Preparation and evaluation of tetrabenazine enantiomers and all eight stereoisomers of dihydrotetrabenazine as VMAT2 inhibitors

Article abstract of DOI:10.1016/j.ejmech.2011.02.046

Tetrabenazine (TBZ) ((±)-1) and dihydrotetrabenazines (DHTBZ) are potent inhibitors of VMAT2. Herein, a practical chemical resolution of (±)-1 and stereoselective synthesis of all eight DHTBZ stereoisomers are described. The result of VMAT2 binding assay revealed that (+)-1 (K_i = 4.47 nM) was 8000-fold more potent than (-)-1 (K_i = 36,400 nM). Among all eight DHTBZ stereoisomers, (2R,3R,11bR)-DHTBZ ((+)-2: K_i = 3.96 nM) showed the greatest affinity for VMAT2. The (3R,11bR)-configuration appeared to play a key role for VMAT2 binding. In summary, (+)-1, (+)-2, and their derivatives warrant further studies in order to develop more potent and safer drugs for the treatment of chorea associated with Huntington's disease and other hyperkinetic disorders.

Full text of DOI:10.1016/j.ejmech.2011.02.046

European Journal of Medicinal Chemistry 46 (2011) 1841e1848
Contents lists available at ScienceDirect
European Journal of Medicinal Chemistry
journal homepage: http://www.elsevier.com/locate/ejmech
Original article
Preparation and evaluation of tetrabenazine enantiomers and all eight
stereoisomers of dihydrotetrabenazine as VMAT2 inhibitors
Zhangyu Yao a, Xueying Wei a, Xiaoming Wu a, Jonathan L. Katz b, Theresa Kopajtic b, Nigel H. Greig c,
,
Hongbin Sun a
*
a Center for Drug Discovery, College of Pharmacy, China Pharmaceutical University, 24 Tongjiaxiang, Nanjing 210009, China
b Psychobiology Section, Intramural Research Program, National Institute on Drug Abuse, National Institutes of Health, Baltimore, MD 21224, USA
c Drug Design and Development Section, Laboratory of Neurosciences, Intramural Research Program, National Institute on Aging, National Institutes of Health,
Baltimore, MD 21224, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Tetrabenazine (TBZ) ((ꢀ)-1) and dihydrotetrabenazines (DHTBZ) are potent inhibitors of VMAT2. Herein,
a practical chemical resolution of (ꢀ)-1 and stereoselective synthesis of all eight DHTBZ stereoisomers
are described. The result of VMAT2 binding assay revealed that (þ)-1 (Ki ¼ 4.47 nM) was 8000-fold more
potent than (ꢁ)-1 (Ki ¼ 36,400 nM). Among all eight DHTBZ stereoisomers, (2R,3R,11bR)-DHTBZ ((þ)-2:
Ki ¼ 3.96 nM) showed the greatest affinity for VMAT2. The (3R,11bR)-configuration appeared to play
a key role for VMAT2 binding. In summary, (þ)-1, (þ)-2, and their derivatives warrant further studies in
order to develop more potent and safer drugs for the treatment of chorea associated with Huntington’s
disease and other hyperkinetic disorders.
Received 23 November 2010
Received in revised form
16 February 2011
Accepted 17 February 2011
Available online 23 February 2011
Keywords:
Tetrabenazine enantiomers
Dihydrotetrabenazine stereoisomers
Resolution
Ó 2011 Elsevier Masson SAS. All rights reserved.
Huntington’s chorea
VMAT2
Hyperkinetic disorders
1. Introduction
its active metabolites is to deplete the levels of monoamines (e.g.
dopamine, serotonin, and norepinephrine) within the central
Tetrabenazine (TBZ, Fig. 1) is a drug used for the treatment of
Huntington’s chorea and other hyperkinetic movement disorders
[1e3]. TBZ ((ꢀ)-1) was first approved in Europe in 1971 for the
treatment of Huntington’s disease (HD), and in 2008, TBZ gained
FDA approval for the treatment of chorea associated with HD. TBZ
has two chiral centers, and hence can theoretically exist in four
isomeric forms, however, the marketed drug TBZ (Nitoman, Xena-
zine) is only a racemic mixture of (þ)-(3R,11bR)-TBZ ((þ)-1) and
(ꢁ)-(3S,11bS)-TBZ ((ꢁ)-1) due to the thermodynamic instability of
the cis-isomer of TBZ.
