Rac- and R-(+)-[4,4′,5,5′-2H4]-2- (1′-[2″,6″-dichlorophenoxy]-ethyl)-Δ2- imidazoline (lofexidine)

Article abstract of DOI:10.1002/jlcr.1655

The synthesis of the d₄-forms of rac- and R-lofexidine was accomplished. Two methods are described; one method is a two-step synthesis of rac-d₄-lofexidine from 2-chloropropionitrile, the second method is a three-step preparation of R-d₄-lofexidine in absolute enantiomeric purity from S-methyl lactate. The commercial availability of R-methyl lactate makes this latter enantioselective synthesis applicable also to the synthesis of S-d₄- lofexidine. These procedures also conserve the utilization of the relatively expensive [1,1′,2,2′-²H ₄]ethylene diamine precursor. The availability of S- and R-d ₄-lofexidines will enable pharmacokinetic studies to be carried out to determine if differential in vivo metabolism of the two enantiomers of lofexidine occurs. Copyright

Full text of DOI:10.1002/jlcr.1655

Research Article
Received 6 March 2009,
Revised 21 May 2009,
Accepted 3 June 2009
Published online 13 July 2009 in Wiley Interscience
(www.interscience.wiley.com) DOI: 10.1002/jlcr.1655
Rac- and R-(1)-[4,40,5,50-2H4]-2-(10-[200,600-
dichlorophenoxy]-ethyl)-D2-imidazoline
(lofexidine)
Ã
Ashish P. Vartak, Vijayakumar Sonar, and Peter A. Crooks
The synthesis of the d4-forms of rac- and R-lofexidine was accomplished. Two methods are described; one method is a two-
step synthesis of rac-d4-lofexidine from 2-chloropropionitrile, the second method is a three-step preparation of R-d4-
lofexidine in absolute enantiomeric purity from S-methyl lactate. The commercial availability of R-methyl lactate makes this
latter enantioselective synthesis applicable also to the synthesis of S-d4- lofexidine. These procedures also conserve the
utilization of the relatively expensive [1,10,2,20-2H4]ethylene diamine precursor. The availability of S- and R-d4-lofexidines
will enable pharmacokinetic studies to be carried out to determine if differential in vivo metabolism of the two enantiomers
of lofexidine occurs.
Keywords: lofexidine; deuterated; enantioselective
hydrochloride salt. A yield of 68%, relative to the amount of
labeled precursor, was obtained.
We have recently developed a scalable enantioselective
Introduction
Lofexidine, or 2 - (2,6 - dichlorophenoxy)ethyl -D2 - imidazoline (1)
(Figure 1), is an a2-adernergic agonist that is currently utilized in
Europe as Britlofexs for the treatment of the symptoms of opiate
addiction. A phase-III clinical trial has recently been conducted in the
United States.1 In addition, US WorldMeds is currently developing
the enantiomeric forms of lofexidine as second-generation ther-
apeutics with potentially superior side-effect profiles.
During our ongoing investigations into the metabolism of
rac-, R- and S-lofexidine,2,3 we were faced with the need for an
isotopically labeled internal standard for HPLC-MS-MS quantifi-
cation of lofexidine enantiomers in biological fluids. Rac-d4-
lofexidine (2) has been previously synthesized by Closter and
Odenthal,3 however, this synthetic methodology is not easily
adaptable for the preparation of enantiomers of d4-lofexidine
(i.e. it is an eight-step procedure). Thus, we were in need of an
efficient and versatile synthetic route that could afford the
enantiopure S- and R-products in reasonable overall yields. We
also were interested in improving the utilization of the relatively
expensive deuterated starting material, [1,10,2,20-2H4]ethylene
diamine (d4-ED) in these processes.
synthetic approach toward the enantiomers of lofexidine that
relies on the electrophilic alkylation of the primary amide 7
(Scheme 3) by trialkyloxonium salts to transform this carbox-
amide into imidazoline 2 in a one-pot sequence.4 Reported
herein is the applicability of this sequence to the synthesis of 2,
with particular attention being paid to maximizing the utiliza-
tion of the relatively expensive d4-ED precursor.
