. / Bioorg. Med. Chem. 22 (2014) 1227–1235
were also synthesized from p-carborane. Hydrogenation of com-
2. Results and discussion
2.1. Design and synthesis
pound 23 afforded bis(hydroxyethyl)carborane 24. Introduction
of 1,3-diol substructure gave compound 25, and construction of
side chain moieties by reaction with ethyl bromoacetate and then
Grignard reagent gave compound 27. Finally, removal of the ben-
zylidene protective group afforded compound 6. Pinacolone deriv-
ative 9 was synthesized via compound 28, and compound 8 was
prepared by reduction of compound 9 (Scheme 1). Compound 7
was also synthesized from p-carborane as a starting material. Con-
struction of the side chain moiety in the manner described for
preparation of compound 6 gave compound 31, and then protec-
tion of alcohol with TES gave compound 32. 1,3-Diol substructure
was constructed to afford compound 7. Introduction of a hydroxy-
ethyl moiety gave alcohol 34, and introduction of 1,3-diol sub-
structure gave compound 35. Removal of protective groups under
acidic conditions gave compound 7.
Synthesis of compounds 12 and 14 is illustrated in Scheme 3.
Introduction of the side chain moiety into compound 18 gave 36,
and then catalytic hydrogenation afforded alcohol 37. Introduction
of 1,2-diol substructure gave 38, and removal of protective groups
afforded compound 12. Compound 14 was also synthesized from
p-carborane. Reaction of lithiated carborane and trimethylene
oxide gave 3-hydroxypropylcarborane 39. Introduction of 1,3-diol
substructure into 39 gave 40, and then introduction of the side
chain moiety gave 41. Removal of protective groups under acidic
conditions afforded compound 14. Preparation of compounds 10,
11, 13, 15 and 16 was reported previously.5,6
In order to uncover the structure–activity relationship of non-
secosteroidal vitamin D analogs, we planned to investigate the
activity of two series of carborane derivatives. One series has mod-
ifications in the mono-hydroxylated alkyl chain, which corre-
sponds to the side-chain structure on the D ring of 1. In this
series, we introduced ether functionalities (5–7) based on the sub-
structure of 2. Neopentyl alcohol 8 and pinacolone derivative 9
were also designed based on structure–activity relationship stud-
ies of diphenylmethane derivatives such as 3.4 The other series
has modifications in the dihydroxyalkyl chain, which corresponds
to the A-ring attached to the conjugated triene structure of 1.
Our previous studies and structure–activity relationship analysis
of diphenylmethane derivative 3 revealed that 1,2-diol substruc-
ture, as well as 1,3-diol (corresponding to the A-ring of 1), could
be a hydrophilic pharmacophore of VDR ligands. Therefore, various
compounds bearing 1,2- or 1,3-diol substructure and/or with ether
oxygen at various positions, including compounds reported previ-
ously (10–13, 15, 16),5,6 were designed and synthesized to investi-
gate their vitamin D activity (Fig. 2). Since we found that the
stereochemistry of the secondary hydroxyl group of the carborane
derivative 4 was not critical for the vitamin D activity,5 these com-
pounds were evaluated as racemic mixtures.
Syntheses of compounds 5–9 are summarized in Schemes 1 and
2. A benzyloxyethyl group was introduced into the carborane cage
via C-lithiated form of p-carborane (17)10 to afford benzyl ether 18,
and removal of the benzyl group by catalytic hydrogenation affor-
ded alcohol 19. Introduction of 1,3-diol substructure afforded 20,
and then a hydroxymethyl group was introduced to give 21. Con-
struction of the side chain moiety gave 22, and removal of protec-
tive groups gave the target compound 5. Compounds 6, 8 and 9
2.2. Biological evaluation
Vitamin D activity of carborane derivatives was evaluated in
terms of cell differentiation-inducing activity toward human acute
promyelocytic leukemia cell line HL-60.11 Table 1 shows structure–
activity relationship data for the mono-hydroxylated alkyl chain
Figure 1. Structures of developed VDR ligands.
Figure 2. Structures of carborane-based vitamin D analogs investigated in this study.

S. Fujii et al. / Bioorg. Med. Chem. 22 (2014) 1227–1235
1229
Scheme 1. Synthesis of designed carborane derivatives 5, 6, 8 and 9. Reagents and conditions: (a) n-BuLi, benzyl 2-bromoethyl ether, ether, THF, 46%; (b) H2, Pd/C, EtOH; (c)
NaH, 4-(p-toluenesulfonyloxymethyl)-2-phenyl-1,3-dioxane, DMF, 32%; (d) n-BuLi, paraformaldehyde, benzene–ether, 70%; (e) NaH, 1-bromo-3-ethyl-3-(triethylsilyl-
oxy)pentane, DMF, 15%; (f) HCl, H2O, MeOH, THF, 25%; (g) H2, Pd/C, EtOH, 87%; (h) NaH, 4-(p-toluenesulfonyloxymethyl)-2-phenyl-1,3-dioxane, DMF, 29%; (i) NaH, ethyl
bromoacetate, DMF, 36%; (j) EtMgBr, THF, 42%; (k) HCl, H2O, MeOH, THF, 44%; (l) NaH, 1-chloropinacolone, DMF, 17%; (m) HCl, H2O, MeOH, THF, 44%; (n) LiAlH4, Et2O, quant.
Scheme 2. Synthesis of designed carborane derivative 7. Reagents and conditions: (a) n-BuLi, paraformaldehyde, benzene, ether, 66%; (b) NaH, ethyl bromoacetate, DMF, 75%;
(c) EtMgBr, THF, 96%; (d) TESOTf, 2,6-lutidine, CH2Cl2, 46%; (e) n-BuLi, benzyl 2-bromoethyl ether, ether, THF, 66%; (f) H2, Pd/C, EtOH, 79%; (g) NaH, 4-(p-
toluenesulfonyloxymethyl)-2-phenyl-1,3-dioxane, DMF, 24%; (h) HCl, H2O, MeOH, THF, 33%.
corresponding to the side chain of 1
a
,25(OH)2D3 (1). Compounds 5
that of alcohol 8. Thus, introduction of ether oxygen into the side
chain moiety of the parent compound 4 decreased the HL-60 cell
differentiation-inducing activity. In the case of secosteroidal com-
pounds, it was expected that the less hindered oxa tethering chain
would result in lower steric repulsion between the side chain and
the CD ring moiety, and therefore 2 might exhibit potent vitamin D
activity.12 On the other hand, our previous X-ray crystallography
and 6 bearing an ether oxygen atom in place of a methylene group
of compound 4 exhibited moderate HL-60 cell differentiation-
inducing activity. Ether derivative 7 bearing a shorter side chain
moiety exhibited lower potency than 5 and 6. Compound 8 bearing
a tert-butyl group exhibited moderate potency similar to that of
the diethyl derivative 5. Ketone 9 exhibited lower potency than

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S. Fujii et al. / Bioorg. Med. Chem. 22 (2014) 1227–1235
Scheme 3. Synthesis of carborane derivatives 12 and 14. Reagents and conditions: (a) n-BuLi, 7-bromo-3-ethyl-3-triethylsilyloxyheptane, THF, ether, 96%; (b) H2, Pd/C, EtOH,
87%; (c) NaH, 4-{2-(p-toluenesulfonyloxy)ethyl)}-2,2-dimethyl-1,3-dioxolane, DMF, 54%; (d) HCl, H2O, MeOH, THF, 81%; (e) n-BuLi, trimethylene oxide, ether, 36%; (f) NaH, 4-
(p-toluenesulfonyloxymethyl)-2-phenyl-1,3-dioxane, DMF, 22%; (g) n-BuLi, 7-bromo-3-ethyl-3-triethylsilyloxyheptane, THF, ether, 48%; (h) HCl, H2O, MeOH, THF, 54%.
