Efficient and selective deuteration of phenylalanine derivatives catalyzed by Pd/C

Article abstract of DOI:10.1055/s-2005-863730

A facile and efficient deuteration method of phenylalanine derivatives using a Pd/C-H₂-D₂O system has been developed. Selective deuteration at the β-position of phenylalanine derivatives occurred using Pd/C as a catalyst with high deuterium efficiency without racemization at 110°C. Also, the α-position was deuterated at higher temperature.

Full text of DOI:10.1055/s-2005-863730

LETTER
845
Efficient and Selective Deuteration of Phenylalanine Derivatives Catalyzed
by Pd/C
Selective
Deute
o
ration of Phe
m
nylalanine
D
erivati
oves hiro Maegawa, Akira Akashi, Hiroyoshi Esaki, Fumiyo Aoki, Hironao Sajiki,* Kosaku Hirota
Laboratory of Medicinal Chemistry, Gifu Pharmaceutical University, Mitahora-higashi, Gifu 502-8585, Japan
Fax +81(58)2375979; E-mail: sajiki@gifu-pu.ac.jp
Received 12 January 2005
C-H bonds using the Pd/C–H2–D2O system under the
heating conditions.9 Herein, we report efficient and regio-
selective deuterium incorporation into the b-position of
L-phenylalanine derivatives by thermal control using a
Abstract: A facile and efficient deuteration method of phenylala-
nine derivatives using a Pd/C–H2–D2O system has been developed.
Selective deuteration at the b-position of phenylalanine derivatives
occurred using Pd/C as a catalyst with high deuterium efficiency
without racemization at 110 °C. Also, the a-position was deuterated heterogeneous Pd/C–H2–D2O system.
at higher temperature.
First, we examined the deuteration of L-phenylalanine
Key words: palladium, catalysis, amino acids, deuteration, regio-
with the Pd/C–H2–D2O system at various temperatures. L-
Phenylalanine (0.25 mmol) and 10% Pd/C [purchased
from Aldrich; 4.3 mg (10 wt%): 0.004 mmol of Pd metal]
in D2O (1 mL) were stirred at room temperature under a
small amount of hydrogen (ca. 40 mL) in a test tube for 24
hours although no deuterium was incorporated into L-phe-
nylalanine (Table 1, entry 1). On the other hand, when the
reaction was performed at 40 °C for 24 hours, deuteration
proceeded at the b-position in 19% deuterium efficiency
(entry 2). It was found that the deuterium efficiency (D
content) increased with an increase in temperature. Selec-
tive deuteration at the b-position of L-phenylalanine with
nearly quantitative deuterium efficiency was ultimately
accomplished when the reaction was performed at 110 °C
for 24 hours. Partial hydrogenation of the benzene ring of
L-phenylalanine was observed along with the deuteration
on heating at more than 80 °C (Table 1, entries 4 and 5).
selectivity
Amino acids labeled with deuterium, a stable isotope, are
applied for a wide range of studies for metabolism, struc-
tural analysis and dynamics of peptides and proteins.1
Although a number of methods for the preparation of deu-
terium-labeled amino acids are reported, appropriately la-
beled amino acids are extremely expensive and rarely
commercially available. While many methods for the
preparation of deuterated L-phenylalanine and L-tyrosine
have been reported,2,3 very few methods for the selective
preparation of b-deuterated derivatives4 are available.
Multistep synthetic methods starting from an originally
deuterium-labeled synthon such as PhCD2Br or PhCDO
are generally applicable for the incorporation of deuteri-
um(s) at the b-position of aromatic amino acids.4 A hydro-
genation reaction using D2 gas is also another effective Table 1 Temperature Dependence of Deuterium Efficiency of L-
Phenylalanine
method for the preparation of b-labeled amino acids al-
though it is necessary to carry out tedious and multistep
D
D
CO2H
NH2
10% Pd/C, D2O
CO2H
synthesis for the preparation of the starting material such
as a,b-unsaturated or b-halogenated amino acids.4 Fur-
thermore, deuterium-labeled synthons and D2 gas are ex-
tremely expensive.5 Only one method is applicable for the
direct deuterium introduction into the b-position of phe-
nylalanine by radiolysis in the presence of N2O in D2O
with moderate deuterium efficiency.6 A few specifically
labeled L-phenylalanine derivatives are commercially
available even though they are quite expensive. For exam-
ple, DL-phenylalanine-b,b-d2 was available from Aldrich
(48,909-3: $ 251.50/500 mg). During our effort to develop
chemoselective hydrogenation methods using a heteroge-
neous Pd/C catalyst,7 we found that a Pd/C–H2–D2O sys-
tem catalyzed the H-D exchange reaction at the benzylic
position of organic compounds effectively without using
D2 gas at room temperature.8 Furthermore, we developed
efficient H-D exchange reaction method of non-activated
H2, ∆, 24 h
NH2
Entry
Temp (°C)
D content (%)
Yield (%)
1
2
3
4
5
R.t.
40
0
19
54
81
96
100
100
100
96a
93a
60
80
110
a Partial hydrogenation of the benzene ring was observed.
The partial hydrogenation of the benzene ring was nearly
suppressed with shorter reaction time (6 h) without de-
pression of the deuterium efficiency because the hydro-
genation of the benzene ring is slow and time-dependent
(Table 2).
SYNLETT 2005, No. 5, pp 0845–0847
2
1.
0
3.
2
0
0
5
Advanced online publication: 09.03.2005
DOI: 10.1055/s-2005-863730; Art ID: U01405ST
© Georg Thieme Verlag Stuttgart · New York