The synthetic sequence we have developed begins with a
Mitsunobu reaction of 2,6-dichlorophenol on the a-carbon of
rac-methyl lactate (6), followed by amidation of the methyl
ester, to yield 7. Carboxamide 7 is then reacted with an
equivalent of trimethyloxonium tetrafluoroborate in CH2Cl2 to
yield the imino ether tetrafluoroborate 8, which is used directly
in the subsequent cyclization step to afford 2. Addition of an
ethanolic solution of d4-ED (0.95 equiv.) to a solution of 8 in
CH2Cl2 led to complete scavenging of the deuteron-label. After
aqueous workup and crystallization, 2 was isolated in 94% yield
based on the amount of d4-ED employed (for comparison, the
method previously described in Reference5 yields rac-d4-
lofexidine in 39% yield, based on the amount of d4-ED
employed). Free base 2 was then converted to its hydrochloride
salt in quantitative yields and isolated in an analytically pure
form after crystallization. An identical reaction sequence carried
Results and discussion
We utilized two distinct approaches to the synthesis of (7)-2
(Figure 1). In our first approach (Scheme 2), 2,6-dichlorophenol
(3) was alkylated with (7)-a-chloropropionitrile in 2-butanone
with K2CO3 as the base, yielding the nitrile (4), which was then
subjected to imidazoline formation by the methodology of
Biedermann et al., by first converting the nitrile into the
corresponding ethylimino ether hydrochloride 5, followed
by condensation with d4-ED in place of the unlabeled
precursor. The resulting free base (7)-2 was converted to the
Department of Pharmaceutical Sciences, College of Pharmacy, University of
Kentucky, 725 Rose Street, 501A, Lexington, KY 40536, USA
*Correspondence to: Peter A. Crooks, Department of Pharmaceutical Sciences,
College of Pharmacy, University of Kentucky, 725 Rose Street, 501A, Lexington,
KY 40536, USA.
E-mail: pcrooks@email.uky.edu
J. Label Compd. Radiopharm 2009, 52 431–434
Copyright r 2009 John Wiley & Sons, Ltd.

A. P. Vartak et al.
out on (–)-methyl lactate (6) proceeded essentially with a net H1D; 4.37%, N; 9.35%, found: C; 44.05%; H; 4.29%; N; 9.19%. EI-LRMS
inversion of configuration about the a-carbon, yielding R-(1)-2 m/z 263.0 (M1H)1 and 265 (M121H)1. Gram at % deuter-
and R-(1)-2.HCl. Enantiomeric purities of R-(1)-2 and R-(1)-2. ium = 99.8%;
HCl were determined by coupling of the free base to 2S-tert-
butyldimethylsiloxy propanoic acid followed by gas chromato- (7)-2-(2,6-Dichlorophenoxy)propionamide (7)
graphic analysis of the product(s). In each case, the enantiomeric
To a solution of (7)-methyl lactate (6) (5.00 g, 48 mmol), Ph3P
(12.6 g, 481 mmol) and 2,6-dichlorophenol (7.83 g, 48 mmol) in THF
excess was determined to be 100%.
(40 mL) at 01C was added drop-wise DIAD (9.8 g, 48 mmol) over a
period of 15 min. The solution was warmed to ambient
temperature over 4 h. Evaporation under reduced pressure
Experimental
[1,10,2,20-2H4]ethylene diamine (98 g at%) was purchased from C.D.N.