Table 1
Table 2
HL-60 cell differentiation-inducing potency of carborane derivatives with modifica-
tion in the mono-hydroxylated alkyl chain
HL-60 cell differentiation-inducing potency of carborane derivatives with modifica-
tion in the diol moiety
a
a
R1
R2
R3
EC50 (nM)
Compound
x
y
z
EC50 (nM)
Compound
X
4
2
1
1
2
3
3
2
1
1
2
3
2
1
1
1
1
2
2
1
1
1
2
1
2
47
110
150
180
580
350
250
2900
4
5
6
7
8
9
–(CH2)4–
Et
Et
Et
Et
t-Bu
t-Bu
Et
Et
Et
Et
H
OH
OH
OH
OH
OH
47
230
460
850
310
860
10
11
12
13
14
15
16
–(CH2)2–O–CH2–
–CH2–O–(CH2)2–
–CH2–O–CH2–
–(CH2)2–O–CH2–
–(CH2)2–O–CH2–
@O
a
HL-60 cell differentiation-inducing potency was evaluated over the concen-
tration range of 10À9 to 10À5 M. Cell differentiation was determined as the ratio of
NBT-positive cells. The EC50 value was calculated as the concentration of each
compound exhibiting 50% of the activity induced by 10À7 M 1.
a
HL-60 cell differentiation-inducing potency was evaluated in the concentration
range of 10À9 to 10À5 M. Cell differentiation was determined as the ratio of NBT-
positive cells. EC50 value was calculated as the concentration of compound exhib-
iting 50% of the activity induced by 10À7 M 1.
study of the complex of the (S)-isomer of 4 and the VDR-LBD
showed that the methylene side chain of compound 4 adopts
straight, continuous anti conformation in the complex. Based on
these observations, a possible explanation of the low activity of
ether compounds 5 and 6 is that the comparative flexibility of
the ether side chain is unfavorable for adopting a straight confor-
mation. The polar property of the ether moiety could also be unfa-
vorable for hydrophobic interaction in the ligand-binding pocket.
In addition, insertion of ether oxygen yields highly symmetrical
structure, therefore the compounds also could induce weaker bind-
ing mode with upside-down direction. The low potency of com-
pound 9 suggested that a hydroxyl group at the terminal of the
side chain is required for potent vitamin D activity of both carbo-
rane derivatives and secosteroidal VDR ligands. Comparison of
compounds 5 and 8 indicates that a tert-butyl group and geminal
diethyl substructure are suitable hydrophobic substructures of
the side chain, and hydrophobicity corresponding to C4 hydrocar-
bon is reasonable for the terminal region (Table 1).
are isomers concerning the position of the ether oxygen atom, indi-
cated that compound 4 exhibited higher HL-60 cell differentiation-
inducing potency than did compound 10. Regarding compounds
11, 12 and 13, which are also isomers regarding the position of
the ether oxygen atom, compounds 11 and 12 exhibited almost
equal vitamin D potency, whereas compound 13 was less potent.
On the basis of these results, it is suggested that a suitable distance
between the carborane cage and ether oxygen atom (x in Table 2) is
two atoms distance, though one atom distance is also acceptable,
whereas three atoms distance is not suitable. Compound 10 bear-
ing 1,3-diol and compound 11 bearing 1,2-diol exhibited similar
activity. Compound 14, which has the longest dihydroxyalkyl func-
tionality (8 atoms distance between carborane and terminal hydro-
xyl group), exhibited moderate activity, and compound 15 bearing
the shortest chain (5 atoms distance) also exhibited moderate
activity. On the other hand, compound 16, which has the shortest
distance between the secondary hydroxyl group and carborane
cage (4 atoms distance) exhibited quite low activity. These results
The structure–activity relationship for the diol moiety is sum-
marized in Table 2. Comparison of compounds 4 and 10, which

S. Fujii et al. / Bioorg. Med. Chem. 22 (2014) 1227–1235
1231
suggested that the position of the secondary hydroxyl group is
4.2. Synthesis
more important than the position of the terminal primary hydroxyl
group. The reason for this, at least in part, could be the less flexible
nature of the secondary hydroxyl group as compared with the
terminal hydroxyl group. These considerations suggest that both
1,2-diol and 1,3-diol derivatives bearing a secondary hydroxyl
group at a suitable position could exhibit vitamin D activity.
The structure–activity relationship studies indicated that com-
pound 4 was the most potent among the synthesized carborane-
based triols. It is noteworthy that the compounds with different
lengths of dihydroxylated side chain or a different position of
the dihydroxyl group also showed activities comparable to that
of compound 4, whereas modification of the A ring, including
change of stereochemistry of hydroxyl groups, in secosteroidal
vitamin D analogs causes significant decrease or loss of activity.
The bulky spherical structure of carborane appears to be an effec-
tive hydrophobic core structure that allows these flexible dihy-
droxylated side chains to be arranged at the proper spatial
positions for interaction with amino acid residues of the VDR.
On the other hand, structure–activity relationship study regarding
the monohydroxylated alkyl chain of carborane derivatives, corre-
4.2.1. 1-(2-Benzyloxyethyl)-1,12-dicarba-closo-dodecaborane
(18)
Under Ar atmosphere, n-BuLi (1.58 M in n-hexane, 4.1 mL,
38.1 mmol) was added to a solution of p-carborane (5.00 g,
34.7 mmol) in ether (120 mL) at 0 °C and stirred for 1 h at room
temperature. Then benzyl 2-bromoethyl ether (8.95 g, 41.6 mmol)
was added to the reaction mixture, and stirred for 16 h at room
temperature. The reaction was quenched with water at 0 °C and di-
luted with AcOEt. The organic layer was washed with water and
brine, dried with Na2SO4 and concentrated, then purified by silica
gel column chromatography (eluent; hexane/ethyl acetate, 20:1)
to give 4.45 g of 18 (46%) as colorless oil. 1H NMR (CDCl3,
400 MHz) d 7.30–7.20 (m, 5H), 4.35 (s, 2H), 3.22 (t, J = 7.1 Hz,
2H), 3.0–1.3 (br m, 10H), 2.59 (s, 1H), 1.91 (t, J = 7.0 Hz, 2 H).
