Synthesis and in vivo biodistribution of BPA-Gd-DTPA complex as a potential MRI contrast carrier for neutron capture therapy

Article abstract of DOI:10.1016/j.bmc.2004.10.046

p-Boronophenylalanine (BPA) conjugated Gd-DTPA complex (3) was synthesized from the active methyne compound 6, the allylic carbonate 7, and BPA by the palladium-catalyzed allylation reaction followed by the DCC coupling reaction. The in vivo biodistribution of complex 3 was evaluated by prompt gamma-ray analysis and α-autoradiography using the tumor-bearing rats. High accumulation of gadolinium was observed in the kidney and the %ID values were 0.17 and 0.088 at 20 and 60 min after injection of 3, respectively. The accumulation was also observed in the tumor and the %ID values were 0.010 and 0.0025 at 20 and 60 min after injection, respectively. The visualization experiment of boron distribution in the tumor-bearing rat by α-autoradiography indicates that boron was accumulated in the tumor and the intestines at 20 min after injection.

Full text of DOI:10.1016/j.bmc.2004.10.046

Bioorganic & Medicinal Chemistry 13 (2005) 735–743
Synthesis and in vivo biodistribution of BPA–Gd–DTPA
complex as a potential MRI contrast carrier
for neutron capture therapy
Kazunori Takahashi,a Hiroyuki Nakamura,b Shozo Furumoto,c Kazuyoshi Yamamoto,d
Hiroshi Fukuda,c,* Akira Matsumurae and Yoshinori Yamamotoa,*
aDepartment of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
bDepartment of Chemistry, Faculty of Science, Gakushuin University, Tokyo 171-8588, Japan
cInstitute of Development, Aging and Cancer, Tohoku University, Sendai 980-8575, Japan
dJapan Atomic Energy Research Institute at Tokai, Ibaragi 319-1195, Japan
eDepartment of Neurosurgery, Institute of Clinical Medicine, University of Tsukuba, Ibaraki 305-8575, Japan
Received 27 September 2004; revised 21 October 2004; accepted 21 October 2004
Available online 21 November 2004
Abstract—p-Boronophenylalanine (BPA) conjugated Gd–DTPA complex (3) was synthesized from the active methyne compound 6,
the allylic carbonate 7, and BPA by the palladium-catalyzed allylation reaction followed by the DCC coupling reaction. The in vivo
biodistribution of complex 3 was evaluated by prompt gamma-ray analysis and a-autoradiography using the tumor-bearing rats.
High accumulation of gadolinium was observed in the kidney and the %ID values were 0.17 and 0.088 at 20 and 60min after injec-
tion of 3, respectively. The accumulation was also observed in the tumor and the %ID values were 0.010 and 0.0025 at 20 and 60min
after injection, respectively. The visualization experiment of boron distribution in the tumor-bearing rat by a-autoradiography indi-
cates that boron was accumulated in the tumor and the intestines at 20min after injection.
ꢀ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
tions in the tumor and the surrounding tissues before
irradiation of neutron in order to improve and optimize
efficient NCT treatment. Positron emission tomography
(PET) using fluorine-18-labeled fluoroboronophenylala-
nine (18F-BPA) has been developed for this purpose and
18F-BPA is now utilized not only for the estimation of
the boron distribution but also for the prognostic indi-
cator in patients with gliomas.12 As an alternative moni-
toring method of the distribution of a carrier, magnetic
resonance imaging (MRI) of boron-11 has been exam-
ined.13–16 However, the actual nucleus for neutron cap-
ture reaction is boron-10, therefore, the administration
of extra amounts of boron-11 compound, which does
not undergo the nucleic reaction on NCT, is needed
for MR imaging. Kahl and co-workers have succeeded
in the synthesis of Mn-boronated porphyrin (BOPP)
complex and visualizing the distribution of Mn by
MRI.17,18
Neutron capture therapy (NCT), as one of the radio-
therapies of cancers, has gained intense interest in recent
years.1–4 This therapy is a binary system, which involves
the use of two components: thermal neutrons and nuc-
lides possessing high thermal neutron capture cross sec-
tion. Although each component should be relatively
innocuous to mammalian cells, their combination gener-
ates a highly lethal cytocidal effect. Among various nuc-
lides that possess usual capacity for absorbing thermal
neutrons, boron-10 and gadolinium-157 have been
focused on NCT as a nonradioactive nuclide. So far,
p-boronophenylalanine (BPA)5–7 and mercaptoundeca-
hydrododecaborate (BSH)8–11 have been utilized for
NCT and it is required to monitor the boron concentra-
Keywords: MRI; Neutron capture therapy; p-Boronophenylalanine;
Gadolinium complex; Gd–DTPA complex; Biodistribution.
Gd–DTPA (Magnevistꢁ) (1) is known as a MRI con-
trast agent and has been utilized for the diagnosis of
patients with gliomas because this complex is able to
*
Corresponding authors. Tel.: +81 222176581; fax: +81 222176784;
e-mail: tl@yamamoto1.chem.tohoku.ac.jp
0968-0896/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2004.10.046

736
K. Takahashi et al. / Bioorg. Med. Chem. 13 (2005) 735–743
BPA-conjugated Gd-DTPA (3)
2-
N
N
B(OH)2
H
CO2
CO2
d
G
N
N
2-
2H+
N
Gd
N
= BH
= C
N
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
N
H
N
N
Gd
2Na+
Gd-DTPA (1)
Figure 1.
CO2
Carborane-Gd-DTPA Complex (2)
CO2
CO2
CO2
CO2Na
O
MR imaging
moiety
linker
BPA
(biofunctional group)
pass through the disrupted blood–brain barrier and
enter into the tumor tissue (Fig. 1).19–24 Gadolinium is
paramagnetic, and its effect on the contrast of the
images can be best seen on T1-weighted images with
short repetition times. Furthermore, the thermal
neutron capture cross section of gadolinium-157 is the
largest (255,000b) among all the stable isotopes, and it
is about 66 times as large as that of boron-10 (3838b).
Matsumura and co-workers examined the visualization
of Mn–metalloporphyrin–Gd–DTPA in rats using
MRI for the possibility of Gd-NCT.25–27
B(OH)2
CO2H
EtO2C
EtO2C
EtO2C
CO2Et
H2N
N
N
N
+
CO2Et
CO2Et
CO2Et
5
4
We recently developed a method for the preparation of
DTPA-bifunctional chelating agents in which a second
functional group is attached to DTPA carbon frame-
work through C–C bond.28–31 This method enables us
to introduce a second functional group into DTPA
framework without losing one of the five carboxyl
groups, whereas a general method for coupling DTPA
with the second functional group is based on the
conversion of one of five carboxyl groups of DTPA into
an ester or an amide to produce the corresponding
tetraacid derivatives.32 Reduction of the number of
carboxylic acids that are capable of coordinating to a
metal ion, may cause the liberation of a metal ion in
vivo.32,33 We previously reported the synthesis of the
carborane conjugated Gd–DTPA complex (2)28 through
C–C bond and succeeded in the visualization of the
biodistribution of complex 2 in rats by MRI (Fig. 1).
