Facile and efficient postsynthetic tritium labeling method catalyzed by Pd/C in HTO

Article abstract of DOI:10.1021/jo0517545

We have developed a facile and efficient tritium labeling method using a Pd/C-HTO-H₂ system. This method can provide multitritium-labeled compounds in highly diluted HTO under T₂ gas-free conditions, and is environmentally benign sin

Full text of DOI:10.1021/jo0517545

the hydrogen bound to the carbon of an unlabeled
compound can postsynthetically introduce tritiums into
various compounds with high specific activity.2,4 While
available carrier free T2 gas has extremely high specific
radioactivity and must be handled in a tracer scale, most
of the conventional labeling methods use T2 gas. Conse-
quently, the development of a facile and efficient tritium
labeling method without T2 gas has been desired. Several
H-T exchange reactions in HTO catalyzed by Rh,7 Pt,8
Ru,7b,9 and so on7a,10 have been reported. Especially,
Garnett et al. reported an efficient and pioneering H-T
exchange reaction in HTO catalyzed by platinum black
prepared by the in situ reduction of PtO2 with NaBH4.8
However, the magnitude of the tritium efficiency was
widely varied.
Facile and Efficient Postsynthetic Tritium
Labeling Method Catalyzed by Pd/C
in HTO
Tomohiro Maegawa,† Kosaku Hirota,†
Kenjiro Tatematsu,‡ Yukio Mori,‡ and Hironao Sajiki*,†
Laboratory of Medicinal Chemistry and Laboratory of
Radiochemistry, Gifu Pharmaceutical University, 5-6-1,
Mitahora-higashi, Gifu, 502-8585, Japan
sajiki@gifu-pu.ac.jp
Received August 19, 2005
We have recently disclosed a chemoselective C-H/C-D
exchange reaction on the benzylic carbon in D2O using
Pd/C as a heterogeneous catalyst in the presence of a
catalytic amount of H2 gas at room temperature5 and
multideuterium introduction including the nonactivated
carbon under the heating conditions.6 As an extension
of these methods, we planned to apply them to the H-T
exchange reaction in HTO. In this paper we describe an
efficient and facile H-T exchange reaction catalyzed by
heterogeneous Pd/C in highly diluted HTO as a tritium
source.
We have developed a facile and efficient tritium labeling
method using a Pd/C-HTO-H2 system. This method can
provide multitritium-labeled compounds in highly diluted
HTO under T2 gas-free conditions, and is environmentally
benign since purification by silica gel column chromatogra-
phy is not necessary, which causes a large quantity of
radioactive waste such as silica gel and eluent.
In the deuteration reactions, the use of high-purity D2O
is essential to achieve high deuterium efficiency.5,6 On
the other hand, T2O with high specific activity is highly
Tritium (T) is a most versatile radionuclide and the
detection sensitivity of tritium is roughly a million times
higher than that of deuterium. Organic compounds
labeled with tritiums are widely used in the investigation
for life sciences as an important tracer, such as biosyn-
thetic pathways, analysis of drug metabolism, autorad-
iography, radioassay, and so on.1 While some tritium-
labeled compounds are commercially available, they are
relatively expensive and it is quite difficult to obtain a
desired compound. There are mainly two types of meth-
ods to prepare tritium-labeled organic molecules as
follows: H-T exchange methods2 and reductive synthetic
methods.2,3 The latter reductive methods starting from
a reducible precursor are effective for the incorporation
of tritium atoms onto specific positions with high specific
radioactivity using a stoichiometric amount of tritiated
reducing reagents such as LiBT4 or LiAlT4 and transition
metal-catalyzed reductive dehalogenation and hydroge-
nation using T2 gas. On the other hand, the transition
metal-catalyzed H-T exchange replacement by T2 gas of
(4) Recent reports for H-T exchange reaction with T2 gas catalyzed
by transition metal; (a) Skaddan, M. B.; Yung, C. M.; Bergman, R. G.
Org. Lett. 2004, 6, 11-13. (b) Augustyniak, W.; Kanski, R.; Kanska,
M. J. Labelled Compd. Radiopharm. 2004, 47, 977-981. (c) Salter,
R.; Bosser, I. J. Labelled Compd. Radiopharm. 2003, 46, 489-498. (d)
Shevchenko, V. P.; Nagaev, I. Y.; Myasoedov, N. F. Russ. Chem. Rev.
