Synthesis of [2H5]-ebastine fumarate and [ 2H5]-hydroxyebastine

Article abstract of DOI:10.1002/jlcr.1877

This study describes the synthesis of deuterium-labelled ebastine fumarate and its deuterium-labelled metabolite hydroxyebastine. The synthesis of the two desired compounds both used [²H₅]-bromodiphenylmethane as deuterium-labelled reagent, which was synthesized beforehand in three steps. [²H₅]-ebastine was synthesized in further three steps with a 27% overall yield and [²H₅]-hydroxyebastine was synthesized in further seven steps with a 13% overall yield.

Full text of DOI:10.1002/jlcr.1877

Research Article
Received 15 September 2010,
Revised 6 November 2010,
Accepted 20 December 2010 Published online 25 March 2011 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.1877
Synthesis of [2H5]-ebastine fumarate and
[2H5]-hydroxyebastine
Ã
Zhoujie Yu,a Wei Wang,b and Liqin Chenc
This study describes the synthesis of deuterium-labelled ebastine fumarate and its deuterium-labelled metabolite
hydroxyebastine. The synthesis of the two desired compounds both used [2H5]-bromodiphenylmethane as deuterium-
labelled reagent, which was synthesized beforehand in three steps. [2H5]-ebastine was synthesized in further three steps
with a 27% overall yield and [2H5]-hydroxyebastine was synthesized in further seven steps with a 13% overall yield.
Keywords: deuterium-labelled; ebastine; hydroxyebastine; metabolite
completion (2 h). After completion, the reaction was quenched
slowly with 10% HCl at 0–101C and the mixture was extracted
with dichloromethane (3 Â 50 mL). The combined dichloro-
Introduction
Histamine plays a very important role in the development of
allergic symptoms and the most widely used treatments for
disease relief are H1-antihistamines.1 Ebastine is a selective
histamine H1-receptor antagonist which belongs to second
generation of nonsedating H1-antihistamine.2 It is an effective
and well-tolerated treatment for allergic rhinitis and chronic
idiopathic urticaria and has no clinically relevant adverse effects
on cognitive function, psychomotor performance or on cardi-
ovascular function.3 Ebastine undergoes extensively sequential
metabolism in the liver and one of the major primary
metabolites identified in the human body is hydroxyebastine.4–6
Ebastine labelled with stable isotopes of hydrogen were
required for drug metabolism (excretion, distribution and
absorption) studies and to develop bioanalytical methods to
support clinical studies, while deuterium-labelled hydroxyebas-
tine could be used as an important internal standard for
bioavailability studies of ebastine.
methane extracts were washed with brine (50 mL), dried over
MgSO4 and concentrated under reduced pressure to afford (2)
as a white solid (3.77 g, 89%).
[2H5]-diphenylmethane (3)
[2H5]-benzophenone (2) (3.77 g, 20.1 mmol) was hydrogenated
in methanol (50 mL) in the presence of 10% Pd/C (0.35 g) at
ambient temperature for 12 h. The Pd/C catalyst was then
filtered and washed with methanol (3 Â 5 mL). The solvent was
removed under reduced pressure to give the title compound as
a colorless liquid with pleasant aroma (3.37 g, 96%).
1HNMR (CDCl3): d 3.99 (s, 2H), d 7.18–7.22 (m, 3H), d 7.26–7.31
(m, 2H).
[2H5]-bromodiphenylmethane (4)
To a solution of compound (3) (3.37 g, 19.4 mmol) in carbon
tetrachloride (30 mL) was added benzoyl peroxide (0.24 g,
1.0 mmol) and 0.5 Eq of N-bromosuccinimide (1.73 g, 9.7 mmol).
The reaction mixture was refluxed at 801C for 0.5 h and another
0.5 Eq of N-bromosuccinimide (1.75 g, 9.8 mmol) was added.
Experimental
General
[2H6]-benzene was purchased from Cambridge Isotope Labora-
tories. All other reagents were obtained from Sigma-Aldrich. All
solvents were of analytical grade and used without further
purification. Mass spectra were obtained with a Quattro micro
API mass spectrometer. 1H NMR spectra was acquired on a
Bruker 300 MHz spectrometer with TMS as internal standard.
aCollege of Material Science and Technology, Nanjing University of Aeronautics
and Astronautics, 29 Yudao Street, Nanjing, Jiangsu 210016, People’s Republic of
China
Chemical purities were determined by an agilent 1200 series bCollege of Pharmaceutical Sciences, Nanjing University of Technology, 5
Xinmofan Road, Nanjing, Jiangsu 210009, People’s Republic of China
HPLC with an Agilent XDB-C18 column, 5 mm, 4.6 Â 150 mm.