4.2.2. 1-{2-[(2-Phenyl-1,3-dioxan-4-yl)methoxy]ethyl}-1,12-
dicarba-closo-dodecaborane (20)
Catalytic hydrogenation of 18 using 5% Pd on carbon in EtOH
gave alcohol 19. NaH (234 mg, 5.84 mmol) was added to a solution
of 19 (1.00 g, 5.84 mmol) in DMF (6.0 mL) at 0 °C, and stirred for
10 min at 0 °C. Then 4-(p-toluenesulfonyloxymethyl)-2-phenyl-
1,3-dioxane (2.03 g, 5.84 mmol) was added to the reaction mixture
and stirred for 2 day at 90 °C. The reaction was quenched with
water at 0 °C, and then diluted with AcOEt. The organic layer was
washed with water and brine, dried with Na2SO4 and concentrated,
then purified by silica gel column chromatography (eluent; hex-
ane/ethyl acetate, 8:1) to give 678 mg of 20 (32%) as colorless oil.
1H NMR (CDCl3, 400 MHz) d 7.48–7.45 (m, 2H), 7.36–7.30 (m,
3H), 5.49 (s, 1H), 4.27 (dd, J = 11.4, 3.8 Hz, 1H), 4.02–3.92 (m,
2H), 3.47 (dd, J = 10.3, 5.9 Hz, 1H), 3.37 (dd, J = 10.4, 4.8 Hz, 1H),
3.3–1.3 (br m, 10H), 3.29–3.23 (m, 2H), 2.63 (s, 1H), 1.89 (t,
J = 7.0 Hz, 2H), 1.82 (dq, J = 12.3, 5.1 Hz, 1H), 1.53 (m, 1H).
sponding to the side chain of 1a,25(OH)2D3 (1), revealed that
introduction of ether substructure reduced the vitamin D potency
of compounds. This finding suggested that flexibility or polarity of
the side chain moiety of carborane derivatives reduces the activity
of the compounds. We previously reported a structural analysis of
VDR ligand-binding domain complexed with carborane derivative
4, and revealed that the three hydroxyl groups of compound 4
function as a hydrogen-bonding pharmacophore corresponding
to the three hydroxyl groups of 1a,25(OH)2D3 (1). On the other
hand, the structure–activity relationships obtained in this study
seemed quite characteristic of carborane-based VDR ligands. This
structure–activity relationship information and structural analyses
of carborane derivatives are expected to be helpful in the develop-
ment of non-secosteroidal VDR ligands.
4.2.3. 1-Hydroxymethyl-12-{2-[(2-phenyl-1,3-dioxan-4-
yl)methoxy]ethyl}-1,12-dicarba-closo-dodecaborane (21)
A 1.56 M solution of n-BuLi in n-hexane (1.38 mL, 2.16 mmol)
was added to a solution of 20 (606 mg, 1.66 mmol) in benzene
(2.0 mL) and ether (1.6 mL) at 0 °C, and the mixture was stirred
at room temperature for 1 h. Paraformaldehyde (80 mg,
2.16 mmol) was added to the mixture at 0 °C in one portion, and
then the stirring was continued at room temperature for 16 h.
The reaction was quenched with saturated aqueous ammonium
chloride and extracted with ethyl acetate. The organic layer was
washed with brine, dried over sodium sulfate and then concen-
trated. Purification by silica gel column chromatography (eluent;
hexane/ethyl acetate, 4:1) gave 21 (70%) as colorless oil. 1H NMR
(CDCl3, 400 MHz) d 7.47–7.45 (m, 2H), 7.36–7.30 (m, 3H), 5.49 (s,
1H), 4.27 (dd, J = 11.5, 4.0 Hz, 1H), 4.02–3.92 (m, 2H), 3.48 (dd,
J = 10.3, 5.9 Hz, 1H), 3.45 (s, 2H), 3.36 (dd, J = 10.3, 5.9 Hz, 1H),
3.45 (s, 2H), 3.36 (dd, J = 10.3, 4.6 Hz, 1H), 3.3–1.3 (br m, 10H),
3.29–3.21 (m, 2H), 1.91 (t, J = 7.0 Hz, 2H), 1.82 (dq, J = 12.5,
5.1 Hz, 1H), 1.52 (m, 1H).
3. Conclusion
We have investigated the structure–activity relationship of a
series of novel non-secosteroidal VDR ligands bearing a carborane
cage as a hydrophobic pharmacophore. The structure–activity rela-
tionships of the carborane derivatives described here are different
from those of secosteroidal vitamin D derivatives, and there is
greater flexibility concerning the length and the substituent posi-
tion of the dihydroxylated side chain in the carborane derivatives.
We consider that carborane is a key hydrophobic core structure
upon which two acyclic side chains can be arranged so as to ensure
effective hydrogen bonding interactions with VDR in the ligand-
binding pocket. The structure–activity relationship information
obtained here should be helpful in further development of non-
secosteroidal VDR ligands, as well as in the development of novel
carborane-based bioactive substances.
4. Experimental
4.1. General
4.2.4. 1-{2-[(2-Phenyl-1,3-dioxan-4-yl)methoxy]ethyl}-12-(3-
ethyl-3-triethylsilyloxylpentyloxy)-methyl-1,12-dicarba-closo-
dodecaborane (22)
All reagents were purchased from Sigma–Aldrich Chemical Co.,
Tokyo Kasei Kogyo Co., Wako Pure Chemical Industries, and Kanto
Chemical Co., INC. p-Carborane was purchased from Katchem s.r.o.
(Prague, Czech Republic). Silica gel for column chromatography
was purchased from Kanto Chemical Co., INC. 1H NMR spectra were
recorded on at 500 MHz on a BRUCKER AVANCE 500 spectrometer
or at 400 MHz on a BRUCKER AVANCE 400 spectrometer, JEOL
JNM-LA-400 or JNM-FX-400 spectrometers.
NaH (56.0 mg, 1.39 mmol) was added to a solution of 21
(457 mg, 1.16 mmol) in DMF (4.0 mL) at 0 °C, and stirred for
10 min a 0 °C. Then the reaction mixture was added (3-bromo-
1,1-diethylpropoxy)-diethylsilane (467 mg, 1.51 mmol) in DMF
(4.0 mL) and stirred for 18 h at room temperature. Then the reac-
tion was quenched with water at 0 °C, and then diluted with AcOEt.
The organic layer was washed with water and brine, dried with
Na2SO4 and concentrated, then purified by silica gel column

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S. Fujii et al. / Bioorg. Med. Chem. 22 (2014) 1227–1235
chromatography (eluent; hexane/ethyl acetate, 8:1) to give 88.5 mg
of 22 (15%) as colorless oil. 1H NMR (CDCl3, 400 MHz) d 7.47–7.45
(m, 2H), 7.36–7.28 (m, 3H), 5.49 (s, 1H), 4.27 (dd, J = 11.5, 4.9 Hz,
1H), 4.00–3.92 (m, 2H), 3.47 (dd, J = 10.3, 6.1 Hz, 1H), 3.38–3.23
(m, 5H), 3.3–1.3 (br m, 10H), 3.24 (s, 2H), 1.89 (t, J = 7.1 Hz, 2H),
1.82 (dq, J = 12.6, 4.9 Hz, 1H), 1.62–1.35 (m, 7H), 0.91
(t, J = 8.1 Hz, 9H), 0.80 (t, J = 7.5 Hz, 6H), 0.53 (q, J = 7.9 Hz, 6H).