In vivo, (ꢀ)-1 is rapidly and extensively metabolized by first-pass
metabolic reduction of the 2-keto group, providing four isomers of
dihydrotetrabenazines (DHTBZ), which include (2R,3R,11bR)-
DHTBZ ((þ)-2), (2S,3S,11bS)-DHTBZ ((ꢁ)-2), (2S,3R,11bR)-DHTBZ
((þ)-3), and (2R,3S,11bS)-DHTBZ ((ꢁ)-3) (Fig. 2) [4e6]. The four
TBZ metabolites are likely the major pharmacologically active
substances in vivo. The primary pharmacological action of TBZ and
nervous system by inhibiting the human vesicular monoamine
transporter 2 (VMAT2) [7e9]. This transporter is predominantly
expressed in the brain, and translocates monoamines from cyto-
plasm into synaptic vesicles, where they are both stored and
protected from metabolism prior to their synaptic release. Multiple
lines of evidence indicate that the binding of the TBZ metabolites to
VMAT2 is stereospecific [10e12]. In the VMAT2 binding assay, (þ)-2
was more potent (Ki ¼ 1.9 nM) [13] than (ꢁ)-2 (Ki ¼ 202 nM) or
(ꢁ)-3 (Ki ¼ 714 nM). On the other hand, besides the four DHTBZ
metabolites with relative trans-configuration at C-11b vs. C-3
(Fig. 2), there should exist four DHTBZ stereoisomers with relative
cis-configuration at C-11b vs. C-3 (Fig. 3) as three chiral centers exist
in the core structure of DHTBZ. The four isomers of 3,11b-cis-DHTBZ
include (2R,3S,11bR)-DHTBZ ((þ)-4), (2S,3R,11bS)-DHTBZ ((ꢁ)-4),
(2S,3S,11bR)-DHTBZ ((þ)-5), and (2R,3R,11bS)-DHTBZ ((ꢁ)-5)
(Fig. 3). In this regard, Clarke et al. claimed the preparation of the
four isomers of 3,11b-cis-DHTBZ by using Mosher’s ester method-
ology; however, the absolute configuration and optical purity of the
four isomers were not reported [14]. Interestingly, two of the four
3,11b-cis-DHTBZ isomers exhibited high affinity for VMAT2 but
negligible binding at dopamine receptors, indicating that they
* Corresponding author. Tel./fax: þ86 25 83271198.
E-mail address: hbsun2000@yahoo.com (H. Sun).
0223-5234/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.ejmech.2011.02.046

1842
Z. Yao et al. / European Journal of Medicinal Chemistry 46 (2011) 1841e1848
8
O
7
O
O
O
O
6
N
N
O
11b
1
N
4
H
H
11
H
3
2
OH
OH
O
(2S, 3R, 11bS)-DHTBZ
(2R, 3S, 11bR)-DHTBZ
TBZ (3,11b-trans)
(+)-4
(-)-4
O
O
O
O
O
O
N
N
O
N
N
O
O
H
H
H
H
OH
O
OH
(3S,11bS)-TBZ
(2S, 3S, 11bR)-DHTBZ
(2R, 3R, 11bS)-DHTBZ
(3R,11bR)-TBZ
(-)-1
(+)-1
5
5
(+)-
(-)-
Fig. 1. Structures of tetrabenazine (TBZ, (ꢀ)-1) and tetrabenazine enantiomers.
Fig. 3. Structures of DHTBZ stereoisomers with cis-configuration at C-3 vs. C-11b.
not suitable for large-scale preparation [13]. Surprisingly, since the
initial launch of (ꢀ)-1 in 1971, direct chemical resolution of (ꢀ)-TBZ
has seen little success [16]. Recently, Yu et al. [17] reported the
preparation of (þ)-1 and (ꢁ)-1 by a chemical resolution procedure
that is similar to ours [18]. This publication prompts us to report our
might be unlikely to give rise to the dopaminergic side effects
encountered with (ꢀ)-1 [14]. Moreover, they also lacked the
unwanted sedative effects associated with (ꢀ)-1 [14]. The stereo-
specific binding profile of DHTBZ indicates that TBZ enantiomers
and DHTBZ stereoisomers may have different pharmacological
and/or toxicological profiles, which still remain to be determined.
Therefore, it is highly desirable to develop a practical access to
optically pure TBZ enantiomers and DHTBZ stereoisomers
to support the development of more potent and safer drugs for the
treatment of Huntington’s disease and hyperkinetic disorders.
Rishel et al. completed the first asymmetric synthesis of (þ)-1
by the use of Sodeoka’s palladium-catalyzed asymmetric malonate
addition as the key step [15]. High optical purity (ee >97%) and
reasonable yields were achieved in this eight-step synthesis.
However, this method suffers from several practical limitations,
such as the use of an expensive chiral catalyst, long reaction
sequences, and a low total yield, and thus is not suitable for
industrial production. Recently, Boldt et al. developed a synthesis of
results describing
a practical route to TBZ enantiomers via
a chemical resolution and the efficient synthesis of all eight DHTBZ
stereoisomers. This allowed our assay of the VMAT2 binding affinity
and SAR analysis of (ꢀ)-TBZ, TBZ enantiomers and all eight DHTBZ
stereoisomers.