cHi-Tech Research Institute and State Key Laboratory of Materials-Oriented
Chemical Engineering, Nanjing University of Technology, 5 Xinmofan Road,
Nanjing, Jiangsu 210009, People’s Republic of China
[2H5]-benzophenone (2)
To [2H6]-benzene (1.85 g, 22.0 mmol) was added aluminium
chloride (36.2 g, 27.1 mmol) and the slurry was stirred under N2
atmosphere for 0.5 h at 01C. Benzoyl chloride (3.04 g, 21.6 mmol)
*Correspondence to: Liqin Chen, Hi-Tech Research Institute and State Key
Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of
Technology, 5 Xinmofan Road, Nanjing, Jiangsu 210009, Peoples’ Republic of
China.
was added slowly at 0–101C. The mixture was warmed to
ambient temperature and monitored by TLC analysis for E-mail: liqin_chen@hotmail.com
J. Label Compd. Radiopharm 2011, 54 352–356
Copyright r 2011 John Wiley & Sons, Ltd.

Z. Yu et al.
After stirring for further 0.5 h, the reaction mixture was cooled to
ambient temperature, filtered and filtercake was washed with
carbon tetrachloride (3 Â 5 mL). The filtrate was washed with
saturated sodium bicarbonate solution, dried over MgSO4 and
evaporated to afford compound (4) as a brown solid (4.35 g,
90%). The compound (4) was used directly without further
purification.
Methyl-2,2-dimethyl-2-phenyl acetate (8)
Methanol (25 mL), 2,2-dimethyl-2-phenyl-aceic acid (5.03 g,
30.6 mmol) and concentrated sulphuric acid (98%, 1 mL) were
charged to a flask at ambient temperature. The temperature was
raised to reflux at 701C and maintained for 12 h. After
completion, the mixture was cooled to ambient temperature,
neutralized with aqueous ammonia and extracted with dichlor-
omethane (3 Â 50 mL). The combined dichloromethane layers
were dried over MgSO4 and concentrated to obtain compound
(8) as a colorless liquid (5.12 g, 94%).
1-[4-(1,1-dimethylethyl)phenyl]-4-[4-hydroxy-1-piperidi-
neyl]-butanone (5)
To a suspension of anhydrous Na2CO3 (2.68 g, 25.2 mmol) and KI
(4.25 g, 25.6 mmol) in acetone (50 mL) was added 4-chloro-1-[4-
(1,1-dimethylethyl)phenyl]-1-butanone (5.97 g, 25.0 mmol) and
4-hydropiperidine (5.06 g, 50.1 mmol). The reaction mixture was
refluxed at 701C overnight. After completion, the mixture was
cooled to ambient temperature and filtered. The filter cake
was washed with acetone (3 Â 5 mL) and the filtrate was
evaporated to afford a brown sticky liquid. The crude product
was purified by chromatography on silica gel (120 g) column,
eluted with 0–5% MeOH in dichloromethane. The title
compound was obtained as a pale yellow solid (4.57 g, 60%).
1HNMR (CDCl3): d 1.34 (s, 9H), d 1.55–1.65 (m, 2H), d 1.93–2.02
(m, 4H), d 2.27 (t, J = 12.0 Hz, 2H), d 2.49 (t, J = 9.0 Hz, 2H),
d 2.81–2.88 (m, 2H), d 3.00 (t, J = 6.0 Hz, 2H), d 3.71-3.76 (m, 1H),
d 7.46 (d, J = 9.0 Hz, 2H), d 7. 89 (d, J = 9.0 Hz, 2H).
1H-NMR (CDCl3): d 1.58 (s, 6H), d 3.65 (s, 3H), d 7.21–7.26
(m, 2H), d 7.33 (d, J = 3.0 Hz, 3H).
2-methyl-2-phenyl propanol (9)
To a chilled solution of compound (8) (5.12 g, 28.7 mmol) in dry
THF (20 mL) was added sodium borohydride (1.06 g, 28.0 mmol)
in portions within 0.5 h. The mixture was stirred at ambient
temperature for 3 h. After completion, the PH was adjusted to
3–4 with 10% HCl and the mixture was filtered. The filtrate
was extracted with dichloromethane (3 Â 50 mL) and the
combined extracts were dried over MgSO4, concentrated under
reduced pressure to obtain compound (9) as a colorless liquid
(3.95 g, 92%).