The visualization experiments using a-autoradiography
also indicated that complex 2 was accumulated into
the necrosis part of a tumor tissue.29,30 We therefore
focused on BPA as an alternative second functional
group of a Gd–DTPA complex for a potential MRI con-
trast carrier. In this paper, we report the synthesis of
p-boronophenylalanine (BPA) conjugated Gd–DTPA
complex (3) and its biodistribution in the tumor-bearing
rats.
EtO2C
N
EtO2C
EtO2C
CO2Et
CO2Et
OTBS
+
N
N
OCO2Et
CO2Et
6
7
Scheme 1. Design of BPA-conjugated Gd–DTPA complex 3.
allylic carbonate 7, since this strategy can be utilized for
introduction of various amines and amino acids as a
biofunctional group into the Gd–DTPA complex by
the amide bond formation.
2.1.2. Synthesis of the allylic carbonate 7. We first syn-
thesized the allylic carbonate 7, as a linker, from com-
mercially available 1,5-pentanediol as shown in
Scheme 2. Selective protection of 1,5-pentanediol with
TBSCl was carried out in DMF at 0ꢂC to give the cor-
responding monosilylated alcohol 8 in 74% yield. The
alcohol 8 was oxidized with 2,2,6,6-tetramethyl-1-pipe-
ridinyloxide (TEMPO) to give the aldehyde 9 in 92%
yield. The addition of vinyl magnesium bromide to 9,
followed by the treatment with ethyl chloroformate gave
the allylic carbonate 7 in 78% yield.
2. Result and discussion
2.1. Chemistry
2.1.3. Synthesis of the precursor 4. The synthesis of the
precursor 4 of the BPA–DTPA conjugate is shown in
Scheme 3. The DTPA hexaethyl ester 6, which has an
active methyne moiety in a molecule, was treated with
7 under the Tsuji–Trost conditions34 to give the allyla-
tion product 10 in 83% yield. Desilylation of 10 with
TBAF afforded the alcohol 11 in 90% yield. Oxidation
of 11 using SO3Æpyridine complex gave the aldehyde 12
in an essentially quantitative yield. It should be noted
that Swern oxidation and TEMPO oxidation were not
effective for the synthesis of 12. Further oxidation was
carried out with NaClO2 to give the precursor 4 in
59% yield.
2.1.1. Design of BPA-conjugated Gd–DTPA complex (3).
Our molecular design of BPA-conjugated Gd–DTPA
complex is shown in Scheme 1. The molecule consists
of three parts; a Gd–DTPA complex as an MR imaging
moiety, a BPA as a biofunctional group, and a linker,
and these three moieties can be connected by the palla-
dium-catalyzed Tsuji–Trost type allylation and the
amide bond formation. Therefore, we decided to synthe-
size the precursors 4 from the hexaethyl ester 628 and the

K. Takahashi et al. / Bioorg. Med. Chem. 13 (2005) 735–743
737
2.1.4. Optimization of protective groups for a boronic acid
OTBS
of BPA. Since the amide bond formation of BPA ethyl
ester 13 and the precursor 4 did not proceed, we investi-
gated suitable protective groups for a boronic acid of
BPA (Fig. 2). A proper choice of protective groups of
a boronic acid is important for the synthesis of biologi-
cally active boron compounds because the deprotection
process is not an easy task at the end of the synthe-
sis.35,36 Although the deprotection reaction of the pina-
col ester 14 did not proceed by the oxidative cleavage
(NaIO4, AcONH4, acetone–H2O), the fluoride ion
(KHF2), and the transesterification (phenylboronic acid,
Et2O–H2O), pinanediol was found to be an effective pro-
tective group for BPA.37–40 The pinanediol ester 15
underwent the transesterification with phenylboronic
acid in Et2O–H2O to afford 13 in 34% yield and the yield
increased up to 48% when the reaction was carried out
in hexane–H2O under pH ꢀ 3.41
EtO2C
N
a
EtO2C
EtO2C
CO2Et
CO2Et
6
N
N
CO2Et
10
OH
EtO2C
N
EtO2C
EtO2C
CO2Et
CO2Et
b
N
N
CO2Et
11
CHO
d
EtO2C
2.1.5. Synthesis of BPA–Gd–DTPA complex. Synthesis
of the BPA–Gd–DTPA complex is shown in Scheme 4.
The amide formation reaction of the precursor 4 with
the pinanediol-protected BPA 15 was carried out with
1,3-dicyclohexylcarbodiimide (DCC) in dichlorometh-
ane to give the desired product 16 in 84% yield. All ethyl
ester groups of 16 were hydrolyzed with excess amounts
of LiOH monohydrate and then the reaction mixture
was acidified with 10% HCl (pH ꢀ 2) to give the corre-
sponding hexacarboxylic acids 17 in 29% yield. Depro-
tection of the pinanediol group of 17 was carried out
in the presence of an equivalent of phenylboronic acid
in hexane–H2O to afford the corresponding boronic acid
18 in 67% yield. The yield of 18 increased up to 89%
when 2equiv of phenylboronic acid were employed. Fi-
nally, the treatment of 18 with 1equiv of gadolinium(III)
chloride hexahydrate and sodium carbonate gave the
desired BPA–Gd–DTPA complex 3 in 68% yield. The
structure of 3 was confirmed by ESI-MS and elemental
analysis.
c
EtO2C
EtO2C
CO2Et
4
N
N
N
CO2Et
CO2Et
12
Scheme 3. Reagents and conditions: (a) 7, Pd(dba)2, dppe, THF,
reflux; (b) TBAF, THF; (c) SO3Æpyridine, TEA, DMSO; (d) NaClO2,
NaH2PO4, 2-methyl-2-butene, THF–t-BuOH–H2O.
cle, brain, and tumor were excised. Gadolinium concen-
tration in each tissue was determined by PGA. The
results are shown in Figure 3a. The percentages of
injected dose per 100mg of tissue (%ID/100mg tissue)
are indicated in the vertical axes. The %ID values in
the blood and the liver were 0.027 and 0.078, respec-
tively, at 20min after injection, and those values
dropped to 0.0032 and 0.013, respectively, at 60min.