2. Results and discussion
2.1. Chemistry
2.1.1. Chemical resolution of tetrabenazine
In our search for an efficient method to resolve TBZ, which was
readily prepared according to literature procedures [1,19], we
investigated several variations of conditions, including the
resolving agents, the solvents and reflux time, crystallization
temperature, and seed crystals, etc. After extensive
experimentation, resolution of (ꢀ)-1 was achieved by using (1S)-
(þ)-1 from the resolution of
a-DHTBZ, followed by Swern oxidation
of the resulting (þ)- -DHTBZ [16]. This indirect methodology
a
suffers from a low atom-economy due to additional reduction/
oxidation steps. Another access to optically pure TBZ employs the
resolution of (ꢀ)-1 with preparative chiral HPLC that, similarly, is
(þ)-10-camphorsulfonic acid ((þ)-CSA) as
a resolving agent
(Scheme 1) [18]. Other chiral acids, including (þ)-tartaric acid,
(þ)-dibenzoyl-
D
-tartaric acid, and (2R,3R)-20-nitrotartranilic acid,
gave disappointing results. Consequently, (þ)-CSA was chosen as
the resolving agent for further optimization studies, which subse-
quently showed that the molar equivalents of the resolving agent,
the resolving solvents and crystallization temperature were key
factors to accomplish successful resolution of (ꢀ)-1. Specifically, the
equivalents of (þ)-CSA played a key role in the efficient resolution
of (ꢀ)-1. When 1 equivalent of (þ)-CSA was used, only moderate ee
(approximately 40%) was obtained after one-time crystallization of
the (þ)-CSA.(þ)-1 salt from acetone, even though a high ee could be
achieved by repeated recrystallization of this salt in acetone. When
the amount of (þ)-CSA was reduced to 0.5 equivalent, the initial
batch of crystals of the (þ)-CSA.(þ)-1 salt precipitating from
acetone possessed up to a 96% ee upon the release of (þ)-1 as a free
base. One-time recrystallization of this salt in acetone led to more
than a 98% ee (see Supporting information). Among the various
resolving solvents, acetone gave the best results in terms of yield
and high ee value. The proportion of acetone vs. (ꢀ)-1 was found to
be critical for successful resolution, and the best result was
obtained when the ratio of (ꢀ)-1 to acetone was in the order of
1 g:13 mL. Crystallization temperature also played an important
O
O
O
O
N
N
H
H
OH
OH
(2S,3S,11bS)-DHTBZ
(2R,3R,11bR)-DHTBZ
(+)-2
(-)-2
O
O
O
N
N
O
H
H
OH
OH
(2S,3R,11bR)-DHTBZ
(2R,3S,11bS)-DHTBZ
(+)-3
(-)-3
Fig. 2. Structures of the TBZ metabolites (DHTBZ) with trans-configuration at C-3 vs.
C-11b.

Z. Yao et al. / European Journal of Medicinal Chemistry 46 (2011) 1841e1848
1843
O
N
O
H
O
a
b
( )-1
O
O
O
O
(+)-CSA
(-)-CSA
N
N
H
H
O
O
(-)-6
(+)-6
c
c
O
O
N
N
O
O
O
H
H
O
(-)-1
(+)-1
Scheme 1. Resolution of tetrabenazine ((ꢀ)-1). Reagents and conditions: (a) (1S)-(þ)-10-camphorsulfonic acid, acetone; (b) (1R)-(ꢁ)-10-camphorsulfonic acid, acetone; (c) NH4OH,
MeOH.
function. When crystallization temperature was kept between 15
and 20 ꢂC, high ee values and high yields could be achieved. In
the event that the crystallization temperature was below 10 ꢂC, the
enantiomeric excess dropped significantly. Using (ꢁ)-CSA in place
of (þ)-CSA as a resolving agent, (ꢁ)-1 was obtained by the same
methodology (Scheme 1).
Stereoselective reduction of (þ)-1 with L-selectrideÒ at 0 ꢂC fur-
nished (þ)-3 in 43% yield. Dehydration of (þ)-3 with PCl5 at 0 ꢂC
afforded alkene (þ)-7 in 57% yield. Thereafter, epoxidation of (þ)-7
with m-chloroperoxybenzoic acid (mCPBA) in the presence of
perchloric acid gave epoxide (þ)-8. The relative configuration of
(þ)-8 was confirmed by a 2D-NMR (NOESY) study (see Supporting
information), and demonstrated that the hydrogen atoms at the C-2
and C-11b positions both had nuclear overhauser effects (NOE) with
the same hydrogen atom at the C-1 position. Regioselective and
stereoselective reductive ring-opening of (þ)-8 with borane
[22,23], followed by oxidation with hydrogen peroxide in the
presence of NaOH, afforded (þ)-4 in 31% yield. Regioselective and
stereoselective hydroborationeoxidation of (þ)-7 provided alcohol
(þ)-5 in 41% yield. In a similar fashion, (2S,3S,11bS)-DHTBZ ((ꢁ)-2),
(2R,3S,11bS)-DHTBZ ((ꢁ)-3), (2S,3R,11bS)-DHTBZ ((ꢁ)-4), and
(2R,3R,11bS)-DHTBZ ((ꢁ)-5) were synthesized starting with (ꢁ)-1
(Scheme 3).
Based on the premise of the existence of an interconversion
between benzo[a]quinolizine and isoquinolinium upon exposure to
an acid [20], we hypothesized that (ꢀ)-1 could be recovered from
the resolution mother liquid upon acid-catalyzed racemization, and
then resolved again. As expected, after adding additional (þ)-CSA to
the resolution mother liquid and refluxing overnight, complete
racemization took place, and when the reaction mixture was cooled
to 15 ꢂC with stirring, the camphorsulfonate salt of (þ)-1 was
formed as crystals with a moderate ee value. The resulting salt was
further purified by repeated recrystallization with acetone until
more than 98% ee was reached upon release of the (þ)-1 as a free
base. This racemizationeresolution process could be repeated for
several times leading to a high-resolution yield.