2-methyl-2-phenylpropyl acetate (10)
[2H5]-ebastine (6)
To a chilled solution of compound (9) (3.95 g, 26.3 mmol) in
pyridine (20 mL) was added acetic anhydride (5.31 g, 52.1 mmol)
slowly. The mixture was stirred for 2 h at 10–201C, then ethyl
acetate (50 mL) and chilled water (25 mL) were added to quench
the reaction. After stirring for further 0.5 h, 10% chilled HCl
(50 mL) was added to the mixture. The organic layer was
separated and washed with saturated sodium bicarbonate
solution until PH of 7–8 was obtained. The separated organic
layer was dried over MgSO4 and concentrated under reduced
pressure to obtain 2-methyl-2-phenylpropyl acetate (10) as a
pale green liquid (4.45 g, 88%).
To a suspension of anhydrous Na2CO3 (1.23 g, 11.6 mmol) in
toluene (10 mL) was added [2H5]-bromodiphenylmethane (4)
(0.91 g, 3.6 mmol) and compound (5) (1.12 g, 3.7 mmol). The
reaction mixture was refluxed at 1251C for 6 h and then cooled
to amibent temperature. The precipitate were filtered and
filtercake was washed with toluene (3 Â 2 mL). The solvent
was removed under reduced pressure to give the product as a
dark brown sticky liquid. The crude product was purified by
chromatography on silica gel (60 g) column, eluted
with 0–40% EtOAc in hexane to afford (6) as a white solid
(1.10 g, 65%).
1HNMR (CDCl3): d 1.34 (s, 9H), d 1.67–1.73 (m, 2H), d 1.87–1.97
(m, 4H),d 2.12 (s, 2H), d 2.38 (t, J = 9.0, 2H), d 2.76 (t, J = 6.0, 2H),
d 2.97 (t, J = 9.0, 2H), d 3.41–3.43 (m, 2H), d 5.51 (s, 1H), d 7.21–7.35
(m, 5H), d 7.45 (d, J = 9.0 Hz, 2H), d 7. 89 (d, J = 9.0 Hz, 2H).
Ethyl-2-[4-(4-chloro-butyryl)phenyl]-2-methyl propanoate (11)
Dry Dichloromethane (30 mL) and aluminium chloride (4.61 g,
34.6 mmol) were charged to a flask and chilled to À101C
in a ice–salt bath. Compound (10) (4.45 g, 23.1 mmol) was
stripped with toluene (2 Â 10 mL), dissolved in dry dichloro-
methane (10 mL) and added slowly. The mixture was stirred
for 0.5 h at À101C under N2 atmosphere. 4-chloro butyryl
chloride (3.90 g, 27.7 mmol) was dissolved in dry dichloro-
methane (10 mL) and added slowly to the reaction mixture.
The mixture was stirred for additional 2 h at 01C. After
completion, the mixture was quenched slowly with 10% HCl
at 10–201C. The dichloromethane layer was separated and the
aqueous layer was extracted with dichloromethane (2 Â 80 mL).
The combined dichloromethane layers were dried over MgSO4
and concentrated under vacuum to obtain crude product as a
brown sticky liquid. The crude product was purified by
chromatography on silica gel (120 g) column, eluted with
EtOAc/Hexanes (1:10) to obtain (11) as a pale brown liquid
(4.78 g, 70%).
[2H5]-ebastine fumarate(7)
[2H5]-ebastine (6) (1.10 g, 2.3 mmol) was dissolved in ethanol
(2 mL) and stirred at amibent temperature. Fumaric acid (0.26 g,
2.3 mmol) was dissolved in ethanol (4 mL) and added to
the reaction flask drop wise. The reaction mixture was stirred
for 20 min. Some white precipitate started to form. The
precipitate was filtered and washed with ethanol (2 Â 1 mL).
The solid was dried under vacuum to afford (7) as a white solid
(1.26 g, 92%).
1HNMR (DMSO-d6): d 1.29 (s, 9H), d 1.54–1.62 (m, 2H), d
1.80–1.84 (m, 4H), d 2.34 (s, 2 H), d 2.50 (t, J = 3.0 Hz, 2H), d 2.84
(t, J = 9.0 Hz, 2H), d 3.00 (t, J = 6.0 Hz, 2H), d 3.39 (s, 1H), d 5.51
(s, 1H), d 6.57 (s, 2H), d 7.29–7.38 (m, 5H), d 7.51 (d, J = 9.0 Hz, 2H),
d 7.87 (d, J = 9.0 Hz, 2H). MS-EI (m/z): 474.2 (5), 475.2 (M1, 100),
476.2 (40), 477.2 (10), 487.2 (2). HPLC (XDB-C18, CH3OH/
10mmol/L CH3COONH4 10.05% TEA = 90/10, 1.0 mL/min):
tR 6.1 min (499.5%).