846
T. Maegawa et al.
LETTER
Table 2 Time Dependence of the Partial Hydrogenation of the
Benzene Ring
To explore the further deuterium incorporation at the a-
position of L-phenylalanine in addition to the b-position,
we examined the reaction at higher temperature. As the re-
sult of the investigation, the Pd/C–H2–D2O system led to
the further deuteration at the a-position of L-phenylala-
nine with a rise in temperature up to 160 °C using a sealed
tube with 10% Pd/C purchased from Merck (Merck prod-
uct number 807104-0010, Table 4, entries 1 and 3), not ef-
fective with Aldrich’s catalyst (Aldrich product number
20,569-9, Table 4, entry 2). The difference in reactivity
between the catalysts may be explained by their acidity.12
Although the deuterium incorporation at the a-position
partly proceeded (30%) at 160 °C without the Pd/C cata-
lyst, the reaction using the catalyst provides better deute-
rium efficiency (65%). Additional optimization revealed
that the deuterium efficiency can be reaction time-depen-
dently improved. Unfortunately, the deuterium-labeled L-
phenylalanine was partially racemized (17% ee)13 under
the reaction conditions.
10% Pd/C,
D2O
D
D
D
D
CO2H
NH2
CO2H
CO2H
+
H2, 110 °C,
time
NH2
2
NH2
1
Entry
Time (h)
24
Ratio (1:2)
93:7
D content (%)
1
2
3
4
96
97
96
91
12
6
94:6
97:3
3
99:1
It is noteworthy that no racemization of L-phenylalanine
was observed even at 110 °C (Scheme 1), and no protec-
tive group is necessary to carry out the b-specific deuter-
ation reaction.
Table 4 Deuterium Incorporation into L-Phenylalanine Using
CO2H
∗
10% Pd/C, H2O
CO2H
∗
Pd/C–H2–D2O System at 160 °Ca
H2, 110 °C, 6 h
NH2
96% ee
NH2
96% ee
D
D
β
no racemization
D
10% Pd/C, D2O
α
CO2H
CO2H
H2, 160 °C, sealed tube
NH2
NH2
Scheme 1 Retention of the a-stereocenter
Entry D content (%)
Time (h)
Yield (%)
The Pd/C–H2–D2O system is also applicable to the b-
specific deuteration of L-phenylalanine-4-d and tyrosine
derivatives (Table 3).10 A literature unknown L-phenyl-
alanine-4,b,b-d3 could be easily obtained from L-phenyl-
alanine-4-d (entry 2), which was synthesized using the
method previously reported by us.11 Additionally, O-tert-
butyl-L-tyrosine was also highly deuterated at the b-posi-
tion (entry 3). On the other hand, deuterium efficiency of
L-tyrosine was lower at the b-position and a small quantity
of the deuterium incorporation on the ortho-positions of
the phenolic hydroxy group was observed (entry 4).
b
a
1
97
97
97
0
65
38
87
30
24
24
48
24
85
94
2b
3
70
4c
100
a The reaction was performed with 10% Pd/C purchased from Merck
(807104-0010).
b The reaction was performed with 10% Pd/C purchased from Aldrich
(20,569-9).
c In the absence of 10% Pd/C.
Table 3 10% Pd/C–H2–D2O-Catalyzed Deuterium Incorporation
into Phenylalanine and Tyrosine Derivatives10
In summary, we have developed a facile and efficient
method for deuteration of phenylalanine derivatives using
the Pd/C–H2–D2O system under neutral conditions. The
deuteration sites can be controlled by the reaction temper-
ature, and the specifically deuterium-labeled phenylala-
nine derivatives such as L-phenylalanine-b,b-d2, L-
phenylalanine-4,b,b-d3 and phenylalanine-a,b,b-d3 were
provided in quantitative yields. Furthermore, the b-selec-
tive deuteration method does not cause racemization.
Since D2O is the cheapest deuterium-labeled compound,
the present method provides inexpensive and readily
available deuterium-labeled aromatic amino acids. Fur-
ther application to other amino acids is now underway.
Entry Substrate
1
Product
Yield
(%)
100
CO2H
NH2
D
D
CO2H
96
NH2
2
3
4
97
CO2H
NH2
D
D
CO2H
97
D
NH2
D
100
96a,b
D
D
CO2H
NH2
CO2H
NH2
CO2H
NH2
90
t-BuO
HO
t-BuO
17
54
CO2H
NH2
HO
17
a The deuterated product was isolated as sodium salt because of low
solubility of L-tyrosine.
b Hydrogenation of the benzene ring was slightly proceeded.
Synlett 2005, No. 5, 845–847 © Thieme Stuttgart · New York