High accumulation was observed in the kidney and
%ID values were 0.17 and 0.088 at 20 and 60min after
2.2. Biodistribution study in vivo
2.2.1. Gadolinium concentrations in various tissues of
tumor-bearing rats using prompt gamma-ray analysis
(PGA). Biodistribution of gadolinium in vivo was exam-
ined using rats with a tumor implanted on their back.
Compound 3 (0.048M, 0.30mL) was injected intrave-
nously into a rat via the tail vein. At 20 and 60min after
injection, the rats were sacrificed by dislocation of the
cervical vertebrae, and blood, liver, kidney, thigh mus-
OH
B
O
B
O
B
O
O
HO
EtO2C NH2
EtO2C NH2
Et
O2C NH2
14
13
15
Figure 2.
a
b
HO
OTBS
HO
OH
8
OTBS
c
OHC
OTBS
OCO2Et
9
7
Scheme 2. Reagents and conditions: (a) i. TBSCl, imidazole, DMF, 0ꢂC; (b) TEMPO, NaClO, KBr, CH2Cl2–H2O, 0ꢂC; (c) i. vinyl magnesium
bromide, THF, À78ꢂC; ii. ethyl chloroformate.

738
K. Takahashi et al. / Bioorg. Med. Chem. 13 (2005) 735–743
O
B
O
EtO2C
O
a
4
N
H
EtO2C
N
EtO2C
EtO2C
CO2Et
CO2Et
N
N
16
CO2Et
O
B
O
HO2C
O
N
H
b
HO2C
HO2C
CO2H
CO2H
N
N
N
CO2H
17
HO
B
OH
HO2C
O
N
H
c
d
HO2C
HO2C
CO2H
CO2H
3
N
N
N
CO2H
18
Scheme 4. Reagents and conditions: (a) 15, DCC, CH2Cl2, 0ꢂC; (b) i. LiOH, H2O; ii. 10% HCl (pH ꢀ 2); (c) phenylboronic acid (2equiv) hexane–
H2O; (d) GdCl3Æ6H2O, Na2CO3, MeOH–H2O.
injection, respectively. Although gadolinium accumula-
tion was very low in the muscle and the brain, the accu-
mulation was observed in the tumor and the %ID values
were 0.010 and 0.0025 at 20 and 60min after injection,
respectively. Figure 3b shows the biodistribution of the
carborane–Gd–DTPA complex 2, which was reported
previously.29,30 The gadolinium concentration of 2 in
the blood was the same level as the BPA–Gd–DTPA
3. The %ID values of 2 in the liver and the kidney were
much lower than those of 3 and gadolinium accumul-
ation was not observed in the brain and the tumor.
4a and b show the cross section of the rat and the boron
distribution visualized by a-ARG, respectively, at
20min after injection of compound 3. The parts visual-
ized as white indicate the distribution of boron in Figure
4b. Since the tumor on the back of the rat was visual-
ized, boron was accumulated significantly in the tumor.
Furthermore, high accumulation of boron was observed
in the intestines.
3. Discussion
2.2.2. Boron distribution of tumor-bearing rats visualized
by a-autoradiography (a-ARG). Compound 3 was dis-
solved in saline, and then the solution (0.048M,
0.30mL) was injected intravenously into a rat via the tail
vein under the anesthesia by ether. At 20 and 60min
after the injection, the rats were sacrificed by an over-
dose of CHCl3. The rats were coated completely with
the carboxymethylcellulose (CMC–Na), and then frozen
at À80ꢂC for an hour. After sectioning the frozen rats,
the sections of 50lm thick were obtained on adhesive
tapes with Auto Cryotome at À20ꢂC, attached to the
special tapes (CR-39, Baryotrack, 2mm thick) for a-
ARG, and then irradiated with thermal neutrons. Figure
The %ID values of 3 and 2 in the tumor at 20min after
injection were 0.0100 and 0.000878, respectively, and the
tumor/blood ratios of those compounds were 0.380 and
0.0750, respectively. The tumor/blood ratio of 3 in-
creased to 0.789 at 60min, although the gadolinium con-
centration in the tumor was not detected at 60min in the
case of compound 2. These results indicate that com-
pound 3 possess higher tumor-selectivity than com-
pound 2. Compound 3 was highly accumulated into
the liver and the kidney at 20min after injection. How-
ever, the gadolinium concentrations of 3 in the liver
and the kidney decreased at 60min, and 16% and 53%
of the gadolinium concentrations obtained at 20min

K. Takahashi et al. / Bioorg. Med. Chem. 13 (2005) 735–743
739
0.25
0.25
0.2
0.15
0.1
0.05
0
(a)
(b)
0.2
20min.
60min.
20min.
60min.
0.15
0.1
0.05
0
Blood Liver Kidney Muscle Brain Tumor
Blood Liver Kidney Brain Tumor
Figure 3. The distribution of Gd incorporated into various tissues of rats at 20 and 60min after injection of BPA–Gd–DTPA 3 (a) and carborane–
Gd–DTPA 2 (b).
were observed in the liver and the kidney, respectively.
The gadolinium concentrations of 2 in the liver and
the kidney at 60min were 25% and 67% of those ob-
served at 20min. Therefore, it is considered that com-
pound 3 would be metabolized and excreted more
rapidly in comparison with compound 2. These results
correspond to the observation by a-autoradiography
in Figure 4b. Perhaps, the compound 3 and its metabo-
lite would be excreted directly in the bile via the liver or
in the intestines. The tumor/muscle ratios of 3 were 2.73
at 20min and 2.89 at 60min, respectively, and no accu-
mulation of 3 was observed in the brain.
Inc., Okayama, Japan. Most chemicals and solvents
were analytical grade and used without further
purification.
5.1.1. 5-(tert-Butyldimethylsilyloxy)pentanol 8. To a
solution of imidazole (0.32g, 4.6mmol) in DMF
(7.2mL) at rt under Ar was added 1,5-pentanediol
(2.1mL, 20mmol), and the mixture was stirred at 0ꢂC.
To the mixture was added TBSCl (0.54g, 3.6mmol) at
0ꢂC, and the mixture was stirred for 15min. The mixture
was diluted with ether, and washed with water and
brine. The organic layer was dried over anhydrous
MgSO4 and concentrated in vacuo. Purification by silica
gel column chromatography (hexane/AcOEt = 4:1) gave
8 (0.55g, 2.7mmol, 74% yield): colorless oil; IR (neat)
4. Conclusion
3500–3000, 2931, 1255, 1101, 835cmÀ1
;
1H NMR
We have succeeded in the synthesis of a new boron–
gadolinium conjugated compound, BPA–Gd–DTPA
complex (3), as an MRI contrast carrier for NCT. The
higher accumulation of complex 3 was observed in the
tumor tissue in comparison with the case of the carbo-
rane–Gd–DTPA complex 2 by the in vivo biodistribu-
tion experiments using PGA and a-autoradiography.