2.2. VMAT2 binding assay and SAR analysis
2.1.2. Synthesis of eight DHTBZ stereoisomers
Synthesis of (2R,3R,11bR)-DHTBZ ((þ)-2), (2S,3R,11bR)-DHTBZ
((þ)-3), (2R,3S,11bR)-DHTBZ ((þ)-4), and (2S,3S,11bR)-DHTBZ
((þ)-5) are shown in Scheme 2. Reduction of (þ)-1 with NaBH4 [21]
yielded a mixture of (2R,3R,11bR)-DHTBZ ((þ)-2) and (2S,3R,11bR)-
DHTBZ ((þ)-3) in a 4:1 ratio. It proved extremely difficult to purify
either (þ)-2 or (þ)-3 by column chromatography or recrystalliza-
tion, although (ꢀ)-2 could be separated from (ꢀ)-3 by recrystalli-
zation of the crude product resulting from NaBH4 reduction of
(ꢀ)-1 [21]. When borane was used as a reducing reagent and the
reaction temperature was maintained at ꢁ20 ꢂC, good stereo-
selectivity was achieved to provide a 19:1 mixture of (þ)-2 and
(þ)-3, which was readily purified by recrystallization from aceto-
neewater to afford pure (þ)-2 (see Supporting information).
Following Clarke’s methodology [14] for the generation of (ꢀ)-3,
(ꢀ)-4 and (ꢀ)-5 by starting with (ꢀ)-1, stereoselective preparation
of (þ)-3, (þ)-4 and (þ)-5 was achieved from (þ)-1 (Scheme 2).
Eleven compounds, including (ꢀ)-TBZ, (þ)-TBZ, (ꢁ)-TBZ, and
eight DHTBZ stereoisomers, were evaluated for VMAT2 binding
affinity by quantifying their ability to displace [3H]dihydrote-
trabenazine ([3H]DHTBZ) from rat striatum [17]. The assay results
are summarized in Table 1, and VMAT2 binding affinity is described
as a dissociation constant (Ki) value.
As expected, (ꢀ)-TBZ ((ꢀ)-1) exhibited
a high affinity
(Ki ¼ 7.62 ꢀ 0.20 nM) for VMAT2, and this value was in the range of
the literature value [17]. Surprisingly, whereas (þ)-TBZ ((þ)-1)
possessed a high affinity (Ki ¼ 4.47 ꢀ 0.21 nM) for VMAT2, the
(ꢁ)-form of TBZ ((ꢁ)-1) (Ki ¼ 36,400 ꢀ 4560 nM) proved to be
8000-fold less potent than its (þ)-enantiomer ((þ)-1). In contrast,
in the previous study [17], (ꢁ)-TBZ proved only 3-fold less potent
than (þ)-TBZ. In this prior study, the VMAT2 affinity of (ꢁ)-TBZ and
(þ)-TBZ was assessed in the form of their camphorsulfonate salts
rather than their corresponding free bases, and although X-ray

1844
Z. Yao et al. / European Journal of Medicinal Chemistry 46 (2011) 1841e1848
O
O
O
a
N
O
N
O
H
H
1
(+)-2
(+)-
OH
b
O
O
O
O
N
N
N
c
H
H
(+)-7
3
(+)-
OH
e
d
e
O
O
O
O
N
H
H
O
(+)-8
(+)-5
OH
O
O
N
H
OH
(+)-4
Scheme 2. Synthesis of (þ)-DHTBZ stereoisomers (þ)-2, (þ)-3, (þ)-4 and (þ)-5. Reagents and conditions: (a) BH3eMe2S, THF, ꢁ20 ꢂC; (b) L-SelectrideÒ, EtOH/THF, 0 ꢂC; (c) PCl5,
CH2Cl2; (d) HClO4, mCPBA, MeOH; (e) i. BH3, THF; ii. NaOH, H2O2.
crystallographic analysis and optical rotation were provided and
matched the literature [16], their optical purity was not reported.
There is discrepancy in the VMAT2 Ki values of TBZ metabolites.
Kilbourn et al. reported that (2R,3R,11bR)-DHTBZ ((þ)-2)
(Ki ¼ 0.97 ꢀ 0.48 nM) was 2000-fold more potent than (2S,3S,11bS)-
affinity. By contrast, compounds with a (3S,11bR) configuration
demonstrated moderate to weak potency (e.g. (þ)-4: Ki ¼ 71.1
ꢀ 6.66 nM; (þ)-5: Ki ¼ 593 ꢀ 69.7 nM). Interestingly, all the
compounds with an 11bS configuration (e.g. (ꢁ)-1 Ki ¼ 36,400
ꢀ 4560 nM; (ꢁ)-2: Ki ¼ 23,700 ꢀ 2350 nM; (ꢁ)-3: Ki ¼ 2460 ꢀ
333 nM;(ꢁ)-4: Ki ¼ 4630 ꢀ 350nM; (ꢁ)-5:Ki ¼ 1253 ꢀ314 nM) were
essentially inactive. In summary, all dextrorotatory enantiomers
exhibited dramatically more potent VMAT2 binding affinity than
their corresponding levorotatory isomers.