1H-NMR (CDCl3): d 1.39 (s, 6H), d 2.00 (s, 3H), d 2.19–2.28
(m, 2H), d 3.18 (t, J = 9.0 Hz, 2H), d 3.70 (t, J = 3.0 Hz, 2H), d 4.16
(s, 2H), d 7.46 (d, J = 6.0 Hz, 2H), d 7.93 (d, J = 9.0 Hz, 2H).
J. Label Compd. Radiopharm 2011, 54 352–356
Copyright r 2011 John Wiley & Sons, Ltd.
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Z. Yu et al.
CH3COONH4 1 0.03% TEA = 83/17, 1.0 mL/min): tR 4.7 min
(499.1%).
Ethyl-2-[4-(4-(4-hydroxy-1-piperidineyl)butyryl)phenyl]-2-
methyl propanoate (12)
To a suspension of anhydrous Na2CO3 (1.75 g, 16.5 mmol) and KI
(2.70 g, 16.3 mmol) in acetone (30 mL) were added compound
Results and discussion
(11) (4.78 g, 16.2 mmol) and 4-hydropiperidine (3.28 g, The synthesis of ebastine has been readily reported,7–9 while the
32.5 mmol). The reaction mixture was refluxed at 701C over- synthesis of [2H5]-ebastine fumarate has not been described
night, and then cooled to ambient temperature. The precipitate previously. Although, several reports have covered the like-
was filtered and filter cake was washed with acetone (3 Â 5 mL). lihood of transformation of ebastine into hydroxyebastine by
The filtrate was evaporated to afford a sticky liquid. The crude cytochrome P450 enzymes in human liver and Buisson et al.
product was purified by chromatography on silica gel (100 g) described biooxidation of ebastine to hydroxyebastine on
column, eluted with 0–5% MeOH in dichloromethane to obtain 100 mg scale using streptomyces platensis,4–6,10,11 the synthesis
the title compound as pale yellow solid (3.75 g, 64%).
1H-NMR (CDCl3): d 1.24 (t, J = 9.0 Hz, 1H), d 1.40 (s, 6H), d
of [2H5]-hydroxyebastine has also not been covered before.
Scheme 1 presents the synthetic route of preparing [2H5]-
1.60–1.63 (m, 2H), d 1.99–2.05 (m, 6H), d 2.36 (s, 2H), d 2.54 bromodiphenylmethane (4). Compound (4) was synthesized in
(s, 2H), d 2.88 (s, 2H), d 3.03 (t, J = 6.0 Hz, 2H), d 3.79 (s, 1H), d 4.15 three steps and the overall yield was a satisfactory 76%.12–14 It
(s, 3H), d 7.44 (d, J = 9.0 Hz, 2H), d 7.92 (d, J = 9.0 Hz, 2H).
began from Friedel–Crafts acylation of [2H6]-benzene and benzyl
chloride to obtain [2H5]-benzophenone (2) as a white solid.
Catalytic hydrogenation of compound (2) with 10% Pd/C gave
[2H5]-diphenylmethane (3) as a colorless liquid with pleasant
aroma. Compound (3) was then brominated by NBS in the
presence of benzoyl peroxide to afford Compound (4), which
was used directly without further purification in the following
reactions.
Scheme 2 describes the synthetic route of preparing [2H5]-
ebastine fumarate (7).7–9 We chose the most concise synthesis
route began with condensation of commercially available 4-
hydropiperidine and 4-chloro-1-[4-(1,1-dimethylethyl)phenyl]-1-
butanone to form compound (5), followed by reaction with
[2H5]-bromodiphenylmethane (4) to give [2H5]-ebastine (6) as a
white solid. Compound (6) was treated with fumaric acid to form
[2H5]-ebastine fumarate (7) as a white solid. The overall yield was
a reasonable 27%.
Ethyl-2-[4-(4-(4-[2H5]-diphenylmethoxy-1-piperidineyl)bu-
tyryl)phenyl]-2-methylpropanoate (13)
To a suspension of anhydrous Na2CO3 (1.65 g, 15.6 mmol) in
toluene (30 mL) was added compound (4) (2.60 g, 10.3 mmol)
and compound (12) (3.75 g, 10.4 mmol). The reaction mixture
was refluxed at 1251C for 6 h, then cooled to ambient
temperature and filtered. The filter cake was washed with
toluene (3 Â 5 mL) and filtrate was evaporated to afford a dark
brown sticky liquid. The crude product was purified by
chromatography on silica gel (100 g) column, eluted with
0–40% EtOAc in hexane to obtain pure product as a white
solid (3.22 g, 57%).