In this regard, the BPA moiety is considered to play
an important role for the accumulation of the Gd–
DTPA complex in the tumor tissue. Since the various
amino acids instead of BPA can be introduced into the
DTPA framework through the amide bond formation,
we believe that the current method described in this
paper would be a very useful strategy not only for the
synthesis of boron carriers on NCT but also for
the development of biofunctional Gd–DTPA
complexes.
(300MHz, CDCl3): d 3.60 (t, J = 6.3Hz, 2H), 3.57 (t,
J = 6.3Hz, 2H), 1.56–1.45 (m, 4H), 1.40–1.32 (m, 2H),
0.84 (s, 9H), 0.00 (s, 6H); 13C NMR (400MHz, CDCl3):
d 62.9, 62.0, 32.3, 32.1, 25.8, 25.7, 21.8, 18.1, À5.4, À5.5.
Anal. Calcd for C11H26O2Si: C, 60.49; H, 12.00. Found:
C, 60.33; H, 11.97.
5.1.2. 5-(tert-Butyldimethylsilyloxy)pentanal 9. To a
solution of 8 (3.4g, 15.6mmol) in CH2Cl2 (63mL) at
0ꢂC were added aqueous KBr (1.0M, 1.56mL,
1.56mmol), 2,2,6,6-tetramethyl-1-piperidinyloxy, free
radical (TEMPO) (0.049g, 0.31mmol), and a freshly
prepared bleach solution (0.3M, a 1:1 mixture of com-
mercial bleach solution and saturated NaHCO3,
62mL, 18.7mmol). After vigorous stirring for 2.5h,
the mixture was diluted with ether and washed with
water and brine. The organic layer was dried over anhy-
drous MgSO4, and concentrated in vacuo. Purification
by silica gel column chromatography (hexane/
AcOEt = 4:1) gave 9 (3.1g, 14.3mmol, 92% yield): color-
5. Experimental
5.1. Chemistry
1
less oil; IR (neat) 2952, 1749, 1255, 1101, 835cmÀ1; H
NMR (300MHz, CDCl3): d 9.72 (t, J = 1.8Hz, 1H),
3.58 (t, J = 6.3Hz, 2H), 2.41 (dt, J = 6.7, 1.8Hz, 2H),
1.70–1.60 (m, 2H), 1.54–1.45 (m, 2H), 0.84 (s, 9H),
0.00 (s, 6H); 13C NMR (400MHz, CDCl3): d 201.0,
62.0, 43.1, 31.7, 25.5, 18.2, 17.9, À5.7, À5.8. Anal. Calcd
for C11H24O2Si: C, 61.05; H, 11.18. Found: C, 60.94; H,
11.32.
1H NMR and 13C NMR spectra were measured with
JEOL JNM-GSX-270, JEOL JNM-AL-300, and JEOL
JNM-AL-400 spectrometer. Chemical shifts of 1H
NMR and 13C NMR were expressed in parts per mil-
lion. IR spectra were measured with SHIMADZU
FTIR-8200A. HPLC was performed with Mightysil
RP-18 250-20 (5lm) column. BPA (L/D = 72:28) was
obtained from Hayashibara Biochemical Laboratories,
5.1.3. The allylic carbonate 7. To a solution of 9 (2.7g,
12.5mmol) in THF (60mL) at À78ꢂC under Ar was

740
K. Takahashi et al. / Bioorg. Med. Chem. 13 (2005) 735–743
Figure 4. Cross section (a) and a-autoradiography (b) of a rat bearing AH109A tumor on the back at 20min after injection of the compound 3.
added vinyl magnesium bromide (1.0M in THF solu-
tion, 15.0mL, 15.0mmol) over a period of 25min. After
being stirred for an hour at the same temperature, ethyl
chloroformate (1.4mL, 15.0mmol) was added to the
solution over a period of 10min. After being stirred
for an hour at rt, the mixture was quenched with satu-
rated NH4Cl at 0ꢂC, diluted with ether, and then
washed with water and brine. The organic layer was
dried over anhydrous MgSO4 and concentrated
in vacuo. Purification by silica gel column chromato-
graphy (hexane/AcOEt = 4:1) gave 7 (3.1g, 9.8mmol,
78% yield): yellow oil; IR (neat) 2952, 1747, 1647,
1257, 1101, 837, 775cmÀ1; 1H NMR (300MHz, CDCl3):
d 5.75 (ddd, J = 17.4, 10.5, 6.9Hz, 1H), 5.25 (d,
J = 17.4Hz, 1H), 5.16 (d, J = 10.5Hz, 1H), 5.04–4.99
(m, 1H), 4.14 (q, J = 6.9Hz, 2H), 3.56 (t, J = 6.3Hz,
2H), 1.70–1.35 (m, 6H), 1.26 (t, J = 6.9Hz, 3H), 0.84
(s, 9H), 0.00 (s, 6H); 13C NMR (400MHz, CDCl3): d
154.2, 135.8, 116.8, 78.3, 63.3, 62.4, 33.7, 25.7, 25.6,
21.1, 18.0, 14.0, À5.4, À5.5. Anal. Calcd for
C16H32O4Si: C, 60.72; H, 10.19. Found: C, 59.87; H,
9.88.
1.94 (m, 2H), 1.47–1.42 (m, 2H), 1.32–1.27 (m, 2H),
1.21 (t, J = 7.2Hz, 18H), 0.84 (s, 9H), 0.00 (s, 6H); 13C
NMR (270MHz, CDCl3): d 171.5, 171.2, 170.9, 169.3,
133.9, 123.9, 75.3, 62.9, 61.0, 60.3, 60.2, 60.1, 55.4,
55.3, 55.2, 53.9, 52.8, 52.4, 50.5, 37.9, 32.3, 25.9, 25.6,
18.3, 14.2, À5.2; MS (FAB), calcd for C40H74SiN3O13
(M+H) 832.4991, found: 832.5254. Anal. Calcd for
C40H73N3O13Si: C, 57.74; H, 8.84; N, 5.05. Found: C,
57.60; H, 8.73; N, 4.92.