J = 10.4, 4.5 Hz, 1H), 3.3–1.3 (br m, 10H), 3.25 (m, 2H), 2.03–1.87
(m, 5H), 1.52 (m, 1H), 1.25 (t, J = 7.1 Hz, 3H).
4.2.9. 1-{2-[(2-Phenyl-1,3-dioxan-4-yl)methoxy]ethyl}-12-[2-(2-
ethyl-2-hydroxybutoxy)ethyl]-1,12-dicarba-closo-
dodecaborane (27)
EtMgBr (0.89 M in THF 0.51 mL 0.460 mmol) was added to a
solution of 26 (75.0 mg, 0.152 mmol) in THF (3.0 mL) at 0 °C,
and stirred for 3 h at room temperature. The reaction was
quenched with saturated aqueous NH4Cl solution at 0 °C and
diluted with ether. The organic layer was washed with water
and brine, dried with Na2SO4 and concentrated. Purification by
silica gel column chromatography (eluent; hexane/ethyl acetate,
3:1) gave 32.5 mg of 27 (42%) as colorless oil. 1H NMR
(CDCl3, 400 MHz) d 7.47–7.45 (m, 2H), 7.36–7.25 (m, 3H), 5.49
(s, 1H), 4.27 (dd, J = 11.4, 4.9 Hz, 1H), 3.95 (m, 2H), 3.47
(dd, J = 10.3, 5.7 Hz, 1H), 3.36 (dd, J = 10.3, 4.6 Hz, 1H), 3.3–1.3
(br m, 10H), 3.27–3.17 (m, 3H), 3.14 (s, 2H), 1.88 (t, J = 7.0 Hz,
4H), 1.80 (dq, J = 12.5, 7.3 Hz, 1H), 1.44 (m, 6H), 0.83
(t, J = 7.5 Hz, 6H).
4.2.5. 1-[2-(2,4-Dihydroxybutoxy)ethyl]-12-(3-ethyl-3-
hydroxylpentyloxy)methyl-1,12-dicarba-closo-dodeca-borane
(5)
2 M HCl (2.0 mL) was added to a solution of 22 (100 mg,
0.161 mmol) in MeOH/THF (3.0 mL), and stirred for 7 days at room
temperature. The reaction mixture was diluted with AcOEt The or-
ganic layer was washed with water and brine, dried with Na2SO4
and concentrated, then purified by silica gel column chromatogra-
phy (eluent; hexane/ethyl acetate, 2:1) to give 16.8 mg of 5 (25%)
as colorless oil. 1H NMR (CDCl3, 400 MHz) d 3.94 (m, 1H), 3.80 (t,
J = 5.9 Hz, 2H), 3.45 (t, J = 6.0 Hz, 2H), 3.31 (dd, J = 9.3, 3.7 Hz,
1H), 3.3–1.3 (br m, 10H), 3.26 (s, 2H), 3.24–3.17 (m, 3H), 1.88 (t,
J = 6.6 Hz, 2H), 1.64 (t, J = 6.1 Hz, 4H), 1.52–1.37 (m, 4H), 0.82 (t,
J = 7.5 Hz, 6H).
4.2.10. 1-[2-(2,4-Dihydroxybutoxy)ethyl]-12-[2-(2-ethyl-2-
hydroxylbutoxy)ethyl]-1,12-dicarba-closo-dodeca-borane (6)
2 M HCl (1.0 ml) was added to a solution of 27 (30 mg,
0.0590 mmol) in MeOH/THF (2.0 mL), and stirred for 7 days at
room temperature. The reaction mixture was diluted with AcOEt
The organic layer was washed with water and brine, dried with
Na2SO4 and concentrated, then purified by silica gel column
chromatography (eluent; hexane/ethyl acetate, 3:1) to give
4.2.6. 1,12-Bis(2-hydroxyethyl)-1,12-dicarba-closo-
dodecaborane (24)
A mixture of 23 (6.38 g, 15.5 mmol) and 10% palladium on car-
bon (1.0 g) in ethanol (150 mL) was stirred at room temperature
for 24 h under atmospheric pressure of hydrogen. Insoluble mate-
rials were filtered off through Celite, then the filtrate was concen-
trated. Purification by silica gel column chromatography (eluent;
hexane/ethyl acetate, 2:1) gave 24 (87%) as white solid. 1H NMR
(CDCl3, 400 MHz) d 3.40 (m, 4H), 3.3–1.3 (br m, 10H), 1.87 (t,
J = 7.1 Hz, 4H), 1.31 (s, 2H).
11.9 mg of
6
(44%) as colorless solid. 1H NMR (CDCl3,
400 MHz) d 3.93 (m, 1H), 3.81 (t, J = 5.5 Hz, 2H), 3.31 (dd,
J = 9.3, 3.6 Hz, 1H), 3.3–1.3 (br m, 10H), 3.26–3.14 (m, 5H),
3.07 (s, 2H), 1.88 (t, J = 6.6 Hz, 4H), 1.71–1.60 (m, 3H), 1.46 (q,
J = 7.7 Hz, 4H), 0.82 (t, J = 7.5 Hz, 6H).
4.2.7. 1-(2-Hydroxyethyl)-12-{2-[(2-phenyl-1,3-dioxan-4-
yl)methoxy]ethyl}-1,12-dicarba-closo-dodecaborane (25)
NaH (207 mg, 5.17 mmol) was added to a solution of 24 (1.00 g,
4.30 mmol) in DMF (2.0 mL) at 0 °C, and stirred for 10 min at
0 °C. Then 4-(p-toluenesulfonyloxymethyl)-2-phenyl-1,3-dioxane
(1.50 g, 4.30 mmol) was added to the reaction mixture, and then
stirred for 18 h at room temperature. Then the reaction was
quenched with water at 0 °C and the mixture was diluted with
AcOEt. The organic layer was washed with water and brine, dried
with Na2SO4 and concentrated, then purified by silica gel column
chromatography (eluent; hexane/ethyl acetate, 4:1) to give
502 mg of 25 (29%) as colorless oil. 1H NMR (CDCl3, 400 MHz) d
7.47–7.44 (m, 2H), 7.36–7.30 (m, 3H), 5.49 (s, 1H), 4.27 (dd,
J = 11.5, 4.2 Hz, 1H), 3.99–3.92 (m, 2H), 3.47 (dd, J = 10.3, 5.9 Hz,
1H), 3.41 (t, J = 7.1 Hz, 2H), 3.36 (dd, J = 10.3, 4.6 Hz, 1H), 3.3–1.3
(br m, 10H), 3.27–3.22 (m, 2H), 1.89–1.76 (m, 5H), 1.54–1.48 (m,
3H).
