Synthesis of (+)-and (-)-tetrabenazine from the resolution of-dihydrotetrabenazine

Article abstract of DOI:10.1080/00397910902788125

Tetrabenazine (1) was reduced with NaBH4 to-dihydrotetrabenazine (2) and then resolved with di-p-toluoyl-L-tartrate and di-p-toluoyl-D-tartrate to subsequently give (+)-and (-)-dihydrotetrabenazine. The enantiomers were oxidized under Swern conditions to prepare samples of (+)-tetrabenazine and (-)-tetrabenazine. The samples were optically pure by chiral HPLC analysis.

Full text of DOI:10.1080/00397910902788125

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Synthesis of (+)- and
(-)-Tetrabenazine from
the Resolution of α-
Dihydrotetrabenazine
Karl G. Boldt a , Michael S. Biggers a , Sharnelle S.
Phifer a , George A. Brine a & Ken S. Rehder a
a Center for Organic and Medicinal Chemistry,
RTI International, Research Triangle Park , North
Carolina, USA
Published online: 08 Sep 2009.
To cite this article: Karl G. Boldt , Michael S. Biggers , Sharnelle S. Phifer , George
A. Brine & Ken S. Rehder (2009) Synthesis of (+)- and (-)-Tetrabenazine from the
Resolution of α-Dihydrotetrabenazine, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 39:20, 3574-3585,
DOI: 10.1080/00397910902788125
To link to this article: http://dx.doi.org/10.1080/00397910902788125
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Synthetic Communications1, 39: 3574–3585, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902788125
Synthesis of (þ)- and (ꢀ)-Tetrabenazine from the
Resolution of a-Dihydrotetrabenazine
Karl G. Boldt, Michael S. Biggers, Sharnelle S. Phifer,
George A. Brine, and Ken S. Rehder
Center for Organic and Medicinal Chemistry, RTI International,
Research Triangle Park, North Carolina, USA
Abstract: Tetrabenazine (1) was reduced with NaBH4 to a-dihydrotetrabenazine
(2) and then resolved with di-p-toluoyl-L-tartrate and di-p-toluoyl-D-tartrate to
subsequently give (þ)- and (ꢀ)-a-dihydrotetrabenazine. The enantiomers were
oxidized under Swern conditions to prepare samples of (þ)-tetrabenazine and
(ꢀ)-tetrabenazine. The samples were optically pure by chiral HPLC analysis.
Keywords: Dihydrotetrabenazine, resolution, tetrabenazine
Tetrabenazine (1) (Nitoman, Xenazine) is a drug used for treatment for
hyperkinetic disorders and is a high-affinity inhibitor of the vesicular
monoamine transporter in the mammalian brain.[1] In rodents and
humans, tetrabenazine is rapidly metabolized by reduction of the 2-keto
group to the a- and b-dihydrotetrabenazines (2 and 3).[2] As part of an
ongoing research program, gram quantities of the optical isomers of
1 and 2 were required. The reported method involved preparative chiral
high-performance liquid chromatographic (HPLC) separation of
a-dihydrotetrabenazine or enzymatic resolution of the acetate ester,
which provided small samples of the enantiomers of 2 but was difficult
to scale up.[3] Reported herein is synthesis of the enantiomers of tetrabe-
nazine and resolution of the enantiomers of a-dihydrotetrabenazine.
Received November 18, 2008.
Address correspondence to Ken S. Rehder, Center for Organic and Medicinal
Chemistry, RTI International, P. O. Box 12194, Research Triangle Park, NC
27709, USA. E-mail: krehder@rti.org
3574

Synthesis of (þ)- and (ꢀ)-Tetrabenazine
3575
Scheme 1.
(ꢁ)-Tetrabenazine (1) was synthesized by the literature method.[4]
Attempts to resolve 1 with chiral acids (di-p-toluoyl-D-tartaric acid,
(þ)-camphoric acid, or 1S-(þ)-10-camphorsulfonic acid) were unsucces-
sful. Taking another approach, 1 was first reduced with NaBH4 in
ethanol (EtOH) to give a mixture of (ꢁ)-a-dihydrotetrabenazine (2) and
(ꢁ)-b-dihydrotetrabenazine (3) in a 4:1 ratio as reported in the literature
(Scheme 1).[5] The isomers were then separated by recrystallization from
methanol (MeOH) to give pure (ꢁ)-2. Additional (ꢁ)-2 was recovered
from the filtrate by column chromatography for an overall yield of 64%.
Initially, a small sample (1 g) of (ꢁ)-2 was treated with 1 equivalent
of di-p-toluoyl-L-tartaric acid, and the salt was recrystallized (2ꢂ)
from MeOH=acetone to give (þ)-a-dihydrotetrabenazine di-p-toluoyl-
L-tartrate [(ꢀ)-4]. [The (þ)-a-dihydrotetrabenazine di-p-toluoyl-L-tartrate
salt [(ꢀ)-4] has a (ꢀ) optical rotation and upon neutralization gives (þ)-2.
The (ꢀ)-a-dihydrotetrabenazine di-p-toluoyl-L-tartrate salt [(þ)-4] has a
(þ) optical rotation and upon neutralization gives (ꢀ)-2]. Chiral HPLC
analysis showed the salt was a single enantiomer and the (þ)-isomer (as

