Synthesis of deuterium-labeled hydroxybupropion

Article abstract of DOI:10.1002/jlcr.3314

This paper describes the synthesis of deuterium-labeled hydroxybupropion. Mass spectrometry analysis of the compound revealed over 98% deuterium enrichment. This paper describes the synthesis of deuterium-labeled hydroxybupropion. Several different methods were applied to introduce deuterium, while Strecker reaction is the successful method to ensure deuterium enrichment. Mass spectrometry analysis of the product revealed over 98% of deuterium enrichment.

Full text of DOI:10.1002/jlcr.3314

Research article
Received 19 March 2015,
Revised 5 May 2015,
Accepted 23 June 2015
Published online 28 July 2015 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.3314
Synthesis of deuterium-labeled
hydroxybupropion
a
b
c
c
*
Jian Li, Lei Tian, Chi Zhang, and Liqin Chen
This paper describes the synthesis of deuterium-labeled hydroxybupropion. Mass spectrometry analysis of the compound
revealed over 98% deuterium enrichment.
Keywords: deuterium-labeled; bupropion; hydroxybupropion; metabolite; synthesis
[2H3] hydroxybupropion. 3-Chlorobenzonitrile (1) was alkylated
Introduction
with [2H3] ethyl magnesium bromide and then quenching with
HCl aqueous solution to afford ketone (2). Bromination of
Bupropion is commonly used for the treatment of depression or
compound (2) with Br2 in carbon tetrachloride, monobromide
smoking cessation and has been studied for a number of other uses
(3) was obtained. Compound (3) was cyclized with 2-amino-2-
methylpropan-1-ol to afforded [2H3] hydroxybupropion (4).
such as bipolar disorder, attention-deficit hyperactivity disorder, and
weight loss.1–3 Bupropion is extensively metabolized in the body with
However, MS showed compound (4) have lost about 12%
less than 0.5% of product recovered intact in urine.4 Three active
metabolites have been identified: hydroxybupropion, resulting from
deuterium label in cyclization process.
Scheme 2 presents the general synthetic scheme for preparing
[2H4] hydroxybupropion. 3-Chlorobenzonitrile (1) was alkylated
cytochrome P450-catalyzed oxidation, and erythrohydrobupropion
with [2H5] ethyl magnesium bromide and then quenched with
and threohydrobupropion, reduced diastereomers formed via non-
P450-dependent pathways (Figure 1).4 Hydroxybupropion is an
HCl aqueous solution to afford ketone (5). Bromination of
active metabolite that contributes to the antidepressant effect of
compound (5) with Br2 in carbon tetrachloride, monobromide
the parent drug. Hydroxybupropion has a longer elimination half-life
(6) was obtained. Compound (6) was cyclized with 2-amino-2-
methylpropan-1-ol to afford [2H4] hydroxybupropion (7).
than the parent drug, peak cerebrospinal fluid, and plasma
concentration exceed those of bupropion by fourfold to sevenfold,
However, MS showed compound (7) have lost about 50%
and the plasma metabolite/parent area under the curve ratio is
deuterium label in cyclization process.
approximately 10 or greater after a single dose.5–9
Identification and quantification of drugs and metabolites by
liquid chromatography-mass spectrometry (LC-MS) relies largely
Scheme 3 presents the general synthetic scheme for preparing
[2H6] hydroxybupropion (14). Treatment of [2H6] acetone with
NaCN and ND4Cl in D2O afford α-amino alcohol (9a) and α-amino
on stable isotope-labeled analogs.10,11 A renewed interest has
nitrile (9b) through the Strecker reaction. The by-product (9a) can
been recently raised to develop a robust and validated LC-MS
method to determine hydroxybupropion, the major metabolite
of bupropion in biological fluids. The preparation of the stable-
be removed by evaporating under the reduced pressure
(0.5 mmHg). The Strecker reaction is a key step that influenced
deuterium enrichment. In order to effectively prevent significant
loss or scrambling of deuterium label, H2O and NH4Cl were
replaced by D2O and ND4Cl. α-Amino nitrile (9a) was hydrolyzed
by DBr in D2O to afford α-amino acid (10). Application of DBr
labeled version of the title compound was requested. Although
3
2H and H have been prepared for pharmacological studies, the
studies of its stable-labeled internal standard have not been
described in detail.12–15 In this paper, the synthetic route to
[2H6] hydroxybupropion is described in detail.
aSchool of Environmental Science, Nanjing Xiaozhuang University, 3601
Hongjing Road, Nanjing, Jiangsu 211171, China
Results and discussion
bCollege of Petroleum Engineering, Yangtze University, 1 Daxue Road, Caidian
Zone, Wuhan, Hubei 434023, China
Although bupropion and hydroxybupropion have been readily
prepared via several synthetic routes,16–19 the synthesis of [2H6]
cHi-Tech Research Institute and State Key Laboratory of Materials-Oriented
hydroxybupropion has not been described previously. Many
approaches can be followed to prepare stable-labeled version Chemical Engineering, Nanjing University of Technology, 5 Xinmofan Road,
Nanjing, Jiangsu 210009, China
of hydroxybupropion. Based on protocols from the literature,
[2H3] hydroxybupropion and [2H4] hydroxybupropion are both
*Correspondence to: Lei Tian, College of Petroleum Engineering, Yangtze
University, 1 Daxue Road, Caidian Zone, Wuhan, Hubei, 434023, China.
synthesized in three steps starting from 3-chlorobenzonitrile.
Scheme 1 presents the general synthetic scheme for preparing E-mail: tianlei4665@163.com
J. Label Compd. Radiopharm 2015, 58 411–413
Copyright © 2015 John Wiley & Sons, Ltd.