afforded an amber gum that was stirred in 1:2 diethyl ether-
hexanes (100 mL) for 30 min, during which time a 1:1 molar
complex of Ph3P = O and diisopropoxycarbonyl hydrazine crystal-
lized. The mixture was filtered, the filter cake washed with hexanes
(50 mL), and the filtrate was evaporated to afford an amber oil. A
solution of this oil in EtOH (30 mL) at 01C was treated with gaseous
NH3 for 30 min and then stirred at ambient temperature. The white
crystals that formed during this time were redissolved by heating
the mixture to reflux for 45 min. The mixture was then filtered to
remove a small amount of a yellow insoluble powder. Cooling of
the filtrate at 41C in the refrigerator for two hours led to the
deposition of white needles, which were isolated by filtration and
dried in air to afford 7 (9.0 g, 80%). 1H NMR (300 MHz, CDCl3) d ppm
7.35ꢀ7.25 (m, 2H, Ar). 7.03 (t, J= 8.4 Hz, 1H, Ar), 6.93 (br, s, NH), 6.08
(br, s, NH), 4.93 (q, 1H, J= 6.9 Hz), 1.50 (d, 3H, J=6.9Hz); 13C NMR
(75 MHz, CDCl3) d ppm 174.2, 148.9, 129.7, 129.4, 125.7, 79.3, 18.1;
mp = 196ꢀ1981C (EtOH) EI-LRMS m/z 233.0 (M)1.
Isotopes, Montreal, Canada. Solvents were of ACS-grade unless
otherwise specified. THF was distilled over Na-benzophenone ketyl
under an atmosphere of argon. All other reagents were used as
received. Gram atom percentages were calculated utilizing PIDC
(Positional Isotopometer Distribution Calculator), Department of
Computer Sciences, University of Helsinki.
2-(2,6-Dichlorophenoxy)propionitrile
This compound was prepared in accordance with the method of
Biedermann et al.5; 1H NMR (300 MHz, CDCl3) d ppm, 7.57 (d, J=8.1,
2H), 7.29 (t, J= 8.25, 1H), 5.34 (q, J= 6.9, 1H), 1.73–1.75 (d, J=6.6, 3H).
(7)-[4,40,5,50-2H4]-2-(10-[200,600-Dichlorophenoxy]-ethyl)-D2-
imidazoline ((7)-2. HCl) (Method 1)
A solution of 2-(2,6-dichlorophenoxy)propionitrile (365 mg, 1.7 mmol)
in absolute ethanol (5 mL) was cooled to 01C, saturated with
dry HCl gas and the mixture was stored at 41C in a refrigerator
overnight. After evaporation of solvent, the resulting imino ester
hydrochloride (not isolated) was treated with an ethanolic solution of
d4-ED (108.8 mg in 5 mL EtOH, 1.7 mmol) and the mixture was
refluxed for 5 h. The crude product was converted to the
hydrochloride salt (a small amount of ester formed by the hydrolysis
of the imino ether hydrochloride was removed during this process,
by washing the hydrochloride salt with diethyl ether). The yield
obtained was 68% (250 mg). mp = 235–2401C; Anal. calc. C; 44.10%,
S-(–)-2-(2,6-Dichlorophenoxy)propionamide ((–)-7)
This compound was synthesized from (–)-methyl lactate,
according to the procedure for (7)-7 described above;
80% yield, [a]2D3 = ꢀ21.41 (c 1.0, acetone)4,5 [a]2D0 = ꢀ19.91 (c 1.0,
acetone) and for (1) ꢀ7 = 120.11 (c 1.0, acetone).
(7)-[4,40,5,50-2H4]-2-(10-[200,600-Dichlorophenoxy]-ethyl)-D2-
imidazoline ((7)-2)
A mixture of 7 (1.00 g, 4.29 mmol) and trimethyloxonium tetra-
fluoroborate (664 mg, 1.1 equiv.) in CH2Cl2 (30 mL) was stirred at
ambient temperature for 48 h, during which time the coarse
suspension changed to a clear solution. Thereupon, the solution
was cooled to–301C (dry ice, glycol-EtOH bath) and treated drop-
wise with a solution of [1,10,2,20-2H4]ethylene diamine (260 mg, 0.95
equiv.) in EtOH (5 mL). The resulting hazy solution was allowed to
stir at ambient temperature for 10 min and then evaporated to
dryness. The pasty white residue obtained was partitioned between
Cl
Cl
O
O
Cl
H3C
Cl
H3C
N
D
D
N
HN
HN
D
2
1
D
Figure 1. Rac-Lofexidine (1) and rac-d4-lofexidine (2).