[2H5]-hydroxyebastine (14)
Scheme 3 shows the synthetic route of preparing [2H5]-
hydroxyebastine (14).15 Methyl-2,2-dimethyl-2-phenyl acetate
(8) was prepared by esterification of 2,2-dimethyl-2-phenyl-aceic
acid and methanol. Reduction of compound (8) by sodium
borohydride gave 2-methyl-2-phenyl propanol (9) as a colorless
oil. Treatment of compound (9) with acetic anhydride at
10–201C obtained ethyl-2-methyl-2-phenyl propanoate (10).
Friedel–Crafts acylation of compound (10) and 4-chloro butyryl
chloride was the key step, in which ortho-substituted by-
product was inavoidable and the crude product was purified by
chromatography on silica gel column to yield compound (11) as
a pale brown liquid. Reaction of compound (11) with 4-
hydroxypiperidine produced compound (12), followed by
condensation with [2H5]-bromodiphenylmethane (4) at reflux
in toluene to give compound (13) as a white solid. Finally,
compound (13) was hydrolyzed to generate an off-white solid
product. After purification by recrystallization from isopropanol,
[2H5]-hydroxyebastine (14) was obtained as a white solid in 13%
overall yield from [2H6]-benzene.
To a solution of Compound (13) (3.22 g, 6.0 mmol) in methanol
(20 mL) was added 10% aqueous sodium hydroxide (8 mL). The
mixture was stirred at ambient temperature overnight. Methanol
was concentrated, then pure water (20 mL) and dichloro-
methane (40 mL) was added to the remaining. The dichloro-
methane layer was separated and the aqueous layer was
extracted with dichloromethane (2 Â 40 mL). The combined
dichloromethane extracts were washed with brine, dried
over MgSO4 and concentrated under reduced pressure to afford
an off-white solid. After purification by recrystallization in
isopropanol, the desired product was obtained as a white solid
(2.45 g, 85%).
1HNMR (CDCl3): d 1.30 (t, J = 9.0 Hz, 1H), d 1.36 (s, 6H), d
1.59–1.68 (m, 2H), d 1.83–1.96 (m, 4H), d 2.08 (t, J = 6.0 Hz, 2H), d
2.36 (t, J = 9.0 Hz, 2H), d 2.72 (t, J = 6.0 Hz, 2H), d 2.99 (t, J = 6.0 Hz,
2H), d 3.35–3.45 (m, 1H), d 3.63 (d, J = 6.0 Hz, 2H), d 5.52 (s, 1H),
d7.26 (d, J = 6.0 Hz, 1H), d 7.31–7.33 (m, 4H), d 7.46 (d, J = 6.0 Hz,
2H), d 7.93 (d, J = 6.0 Hz, 2H). MS-EI (m/z): 490.2 (9), 491.2 (M1,
100), 492.3 (35), 493.3 (7). HPLC (XDB-C18, CH3OH/10 mmol/L
D
D
D
O
D
D
D
D
Br
D
O
D
D
D
D
D
D
D
D
NBS
D
D
AlCl3
Pd/C
H2
Cl
D
D
D
1
2
3
4
Scheme 1.
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Copyright r 2011 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2011, 54 352–356

Z. Yu et al.
OH
O
OH
O
KI
4
Cl
N
N
H
5
D
D
D
D
D
D
D
D
D
D
O
O
fumaric acid
O
N
O
.
N
COOH
HOOC
6
7
Scheme 2.
Scheme 3.
J. Label Compd. Radiopharm 2011, 54 352–356
Copyright r 2011 John Wiley & Sons, Ltd.
www.jlcr.org

Z. Yu et al.
HPLC results showed that both [2H5]-ebastine fumarate (7)
and [2H5]-hydroxyebastine (14) were obtained with over 99%
chemical purity. Mass spectrometry analysis of compound (7)
and compound (14) revealed over 98% deuterium enrichment.
[6] D. Buisson, I. S. Arnaud, A. E. Ouarradi, Tetrahedron 2008, 64(51),
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[10] M. Claire, J. Maryse, S. M. Agnes, D. Buisson, Bioorg. Med. Chem.
2004, 14(21), 5423–5426.
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J. Label Compd. Radiopharm 2011, 54 352–356