5.1.5. Desilylation of 10. To a solution of 10 (0.24g,
0.29mmol) in THF (1.5mL) at rt was added TBAF
(1.0M in THF, 0.44mL, 0.44mmol), and the mixture
was stirred for 10h. The mixture was diluted with ether
and washed with water and brine. The organic layer
was dried over anhydrous MgSO4 and concentrated
in vacuo. Purification by silica gel column chromato-
graphy (hexane/AcOEt = 1:2) gave 11 (0.19g,
0.26mmol, 90% yield): yellow oil; IR (neat) 3550–3200,
2981, 1747, 1650, 1456, 1369, 1031cmÀ1 1H NMR
;
(300MHz, CDCl3): d 5.38 (br s, 2H), 4.13–4.01 (m,
12H), 3.52 (t, J = 6.6Hz, 2H), 3.49 (s, 6H), 3.33 (s,
2H), 2.81–2.62 (m, 10H), 2.00 (br s, 1H), 1.93 (m, 2H),
1.50–1.42 (m, 2H), 1.36–1.29 (m, 2H), 1.18 (t,
J = 6.9Hz, 18H); 13C NMR (300MHz, CDCl3): d
171.2, 170.8, 170.5, 168.9, 133.2, 123.5, 74.6, 60.6,
59.9, 59.8, 59.6, 54.9, 54.8, 54.7, 53.3, 52.3, 51.7, 49.7,
37.3, 31.5, 24.9, 13.7; HRMS (ESI), calcd for
C34H60N3O13 (M+H) 718.4121, found: 718.4121. Anal.
Calcd for C34H59N3O13: C, 56.89, H, 8.28, N, 5.85.
Found: C, 56.48, H, 8.38, N, 5.83.
5.1.4. Allylation of the diethylenetriaminehexaacetic acid
hexaethyl ester 6 with the allylic carbonate 7. A mixture
of 6 (1.6g, 2.6mmol), 7 (1.3g, 4.0mmol), Pd(dba)2
(0.30g, 0.52mmol), and dppe (0.31g, 0.78mmol) in
THF (25mL) was refluxed for two days under Ar. The
mixture was cooled to rt, diluted with ether, and washed
with water and brine. The organic layer was dried over
anhydrous MgSO4 and concentrated in vacuo. Purifica-
tion by silica gel column chromatography (hexane/
AcOEt = 1:1) gave 10 (1.9g, 2.2mmol, 83% yield): yel-
low oil; IR (neat) 2981, 1732, 1652, 1463, 1369, 1190,
5.1.6. The aldehyde 12. To a mixture of 11 (0.10g,
0.14mmol) and Et3N (0.097mL, 0.70mmol) in DMSO
(0.35mL) at rt under Ar was added SO3Æpyridine com-
plex (0.067g, 0.42mmol) in DMSO (0.35mL), and the
mixture was stirred for 30min at rt. The mixture was
1
1097, 1031cmÀ1; H NMR (300MHz, CDCl3): d 5.41
(br s, 2H), 4.17–4.05 (m, 12H), 3.54 (t, J = 6.3Hz,
2H), 3.52 (s, 6H), 3.37 (s, 2H), 2.83–2.64 (m, 10H),

K. Takahashi et al. / Bioorg. Med. Chem. 13 (2005) 735–743
741
quenched with saturated NH4Cl at 0ꢂC, diluted with
ether, and then washed with water and brine. The organic
layer was dried over anhydrous MgSO4 and concen-
trated in vacuo. The pure compound 12 was obtained
quantitatively (0.10g, 0.14mmol): yellow oil; IR (neat)
1.2mL) in CH2Cl2 (0.5mL), and the mixture was stirred
for 2.5h at rt. The mixture was filtrated, concentrated,
and then purified by preparative thin layer chromato-
graphy with AcOEt to give 15 (0.45g, 1.2mmol, >99%
yield): yellow oil; IR (neat) 3384, 2933, 1735, 1612,
1
2981, 1747, 1446, 1369, 1031cmÀ1
;
1H NMR
1236cmÀ1; H NMR (400MHz, CDCl3): d 7.74 (d, J =
(300MHz, CDCl3): d 9.73 (s, 1H), 5.42 (m, 2H), 4.19–
4.06 (m, 12H), 3.53 (s, 6H), 3.38 (s, 2H), 2.83–2.73 (m,
8H), 2.67 (d, J = 5.1Hz, 2H), 2.40 (t, J = 6.6Hz, 2H),
2.00 (m, 2H), 1.66–1.61 (m, 2H), 1.23 (t, J = 7.2Hz,
18H); 13C NMR (270MHz, CDCl3): d 202.3, 171.5,
171.3, 171.0, 169.3, 132.6, 125.2, 75.2, 61.1, 60.3, 60.2,
60.1, 55.4, 55.3, 55.2, 53.9, 52.8, 52.4, 50.4, 43.0, 37.8,
31.7, 21.5, 14.2; HRMS (ESI), calcd for C34H57N3O13-
Na (M+Na) 738.3734, found: 738.3734.
8.0Hz, 2H), 7.20 (d, J = 8.0Hz, 2H), 4.43 (dd, J = 9.0,
1.6Hz, 1H), 4.15 (q, J = 7.0Hz, 2H), 3.70 (dd, J = 8.0,
5.2Hz, 1H), 3.09 (dd, J = 13.2, 5.2Hz, 1H), 2.85 (dd,
J = 13.2, 8.0Hz, 1H), 2.43–2.37 (m, 1H), 2.22–2.20 (m,
1H), 2.13 (t, J = 6.0Hz, 1H), 1.96–1.91 (m, 2H), 1.46
(s, 3H), 1.30 (s, 3H), 1.23 (t, J = 7.0Hz, 3H), 1.19 (s,
1H), 0.87 (s, 3H); 13C NMR (400MHz, CDCl3): d
173.9, 139.9, 134.3, 128.0, 85.3, 77.4, 60.0, 55.1, 50.7,
40.7, 38.9, 37.5, 34.9, 28.1, 26.5, 25.9, 23.4, 13.7; HRMS
(ESI), calcd for C21H30BNO4 (M+H) 372.2340, found:
372.2340.
5.1.7. Synthesis of the precursor 4. To a solution of 12
(3.0g, 4.2mmol) in t-BuOH (21mL) and water (21mL)
at rt were added 2-methyl-2-butene (2.7mL, 25mmol),
NaH2PO4 (1.5g, 13mmol), and NaClO2 (1.1g,
13mmol), and the mixture was stirred vigorously for
11h at rt. The mixture was diluted with AcOEt and
washed with water and brine. The organic layer was
dried over anhydrous Na2SO4 and concentrated
in vacuo. Purification by silica gel column chromato-
graphy (hexane/AcOEt = 1:10) gave 4 (1.8g, 2.5mmol,
59% yield): yellow oil; IR (neat) 3200–2800, 1749,
5.1.10. DCC coupling of 4 with 15. To a solution of 4
(1.6g, 2.2mmol) and 15 (0.81g, 2.2mmol) in CH2Cl2
(7mL) at 0ꢂC under Ar was added DCC (0.45g,
2.2mmol) dissolved in CH2Cl2 (7mL). The mixture
was stirred for 2h at 0ꢂC and then stirred for 5h at rt.