4.2.11. 1-{2-[(2-Phenyl-1,3-dioxan-4-yl)methoxy]ethyl}-12-[2-
(3,3-dimethyl-2-oxobutoxy)ethyl]-1,12-dicarba-closo-
dodecaborane (28)
NaH (22.0 mg, 0.540 mmol) was added to a solution of 28
(200 mg, 0.490 mmol) in DMF (2.0 mL) and stirred for 10 min
at 0 °C. Then 1-chloropinacolin (132 mg, 0.980 mmol) was added
to the mixture and stirred for 18 h at room temperature. Then
the reaction was quenched with water and diluted with AcOEt.
The organic layer was washed with water and brine, dried with
Na2SO4 and concentrated, then purified by silica gel column
chromatography (eluent; hexane/ethyl acetate, 4:1) to give
42.2 mg of 28 (17%) as colorless oil. 1H NMR (CDCl3, 500 MHz)
d 7.47–7.45 (m, 2H), 7.36–7.30 (m, 3H), 5.48 (s, 1H), 4.27 (dd,
J = 5.1, 11.4 Hz, 1H), 4.19 (s, 2H), 3.97–3.88 (m, 2H), 3.46 (dd,
J = 5.9, 10.4 Hz, 1H), 3.35 (dd, J = 4.6, 10.3 Hz, 1H), 3.26–3.19
(m, 4H), 1.97–1.79 (m, 6H), 1.12 (s, 9H), 2.80–1.50 (br m, 10H).
4.2.8. 1-{2-[(2-Phenyl-1,3-dioxan-4-yl)methoxy]ethyl}-12-[2-
(ethoxycarbonylmethoxy)ethyl]-1,12-dicarba-closo-
dodecaborane (26)
4.2.12. 1-[2-(2,4-Dihydroxybutoxy)ethyl]-12-[2-(3,3-dimethyl-
2-oxobutoxy)ethyl]-1,12-dicarba-closo-dodeca-borane (9)
2 M HCl (1.0 ml) was added to a solution of 28 (39.0 mg,
0.0770 mmol) in MeOH/THF (2.0 mL), and stirred for 7 days at
room temperature. The reaction mixture was diluted with AcOEt.
The organic layer was washed with water and brine, dried with
Na2SO4 and concentrated, then purified by silica gel column
chromatography (eluent; hexane/ethyl acetate, 3:1) to give
14.2 mg of 9 (44%) as colorless oil. 1H NMR (CDCl3, 500 MHz) d
4.19 (s, 2H), 3.93 (m, 1H), 3.80 (t, J = 6.1 Hz, 2H), 3.30 (dd,
J = 3.7, 9.3 Hz, 1H), 3.23–3.17 (m, 5H), 1.94 (t, J = 7.3 Hz, 2H),
1.87 (t, J = 6.8 Hz, 2H), 1.65 (m, 2H), 1.11 (s, 9H), 2.80–1.50 (br
m, 10H).
NaH (39.0 mg, 0.979 mmol) was added to a solution of 26
(200 mg, 0.490 mmol) in DMF (2.0 mL) at 0 °C, and stirred for
10 min at 0 °C. Then ethyl bromoacetate (200 mg, 0.490 mmol)
was added to the reaction mixture, and then stirred for 18 h at
room temperature. Then the reaction was quenched with water
and diluted with AcOEt. The organic layer was washed with water
and brine, dried with Na2SO4 and concentrated. Purification by sil-
ica gel column chromatography (eluent; hexane/ethyl acetate, 4:1)
gave 174 mg of 26 (36%) as colorless oil. 1H NMR (CDCl3, 400 MHz)
d 7.47–7.45 (m, 2H), 7.35–7.32 (m, 3H), 5.48 (s, 1H), 4.24–4.11 (m,
5H), 3.98–3.93 (m, 4H), 3.46 (dd, J = 10.4, 6.0 Hz, 1H), 3.36 (dd,

S. Fujii et al. / Bioorg. Med. Chem. 22 (2014) 1227–1235
1233
4.2.13. 1-[2-(2,4-Dihydroxybutoxy)ethyl]-12-[2-(3,3-dimethyl-
2-hydroxylbutoxy)ethyl]-1,12-dicarba-closo-dodecaborane (8)
LiAlH4 (excess) was added to 9 (10.0 mg, 0.024 mmol) in ether
(1.0 mL) at 0 °C, and stirred for 3 h at room temperature. The reac-
tion was quenched with saturated aqueous NH4Cl at 0 °C, and the
mixture was diluted with ether. The organic layer was washed
with water and brine, dried with Na2SO4. Concentration gave
10.1 mg of 8 (quant.) as colorless oil. The two diastereomers could
not be separated, and the ratio could not be determined by 1H
Purification by silica gel column chromatography (eluent; hexane)
gave 32 (46%) as colorless oil; 1H NMR (CDCl3, 400 MHz) d 3.3–1.3
(br m, 10H) 3.22 (s, 2H), 3.09 (s, 2H), 1.43 (q, J = 7.5 Hz, 4H), 0.90 (t,
J = 8.1 Hz, 9H), 0.78 (t, J = 7.5 Hz, 6H), 0.52 (q, J = 8.1 Hz, 6H).
4.2.18. 1-(2-Benzyloxyethyl)-12-(2-ethyl-2-
triethylsilyloxylbutoxy)methyl-1,12-dicarba-closo-
dodecaborane (33)
Under Ar atmosphere, n-BuLi (1.58 M in n-hexane, 0.64 mL,
1.00 mmol) was added to a solution of 32 (300 mg, 0.773 mmol)
in ether (10 mL) at 0 °C, and stirred for 1 h at room temperature.
Then benzyl 2-bromoethyl ether (249 mg, 1.16 mmol) was added
to the mixture and stirred for 18 h at room temperature. Then
the reaction was quenched with water, and the reaction mixture
was diluted with AcOEt. The organic layer was washed with water
and brine, dried with Na2SO4 and concentrated. Purification by sil-
ica gel column chromatography (eluent; hexane/ethyl acetate,
20:1) gave 265 mg (66%) of 33 as colorless oil. 1H NMR (CDCl3,
400 MHz) d 7.33–7.25 (m, 5H), 4.37 (s, 2H), 3.3–1.3 (br m, 10H),
3.23 (t, J = 7.1 Hz, 2H), 3.23 (s, 2H), 3.08 (s, 2H), 1.93 (t, J = 7.1 Hz,
2H), 1.42 (q, J = 7.5 Hz, 4H), 0.89 (t, J = 8.1 Hz, 9H), 0.78 (t,
J = 7.5 Hz, 6H), 0.51 (q, J = 7.7 Hz, 6H).