3576
K. G. Boldt et al.
the free base). The supernatant was evaporated under vacuum, and the
residue was converted to the free base, which was treated with 1 equivalent
of di-p-toluoyl-D-tartaric acid, and the salt was recrystallized (3ꢂ) from
MeOH=acetone to give (ꢀ)-a-dihydrotetrabenazine di-p-toluoyl-D-tartrate
[(þ)-4]. Chiral HPLC analysis showed the salt was a single enantiomer and
the (ꢀ)-isomer (as the free base). These resolved salt samples were used as
seed crystals in larger scale resolutions. [The structural assignment of the
absolute stereochemistry of (þ)-2 and (ꢀ)-2, and by analogy (þ)-1 and
(ꢀ)-1, was made based on Ref. 3. However, this assignment has been
questioned.[6] Because of this ambiguity, R=S designations were not used
for the compounds herein.]
The resolution was repeated on a larger scale [22 g of (ꢁ)-2) to yield
(ꢀ)-4 and (þ)-4. Each salt was ꢃ95% enantiomerically pure by chiral
HPLC analysis of the corresponding free base. The individual salts from
two such large-scale-resolution batches were combined, recrystallized, and
determined to be enantiomerically pure by chiral HPLC (as the free base).
Overall, resolved (ꢀ)-4 and (þ)-4 were obtained in 26% (52% theoretical)
and 17% (34% theoretical) yield, respectively, from two batches of (ꢁ)-2.
Each salt was converted separately to the free base using concen-
trated aqueous NH4OH followed by extraction. Compound (ꢀ)-4 gave
(þ)-a-dihydrotetrabenazine [(þ)-2] in 80% yield. Compound (þ)-4 gave
(ꢀ)-a-dihydrotetrabenazine [(ꢀ)-2] in 74% yield. Chiral HPLC analysis
showed each sample was 100% enantiomerically pure.
Samples of (þ)-2 and (ꢀ)-2 were individually oxidized using
Swern conditions[7] to give (þ)-tetrabenazine [(þ)-1] in 72% yield and
(ꢀ)-tetrabenazine [(ꢀ)-1] in 74% yield. Chiral HPLC analysis showed
each sample was 100% enantiomerically pure.
In summary, a resolution of (ꢁ)-a-dihydrotetrabenazine (2) using
di-p-toluoyl-L-tartaric acid and di-p-toluoyl-D-tartaric acid followed by
regeneration of optically pure samples of (þ)-a-dihydrotetrabenazine
[(þ)-2] and (ꢀ)-a-dihydrotetrabenazine [(ꢀ)-2] is reported. The resolution
is easily scaled up to produce gram quantities of the enantiomers. Sub-
sequent Swern oxidation of the enantiomers of 2 to the enantiomers of
1 allows for the preparation of the optical isomers of tetrabenazine
(1), which have not been previously reported in the literature.
EXPERIMENTAL
General
Melting points were obtained on a Thomas Hoover capillary apparatus
and are corrected. Proton and carbon-13 nuclear magnetic resonance

Synthesis of (þ)- and (ꢀ)-Tetrabenazine
3577
(NMR) spectra were run on a Bruker Avance 300-MHz NMR spectro-
meter or a Varian AMX-500 NMR spectrometer. Mass spectra (MS)
were run on a Perkin-Elmer Sciex API 150 EX mass spectrometer
equipped with APCI (atmospheric pressure chemical ionization) or ESI
(turbospray) sources. MS sample introduction was accomplished by
means of an Agilent 1100 series liquid-chromatography system. Thin-
layer chromatography (TLC) analyses were run on commercial precoated
analytical silica-gel 60 F254 glass plates (E. Merck: 5 ꢂ 10 cm) using the
solvent systems indicated. Spot visualization was achieved using a combi-
nation of 5% phosphomolybdic acid in EtOH and 10% ceric sulfate in
10% sulfuric acid followed by heating on a hot plate or with I2. HPLC
analysis were performed on a Rainin HPLC system utilizing two Rainin
solvent pumps (25-mL pump heads), Rainin solvent mixer, Rheodyne
injector, Varian dual-wavelength detector, and Power Macintosh 7200
running Rainin Dynamax software for gradient control and data hand-
ling. Chiral HPLC analysis was performed on a Varian system using a
Varian Prostar 335 binary pump system, a Rheodyne injector, a Varian
photodiode array (PDA) detector, and Varian Galaxie software (run on a
Dell computer) for system operation and data handling. The column and
solvents used for chiral purity analysis is defined hereinafter as system A:
Phenomenex Chirex (S)-Valine (Val) and (R)-1-(a-naphthy)ethylamine
(NEA) (250 ꢂ 4.6 mm) using isocratic 90% A=10% B at 1.0 mL=min with
ultraviolet (UV) detection at 254 nm with solvent A being hexane=
1,2-dichloroethane (5:1) and solvent B being 0.1% trifluoroacetic acid
(TFA)=EtOH. Optical rotations were obtained using an Autopol IV auto-
matic polarimeter (Rudolph Research Analytical Corporation) and a glass
cell (1.5 mL volume, 100 mm pathlength). Column chromatography was
performed on E. Merck silica gel 60, 230–400 mesh. In-house nitrogen gas
(produced from liquid nitrogen) was utilized to supply an inert atmosphere.
Elemental analyses were done by Atlantic Microlab Inc. (Norcross, GA).
(ꢁ)-a-Dihydrotetrabenazine (2)
Compound 1 (34.1 g, 0.107 mol) was dissolved in absolute EtOH (3.40 L)
by warming. The resulting solution was stirred and cooled to 0 ꢄC; then
sodium borohydride was added (14.2 g, 0.376 mol). The reaction mixture
was stirred at 0–3 ꢄC for 3 h under N2. The reaction mixture was checked
by TLC (silica gel, EtOAc), which showed all the starting material
had reacted. The reaction mixture was evaporated under vacuum to
give a white solid. The solid was partitioned between CH2Cl2 (300 mL)
and H2O (300 mL), and the mixture was stirred overnight. The layers
were separated, and the aqueous layer was extracted with CH2Cl2