DHTBZ ((ꢁ)-2) (Ki ¼ 2.2 ꢀ 0.3
mM) in their VMAT2 binding assay [10],
whereas Gano claimed that (þ)-2 (Ki ¼ 1.9 nM) showed only 100-
fold more potent binding affinity than (ꢁ)-2 (Ki ¼ 202 nM) [13]. In
this study, (þ)-2 (Ki ¼ 3.96 ꢀ 0.40 nM) proved to be 6000-fold more
potent than (e)-2 (Ki ¼ 23.70 ꢀ 2.35
mM), which is in line with Kil-
bourn’s report [10]. In Gano’s study [13], (2S,3R,11bR)-DHTBZ
((þ)-3) exhibited a potent VMAT2 affinity (Ki ¼ 13 nM) that is similar
to our value (Ki ¼ 13.4 ꢀ 1.36 nM). In contrast with Gano’s result on
(2R,3S,11bS)-DHTBZ ((ꢁ)-3) (Ki ¼ 714 nM), in our hands ((ꢁ)-3) had
Ki value of 2460 ꢀ 333 nM. Amongthe four DHTBZisomerswith a cis-
configuration at C-3 vs. C-11b, (2R,3S,11bR)-DHTBZ ((þ)-4)
(Ki ¼ 71.1 ꢀ 6.66 nM) showed the most potent binding affinity,
whereas (2S,3R,11bS)-DHTBZ ((ꢁ)-4) (Ki ¼ 4630 ꢀ 350 nM) was the
least potent.
3. Conclusion
In the current study, a practical chemical resolution of tetrabe-
nazine with camphorsulfonic acids has been carried out to produce
optically pure tetrabenazine enantiomers in high yields. With both
tetrabenazine enantiomers in hand, all eight stereoisomers of
dihydrotetrabenazines were synthesized with high stereo-
selectivity. Analysis of VMAT2 binding revealed that (þ)-tetrabe-
nazine ((þ)-1) was 8000-fold more potent than (ꢁ)-tetrabenazine
((ꢁ)-1). Among all eight dihydrotetrabenazine stereoisomers,
(2R,3R,11bR)-dihydrotetrabenazine ((þ)-2: Ki ¼ 3.96 nM) showed
the highest affinity for VMAT2, and proved slightly more potent
than (þ)-tetrabenazine ((þ)-1: Ki ¼ 4.47 nM). The (3R,11bR)-
SAR analysis revealed that the (3R,11bR)-configuration made the
most important contribution to VMAT2 binding affinity, and all the
corresponding compounds ((þ)-1: Ki ¼ 4.47 ꢀ 0.21 nM; (þ)-2:
Ki ¼ 3.96 ꢀ 0.40 nM; (þ)-3: Ki ¼ 13.4 ꢀ 1.36 nM) exhibited very high

Z. Yao et al. / European Journal of Medicinal Chemistry 46 (2011) 1841e1848
1845
O
O
O
a
N
O
N
O
H
H
1
(-)-2
(-)-
OH
b
O
O
O
O
N
N
N
c
H
H
(-)-7
3
(-)-
OH
e
d
e
O
O
O
O
N
H
H
O
(-)-8
(-)-5
OH
O
O
N
H
OH
4
(-)-
Scheme 3. Synthesis of (ꢁ)-DHTBZ stereoisomers (ꢁ)-2, (ꢁ)-3, (ꢁ)-4 and (ꢁ)-5. Reagents and conditions: (a) BH3eMe2S, THF, ꢁ20 ꢂC; (b) L-SelectrideÒ, EtOH/THF, 0 ꢂC; (c) PCl5,
CH2Cl2; (d) HClO4, mCPBA, MeOH; (e) i. BH3, THF; ii. NaOH, H2O2.
configuration played a key role in delineating the affinity of tetra-
benazine and dihydrotetrabenazines binding to VMAT2. In
summary, (þ)-1, (þ)-2, and their derivatives warrant further
studies in order to develop more potent and safer drugs to
ameliorate chorea associated with Huntington’s disease and other
hyperkinetic disorders.
4. Experimental
4.1. Chemistry
All commercially available solvents and reagents were used
without further purification. Melting points were determined with
a Büchi capillary apparatus and were not corrected. 1H and 13C NMR
spectra were recorded on an ACF* 300Q Bruker, spectrometer in
CDCl3, with Me4Si as the internal reference, or in d6-DMSO. Low-
and high-resolution mass spectra (LRMS and HRMS) were recorded
in electron impact mode. Reactions were monitored by TLC on Silica
Gel 60 F254 plates (Qingdao Ocean Chemical Company, China).
Column chromatography was carried out on silica gel
(200e300 mesh, Qingdao Ocean Chemical Company, China).
Table 1
VMAT2 binding affinity of TBZ, TBZ enantiomers, and DHTBZ stereoisomers.
No.