2003, 72, 423-446.
(5) Sajiki, H.; Hattori, K.; Aoki, F.; Yasunaga, K.; Hitora, K. Synlett
2002, 1149-1151.
(6) (a) Sajiki, H.; Aoki, F.; Esaki, H.; Maegawa, T.; Hirota, K. Org.
Lett. 2004, 6, 1485-1487. (b) Maegawa, T.; Akashi, A.; Esaki, H.; Aoki,
F.; Sajiki, H.; Hirota, K. Synlett 2005, 845-847. (c) Sajiki, H.; Esaki,
H.; Aoki, F.; Maegawa, T.; Hirota, K. Synlett 2005, 1385-1388.
(7) (a) Al-Rawi, J. M. A.; Elvidge, J. A.; Jones, J. R.; Mane, R. B.;
Saieed, M. J. Chem. Res. (S) 1980, 298-299. (b) Hesk, D.; Jones, J. R.
J. Labelled Compd. Radiopharm. 1990, 28, 1427-1436. (c) Hesk, D.;
Jones, J. R.; Lockley, W. J. S. J. Pharm. Sci. 1991, 80, 887-890. (d)
Oohashi, K.; Seki, T. J. Radioanal. Nucl. Chem. 1994, 187, 303-311.
(8) (a) Garnett, J. L.; Long, M. A.; Lukey, C. A. J. Chem. Soc., Chem.
Commun. 1979, 634-635. (b) Garnett, J. L.; Long, M. A.; Lukey, C.
A.; Williams, P. G. J. Chem. Soc., Perkin Trans. 2 1982, 287-289. (c)
Garnett, J. L.; Long, M. A.; Than, C.; Williams, P. G. J. Chem. Soc.,
Faraday Trans. 1990, 86, 875-879. (d) Williams, P. G.; Lukey, C. A.;
Long, M. A.; Garnett, J. L. J. Labelled Compd. Radiopharm. 1990,
29, 175-192. (e) Hesk, D.; Duelfer, T.; Hickey, S.; Hochman, D.;
Koharski, D.; MaNamura, P.; Saluja, S. J. Labelled Compd. Radio-
pharm. 1994, 34, 681-689. (f) Hesk, D.; Bowlen, C.; Hendershot, S.;
Koharski, D.; McNamura, P.; Rettig, D.; Saluja, S. J. Labelled Compd.
Radiopharm. 1996, 38, 1039-1046.
(9) Hesk, D.; Cesarz, D.; Magatti, C.; Voronin, K.; Lavey, C.;
McNamara, P.; Koharski, D.; Saluja, S.; Hendershot, S.; Pham, H.;
Truong, V. J. Labelled Compd. Radiopharm. 2005, 48, 11-23.
(10) (a) Mantescu, C.; Balaban, A. T. Can. J. Chem. 1963, 41, 2120-
2121. (b) Mantescu, C.; Genunche, A.; Balaban, A. T. J. Labelled
Compd. 1965, 1, 178-181. (c) Mantescu, C.; Genunche, A.; Balaban,
A. T. J. Labelled Compd. 1966, 2, 261-266. (d) Mantescu, C.;
Genunche, A.; Duta-Cristu, D.; Balaban, A. T. J. Labelled Compd.
1966, 2, 267-271. (e) Long, M. A.; Garnett, J. L.; Vining, R. F. W.;
Mole, T. J. Am. Chem. Soc. 1972, 94, 8632-8633. (f) Long, M. A.;
Garnett, J. L.; Vining, R. F. W. J. Chem. Soc., Perkin Trans. 2 1975,
1298-1303.
* To whom correspondence should be addressed. Phone: +81-58-
237-3931. Fax: +81-58-237-5979.
† Laboratory of Medicinal Chemistry.
‡ Laboratory of Radiochemistry.
(1) (a) Feinendegen, L. E. Tritium-labeled Molecules in Biology and
Medicine; Academic Press: New York, 1967. (b) Evans, E. A. Tritium
and Its Compounds, 2nd ed.; John Wiley and Sons: London, UK, 1974.