3578
K. G. Boldt et al.
(2 ꢂ 150 mL). The CH2Cl2 layers were combined, dried (Na2SO4),
filtered, and evaporated under vacuum to give a white solid (34.4 g). The
solid was recrystallized from MeOH (50 mL) to give 17.9g (52%) of (ꢁ)-2
as white crystals: mp 164–168 ꢄC (Lit.[8] 168 ꢄC); TLC single spot, Rf 0.28,
1
(silica gel, EtOAc); H NMR (CDCl3) d 0.92 (3H, d, J ¼ 6.5 Hz), 0.94
(3H, d, J ¼ 6.5 Hz),1.05 (1H, m), 1.49 (1H, q, J ¼ 11.7 Hz), 1.62 (1H, m),
1.75 (2H, m), 1.98 (1H, dd, J ¼ 11.4, 11.3 Hz), 2.45 (1H, ddd, J ¼ 11.5,
11.1, 4.6 Hz), 2.60 (1H, m), 2.63 (1H, m), 3.01 (1H, m), 3.06 (1H, m), 3.12
(1H, d, J ¼ 11.0 Hz), 3.39 (1H, m), 3.84 (6H, s), 6.58 (1H, s), 6.68 (1H, s).
The filtrate was evaporated under vacuum to recover a yellow foamy
solid (16.0 g). The solid was chromatographed on silica gel 60 (300 g)
using 2% MeOH=CH2Cl2. Fractions were collected and checked by
TLC (silica gel, EtOAc) before combination. Three samples resulted from
the chromatography: 3.95 g (12%) of (ꢁ)-2 as a white solid, 4.65 g (14%)
of mixed a=b-dihydrotetrabenazine (2=3) as a white solid, and 2.74 g (8%)
of b-dihydrotetrabenazine (3) as a white solid.
Resolution of (ꢁ)-a-Dihydrotetrabenazine (2)
Chiral HPLC Analysis Procedure
The resolution of (ꢁ)-2 was monitored using system A. Samples of the
di-p-toluoyltartrate salts were converted to the corresponding free bases
for analysis. (þ)-a-Dihydrotetrabenazine had a longer retention time
than (ꢀ)-a-dihydrotetrabenazine under the analysis conditions. A stan-
dard of (ꢁ)-a-dihydrotetrabenazine was injected during each analysis
to verify retention times and separation.
Small-Scale Resolution (Seed Crystals)
A solution of (ꢁ)-2 (1.00 g, 3.13 mmol) in MeOH (40 mL) was obtained
by warming on a steam bath. Di-p-toluoyl-L-tartaric acid (1.21 g,
3.13 mmol) was added to the solution. The resultant solution was evapo-
rated to a white solid (2.30 g). This solid was recrystallized twice from
acetone=MeOH to provide 0.551 g (25%) of (þ)-a-dihydrotetrabenazine
di-p-toluoyl-L-tartrate [(ꢀ)-4] as a white solid: mp 172–173 ꢄC foaming,
chiral HPLC single peak, tR 10.2 min, (þ)-isomer (99.6%). The filtrates
from the recrystallizations were combined and evaporated to obtain a
foamy pale yellow solid (1.33 g), which was suspended in H2O (50 mL).
Concentrated NH4OH (20 drops) was added, and the mixture extracted
with CH2Cl2 (2 ꢂ 50 mL). The combined CH2Cl2 layers were dried
(Na2SO4) and evaporated to recover a yellow solid (0.70 g). The solid