Compound
Ki ꢀ SEM (nM)a
(ꢀ)-1
(þ)-1
(ꢁ)-1
(þ)-2
(ꢁ)-2
(þ)-3
(ꢁ)-3
(þ)-4
(ꢁ)-4
(þ)-5
(ꢁ)-5
(ꢀ)-TBZ
7.62 ꢀ 0.20
4.47 ꢀ 0.21
36,400 ꢀ 4560
3.96 ꢀ 0.40
23,700 ꢀ 2350
13.4 ꢀ 1.36
2460 ꢀ 333
71.1 ꢀ 6.66
4630 ꢀ 350
593 ꢀ 69.7
(þ)-TBZ
(ꢁ)-TBZ
(2R,3R,11bR)-DHTBZ
(2S,3S,11bS)-DHTBZ
(2S,3R,11bR)-DHTBZ
(2R,3S,11bS)-DHTBZ
(2R,3S,11bR)-DHTBZ
(2S,3R,11bS)-DHTBZ
(2S,3S,11bR)-DHTBZ
(2R,3R,11bS)-DHTBZ
4.1.1. (3R,11bR)-Tetrabenazine ((þ)-1)
A warm solution of tetrabenazine ((ꢀ)-1) (17 g, 53.6 mmol) in
acetone (230 mL) was treated with (1S)-(þ)-10-camphorsulfonic
acid (6.2 g, 26.7 mmol). After cooling to room temperature with
stirring, the mixture was left at room temperature for 48 h and the
resulting crystals were collected (8 g, ee 96.5% for released free
base). These crystals were then recrystallized from acetone (80 mL)
to give (1S)-(þ)-10-camphorsulfonic acid salt of (þ)-(3R,11bR)-
1253 ꢀ 314
a
Ki values represent experimental n ¼ 3 or greater (p < 0.05). All experiments
were undertaken in triplicate.

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Z. Yao et al. / European Journal of Medicinal Chemistry 46 (2011) 1841e1848
tetrabenazine (6.5 g, ee 98.9% for released free base). This salt was
dissolved in MeOH (26 mL) and neutralized with NH4OH to pH ¼ 8.
Thereafter water (190 mL) was added, and the solids were collected
to afford (þ)-1 as a white solid (3.4 g), ee 98.7% (Chiral HPLC
analytical conditions: Chiralpak IC column, 4.6 mm ꢃ 250 mm,
1.14e1.01 (m, 1H), 0.94 (d, 3H, J ¼ 6.5 Hz), 0.92 (d, 3H, J ¼ 6.5 Hz);
13C NMR (75 MHz, CDCl3):
147.6, 147.3, 129.4, 126.5, 111.6, 108.1,
74.6, 60.9, 60.1, 56.0, 55.9, 51.9, 41.7, 40.6, 39.7, 29.2, 25.4, 24.1, 21.8;
ESI-MS m/z 320.3 [M þ H]þ; HRMS calcd for C19H30NO3 [M þ H]þ m/
z 320.2226, found 320.2240.
d
eluting with 100% EtOH þ 0.1% Et2NH, flow rate 0.5 mL/min, oven
22
temperature 35 ꢂC, detection UV 220 nm). [
a
]
D
¼ þ66.3ꢂ (c 1.04,
4.1.4. (2S,3S,11bS)-3-Isobutyl-9,10-dimethoxy-1,3,4,6,7,11b-
hexahydro-2H-pyrido[2,1-a]isoquinolin-2-ol ((ꢁ)-2)
MeOH). 1H NMR (300 MHz, CDCl3):
d 6.62 (s, 1H), 6.55 (s, 1H), 3.85
(s, 3H), 3.83 (s, 3H), 3.51 (d, 1H, J ¼ 10.0 Hz), 3.28 (dd, 1H, J ¼ 6.2,
11.5 Hz), 3.14e3.09 (m, 2H), 2.89 (dd,1H, J ¼ 3.2,13.7 Hz), 2.75e2.49
(m, 4H), 2.35 (t, 1H, J ¼ 11.6 Hz), 1.85e1.66 (m, 2H), 1.07e1.01 (m,
Reduction of (3S,11bS)-tetrabenazine with boraneemethyl
sulfide in THF was carried out following the procedure for prepa-
ration of (2R,3R,11bR)-dihydrotetrabenazine to afford (ꢁ)-2 as
a white solid, mp: 98e100 ꢂC, ee >99% (Chiral HPLC analytical
conditions: Chiralpak IC column, 4.6 mm ꢃ 250 mm, eluting with
1H), 0.90 (m, 6H); 13C NMR (75 MHz, CDCl3):
d 210.0, 147.8, 147.5,
128.6, 126.0, 111.5, 107.9, 62.5, 61.5, 56.0, 55.9, 50.5, 47.6, 47.5, 35.0,
29.3, 25.4, 23.2, 22.1; ESI-MS m/z 318.2 [M þ H]þ, 340.2 [M þ Na]þ.
The acetone mother liquid from the above resolution was
evaporated to dryness to give a yellow solid (14 g) that was again
dissolved in acetone (240 mL), and (1S)-(þ)-10-camphorsulfonic
acid (7 g, 30.1 mmol) was added. After stirring at reflux overnight,
the mixture was cooled with stirring and left at room temperature
for 48 h. The resulting crystals were collected (8.6 g, ee 70.8% for
released free base), and recrystallized twice from acetone to give
optically pure (1S)-(þ)-10-camphorsulfonic acid salt of
(þ)-(3R,11bR)-tetrabenazine as a white solid (4.1 g, ee 98.7% for
released free base). The salts were dissolved in MeOH (17 mL) and
neutralized with NH4OH to pH ¼ 8. Water (100 mL) was then
100% MeOH þ 0.1% Et2NH, flow rate 0.5 mL/min, oven temperature
25
35 ꢂC, detection UV 220 nm). [
NMR (300 MHz, CDCl3):
a
]
¼ ꢁ56.1ꢂ (c 0.23, MeOH). 1H
D
d 6.67 (s, 1H), 6.58 (s, 1H), 3.87 (s, 6H),
3.43e3.34 (m, 1H), 3.14e2.96 (m, 4H), 2.65e2.54 (m, 2H), 2.53
(ddd, 1H, J ¼ 11.1, 11.1, 3.6 Hz), 2.01e1.94 (m, 1H), 1.77e1.64 (m, 2H),
1.62e1.36 (m, 2H), 1.10e0.99 (m, 1H), 0.95e0.70 (m, 6H); 13C NMR
(75 MHz, CDCl3):
d 147.50, 147.23, 129.36, 126.42, 111.49, 107.96,
74.63, 60.88, 60.08, 55.94, 55.84, 51.89, 41.65, 40.58, 39.70, 29.18,
25.35, 24.12, 21.76; ESI-MS m/z 320.2 [M þ H]þ, 342.2 [M þ Na]þ;
HRMS calcd for C19H30NO3 m/z 320.2226, found 320.2242.