NMR. 1H NMR (CDCl3, 500 MHz)
d 3.94 (m, 1H), 3.81 (t,
J = 5.3 Hz, 2H), 3.42–3.30 (m, 3H), 3.26–3.14 (m, 6H), 1.88 (t,
J = 6.8 Hz, 4H), 1.72–1.62 (m, 2H), 0.88 (s, 9H), 2.80–1.50 (br m,
10H).
4.2.14. 1-Hydroxymethyl-1,12-dicarba-closo-dodecaborane (29)
A 1.60 M solution of n-BuLi in n-hexane (10.4 mL, 16.7 mmol)
was added dropwise to
a solution of p-carborane (2.00 g,
13.9 mmol) in benzene (10 mL) and diethyl ether (5 mL) at 0 °C un-
der Ar. The mixture was stirred at room temperature for 30 min,
and paraformaldehyde (620 mg, 20.8 mmol) was added in one por-
tion. After 3 h, the reaction was quenched with 1 M hydrochloric
acid and extracted with ethyl acetate. The organic layer was
washed successively with saturated aqueous sodium bicarbonate,
water and brine, dried over sodium sulfate, and then concentrated.
Purification by silica gel column chromatography (eluent: hexane/
ethyl acetate, 8:1) gave 29 (66%) as a colorless solid. 1H NMR
(CDCl3, 400 MHz) d 3.47 (d, J = 7.3 Hz, 2H), 3.5–1.5 (br m, 10H),
2.7 (br s, 1H), 1.55 (t, J = 7.3 Hz, 1H).
4.2.19. 1-(2-Hydroxyethyl)-12-(2-ethyl-2-
triethylsilyloxylbutoxy)methyl-1,12-dicarba-closo-
dodecaborane (34)
Under H2 atmosphere, 10% Pd/C (30 mg) was added to a solu-
tion of 33 (265 mg, 0.509 mmol) in EtOH (6.0 mL), and stirred for
18 h at room temperature. Insoluble materials were filtrated off
and the filtrate was concentrated. Purification by silica gel column
chromatography (eluent; hexane/ethyl acetate, 6:1) gave 173 mg
(79%) of 34 as colorless oil. 1H NMR (CDCl3, 500 MHz) d 3.42 (t,
J = 7.0 Hz, 2H), 3.24 (s, 2H), 3.08 (s, 2H), 1.88 (t, J = 7.0 Hz, 2H),
1.41 (q, J = 7.5 Hz, 4H), 0.89 (t, J = 8.0 Hz, 9H), 0.77 (t, J = 7.5 Hz,
6H), 0.51 (q, J = 8.0 Hz, 6H), 2.80–1.50 (br m, 10H).
4.2.15. 1-Ethoxycarbonylmethoxymethyl-1,12-dicarba-closo-
dodecaborane (30)
A solution of 29 (4.02 g, 23.1 mmol) in DMF (20 mL) was added
to a suspension of NaH (60% in oil, 1.20 g, 30.0 mmol) in DMF
(60 mL) at 0 °C and the mixture was stirred at room temperature
for 1 h. Ethyl bromoacetate (15.4 g, 92.2 mmol) was added to the
mixture and the mixture was heated at 60 °C for 18 h. The solvent
was removed under reduced pressure, and the residue was poured
into water and extracted with dichloromethane. The organic layer
was washed with brine, dried over magnesium sulfate, and then
concentrated. Purification by silica gel column chromatography
(eluent; hexane/ethyl acetate, 16:1) gave 30 (75%) as colorless
oil. 1H NMR (CDCl3, 400 MHz) d 4.15 (q, J = 7.2 Hz, 2H), 3.3–1.4
(br m, 10H), 3.94 (s, 2H), 3.41 (s, 2H), 2.69 (s, 1H), 1.24 (t,
J = 7.2 Hz, 3H).
4.2.20. 1-[2-(2,4-Dihydroxybutoxy)ethyl]-12-(2-ethyl-2-
hydroxylbutoxy)methyl-1,12-dicarba-closo-dodeca-borane (7)
NaH (48.3 mg, 1.20 mmol) was added to a solution of 34
(400 mg, 0.925 mmol) in DMF (2.0 mL) and stirred for 10 min at
0 °C. Then 4-(p-toluenesulfonyloxymethyl)-2-phenyl-1,3-dioxane
(322 mg, 0.925 mmol) was added to the reaction mixture, and stir-
red for 1.5 h at room temperature. Then the reaction was quenched
with water, and the mixture was diluted with CH2Cl2. The organic
layer was washed with water and brine, dried with Na2SO4 and
concentrated. Purification by silica gel column chromatography
(eluent; hexane/ethyl acetate, 10:1) gave 135 mg of 35 (24%) as
colorless oil. Then 2 M HCl (1.0 mL) was added to a solution of ob-
tained 35 (132 mg, 0.217 mmol) in MeOH/THF (1.0 mL), and stirred
for 4 h at room temperature. The reaction mixture was diluted
with AcOEt. The organic layer was washed with water and brine,
dried with Na2SO4 and concentrated. Purification by silica gel col-
umn chromatography (eluent; hexane/ethyl acetate, 2:1) gave
29.1 mg of 7 (33%) as colorless oil. 1H NMR (CDCl3, 400 MHz) d
3.94 (m, 1H), 3.82 (t, J = 5.5 Hz, 2H), 3.32 (dd, J = 9.3, 3.5 Hz, 1H),
3.3–1.3 (br m, 10H), 3.28 (s, 2H), 3.22 (m, 3H), 3.12 (s, 2H), 2.28
(s, 3H), 1.89 (t, J = 6.8 Hz, 2H), 1.64 (m, 2H), 1.42 (q, J = 7.5 Hz,
4H), 0.80 (t, J = 7.5 Hz, 6H).
4.2.16. 1-(2-Ethyl-2-hydroxylbutoxy)methyl-1,12-dicarba-closo-
dodecaborane (31)
A 0.89 M solution of EtMgBr (63.9 mL, 56.9 mmol) was added to
a solution of 30 (4.49 g, 17.2 mmol) in THF (250 mL) at 0 °C and the
mixture was stirred at room temperature for 2 h. The reaction was
quenched with saturated aqueous ammonium chloride and ex-
tracted with ether. The organic layer was washed with brine, dried
over sodium sulfate and then concentrated. Purification by silica
gel column chromatography (eluent; hexane/ethyl acetate, 6:1)
gave 31 (96%) as colorless oil: 1H NMR (CDCl3, 400 MHz) d 3.36
(s, 2H), 3.3–1.3 (br m, 10H), 3.15 (s, 2H), 2.70 (s, 1H), 1.47 (q,
J = 7.5 Hz, 4H), 0.83 (t, J = 7.5 Hz, 6H).