Synthesis of (þ)- and (ꢀ)-Tetrabenazine
3579
(0.70 g, 2.19 mmol) was dissolved in MeOH (40 mL) by warming on a
steam bath. Di-p-toluoyl-D-tartaric acid (0.847 g, 2.19 mmol) was added
to the solution. The solution was evaporated to a yellow foamy solid
(1.63 g). This solid was recrystallized three times from acetone=MeOH
to give 0.313 g (14%) of (ꢀ)-a-dihydrotetrabenazine di-p-toluoyl-
D-tartrate [(þ)-4] as a white solid: mp 177–178 ꢄC foaming, chiral HPLC
major peak, tR 9.4 min, (ꢀ)-isomer (100%). The analysis also showed a
peak at tR 10.52 min that was not from the (þ)-isomer. The small samples
of (ꢀ)-4 and (þ)-4 were employed as seed crystals.
Large-Scale Resolution
The (ꢁ)-2 (22.0 g, 0.069 mol) was dissolved in MeOH (300 mL) by
warming, and di-p-toluoyl-L-tartaric acid (26.6 g, 0.069 mol) was added
to the solution. The resulting solution was evaporated under vacuum
to give an off-white foam (48.7 g). The foam was dissolved in MeOH
(320 mL) and diluted with acetone (320 mL). The solution was concen-
trated to 320 mL, diluted to 640 mL with acetone, and concentrated to
320 mL. The resulting solution was seeded with (ꢀ)-4 and allowed to
stand at room temperature. After 7 days, the crystals were collected,
washed with acetone, and dried under vacuum to give 14.9 g (31%) of
(ꢀ)-4: mp 177–179 ꢄC foaming; chiral HPLC major peak [95.4%,
(þ)-isomer free base], tR 9.8 min, on system A.
The filtrate from the crystallization was evaporated under vacuum to
give a yellow foamy solid (33.4 g). The solid was suspended in H2O
(100 mL), and concentrated NH4OH (10 mL) was added. The mixture
was extracted with CH2Cl2 (3 ꢂ 200 mL). The combined CH2Cl2 layers
were dried (Na2SO4), filtered, and evaporated to recover a yellow solid
(14.9 g). The solid (14.9 g, 0.047 mol) was dissolved in MeOH (200 mL)
by warming, and di-p-toluoyl-D-tartaric acid (18.0 g, 0.047 mol) was
added to the solution. The resulting solution was evaporated under
vacuum to give a yellow foamy solid (32.7 g). The solid was dissolved
in MeOH (400 mL), concentrated to 300 mL, and diluted with acetone
(300 mL). The solution was concentrated to 300 mL, and diluted to
600 mL with acetone, and concentrated to 300 mL. The resulting solution
was seeded with (þ)-4 and allowed to stand at room temperature. After 8
days, the crystals were collected, washed with acetone, and dried under
vacuum to give 16.5 g (34%) of (þ)-4: mp 179–180 ꢄC foaming, chiral
HPLC major peak [96%, (ꢀ)-isomer free base], tR 9.2 min, on system A.
The filtrate was evaporated under vacuum to give a yellow foamy
solid (16.0 g). The solid was suspended in H2O (100 mL), and concen-
trated NH4OH (5 mL) was added. The mixture was extracted with

3580
K. G. Boldt et al.
CH2Cl2 (3 ꢂ 200 mL). The combined CH2Cl2 layers were dried (Na2SO4),
filtered, and evaporated to recover 7.44 g (34% recovery) of a yellow solid
of slightly enriched (þ)-2.
(ꢀ)-a-Dihydrotetrabenazine Di-p-toluoyl-L-tartrate [(ꢀ)-4]
(Salt of (þ)-a-Dihydrotetrabenazine)
Two batches (14.9 g and 13.2 g) of (ꢀ)-4 from the resolutions were com-
bined, and 28.1 g were dissolved in boiling MeOH (500 mL), concen-
trated to 400 mL, diluted to 800 mL with acetone, concentrated to
400 mL, diluted with acetone to 800 mL, and concentrated to 300 mL.
Crystallization started. After standing for 8 days, the crystals were col-
lected, rinsed with acetone (100 mL), and vacuum dried at room tem-
perature to give 19.9 g of (ꢀ)-4 as white crystals: mp 182–183 ꢄC
foaming, chiral HPLC major peak [100%, (þ)-isomer free base], tR
24ꢄ
10.2 min, on system A; ½aꢅD ꢀ49.4ꢄ (c 1.01, MeOH). Anal. calcd. for
C39H47NO11 . 0.5 H2O: C, 65.53; H, 6.77; N, 1.96. Found: C, 65.68;
H, 6.58; N, 2.03.
The filtrate was concentrated to 80 mL and cooled to room tem-
perature. After standing 2 days, the crystals were collected to give
4.25 g of (ꢀ)-4 as white crystals: mp 178–179 ꢄC foaming, chiral HPLC
major peak [100%, (þ)-isomer free base] tR 9.8 min, on system A.
The filtrate was concentrated to 40 mL and cooled to room tempera-
ture. After standing 3 days, the crystals were collected to give 2.50 g of
(ꢀ)-4 as white crystals: chiral HPLC major peak [100%, (þ)-isomer free
base], tR 10.0 min, on system A. The three recrystallized samples (26.6 g,
95% weight recovery) of (ꢀ)-4 were used in the next reaction without
further purification. A total of 26.6 g (26%) of (ꢀ)-2 di-p-toluoyl-
D-tartrate was prepared from 46.4 g of (ꢁ)-2.
(þ)-a-Dihydrotetrabenazine Di-p-toluoyl-L-tartrate
[(þ)-4] (Salt of (ꢀ)-a-Dihydrotetrabenazine)
Two batches (16.5 g and 17.1 g) of (þ)-4 from the resolutions were com-
bined, and 33.6 g were dissolved in boiling MeOH (550 mL), concentrated
to 400 mL, diluted to 800 mL with acetone, concentrated to 400 mL,
diluted with acetone to 800 mL, and concentrated to 300 mL. The
solution was seeded with a crystal of (þ)-4. After standing 2 days, the
crystals were collected, rinsed with acetone (50 mL), and vacuum dried
at room temperature to give 11.6 g of (þ)-4 as white crystals: mp