4.1.5. (2S,3R,11bR)-3-Isobutyl-9,10-dimethoxy-1,3,4,6,7,11b-
hexahydro-2H-pyrido[2,1-a]isoquinolin-2-ol ((þ)-3)
added, and the solids were collected to afford (þ)-1 as a white solid
(2 g), ee >99%, [
a]
¼ þ66.5ꢂ (c 0.39, MeOH).
L-SelectrideÒ (1 M, 126 mL) was added dropwise to a solution of
(3R,11bR)-tetrabenazine ((þ)-1) (14 g, 44.2 mmol, ee >99%) in EtOH
(70 mL) and THF (70 mL) at 0 ꢂC. After stirring at 0 ꢂC for 40 min, the
mixture was poured into 400 mL of ice-water and extracted with
ether. The organic layer was washed with brine, dried over Na2SO4,
and concentrated under vacuum (<25 ꢂC, protected from light) to
give a yellow oil. To a solution of the crude product in EtOH (25 mL)
was added 1 equivalent of methanesulfonic acid, and the resulting
mixture was stirred overnight at room temperature to give a crude
methanesulfonic acid salt (9.2 g), which was recrystallized from
EtOH to provide methanesulfonic acid salt of (2S,3R,11bR)-dihy-
drotetrabenazine (8.14 g). This salt was dissociated with NH4OH to
afford (þ)-3 as a white solid (6.05 g, 42.9%), ee 100% (Chiral HPLC
analytical conditions: Chiralpak IC column, 4.6 mm ꢃ 250 mm,
22
D
The above racemization-resolution process was repeated three
times, and collectively 10.27 g of (þ)-1 (ee >98%) was obtained
from the resolution of 17 g of tetrabenazine.
4.1.2. (3S,11bS)-Tetrabenazine ((ꢁ)-1)
Resolution of tetrabenazine with (1R)-(ꢁ)-10-camphorsulfonic
acid was carried out following the procedure for preparation of
(3R,11bR)-tetrabenazine to give (ꢁ)-1 as a white solid, ee 99.0%
(Chiral HPLC analytical conditions: Chiralpak IC column,
4.6 mm ꢃ 250 mm, eluting with 100% EtOH þ 0.1% Et2NH, flow rate
0.5 mL/min, oven temperature 35 ꢂC, detection UV 220 nm).
25
[
a]
¼ ꢁ63.7ꢂ (c 0.27, MeOH), 1H NMR (300 MHz, CDCl3):
d 6.62 (s,
D
1H), 6.55 (s, 1H), 3.85 (s, 3H), 3.82 (s, 3H), 3.52 (m, 1H), 3.28 (dd, 1H,
J ¼ 6.3, 11.5 Hz), 3.20e3.05 (m, 2H), 2.89 (dd, 1H, J ¼ 3.0, 13.6 Hz),
2.79e2.65 (m, 2H), 2.48e2.63 (m, 2H), 2.35 (t, 1H,
J ¼ 11.6 Hz),1.85e1.76 (m,1H),1.72e1.57 (m,1H),1.13e0.96 (m,1H,),
eluting with 25% EtOH þ 75% n-hexane, flow rate 0.5 mL/min, oven
25
temperature 35 ꢂC, detection UV 220 nm). [
a
]
¼ þ98.6ꢂ (c 0.27,
D
MeOH).1H NMR (300 MHz, CDCl3):
d 6.66 (s,1H), 6.56 (s,1H), 4.08 (d,
0.90 (m, 6H); 13C NMR (75 MHz, CDCl3):
d
210.0, 147.8, 147.5, 128.6,
1H, J ¼ 2.7 Hz), 3.83 (s, 6H), 3.48 (d, 1H, J ¼ 11.6 Hz), 3.16e3.05 (m,
1H), 2.97e2.92 (m, 1H), 2.67 (dd, 1H, J ¼ 4.2, 11.3 Hz), 2.60e2.36 (m,
7H), 2.00e1.94 (m,1H),1.73e1.62 (m, 2H),1.31e1.10(m, 2H), 0.93 (d,
126.1, 111.5, 107.9, 62.4, 61.5, 56.0, 55.9, 50.5, 47.6, 47.5, 35.0, 29.3,
25.4, 23.2, 22.1; ESI-MS m/z 318.2 [M þ H]þ, 340.2 [M þ Na]þ.