4.2.17. 1-(2-Ethyl-2-triethylsilyloxylbutoxy)methyl-1,12-
dicarba-closo-dodecaborane (32)
4.2.21. 1-(2-Benzyloxyethyl)-12-(5-triethylsilyloxy-5-
Triethylsilyl trifluoromethanesulfonate (2.64 g, 9.99 mmol) and
2,6-lutidine (1.65 g, 15.4 mmol) were added to a solution of 31
(2.11 g, 7.68 mmol) at 0 °C and the mixture was stirred at room
temperature for 16 h. The mixture was poured into water and ex-
tracted with ethyl acetate. The organic layer was washed with
brine, dried over magnesium sulfate and then concentrated.
ethylheptyl)-1,12-dicarba-closo-dodecaborane (36)
A 1.6 M solution of n-BuLi in n-hexane (4.08 mL, 6.54 mmol)
was added to a solution of 18 (1.60 g, 5.75 mmol) in THF (20 mL)
at 0 °C, and the mixture was stirred at room temperature for
15 min under Ar. Then, 7-bromo-3-ethyl(3-triethylsilyloxy)-hep-
tane (1.70 g, 5.04 mmol) was added to the mixture at 0 °C, and

1234
S. Fujii et al. / Bioorg. Med. Chem. 22 (2014) 1227–1235
stirring was continued at room temperature for 40 h. The reaction
mixture was poured into water and extracted with ethyl acetate.
The organic layer was washed with brine, dried over sodium
sulfate, and then concentrated. Purification by silica gel column
chromatography (eluent; hexane/ethyl acetate, 100/1 to 20/1) gave
36 (2.60 g, 4.86 mmol, 96%) as a colorless oil. 1H NMR (CDCl3,
400 MHz) d 7.31–7.25 (m, 5H), 4.37 (s, 2H), 3.3–1.3 (br m, 10H),
3.23 (t, J = 7.3 Hz, 2H), 1.93 (t, J = 7.3 Hz, 2H), 1.56 (m, 2H), 1.37
(q, J = 7.5 Hz, 4H), 1.25 (m, 3H), 1.07 (m, 4H), 0.90 (t, J = 7.9 Hz,
9H), 0.76 (t, J = 7.3 Hz, 6H), 0.52 (q, J = 7.9 Hz, 6H).
with ether. The organic layer was washed with brine, dried over
magnesium sulfate, and then concentrated. Purification by silica
gel column chromatography (eluent; hexane/ethyl acetate, 8:1)
gave 39 (36%) as white wax. 1H NMR (CDCl3, 400 MHz) d 3.45 (t,
J = 6.3 Hz, 2H), 3.0–1.4 (br m, 10H), 2.75 (s, 1H), 1.72–1.68 (m,
2H), 1.44–1.36 (m, 2H).
4.2.26. 1-{3-[(2-Phenyl-1,3-dioxan-4-yl)methoxy]propyl}-1,12-
dicarba-closo-dodecaborane (40)
NaH (60% in oil, 46.8 mg, 1.17 mmol) was added to a solution of
39 (500 mg, 2.47 mmol) in DMF (4.0 mL) and stirred for 1.0 h at
room temperature. Then 4-(p-toluenesulfonyloxymethyl)-2-phe-
nyl-1,3-dioxane (861 mg, 2.47 mmol) in DMF (4.0 mL) was added
to the reaction mixture, and stirred for 20 h at room temperature.
Then the reaction was quenched with water, and the reaction mix-
ture was diluted with AcOEt. The organic layer was washed with
water and brine, dried with Na2SO4 and concentrated. Purification
by silica gel column chromatography (eluent; hexane/ethyl ace-
tate, 10:1) gave 38 (22%) as colorless oil. 1H NMR (CDCl3,
400 MHz) d 7.46–7.44 (m, 2H), 7.35–7.30 (m, 3H), 5.49 (s, 1H),
4.27 (dd, J = 11.5, 5.1 Hz, 1H), 4.01–3.92 (m, 2H), 3.49 (dd,
J = 10.4, 5.9 Hz, 1H), 3.38 (dd, J = 10.4, 4.8 Hz, 1H), 3.3–1.3 (br m,
10H), 3.31 (t, J = 6.2 Hz, 2H), 2.61 (s, 1H), 1.80 (dq, J = 12.8,
5.3 Hz, 1H), 1.71–1.67 (m, 2H), 1.48–1.38 (m, 3H).
4.2.22. 1-(2-Hydroxyethyl)-12-(5-triethylsilyloxy-5-
ethylheptyl)-1,12-dicarba-closo-dodecaborane (37)
A mixture of 36 (1.84 g, 3.44 mmol) and 10% palladium on car-
bon (200 mg) in ethanol (40 mL) was stirred at room temperature
for 20 h under atmospheric pressure of hydrogen. Insoluble mate-
rials were filtered off through Celite, then the filtrate was concen-
trated. Purification by silica gel column chromatography (eluent;
hexane/ethyl acetate, 8:1) gave 37 (87%) as white solid. 1H NMR
(CDCl3, 400 MHz) d 3.42 (t, J = 7.1 Hz, 2H), 3.3–1.3 (br m, 10H),
1.87 (t, J = 7.1 Hz, 2H), 1.58 (m, 2H), 1.37 (q, J = 7.7 Hz, 4H), 1.25
(m, 3H), 1.07 (m, 4H), 0.90 (t, J = 7.7 Hz, 9H), 0.76 (t, J = 7.3 Hz,
6H), 0.52 (q, J = 8.1 Hz, 6H).
4.2.23. 1-{2-[2-(2,2-Dimethyl-1,3-dioxolan-4-yl)ethoxy]ethyl}-
12-(5-triethylsilyloxy-5-ethylheptyl)-1,12-dicarba-closo-
dodecaborane (38)
4.2.27. 1-{3-[(2-Phenyl-1,3-dioxan-4-yl)methoxy]propyl}-12-(5-
triethylsilyloxy-5-ethylheptyl)-1,12-dicarba-closo-
NaH (46.8 mg, 1.17 mmol) was added to a solution of 37
(400 mg, 0.900 mmol) in DMF (2.0 mL) and stirred for 10 min at
0 °C. Then 4-{2-(p-toluenesulfonyloxy)ethyl}-2,2-dimethyl-1,3-
dioxolane (270 mg, 0.900 mmol) in DMF (2.0 mL) was added to
the reaction mixture, and stirred for 18 h at room temperature.