Synthesis of (þ)- and (ꢀ)-Tetrabenazine
3581
185–186 ꢄC foaming, chiral HPLC major peak [100%, (ꢀ)-isomer free
24ꢄ
base], tR 8.9 min, on system A; ½aꢅD þ 46.4ꢄ (c 1.03, MeOH). Anal. calcd.
for C39H47NO11: C, 66.37; H, 6.71; N, 1.98. Found: C, 66.35; H, 6.73;
N, 2.05.
The filtrate was concentrated to 50 mL and cooled to room tempera-
ture. After standing 1 day, the crystals were collected to give 4.64 g of
(þ)-4 as white crystals: mp 186–187 ꢄC foaming, chiral HPLC major peak
[100%, (ꢀ)-isomer free base], tR 9.1 min, on system A.
The filtrate was concentrated to 40 mL and cooled to room tempera-
ture. After standing 1 day, the crystals were collected to give 1.25 g of
(þ)-4 as white crystals: chiral HPLC major peak [100%, (ꢀ)-isomer free
base], tR 9.0 min, on system A. The three recrystallized samples (17.5 g,
52% weight recovery) of (þ)-4 were used in the next reaction without
further purification. A total of 17.5 g (17%) of (þ)-2 di-p-toluoyl-
D-tartrate was prepared from 46.4 g of (ꢁ)-2.
(þ)-a-Dihydrotetrabenazine [(þ)-2]
Compound (ꢀ)-4 (26.6 g, 0.038 mol) was suspended in H2O (350 mL),
and concentrated NH4OH (15 mL) was added, followed by CH2Cl2
(300 mL). The mixture was stirred for 15 min, and the layers were sepa-
rated. The aqueous layer was extracted with CH2Cl2 (2 ꢂ 300 mL). The
CH2Cl2 layers were combined, dried (Na2SO4), filtered, and evaporated
under vacuum to give a yellow foamy solid (12.0 g). The solid was chro-
matographed on silica gel 60 (400 g) using 2% MeOH=CH2Cl2. Fractions
were collected and checked by TLC (silica gel, EtOAc), and similar frac-
tions were combined and evaporated under vacuum to give a pale yellow
solid (10.2 g). The solid was triturated with hexanes to give 9.68 g (80%)
24:7ꢄ
of (þ)-2 as a white solid: mp 106–108 ꢄC, ½aꢅD þ 62.4ꢄ (c 1.066, MeOH),
TLC single spot, Rf 0.74 [silica gel, CHCl3=MeOH=concentrated
NH4OH (90:9:1)], chiral HPLC single peak (100%), tR 9.4 min, on system
A [The retention times during chiral HPLC analysis showed some varia-
bility, thus a co-injection strategy using racemic material was employed.
Method A: chiral HPLC analysis in system A: (þ)-2 single peak, tR
9.4 min; (ꢁ)-2 two peaks tR 9.2 min and 10.0 min; and co-injection
(þ)-2 and (ꢁ)-2 two peaks, tR 9.1 min (small peak) and 9.5 min (large
peak)]; HPLC single peak (100%), tR 7.7 min, on a Varian Dynamax
C18 column (5 m) (4.6 ꢂ 250 mm) using a gradient (25% B ! 65% B over
30 min) at 1.0 mL=min with UV detection at 220 nm, with solvent A being
0.1% TFA=H2O and solvent B being 0.1% TFA=CH3CN; 1H NMR
(CDCl3) (500 MHz) d 0.92 (3H, d, J ¼ 6.5 Hz), 0.94 (3H, d, J ¼ 6.5 Hz),
1.06 (1H, ddd, J ¼ 13.7, 9.7, 4.3 Hz), 1.49 (1H, q, J ¼ 11.4 Hz), 1.58