The solution was filtrated, and the filtrate was diluted
with AcOEt and washed with water and brine. The or-
ganic layer was dried over anhydrous Na2SO4 and con-
centrated in vacuo. Purification by silica gel column
chromatography (hexane/AcOEt = 1:2) gave 16 (0.30g,
0.28mmol, 84% yield): yellow oil; IR (neat) 3375,
1
1652, 1456, 1028cmÀ1; H NMR (400MHz, CDCl3): d
5.42 (br s, 2H), 4.19–4.08 (m, 12H), 3.57 (s, 2H), 3.55
(s, 4H), 3.42 (s, 2H), 2.90–2.72 (m, 10H), 2.34 (t,
J = 7.2Hz, 2H), 2.04 (m, 2H), 1.68 (quint, J = 8.0Hz,
2H), 1.26–1.22 (m, 18H); 13C NMR (400MHz, CDCl3):
d 177.3, 171.4, 170.8, 170.7, 169.1, 132.6, 124.7, 74.8,
60.8, 60.0, 59.9, 59.9, 54.7, 53.3, 52.3, 52.2, 51.6, 49.7,
37.4, 32.9, 31.4, 23.9, 13.8; HRMS (ESI), calcd for
C34H57N3O14Na (M+Na) 754.3733, found: 754.3732.
Anal. Calcd for C34H57N3O14ÆH2O: C, 54.46; H, 7.93;
N, 5.60. Found: C, 54.08; H, 7.63; N, 5.90.
2979, 1749, 1616, 1471, 1224cmÀ1
;
1H NMR
(400MHz, CDCl3): d 7.70 (d, J = 8.0Hz, 2H), 7.14 (d,
J = 8.0Hz, 2H), 6.30 (d, J = 7.6Hz, 1H), 5.49–5.34 (m,
2H), 4.81 (q, J = 7.6Hz, 1H), 4.42 (d, J = 8.4Hz, 1H),
4.16–4.06 (m, 14H), 3.55 (s, 2H), 3.53 (s, 4H), 3.38 (s,
2H), 3.17–3.06 (m, 2H), 2.86–2.67 (m, 10H), 2.42–2.34
(m, 1H), 2.21–2.10 (m, 4H), 1.91–1.94 (m, 4H), 1.65–
1.57 (m, 2H), 1.44 (s, 3H), 1.26 (s, 3H), 1.23–1.17 (m,
22H), 0.87 (s, 3H); 13C NMR (400MHz, CDCl3): d
172.5, 171.5, 171.4, 171.3, 171.1, 170.8, 169.3, 139.4,
134.6, 132.7, 128.42, 124.9, 85.8, 77.8, 74.9, 60.9, 60.0,
59.8, 55.1, 54.9, 53.4, 53.2, 52.9, 52.7, 52.5, 52.0, 51.1,
49.9, 39.2, 37.8, 37.6, 37.4, 35.2, 34.3, 31.0, 28.4, 26.7,
26.1, 24.2, 23.7, 13.9; HRMS (ESI), calcd for
5.1.8. BPA ethyl ester 13. A solution of thionyl chloride
(0.13mL, 1.8mmol) in EtOH (1.5mL) was stirred for an
hour at 0ꢂC under Ar. To the mixture was added BPA
(L/D = 72:28, 0.11g, 0.50mmol), and the solution was
stirred for 5min at 0ꢂC and then stirred for two days
at rt. The reaction was neutralized with saturated NaH-
CO3 at 0ꢂC, and the mixture was diluted with AcOEt
and washed with water and brine. The organic layer
was dried over anhydrous Na2SO4 and concentrated in
vacuo. Purification by recrystallization (CH3Cl/ether)
gave 13 (96mg, 0.40mmol, 81% yield): white solid; IR
C55H85BN4O17Na
(M+Na)
1107.5895,
found:
1107.5905. Anal. Calcd for C55H85BN4O17: C, 60.88;
H, 7.90; N, 5.16. Found: C, 60.52; H, 7.94; N, 5.50.
5.1.11. Deprotection of the ethyl esters 16. To a solution
of 16 (83mg, 0.077mmol) in EtOH (4mL) was added
LiOHÆH2O (52mg, 1.2mmol) dissolved in water
(1mL), and the mixture was stirred for two days at rt.
The mixture was concentrated, diluted with a little
amount of water, and then washed with ether. To the
water layer was added aq 10% HCl until a white solid
was precipitated. The white solid was washed with water
and then dried in vacuo to give 17 (19mg, 0.022mmol,
29% yield): white solid; IR (KBr) 3300–2700, 1732,
(KBr) 3380, 2981, 1732, 1614, 1249cmÀ1 1H NMR
;
(400MHz, CDCl3): d 7.84 (d, J = 7.5Hz, 2H), 7.14 (d,
J = 7.5Hz, 2H), 4.16–4.01 (m, 2H), 3.81 (dd, J = 9.2,
4.4Hz, 1H), 3.16 (dd, J = 13.6, 4.4Hz, 1H), 2.76 (dd,
J = 13.2, 9.6Hz, 1H), 2.34 (br s, 1H), 1.22 (t,
J = 7.2Hz, 3H); 13C NMR (400MHz, CDCl3): d 173.5,
137.3, 134.0, 128.2, 61.3, 58.1, 39.9, 13.8; HRMS
(ESI), calcd for C13H21BNO4 (M+C2H4+H) 266.1558,
found: 266.1558.
1
1650, 1398, 1234cmÀ1; H NMR (400MHz, CD3OD):
d 7.62 (d, J = 8.0Hz, 2H), 7.20 (d, J = 8.0Hz, 2H),
5.40 (br s, 2H), 4.65–4.64 (m, 2H), 4.43 (d, J = 8.0Hz,
1H), 4.00 (br s, 2H), 3.67–3.12 (m, 16H), 2.93 (dd,
J = 13.8, 4.4Hz, 1H), 2.43–2.37 (m, 3H), 2.19–2.05 (m,
4H), 1.89–1.87 (m, 4H), 1.51 (br s, 2H), 1.42 (s, 3H),
5.1.9. Protection of BPA ethyl ester by pinanediol. To a
solution of 13 (0.28g, 1.2mmol) in CH2Cl2 (2mL) at
rt was added (1S,2S,3R,5S)-(+)-pinanediol (0.20g,

742
K. Takahashi et al. / Bioorg. Med. Chem. 13 (2005) 735–743
1.28 (s, 3H), 1.11 (d, J = 10.8Hz, 1H), 0.87 (s, 3H); 13C
NMR (400MHz, CD3OD): d 176.1, 175.9, 175.3, 174.8,
174.7, 174.4, 170.2, 141.9, 135.8, 134.1, 129.7, 127.7,
87.4, 79.4, 66.4, 56.0, 55.7, 54.7, 54.2, 53.9, 52.9, 52.7,
50.8, 50.7, 40.8, 39.2, 38.5, 36.5, 36.0, 34.7, 34.4, 32.7,
29.1, 27.5, 27.3, 26.2, 24.3; MS (ESI), calcd for
C40H57BN4O15 (M+) 844.3921, found: 844.6067; HPLC
analysis: retention time, 34.9min (ODS, MeOH/
H2O = 5:2, 4.5mL/min). Anal. Calcd for C40H57BN4-
O15ÆH2O: C, 55.69; H, 6.89; N, 6.49. Found: C, 55.69;
H, 6.94; N, 6.37.
tion, the tumors grew up to an average weight of 2.7g.