J. Li et al.
1H NMR spectra were recorded on a Bruker 300-MHz instrument (Bruker
Corporation, Germany). Chemical purities were determined by an Agilent
1200 HPLC with a XDB-C18 column, 5 μm, 4.6 × 150 mm (Agilent, USA).
Synthesis of [2H6]2-amino-2-methylpropanoic acid(10)
To a stirred solution of ND4Cl (15.0 g, 0.261 mol) in D2O (40 mL) was
added [2H6] acetone (8) (13.5 g, 0.21 mol) in diethyl ether (40 mL), while
the temperature was maintained at 5–8 °C. Then the solution of NaCN
(11.0 g, 0.224 mol) in D2O (24.5 mL) was added to the aforementioned
solution, while the temperature was maintained at 3–6 °C. After addition,
stirring was continued at 5–10 °C for 1.5 h. The organic phase was
separated, and the aqueous phase was extracted with diethyl ether
(50 mL × 8). The organic layers were combined, dried over Na2SO4, and
concentrated under reduced pressure to give a blue oil residue (18.7 g).
1H NMR showed the residues contain 80% compound (9b) and 20%
compound (9a). The aforementioned residue was dissolved in D2O
(40 mL) containing DBr (40 g, 0.49 mol) to give a light brown solution
under ice–water bath. The solution was heated at 50–60 °C, evaporated
to dryness to afford (9b) as a light yellow semi-solid, and dried under
the reduced pressure (0.5 mmHg). Thin layer chromatography (TLC)
analysis showed that more than 90% by-product was removed.
Compound (9b) was dissolved in D2O (40 mL) containing DBr (40 g,
0.49 mol), and the mixture was stirred at 125 °C for 8 h. The mixture
was evaporated to dryness, co-evaporated with 30% H2O2 (45 mL) three
times, and deeply dried in vacuum to give a white semi-solid. The solid
was dissolved in methanol, followed by filtering inorganic salts. The filtrate
was adjusted to pH 6 by adding pyridine and stirred for 8 h, followed by
filtering the white solid. The filter cake was rinsed with methanol and dried
under vacuum to give (10) as a white solid (12.0 g, 52.3%).
Figure 1. The metabolite routes of bupropion.
and D2O can greatly decrease the loss of deuterium. In addition,
methanol was employed to treat the reaction residue instead of
methanol with ammonia, which avoids aminolysis, and greatly
decreased the loss of deuterium. α-Amino acid (10) was treated
with SOCl2 in methanol to give α-amino ester (11), which was
reduced by LiAlH4 in THF to afford β-amino alcohol (12). Compound
(12) was reacted with 2-bromo-1-(3-chlorophenyl) propan-1-one by
cyclization reaction to give the [2H6] hydroxybupropion (13).
The HPLC results showed that [2H6] hydroxybupropion (13) was
obtained with over 98% chemical purity. MS analysis of [2H6]
hydroxybupropion (13) revealed over 98% deuterium enrichment.
The synthesis provided important internal standards for the
clinical studies of bupropion.
Experimental
Synthesis of [2H6] methyl 2-amino-2-methylpropanoate hydrochloride
(11)
General
All reagents were obtained from Sigma-Aldrich and CDN Isotope. Mass
spectra were recorded using a Quattro micro API mass spectrometer.
To a suspension of compound (10) (2 g, 18.3 mmol) in methanol (15 mL),
was added SOCl2 (5.4 mL) dropwise, and the mixture was stirred at 50 °C
Scheme 1. Synthesis of [2H3] hydroxybupropion.
Scheme 2. Synthesis of [2H4] hydroxybupropion.
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J. Label Compd. Radiopharm 2015, 58 411–413

J. Li et al.
Scheme 3. Synthesis of [2H6] hydroxybupropion.
for 4 h. The reaction mixture was concentrated under reduced pressure
to give a white solid. The solid was redissolved in CH2Cl2 (60 mL),
Conflict of interest
followed by filtering the insoluble matter. The filter cake was rinsed The authors did not report any conflict of interest.
thoroughly with CH2Cl2 (20 mL), and the combined organic layer was
concentrated under reduced pressure to give (11) as a white solid
(2.9 g, 99.2%).
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J. Label Compd. Radiopharm 2015, 58 411–413
Copyright © 2015 John Wiley & Sons, Ltd.
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