D
D
D
D
N
D
D
D
H
N
Cl
Cl
D
H
N
N
O
O
a
( )-2
TBSO
+
O
TBSO
H3C
O
CH3
Rt = 15.19 min.
Rt = 15.19 min. (sole peak), m/ z = 448
Cl
Cl
Rt = 15.37 min.
m/z = 448
a
(+)-2
Reagents and Conditions: a. 2S-tert-butyldimethylsilyl propanoic acid
(10 equiv.), DIC (10 equiv.), CH2Cl2, rt, 2h
Scheme 1. Conversion of rac- and (1)-d4-lofexidine to their diastereomeric 2S-tert-butyldimethylsiloxy propanamide derivatives.
www.jlcr.org
Copyright r 2009 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2009, 52 431–434

A. P. Vartak et al.
129–1311C [a]2D3 = 173.41 (c 1.0, EtOH) (for (1)-2:5 [a]2D0
=
CN
173.71 (c 1.0, EtOH).6 [a]D20 for (–)-2 = –80.71 (c 1.0, EtOH));
LR-EIMS m/z 262 (M)1 and 264 (M12)1.
OH
O
b
Cl
Cl
a
Cl
Cl
For determination of enantiomeric purity, (1)-2 (1.08 mg,
0.00387 mmol) was treated with 2S-tert-butyldimethylsiloxy
propanoic acid (8 mg, 0.0387 mmol, 10 equiv.) and diisopropyl
carbodiimide (5 mg, 0.0387 mmol, 10 equiv.) in CH2Cl2 (100 mL)
at room temperature. After 2 h, the entire mixture was
diluted with CH2Cl2 (1 mL) and injected into a gas chromato-
graph (Agilent 6890 GC system with Agilent 5973
MSD equipped with EI-mass spectrometer). A sample of (7)-2
was similarly treated and the resulting reaction mixture
also injected into the GC. The following were noted: (1)
all of the lofexidine in both reactions had been consumed
under the reaction conditions; (2) the reaction mixture resulting
from (7)-2 gave rise to two distinct base-line resolved peaks
(ratio 1:1) appearing at Rt = 15.19 min (m/z = 448) and
Rt = 15.37 min (m/z = 448); (3) the reaction mixture resulting
from (1)-2 gave rise to only one peak at Rt = 15.19 min
(Scheme 1). Consequently, the enantiomeric excess of (1)-2
was judged to be 100%.
3
4
EtO
NH2Cl
Cl
O
c,d
-2.HCl
Cl
5
Reagents and Conditions: a. K2CO3, 2-butanone, 2-
chloro propionitrile, ref lux, 5h. b. HCl, ethanol, 0 oC,
c. 1,1',2,2'-2d4-ethylene diamine, EtOH, d. HCl, i-
PrOH (68% over 2 steps)
Scheme 2. Synthesis of (7)-2; Method 1.
(7)-[4,40,5,50-2H4]-2-(10-[200,600-Dichlorophenoxy]-ethyl)-D2-
imidazoline ((7)-2. HCl)
Cl
Cl
To a solution of (1)-2 (1.00 g, 3.33 mmol) in 9:1 Et2O/EtOH
(10 mL) was added in a drop-wise fashion conc. aqueous HCl
(0.3 mL, 1.3 equiv.). Upon stirring for 10 min the suspension was
diluted with diethyl ether (10 mL), filtered and the filter-cake was
washed with diethyl ether (3 ꢁ 10 mL) and air-dried, affording
(7)-2.HCl (1.13 g, 100%). 1H NMR (300 MHz, d6-DMSO) d ppm
10.59 (s, 2H, NH), 7.53 (d, J = 8.1 Hz, 2H, Ar), 7.23 (t, J = 8.1 Hz, 1H,
Ar), 5.16 (q, J = 6.6 Hz, 1H) 3.87 (s, br, 4H), 1.63 (d, J = 6.6 Hz, 3H);
13C NMR (75 MHz, d6-DMSO) d ppm 169.5, 149.2, 130.3, 129.1,
127.5, 73.9, 44.6, 19.4; mp = 222ꢀ2231C; gram at % deuterium:
99.2%, Anal. calc. C: 44.10%, H1D: 4.37%, N: 9.35%, found: C:
44.24%: H; 4.12%; N: 9.30%.