A solution of the compound 3 in saline (3mL, 0.048M)
was injected intravenously into a rat via the tail vein
under the anesthesia by ether. At 20 and 60min after
the injection, the rats were sacrificed by a dislocation of
the cervical vertebrae, and blood, liver, kidney, thigh
muscle, brain, and tumor were excised. Gadolinium con-
centrations in those organs were measured with PGA.
5.2.2. Biodistribution of boron using a-autoradiography
(a-ARG). A solution of the compound 3 in saline (3mL,
0.048M) was injected intravenously into a rat via the tail
vein under the anesthesia by ether. At 20min after the
injection, the rat was sacrificed by an overdose of
CHCl3. The rat was coated completely with the sodium
salt of 3.5% carboxymethylcellulose (CMC–Na) and
then frozen at À80ꢂC for an hour. After sectioning the
frozen rats, the sections of 50lm thick were obtained
on adhesive tapes with Auto Cryotome at À20ꢂC and
then dried enough in the Cryotome at À20ꢂC for a
few days. The sections were attached to the special tapes
(CR-39, Baryotrack, 2mm thick) for a-ARG. After irra-
diation with thermal neutrons (1.7 · 1013 n/cm2), the sec-
tions on the Baryotrack were etched with 5N NaOH
aqueous solution for 3h at 60ꢂC, and then boron distri-
bution was visualized by the irradiation of a fluorescent
light.
5.1.12. Deprotection of the pinanediol ester 17. A mixture
of 17 (0.23g, 0.27mmol) and phenylboronic acid (65mg,
0.54mmol) in hexane (100mL), MeOH (35mL), and
water (70mL) was vigorously stirred at rt. After
30min, the hexane layer was removed and hexane
(100mL) was added to the aqueous solution. After
30min of stirring, the hexane layer was removed again.
This procedure was repeated several times until the pin-
anediol protective group of 17 was transferred to phen-
ylboronic acid. After the deprotection proceeded
completely, the aqueous layer was washed with ether
in order to remove excess phenylboronic acid. The mix-
ture was concentrated in vacuo to give 18 (0.17g,
0.24mmol, 89% yield): white solid; IR (KBr) 3350–
2650, 1770, 1652, 1398, 1226, 1037cmÀ1 1H NMR
;
(400MHz, CD3OD): d 7.61 (br s, 2H), 7.21 (d,
J = 7.6Hz, 2H), 5.43 (br s, 2H), 4.68 (br s, 1H), 4.00
(br s, 2H), 3.61–3.20 (m, 16H), 2.97 (d, J = 13.7,
4.2Hz, 1H), 2.53–2.37 (m, 2H), 2.15 (br s, 2H), 1.90
(br s, 2H), 1.55 (br s, 2H); 13C NMR (400MHz,
CD3OD): d 176.0, 175.3, 175.0, 174.4, 170.5, 140.6,
135.0, 134.1, 129.5, 127.7, 66.6, 56.2, 55.9, 54.8, 54.1,
53.8, 53.1, 50.9, 38.4, 36.0, 34.2, 32.7, 26.3; HPLC anal-
ysis: retention time, 22.0min (ODS, MeOH/H2O = 1:2,
4.5mL/min). Anal. Calcd for C30H43BN4O15ÆC2H4: C,
52.04; H, 6.41; N, 7.59. Found: C, 55.08; H, 6.39; N,
7.46.
Acknowledgements
We thank Dr. M. Kawate (Neutron Science Laboratory,
Institute for Solid State Physics, The University of
Tokyo) for experimental supports of the prompt
gamma-ray analysis.
References and notes
1. Sloway, A. H.; Tjarks, W.; Barnum, B. A.; Rong, F.-G.;
Barth, R. F.; Wyzlic, I. M.; Wilson, J. G. Chem. Rev. 1998,
98, 1515–1562.
5.1.13. BPA–Gd–DTPA complex 3. To a solution of 18
(58mg, 0.082mmol) in MeOH (4.0mL) and water
(1.0mL) at rt was added GdCl3Æ6H2O (30mg,
0.082mmol) in water (1.0mL), and the mixture was stir-
red for 10h at rt. The mixture was neutralized with
aqueous Na2CO3 (0.24M) and then stirred for an hour.
The solution was concentrated and the residue was puri-
fied by HPLC (ODS, MeOH/H2O = 1:2, retention time;
12min, 4.5mL/min) to give 3 (53mg, 0.056mmol, 68%
yield): white solid; IR (KBr) 3674–3000, 2977, 1652,
1398, 1272, 1095cmÀ1; HPLC analysis: retention time,
12.3min (ODS, MeOH/H2O = 1:2, 4.5mL/min); MS
(ESI), calcd for C32H41BGdN4Na3O15 (MH+)
960.1688, found: 959.7672. Anal. Calcd for
C30H37BGdN4Na3O15Æ6H2O: C, 34.69; H, 4.75; N,
5.39. Found: C, 35.24; H, 4.45; N, 5.30.
2. Barth, R. F.; Soloway, A. H.; Goodman, J. H.; Gahbauer,
R. A.; Gupta, N.; Blue, T. E.; Yang, W.; Tjarks, W.
Neurosurgery 1999, 44, 433–450.
3. Coderre, J. A.; Morris, G. M. Radiat. Res. 1999, 151, 1–
18.
4. Godwin, J. T.; Farr, L. E.; Sweet, W. H.; Robertsson, J. S.
Cancer 1955, 8, 601–615.
5. Snyder, H. R.; Reedy, A. J.; Lennarj, W. J. J. Am. Chem.
Soc. 1958, 80, 835–838.
6. Ichihashi, M.; Nakanishi, T.; Mishima, Y. J. Invest.
Dermatol. 1982, 78, 215–218.
7. Kulvik, M.; Va¨ha¨talo, J.; Buchar, E.; Fa¨rkkila¨, M.;
Ja¨rviluoma, E.; Ja¨a¨skela¨inen, J.; Krꢀizꢀ, O.; Laakso, J.;
Rasilainen, M.; Ruokonen, I.; Kallio, M. Eur. J. Pharm.