(c) Junk, T.; Catallo, W. J. Chem. Soc. Rev. 1997, 26, 401-406.
(2) Saljoughian, M.; Williams, P. G. Curr. Pharm. Des. 2000, 6,
1029-1056.
(3) (a) Saljoughian, M. Synthesis 2002, 1781-1801. (b) Shevchenko,
V. P.; Nagaev, I. Y.; Myasoedov, N. F. Radiochemistry 2004, 46, 77-
83.
10.1021/jo0517545 CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/08/2005
J. Org. Chem. 2005, 70, 10581-10583
10581

TABLE 1. Multitritium Incorporation with Use of the
SCHEME 1. Tritium Incorporation into
Diphenylmethane with Use of Pd/C-HTO-H2 at
Room Temperature
Pd/C-HTO-H2 System at 160 °Cc
hazardous and labile and is not readily available.1a The
magnitude of the tritium efficiency was expected to
drastically decrease with the highly diluted conditions
because it should be affected by the frequency of contacts
between the substrate and HTO. However, we expected
that the incorporation method of tritium atoms into
organic compounds with highly diluted HTO could be
suitable for practical use since tritium is an extremely
sensitive radioisotope. In an initial attempt, we examined
the incorporation of tritiums on the benzylic site using
HTO diluted to 2.22 kBq/mmol in the presence of 10 wt
% (vs the substrate) of 10% Pd/C and a catalytic amount
of H2 gas at room temperature (Scheme 1).
a In the absence of Pd/C. b Argon atmosphere c Reaction condi-
tions: substrate (0.5 mmol), 10% Pd/C (10 wt %), HTO (2.0 mL).
TABLE 2. Tritium Incorporation with Bioactive
Compounds as a Substratea
As a result, tritium was successfully incorporated into
diphenylmethane in 1.28 kBq/mmol specific activity.11 It
was surprising that the tritium incorporation was
smoothly achieved regardless of the use of highly diluted
HTO (2.22 kBq/mmol, ca. 1 ppb of pure T2O). The specific
activity of diphenylmethane was obviously increased to
250 kBq/mmol with an increase in the tritium concentra-
tion of HTO (333 kBq/mmol). Although it is not deter-
mined where the tritium atoms are incorporated, it is
expected that they are selectively incorporated into the
benzylic position based upon our H-D exchange reac-
tions5 (Scheme 1).
Next, we applied the heating conditions to our tritium
labeling reaction using the Pd/C-HTO-H2 system that
would be expected to promote the tritium introduction.
The tritiation of 5-phenylvaleric acid proceeded with 1.82
kBq/mmol at 160 °C, which indicates the multitritium
incorporation occurs under these conditions since the
obtained specific activity of 5-phenylvaleric acid was
roughly 3 times higher than that of HTO (0.67 kBq/mmol,
Table 1, entry 1).12 The combination of Pd/C-HTO-H2
is indispensable for the progress of an efficient H-T
exchange reaction since no tritium incorporation was
observed in the absence of either Pd/C or H2 gas (Table
1, entries 2 and 3). Further purifications are not neces-
sary except for the simple removal of the heterogeneous
catalyst by filtration and extraction, and the analytically
pure products (>98% purity by HPLC analysis) were
obtained. This is an important advantage of the pre-
a Reaction conditions: substrate (0.5 mmol), 10% Pd/C (10 wt
%) or 5% Pt/C (20 wt %), HTO (2.0 mL).
sented method because silica gel column chromatography
is usually required for purification in the conventional
method and silica gel flowed with the tritiated compounds
formed radioactive waste. These conditions were also
applicable to the labeling reaction of 4-propylbenzoic acid
with 5.50 kBq/mmol, which was more than twice the
specific activity of HTO (2.22 kBq/mmol).
Further application to bioactive compounds was also
examined. As a result, all of the substrates such as
ibuprofen, acetaminophen, adenosine, and phenylalanine
were tritiated (Table 2). Surprisingly, efficient tritium
incorporation was achieved on ibuprofen and 3.42 kBq/
mmol of specific activity is more than 5 times higher than
0.67 kBq/mmol of HTO. On the other hand, when 5% Pt/
C, which was an effective catalyst for the deuteration of
aromatic rings,13 was used in place of 10% Pd/C in 2.22
kBq/mmol of HTO, tritiums were successfully incorpo-
rated into the acetaminophen molecule in 4.26 kBq/mmol.