3582
K. G. Boldt et al.
(1H, ddd, J ¼ 13.5, 10.2, 3.2 Hz), 1.63–1.79 (3H, m), 1.98 (1H, t,
J ¼ 11.5 Hz), 2.46 (1H, td, J ¼ 11.4, 4.1 Hz), 2.58 (1H, ddd, J ¼ 12.4,
4.6, 2.5 Hz), 2.64 (1H, dd, J ¼ 15.7, 3.0 Hz), 3.00 (1H, ddd, J ¼ 11.3,
5.9, 1.6 Hz), 3.04 (1H, dd, J ¼ 11.8, 4.2 Hz), 3.06–3.10 (1H, m), 3.13
(1H, dd, J ¼ 11.8, 3.2 Hz), 3.34–3.44 (1H, m), 3.84 (6H, s), 6.58 (1H, s),
6.68 (1H, s); 13C NMR (CDCl3) (125 MHz) d 21.7, 24.1, 25.3, 29.1,
39.6, 40.5, 41.6, 51.9, 55.8, 55.9, 60.0, 60.9, 74.5, 107.8, 111.4, 126.3,
129.3, 147.1, 147.4; MS (ESI) (positive ion) m=z 320.2 (M þ H). Anal.
calcd. for C19H29NO3: C, 71.44; H, 9.15; N, 4.38. Found: C, 71.33; H,
9.10; N, 4.39.
(ꢀ)-a-Dihydrotetrabenazine [(ꢀ)-2]
Compound (þ)-(4) (17.5 g, 0.025 mol) was suspended in H2O (200 mL),
and concentrated NH4OH (10 mL) was added, followed by CH2Cl2
(200 mL). The mixture was stirred for 15 min, and the layers were sepa-
rated. The aqueous layer was extracted with CH2Cl2 (200 mL). The
CH2Cl2 layers were combined, dried (Na2SO4), filtered, and evaporated
under vacuum to give a yellow foamy solid (6.72 g). The solid (6.72 g)
was chromatographed on silica gel 60 (275 g) using 2% MeOH=CH2Cl2.
Fractions were collected and checked by TLC (silica gel, EtOAc), and
similar fractions were combined and evaporated under vacuum to give
a pale yellow solid (6.19 g). The solid was triturated with hexanes to give
24:6ꢄ
5.85 g (74%) of (ꢀ)-(2) as an off-white solid: mp 107–108 ꢄC; ½aꢅD ꢀ 59.1ꢄ
(c 1.248, MeOH); TLC single spot, Rf 0.74 [silica gel, CHCl3=MeOH=
concentrated NH4OH (90:9:1)]; chiral HPLC single peak (100%), tR
9.3 min, on system A [Co-injection strategy B: chiral HPLC analysis in
system A: (ꢀ)-2 single peak, tR 9.3 min; (ꢁ)-2 two peaks, tR 9.6 min and
10.5 min; and co-injection (ꢀ)-2 and (ꢁ)-2 two peaks tR 9.5 min (large
peak) and 10.5 min (small peak)]; HPLC single peak (100%), tR 7.5 min,
on a Varian Dynamax C18 column (5 m) (4.6 ꢂ 250 mm) using a gradient
(25% B ! 65% B over 30 min) at 1.0 mL=min with UV detection at
220 nm, with solvent A being 0.1% TFA=H2O and solvent B being
0.1% TFA=CH3CN; 1H NMR (CDCl3) (500 MHz) d 0.92 (3H, d,
J ¼ 6.5 Hz), 0.94 (3H, d, J ¼ 6.5 Hz), 1.06 (1H, ddd, J ¼ 13.6, 9.7,
4.2 Hz), 1.49 (1H, q, J ¼ 11.5 Hz), 1.58 (1H, ddd, J ¼ 13.4, 10.2, 3.3 Hz),
1.64–1.78 (3H, m), 1.97 (1H, t, J ¼ 11.4 Hz), 2.46 (1H, td, J ¼ 11.4,
4.1 Hz), 2.58 (1H, ddd, J ¼ 12.3, 4.5, 2.5 Hz), 2.64 (1H, dd, J ¼ 15.7,
2.8 Hz), 3.00 (1H, ddd, J ¼ 11.3, 5.9, 1.6 Hz), 3.04 (1H, dd, J ¼ 11.8,
4.2 Hz), 3.06–3.10 (1H, m), 3.12 (1H, d, J ¼ 11.6 Hz), 3.35–3.43 (1H, m),
3.84 (6H, s), 6.58 (1H, s), 6.68 (1H, s); 13C NMR (CDCl3) (125 MHz) d
21.7, 24.1, 25.3, 29.1, 39.6, 40.5, 41.6, 51.9, 55.8, 55.9, 60.0, 60.9, 74.5,