Sci. 2003, 18, 155–163.
8. Soloway, A. H.; Hatanaka, H.; Davis, M. A. J. Med.
Chem. 1967, 10, 714–717.
9. Hatanaka, H.; Sano, K.; Yasukochi, H. In Progress in
Neutron Capture Therapy for Cancer; Allen, B. J., Moore,
D. E., Harrington, B. V., Eds.; Plenum: New York, 1992;
pp 561–568.
5.2. In vivo biodistribution
5.2.1. Biodistribution of gadolinium using prompt gamma-
ray analysis (PGA). The tumors (AH109A-ascitic hepa-
toma) were implanted into the thighs of the six Donryu
Rats weighing 190–200g. Eight days after transplanta-
10. Hatanaka, H.; Nakagawa, Y. Int. J. Radiat. Oncol. Bio.
Phys. 1994, 28, 1061–1066.
11. Hatanaka, H.; Nakagawa, Y. J. Neuro-Oncol. 1997, 33,
105–115.

K. Takahashi et al. / Bioorg. Med. Chem. 13 (2005) 735–743
743
12. Takahashi, Y.; Imahori, Y.; Mineura, K. Clin. Cancer
Res. 2003, 9, 5888–5895.
Therapy; Mishima, Y., Ed.; Plenum: New York, 1996;
pp 245–250.
13. Kabalka, G. W.; Davis, M.; Bendel, P. Magn. Reson. Med.
1988, 8, 231–237.
14. Kabalka, G. W.; Tang, C.; Bendel, P. Neuro-Oncology
1997, 33, 153–161.
15. Bendel, P.; Zilberstein, J.; Salomon, Y. Magn. Reson.
Med. 1994, 32, 170–174.
27. Matsumura, A.; Shibata, Y.; Yamamoto, T.; Nakagawa,
K.; Yasuda, S.; Nakajima, S.; Sakata, I.; Yoshizawa, T.;
Nose, T. Neurol. Med.-Chir. 1997, 37, 327–331.
28. Nemoto, H.; Cai, J.; Nakamura, H.; Fujiwara, M.;
Yamamoto, Y. J. Organomet. Chem. 1999, 581, 170–
175.
16. Bradshaw, K. M.; Schweizer, M. P.; Glover, G. H.;
Hadley, J. R.; Tippetts, R.; Tang, P.-P.; Davis, W. L.;
Heilbrum, M. P.; Johnson, S.; Ghanem, T. Magn. Reson.
Med. 1995, 34, 48–56.
29. Girald, F.; Nakamura, H.; Fukuda, H.; Yamamoto, Y.;
Yoahida, K. In Recent Advances in Biomedical Imaging;
Ishii, Y., et al., Eds.; Elsevier Science: Amsterdam, 1997;
pp 201–207.
17. Straubinger, R. M.; Kahl, S. B.; Koo, M. S.; Alletto, J. J.;
Mazurchuk, R.; Chau, R. I.; Thamer, S. L.; Fiel, R. J.
J. Magn. Reson. Im. 1993, 3, 351–356.
18. Combs, M. J.; Mulder, R. U.; Berr, S. S.; Oveissi, C.;
Kahl, S. B. In Cancer Neutron Capture Therapy; Mishima,
Y., Ed.; Plenum: New York, 1996; pp 239–244.
19. Weinmann, H. J.; Brasch, R. C.; Press, W. R.; Wesbey, G.
E. AJR 1984, 142, 619–624.
30. Nakamura, H.; Fukuda, H.; Girald, F.; Kobayashi, T.;
Hiratsuka, J.; Akaizawa, T.; Nemoto, H.; Cai, J.; Yosh-
ida, K.; Yamamoto, Y. Chem. Pharm. Bull. 2000, 48,
1034–1038.
31. Yamamoto, Y. Pure Appl. Chem. 2003, 75, 1343–1348.
32. Sieving, P. F.; Watson, A. D.; Rocklage, S. M. Biocon-
jugate Chem. 1990, 1, 65.
33. Gries, H.; Miklautz, H. Physiol. Chem. Phys. Med. NMR
1984, 16, 105.
34. Tsuji, J.; Shimizu, I.; Mikami, I.; Ohashi, Y.; Sugiura, T.;
Takahashi, K. J. Org. Chem. 1985, 50, 1523–1529.
35. Nakamura, H.; Fujiwara, M.; Yamamoto, Y. J. Org.
Chem. 1998, 63, 7529–7530.
36. Nakamura, H.; Fujiwara, M.; Yamamoto, Y. Bull. Chem.
Soc. Jpn. 2000, 73, 231–235.
37. Matteson, D. S.; Kim, G. Y. Org. Lett. 2002, 4, 2153–
2155.
38. Matteson, D. S.; Lu, J. Tetrahedron: Asymmetry 1998, 9,
2423–2436.
39. Decicco, C. P.; Song, Y.; Evans, D. A. Org. Lett. 2001, 3,
1029–1032.
20. Gries, H.; Miklautz, H. Physiol. Chem. Phys. Med. NMR
1984, 19, 105–112.
21. Caravan, P.; Ellison, J. J.; McMurry, T. J.; Lauffer, R. B.
Chem. Rev. 1999, 99, 2293–2352.
´
´
22. Toth, E.; Burai, L.; Merbach, A. E. Coord. Chem. Rev.
2001, 216–217, 363–382.
23. Stack, J. P.; Antoun, N. M.; Jenkins, J. P. R.; Metcalfe,
R.; Isherwood, I. Neuroradiology 1988, 30, 145–154.
24. Runge, V. M.; Jacobson, S.; Wood, M. L.; Kaufman, D.;
Adelman, L. S. Radiology 1988, 166, 835–838.
25. Shibata, Y.; Matsumura, A.; Yamamoto, T.; Nakagawa,
K.; Yoshii, Y.; Nose, T.; Sakata, I.; Nakajima, S.;
Hayakawa, Y.; Ono, K. J. Exp. Clin. Cancer Res. 1998,
17, 285–289.
40. Coutts, S. J.; Adams, J.; Krolikowski, D.; Snow, R. J.
Tetrahedron Lett. 1994, 35, 5109–5112.
41. Asano, T.; Nakamura, H.; Uehara, Y.; Yamamoto, Y.
ChemBioChem 2004, 5, 483–490.
26. Matsumura, A.; Shibata, Y.; Nakagawa, K.; Yamamoto,
T.; Yoshizawa, T.; Yoshii, Y.; Nose, T.; Sakata, I.;
Nakajima, S.; Miwa, N. In Cancer Neutron Capture