Then the reaction was quenched with water, and the reaction mix-
ture was diluted with AcOEt. The organic layer was washed with
water and brine, dried with Na2SO4 and concentrated. Purification
by silica gel column chromatography (eluent; hexane/ethyl ace-
tate, 10:1) gave 277 mg of 38 (54%) as colorless oil. 1H NMR (CDCl3,
400 MHz) d 4.12 (quint, J = 6.1 Hz, 1H), 4.00 (dd, J = 8.1, 5.9 Hz, 1H),
3.49 (t, J = 7.9 Hz, 1H), 3.36 (dd, J = 7.0, 5.5 Hz, 2H), 3.3–1.3 (br m,
10H), 3.13 (t, J = 6.2 Hz, 2H), 1.84 (t, J = 7.0 Hz, 2H), 1.81–1.66 (m,
2H), 1.56 (m, 2H), 1.37 (s, 3H), 1.32 (s, 3H), 1.24 (m, 2H), 1.04
(m, 4H), 0.89 (t, J = 8.1 Hz, 9H), 0.75 (t, J = 7.5 Hz, 6H), 0.50 (q,
J = 7.9 Hz, 6H).
dodecaborane (41)
Under Ar atmosphere, n-BuLi (1.57 M in n-hexane, 0.71 mL,
1.12 mmol) was added to a solution of 18 (326 mg, 0.86 mmol)
in ether (8.0 mL) at 0 °C, and stirred for 30 min at room tempera-
ture. Then 7-bromo-3-ethyl-3-triethylsilyloxyheptane (786 mg,
2.33 mmol) was added to the reaction mixture and stirred for
16 h at room temperature. Then the reaction was quenched with
water, and the mixture was diluted with AcOEt. The organic layer
was washed with water and brine, dried with Na2SO4 and concen-
trated. Purification by silica gel column chromatography (eluent;
hexane/ethyl acetate, 20:1) gave 452 mg of 36 (48%) as colorless
oil. 1H NMR (CDCl3, 400 MHz) d 7.46–7.44 (m, 2H), 7.36–7.30 (m,
3H), 5.49 (s, 1H), 4.27 (dd, J = 11.4, 4.0 Hz, 1H), 4.02–3.95 (m,
1H), 3.95 (dt, J = 11.9, 2.4 Hz, 1H), 3.49 (dd, J = 10.4, 6.1 Hz, 1H),
3.37 (dd, J = 10.3, 4.8 Hz, 1H), 3.30 (t, 6.2 Hz, 2H), 1.80 (dq, 12.6,
5.1 Hz, 1H), 3.3–1.3 (br m, 10H), 1.59–1.40 (m, 7H), 1.36 (q,
J = 7.5 Hz, 4H), 1.27–1.23 (m, 2H), 1.07–1.05 (m, 4H), 0.90 (t,
J = 7.7 Hz, 9H), 0.76 (t, J = 7.3 Hz, 6H), 0.52 (q, J = 7.7 Hz, 6H).
4.2.24. 1-[2-(3,4-Dihydroxybutoxy)ethyl]-12-(5-ethyl-5-
hydroxyheptyl)-1,12-dicarba-closo-dodecaborane (12)
2 M HCl (4.0 mL) was added to a solution of 38 (290 mg,
0.480 mmol) in MeOH/THF (6.0 mL), and the mixture was stirred
for 18 h at room temperature. The mixture was diluted with AcOEt,
and then the organic layer was washed with water and brine, dried
with Na2SO4 and concentrated. Purification by silica gel column
chromatography (eluent; hexane/ethyl acetate, 2:1) gave 163 mg
of 12 (81%) as colorless solid. 1H NMR (CDCl3, 400 MHz) d 3.84
(m, 1H), 3.59 (dd, J = 11.2, 3.5 Hz, 1H), 3.50 (m, 2H), 3.46 (dd,
J = 11.2, 4.8 Hz, 1H), 3.3–1.3 (br m, 10H), 3.18 (m, 2H), 1.87 (t,
J = 7.1 Hz, 2H), 1.78–1.57 (m, 4H), 1.38 (q, J = 7.5 Hz, 4H), 1.27
(m, 2H), 1.09 (m, 4H), 0.80 (t, J = 7.3 Hz, 6H).
4.2.28. 1-[3-(2,4-Dihydroxybutoxy)propyl]-12-(5-ethyl-5-
hydroxyheptyl)-1,12-dicarba-closo-dodecaborane (14)
2 M HCl (3.0 mL) was added to a solution of 41 (317 mg,
0.499 mmol) in MeOH/THF (12 mL), and the mixture was stirred
for 4 h at room temperature. The mixture was diluted with AcOEt,
and then the organic layer was washed with water and brine, dried
with Na2SO4 and concentrated. Purification by silica gel column
chromatography (eluent; hexane/ethyl acetate, 1:1) gave 116 mg
of 14 (54%) as colorless solid. 1H NMR (CDCl3, 400 MHz) d 3.94 (s,
1H), 3.80 (q, J = 5.7 Hz, 2H), 3.34–3.20 (m, 4H), 3.3–1.3 (br m,
10H), 2.60 (d, J = 3.1 Hz, 1H), 2.36 (t, J = 3.1 Hz, 1H), 1.68–1.57
(m, 6H), 1.45–1.35 (m, 2H), 1.38 (q, J = 7.5 Hz, 4H), 1.29–1.24 (m,
2H), 1.10–1.09 (m, 4H), 1.00 (s, 1H), 0.80 (t, J = 7.7 Hz, 6H).
4.2.25. 1-(3-Hydroxypropyl)-1,12-dicarba-closo-dodecaborane
(39)
A 1.57 M solution of n-BuLi in n-hexane (24.3 mL, 38.1 mmol)
was added to a solution of p-carborane (5.00 g, 34.7 mmol) in ether
(120 mL) at 0 °C, and the mixture was stirred at room temperature
for 1 h. Then trimethylene oxide (2.21 g, 38.1 mmol) was added to
the mixture and stirring was continued at room temperature for
16 h. The reaction mixture was poured into water and extracted
4.3. Biology
4.3.1. Assay of HL-60 cell differentiation-inducing activity
HL-60 cells were cultured in RPMI-1640 medium supplemented
with 5% FBS and penicillin G and streptomycin at 37 °C under 5% CO2
in air. The cells were diluted to 8.0 Â 104 cell/mL with RPMI-1640

S. Fujii et al. / Bioorg. Med. Chem. 22 (2014) 1227–1235
1235
Allegretto, E. A. Chem. Biol. 1999, 6, 265; (b) Hosoda, S.; Tanatani, A.;
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(5% FBS), and ethanol solution of a test compound was added to
give 10À9 to 10À6 M final concentration. Control cells were treated
with the same volume of ethanol alone. 1a,25(OH)2D3 was always
5. Fujii, S.; Masuno, H.; Taoda, Y.; Kano, A.; Wongmayura, A.; Nakabayashi, M.; Ito,
N.; Shimizu, M.; Kawachi, E.; Hirano, T.; Endo, Y.; Tanatani, A.; Kagechika, H. J.
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Tanatani, A. Bioorg. Med. Chem. Lett. 2012, 22, 1756–1760.
assayed at the same time as a positive control. The cells were incu-
bated at 37 °C under 5% CO2 in air for 4 days. The percentage of dif-
ferentiated cells was determined by nitro-blue tetrazolium (NBT)
reduction assay. Cells were incubated at 37 °C for 20 min in
RPMI-1640 (5% FBS) and an equal volume of phosphate-buffered
saline (PBS) containing NBT (0.2%) and 12-O-tetradecanoylphorbol
13-acetate (TPA; 200 ng/mL). The percentage of cells containing
blue-black formazan was determined in a minimum of 200 cells.
7. Carboranes; Grimes, R. N., Ed., 2nd ed.; Academic Press: Elsevier, 2011.
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