LETTER
Selective Deuteration of Phenylalanine Derivatives
847
(4) Multistep synthesis of b-deuterated phenylalanine: (a) Ife,
R.; Haslam, E. J. Chem. Soc. C 1971, 2818. (b) Hanson, K.
R.; Wightman, R. H.; Staunton, J.; Battersby, A. R. J. Chem.
Soc., Chem. Commun. 1971, 185. (c) Kirby, G. W.;
Michael, J. J. Chem. Soc., Chem. Commun. 1971, 187.
(d) Yamamoto, D. M.; Upson, D. A.; Linn, D. K.; Hruby, V.
J. J. Am. Chem. Soc. 1977, 99, 1564. (e) Seebach, D.;
Dziadulewicz, E.; Behrendt, L.; Cantoreggi, S.; Fitzi, R.
Liebigs Ann. Chem. 1989, 1215. (f) Easton, C. J.; Hutton, C.
A. J. Chem. Soc., Perkin Trans. 1 1994, 3545.
(5) D2 gas [$ 96.40/10 L of D2 gas (Aldrich 36,186-0) in lecture
bottle] is also purchased as a lecture bottle or cylinder
charged by high-pressure.
(6) Direct H/D exchange of the b-position using gamma rays:
Nukuna, B. N.; Goshe, M. B.; Anderson, V. E. J. Am. Chem.
Soc. 2001, 123, 1208.
(7) (a) Sajiki, H.; Hattori, K.; Hirota, K. J. Org. Chem. 1998, 63,
7990. (b) Sajiki, H.; Hirota, K. Tetrahedron 1998, 54,
13981. (c) Sajiki, H.; Hattori, K.; Hirota, K. Chem.
Commun. 1999, 1041. (d) Sajiki, H.; Hattori, K.; Hirota, K.
Chem.–Eur. J. 2000, 6, 2200. (e) Hattori, K.; Sajiki, H.;
Hirota, K. Tetrahedron 2001, 57, 4817.
(8) Sajiki, H.; Hattori, K.; Aoki, F.; Yasunaga, K.; Hirota, K.
Synlett 2002, 1149.
(9) Sajiki, H.; Aoki, F.; Esaki, H.; Maegawa, T.; Hirota, K. Org.
Lett. 2004, 6, 1485.
(10) Typical Procedure for Selective Deuteration at the b-
position of Phenylalanine Derivatives Using Pd/C–H2–
D2O System and Selected Spectroscopic Data.14
A mixture of phenylalanine derivatives (0.5 mmol) and 10%
Pd/C (Aldrich; 10 wt% of substrate) in D2O (2 mL) was
stirred under ordinary hydrogen pressure at 110 °C for 6 h.
The reaction mixture was filtered through a membrane filter
(Millepore, Millex®-LH, 0.45 mm), and the filtrate was
concentrated in vacuo. The residue was treated with H2O to
replace the deuterium on the heteroatoms (CO2D and ND2)
with a hydrogen to afford the b-deuterated phenylalanine.
Deuterium efficiency was determined by 1H NMR using p-
anisic acid sodium salt as an internal standard. L-Phenyl-
alanine-b,b-d2: 1H NMR (400 MHz, D2O): d = 2.94 (d,
J = 8.3 Hz, 0.04 H), 3.11 (d, J = 5.1 Hz, 0.05 H), 3.82 (s,
1 H), 7.15–7.28 (m, 5 H). 2H NMR (60.7 MHz, H2O):
d = 2.91, 3.08. MS (EI): m/z = 167 [M+].
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Synlett 2005, No. 5, 845–847 © Thieme Stuttgart · New York