Synthesis of (þ)- and (ꢀ)-Tetrabenazine
3583
107.8, 111.4, 126.3, 129.3 147.1, 147.4; MS (ESI) (positive ion) m=z 320.2
(M þ H). Anal. calcd. for C19H29NO3: C, 71.44; H, 9.15; N, 4.38. Found:
C, 71.51; H, 9.23; N, 4.40.
(þ)-Tetrabenazine [(þ)-1]
The chiral HPLC analyses of the tetrabenazine optical isomers were per-
formed on a Regis (S, S) Whelk-01 10=100 column (4.6 ꢂ 250 mm) using
50:50 A=B (isocratic) at 2.0 mL=min with UV detection at 280 nm, and
with solvent A being hexane=1,2-dichloroethane (5:1) and solvent B being
0.5% TFA=2-PrOH.
Under nitrogen in a three-neck, 1000-mL, round-bottom flask,
CH2Cl2 (200 mL) was cooled in a dry ice=2-PrOH bath to ꢀ60 ꢄC (inter-
nal reaction temperature), and 2.0 M oxalyl chloride in CH2Cl2 (8.9 mL,
17.8 mmol) was added by syringe. Afterward, dimethylsulfoxide (1.46 g,
18.7 mmol) was added by syringe, and the reaction mixture was stirred
5 min at ꢀ60 ꢄC. A solution of (þ)-a-dihydrotetrabenazine [(þ)-2]
(4.97 g, 15.5 mmol) in CH2Cl2 (50 mL) was added to the reaction mixture
dropwise at ꢀ60 ꢄC. Following the addition, the resultant reaction mix-
ture was stirred at ꢀ60 ꢄC for 2 h. Afterward, Et3N (7.9 g, 0.078 mol)
was added to the reaction mixture at ꢀ60 ꢄC by syringe, and the mixture
was allowed to warm to room temperature. It was then diluted with H2O
(200 mL), and the layers were separated. The CH2Cl2 layer was washed
with additional H2O (400 mL), dried (Na2SO4), and evaporated to obtain
a yellow solid (5.12 g). This yellow solid was chromatographed on silica
gel 60 (150 g), eluting with EtOAc and monitoring fractions by TLC
(EtOAc). Appropriate fractions were combined and evaporated to
recover a pale yellow solid (4.58 g). The 4.58 g sample was combined with
a similar 1.12 g sample from a probe reaction. The resultant sample
(5.70 g) was recrystallized from MeOH (20 mL) to give 4.46 g (72%) of
23ꢄ
(þ)-1 as white needles: mp 113–115 ꢄC; ½aꢅD þ71.6ꢄ (c 1.022, MeOH);
TLC single spot, Rf 0.60, (EtOAc); chiral HPLC single peak (100%), tR
5.89 min; analytical HPLC single peak (99.7%), tR 6.19 min, on a Dyna-
max C18 column (4.6 ꢂ 250 mm) (5 m) employing a gradient (40%
B ! 85% B over 15 min, then 85% B for 5 min) at 1.0 mL=min with UV
detection at 220 nm, and with solvent A being 0.1% TFA=H2O and
solvent B being 0.1% TFA=CH3CN; 1H NMR (CDCl3) d 0.90 (3H,
d, J ¼ 6.0 Hz), 0.92 (3H, d, J ¼ 6.0 Hz), 1.04 (1H, m), 1.65 (1H, m),
1.80 (1H, m), 2.35 (1H, dd, J ¼ 12.0, 11.5 Hz), 2.54 (1H, m), 2.59
(1H, m), 2.71 (1H, m), 2.74 (1H, m), 2.89 (1H, dd, J ¼ 14.0, 3.5 Hz),
3.09 (1H, m), 3.14 (1H, m), 3.28 (1H, dd, J ¼ 11.0, 6.5 Hz), 3.50 (1H, d,
J ¼ 11.5 Hz), 3.80 (3H, s), 3.85 (3H, s), 6.54 (1H, s), 6.61 (1H, s);