OH
COOCH3
c
a,b
O
O
Cl
H3C
Cl
H3C
H3C
NH2BF4
OCH3
CONH2
8
7
6, rac-methyl
lactate
Cl
Cl
e
d
O
O
Cl
H3C
Cl
H3C
ClH2N
2. HCl
N
N
D
D
D
D
HN
D
D
2
D
D
e
a–d
2
2 HCl
100% ee
R-(+)-
R-(+)- .
9 ( )-methyl lactate
, –
R-(1)-[4,40,5,50-2H4]-2-(10-[200,600-Dichlorophenoxy]-ethyl)-D2-
imidazoline hydrochloride ((1)-2. HCl)
100% ee
a. 2,6-dichlorophenol, Ph3P, DIAD, THF, rt, 4h; b. NH3, EtOH (80% over
two steps); c. Me3OBF4,CH2Cl2; d. [1,1,2,2'-2H4]ethylenediamine, EtOH
94% (over two steps, based on [1,1,2,2'-2H4]ethylenediamine); e. aq. HCl,
Et2O/EtOH, quant.
This compound was synthesized from 70 mg of (1)-2 as
described above for (7)-2. HCl, 40 mg (46 %); mp =
202–2041C, [a]2D3 = 131.81 (c 1.0, EtOH) (for (1)-2.HCl:5 [a]D201
37.91 (c 1.0, EtOH)6 [a]D20 for (–)-2 = –33.41 (c 1.0, EtOH). The
enantiomeric excess of (1)-2.HCl was calculated to be 100% by
conversion to (1)-2 and diastereomer formation followed by gas
chromatographic analysis, in a manner similar to that described
in the preparation of (1)-2.
Scheme 3. Synthesis of (7)-2, R-(1)-2 and their HCl salts; Method 2.
5% aq. K2CO3 (20 mL) and CH2Cl2 (50 mL). The aqueous layer was
washed with another portion of CH2Cl2 (50 mL), and the combined
organic layers were dried (anhydrous MgSO4) and evaporated to
afford a white solid residue that was recrystallized from boiling
hexanes (100 mL) to give (7)-2 (1.00 g, 94%); mp = 131–1331C.
1H NMR (300 MHz, d6-DMSO) d ppm 7.46ꢀ7.44 (m, 2H, Ar), 7.14
(t, J= 7.8 Hz, 1H, Ar), 6.45 (s, 1H, NH), 4.79 (q, J= 6.6 Hz, 1H), 1.47
(d, J= 6.6 Hz, 3H). 13C NMR (75MHz, d6-DMSO) d ppm 168.2, 149.0,
131.2, 129.4, 128.2, 73.1, 19.3. EI-LRMS m/z 262.0 (M)1.
Conclusions
A new route to both rac- and R-d4-lofexidine has been
successfully developed. This route effectively maximizes the
utilization of the expensive labeled precurssor, d4-ED, thereby
providing access to gram quantities of each of the final deutero-
products.
R-(1)-[4,40,5,50-2H4]-2-(10-[200,600-Dichlorophenoxy]-ethyl)-D2-
imidazoline ((1)-2)
Acknowledgement
Financial support from US WorldMeds Inc. Louisville, KY, is
gratefully acknowledged.
This compound was synthesized from (–)-7 (100 mg, 0.42 mmol)
as described above for (7)-2, yield = 70 mg (66%) mp =
J. Label Compd. Radiopharm 2009, 52 431–434
Copyright r 2009 John Wiley & Sons, Ltd.
www.jlcr.org

A. P. Vartak et al.
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Copyright r 2009 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2009, 52 431–434