The reaction with adenosine and phenylalanine as sub-
strates gave rather lower specific activities (0.70 and 0.62
kBq/mmol, respectively, in 2.22 kBq/mmol of HTO). The
reason for the lower efficiency is not clarified, but it may
be caused by the lower reaction temperature.14
(11) Our results indicated that the efficiency of the tritium atom
transfer from HTO to substrate is roughly two orders higher than
Williams’ results.8d
(12) In our previous report of the H-D exchange reaction with the
Pd/C-D2O-H2 system,6 methylenes of 5-phenylvaleric acid were
almost fully deuterated at 160 °C.
In summary, we have developed a facile and efficient
tritiation method catalyzed by Pd/C or Pt/C using HTO
as a tritium source in the presence of a catalytic amount
of H2 gas. The method presented here provides a T2 gas-
free, and totally postsynthetic tritium labeling method
(13) Sajiki, H.; Ito, N.; Esaki, H.; Maesawa, T.; Maegawa, T.; Hirota,
K. Tetrahedron. Lett. 2005, 46, 6995-6998.
(14) In the case of adenosine or phenylalanine, lower temperature
(110 °C) was used for each reaction to avoid cleavage of the glycosyl
bond or racemization.
10582 J. Org. Chem., Vol. 70, No. 25, 2005

saturated aq NaCl and dried with MgSO4, then concentrated in
vacuo. Measurements of the specific activity of the substrate
were made on a liquid scintillation analyzer.
[3H]Diphenylmethane (1). Yield 100% (99.0% pure by
HPLC), specific activity 1.28 kBq/mmol (using 2.22 kBq/mmol
of HTO).
Typical Procedure for the Tritiation Reactions with Pd/
C-HTO-H2 at 160 °C. A suspension of 3-phenylpropionic acid
(0.5 mmol) and 10% Pd/C (10 wt % of substrate) in HTO (2 mL)
was stirred at 160 °C in a sealed tube under H2 atmosphere for
24 h. The mixture was diluted with ether (20 mL) and filtered
with a membrane filter (Millipore, Millex-LH, 0.45 µm). The
filtrate was extracted with ether (2 × 20 mL). The combined
etheral layer was washed with H2O and saturated aq NaCl and
dried with MgSO4, then concentrated in vacuo. Measurements
of the specific activity of substrate were made on a liquid
scintillation analyzer.
under neutral conditions without requiring the chro-
matographic purification. The reaction is applicable to
various substrates and tolerates a wide range of different
functional groups, such as amine, carboxylic acid, amide,
and so on. The multitritium incorporation provides
tritium-labeled compounds with high specific activities,
using HTO possessing low specific activities. Accordingly,
the presented reaction possesses the potential to be an
efficient, applicable, and environmentally benign tritium
labeling method.
Experimental Section
Caution! The compounds containing tritium atoms are
hazardous, especially in the case of inhalation. All experiments
were conducted with groves in the fume food in the control area
for radiation. And care should be taken with the internal
pressure of the reaction, which increases to ca. 2.5 atm in the
sealed tube at 160 °C.
[3H]3-Phenylpropionic acid (2). Yield 98% (99.3% pure by
HPLC), specific activity 1.82 kBq/mmol (using 0.67 kBq/mmol
of HTO).
Typical Procedure for the Tritiation Reactions with Pd/
C-HTO-H2 at Room Temperature. A suspension of diphen-
ylmethane (1.0 mmol) and 10% Pd/C (10 wt % of substrate) in
HTO (0.5 mL) was stirred at room temperature in a test tube
under H2 atmosphere for 72 h. The mixture was diluted with
ether (20 mL) and filtered with a membrane filter (Millipore,
Millex-LH, 0.45 µm). The filtrate was extracted with ether (2 ×
20 mL). The combined Etheral layer was washed with H2O and
Supporting Information Available: Experimental pro-
cedures and spectral data for tritiated compounds and the
deuteration data of each substrate in Table 2 as a reference
of the tritiation reaction. This material is available free of
charge via the Internet at http://pubs.acs.org.
JO0517545
J. Org. Chem, Vol. 70, No. 25, 2005 10583