3584
K. G. Boldt et al.
13C NMR (CDCl3) d 22.0, 23.2, 25.3, 29.3, 35.0, 47.4, 47.5, 50.5, 55.8,
55.9, 61.4, 62.4, 107.7, 111.3, 126.0, 128.4, 147.4, 147.7, 210.1; MS
(ESI, positive ion) m=z 318.1 (M þ H). Anal. calcd. for C19H27NO3: C,
71.89; H, 8.57; N, 4.41. Found: C, 71.77; H, 8.64; N, 4.42.
(ꢀ)-Tetrabenazine [(ꢀ)-1]
Under nitrogen in a three-neck 1000-mL, round-bottom flask, CH2Cl2
(200 mL) was cooled in a dry ice=2-PrOH bath to ꢀ 60 ꢄC (internal
reaction temperature) and 2.0 M oxalyl chloride in CH2Cl2 (11.6 mL,
2.3 mmol) was added by syringe. Afterward, dimethylsulfoxide (1.90 g,
2.4 mmol) was added by syringe, and the reaction mixture was stirred
5 min at ꢀ60 ꢄC. A solution of (ꢀ)-a-dihydrotetrabenazine [(ꢀ)-2]
(6.50 g, 20.3 mmol) in CH2Cl2 (50 mL) was added to the reaction mix-
ture dropwise at ꢀ60 ꢄC. Following the addition, the resultant reaction
mixture was stirred at ꢀ60 ꢄC for 2 h. Afterward, Et3N (10.33 g,
0.102 mol) was added to the reaction mixture at ꢀ60 ꢄC by syringe,
and the mixture was allowed to warm to room temperature. It was
then diluted with H2O (250 mL), and the layers were separated. The
CH2Cl2 layer was washed with additional H2O (500 mL), dried
(Na2SO4), and evaporated to obtain a yellow solid (6.51 g). This yellow
solid was chromatographed on silica gel 60 (450 g), eluting with EtOAc
and monitoring fractions by TLC (EtOAc). Appropriate fractions were
combined and evaporated to recover a pale yellow solid (6.23 g). This
solid was recrystallized from MeOH (20 mL) to provide 4.80 g (74%)
23ꢄ
of (ꢀ)-1 as white needles: mp 113–115 ꢄC; ½aꢅD ꢀ 69.7ꢄ (c 1.008,
MeOH); TLC single spot, Rf 0.60, EtOAc; chiral HPLC single peak
(100%), tR 8.93 min; analytical HPLC single peak (99.7%), tR
6.22 min, on a Dynamax C18 column (4.6 ꢂ 250 mm) (5 m) using a gra-
dient (40% B ! 85% B over 15 min, then 85% B for 5 min) at
1.0 mL=min with UV detection at 220 nm, and with solvent A being
0.1% TFA=H2O and solvent B being 0.1% TFA=CH3CN. 1H NMR
(CDCl3) d 0.90 (3H, d, J ¼ 6.0 Hz), 0.92 (3H, d, J ¼ 6.0 Hz), 1.04
(1H, m), 1.65 (1H, m), 1.80 (1H, m), 2.35 (1H, dd, J ¼ 11.5,
11.5 Hz), 2.54 (1H, m), 2.59 (1H, m), 2.71 (1H, m), 2.74 (1H, m),
2.89 (1H, dd, J ¼ 13.5, 3.0 Hz), 3.09 (1H, m), 3.14 (1H, m), 3.28 (1H,
dd, J ¼ 11.0, 6.5 Hz), 3.50 (1H, d, J ¼ 11.0 Hz), 3.82 (3H, s), 3.85
(3H, s), 6.54 (1H, s), 6.61 (1H, s); 13C NMR (CDCl3) d 22.0, 23.2,
25.3, 29.3, 35.0, 47.4, 47.5, 50.5, 55.9, 55.9, 61.4, 62.4, 107.7, 111.4,
126.0, 128.4, 147.4, 147.7, 210.1; MS (ESI, positive ion) m=z 318.3
(M þ H). Anal. calcd. for C19H27NO3: C, 71.89; H, 8.57; N, 4.41.
Found: C, 71.85; H, 8.61; N, 4.43.

Synthesis of (þ)- and (ꢀ)-Tetrabenazine
3585
ACKNOWLEDGMENT
This work was supported by the National Institute of Mental Health
under Contract N01-MH-32005. The authors gratefully acknowledge
the assistance of Ms. Terri Komar and Ms. Ann Allen in manuscript
preparation.
REFERENCES
1. Zheng, G.; Dwoskin, L. P.; Crooks, P. A. Vesicular monoamine transporter 2:
Role as a novel target for drug development. AAPS J. 2006, 8, E682–E692.
2. Roberts, M. S.; McLean, S.; Milligan, K. S.; Galloway, H. M. The pharmaco-
kinetics of tetrabenazine and its hydroxy metabolite in patients treated for
involuntary movement disorders. Eur. J. Clin. Pharmacol. 1986, 29, 703–708.
3. Kilbourn, M. R.; Lee, L. C.; Heeg, M. J.; Jewett, D. M. Absolute configura-
tion of (þ)-a-dihydrotetrabenazine, an active metabolite of tetrabenazine.
Chirality 1997, 9, 59–62.
4. Whittaker, N. The synthesis of emetine and related compounds. Part VI:
Improvements in the synthesis of 3-alkyl-1,3,4,6,7,11b-hexahydro-9,10-
dimethoxy[a]quinolizine-2-ones and 3-alkyl-1,2,3,4,6,7,12,12b-octahydroindolo
[2,3-a]quinolizine-2-ones: Formation of some related diazabicyclo[3,3,1]nonanes.
J. Chem. Soc. C 1969, 85–89.
5. Lee, L. C.; Borght, T. V.; Sherman, P. S.; Frey, K. A.; Kilbourn, M. R. In
vitro and in vivo studies of benzisoquinoline ligands for the brain synaptic
vesicle monoamine transporter. J. Med. Chem. 1996, 39, 191–196.
6. Flack, H. D.; Bernardinelli, G. Reporting and evaluating absolute-structure and
absolute-configuration determinations. J. Appl. Cryst. 2000, 33, 1143–1148.
7. Mancuso, A. J.; Brownfain, D. S.; Swern, D. Structure of the dimethyl
sulfoxide–oxalyl chloride reaction product: Oxidations of heteroaromatic
and diverse alcohols to carbonyl compounds. J. Org. Chem. 1979, 44, 4148–
4150.
8. Brossi, A.; Chopard-dit-Jean, L. H.; Schneider, O. Syntheseversuche in der
Emetin-Reihe 3: Mitteilung 2-Hydroxy-hydrobenzo[a]chinolizine. Helv. Chim.
Acta 1958, 41, 1793–1806.