3H, J ¼ 5.5 Hz), 0.91 (d, 3H, J ¼ 5.5 Hz); 13C NMR (75 MHz, CDCl3)
d:
4.1.3. (2R,3R,11bR)-3-Isobutyl-9,10-dimethoxy-1,3,4,6,7,11b-
hexahydro-2H-pyrido[2,1-a]isoquinolin-2-ol ((þ)-2)
147.4,147.1,130.0, 126.9, 111.6,108.1, 68.0, 56.4, 56.3, 56.0, 55.8, 52.5,
39.2, 38.9, 37.8, 29.2, 24.9, 23.0, 22.9; ESI-MS m/z 320.3 [M þ H]þ.
HRMS calcd for C19H30NO3 [M þ H]þ m/z 320.2226, found 320.2242.
To a solution of (3R,11bR)-tetrabenazine (1.0 g, 3.2 mmol) in THF
(11 mL) was added dropwise 2 M boraneeMe2SeTHF (3.2 mL,
6.4 mmol) at ꢁ20 ꢂC. After stirring at this temperature for 2 h,
ammonia water (11 mL) was added, and the mixture was warmed
to 35 ꢂC and stirred overnight. The mixture was then diluted with
brine, and extracted with ether. The organic layer was washed
with brine, dried over Na2SO4, and concentrated under vacuum to
give a white solid. This crude product was recrystallized twice from
acetone-water to afford (þ)-2 as a white solid (0.64 g, 64%), mp:
100e102 ꢂC, ee >99% (Chiral HPLC analytical conditions: Chiralpak
IC column, 4.6 mm ꢃ 250 mm, eluting with 100% MeOH þ 0.1%
4.1.6. (2R,3S,11bS)-3-Isobutyl-9,10-dimethoxy-1,3,4,6,7,11b-
hexahydro-2H-pyrido[2,1-a]isoquinolin-2-ol ((ꢁ)-3)
Reduction of (3S,11bS)-tetrabenazine with L-SelectrideÒ was
carried out following the procedure for preparation of (þ)-3 to
afford (ꢁ)-3 as a white solid, ee 100% (Chiral HPLC analytical
conditions: Chiralpak IC column, 4.6 mm ꢃ 250 mm, eluting with
25% EtOH þ 75% n-hexane, flow rate 0.5 mL/min, oven temperature
25
35 ꢂC, detection UV 220 nm). [
NMR (300 MHz, CDCl3):
a
]
D
¼ ꢁ93.8ꢂ (c 0.23, MeOH). 1
H
d 6.68 (s, 1H), 6.59 (s, 1H), 4.09 (bs, 1H),
Et2NH, flow rate 0.5 mL/min, oven temperature 35 ꢂC, detection UV
3.85 (s, 6H), 3.50 (d, 1H, J ¼ 12 Hz), 3.12 (m, 1H), 2.99e2.96 (m, 1H),
2.71e2.53 (m, 3H), 2.46e2.38 (m, 2H), 2.00 (m, 1H), 1.76e1.66 (m,
2H), 1.31e1.15 (m, 2H), 0.96e0.92 (m, 6H); 13C NMR (75 MHz,
21
220 nm). [
CDCl3):
a]
¼ þ58.93ꢂ (c 0.6, MeOH). 1H NMR (300 MHz,
D
d
6.68 (s, 1H), 6.58 (s, 1H), 3.84 (s, 6H), 3.39 (ddd, 1H, J ¼ 4.6,
10.6, 10.6 Hz), 3.15e2.97 (m, 4H), 2.66e2.55 (m, 2H), 2.45 (ddd, 1H,
J ¼ 3.6, 11.0, 11.0 Hz), 1.98 (t, 1H, J ¼ 11.3 Hz), 1.78e1.43 (m, 5H),
CDCl3)
d: 147.39, 147.13, 130.07, 126.94, 111.56, 108.10, 68.03, 56.39,
56.35, 56.00, 55.83, 52.48, 39.20, 38.86, 37.81, 29.22, 24.87, 22.95;

Z. Yao et al. / European Journal of Medicinal Chemistry 46 (2011) 1841e1848
1847
ESI-MS m/z 320.2 [M þ H]þ; HRMS calcd for C19H30NO3 m/z
100% (Chiral HPLC analytical conditions: Chiralpak IC column,
320.2226, found 320.2246.
4.6 mm ꢃ 250 mm, eluting with 100% i-PrOH þ 0.1% Et2NH, flow
rate 0.5 mL/min, oven temperature 35 ꢂC, detection UV 220 nm).
23
4.1.7. (11bR)-1,6,7,11b-Tetrahydro-9,10-dimethoxy-3-(2-
methylpropyl)-4H-benzo[a]quinolizine ((þ)-7)
[a
]
¼ ꢁ113.33ꢂ (c 0.15, MeOH). 1H NMR (300 MHz, CDCl3)
d 6.68
D
(s, 1H), 6.58 (s, 1H), 3.84 (s, 6H), 3.60 (s, 1H), 3.16e3.02 (m, 1H),
2.91e2.76 (m, 2H), 2.63e2.53 (m, 3H), 2.21e2.17 (m, 1H), 1.96e1.42
(m, 5H), 1.29e1.19 (m, 1H), 0.89 (d, 6H, J ¼ 7.0 Hz); 13C NMR
PCl5 (8.2 g, 39.2 mmol) was added in portions over 30 min to
a solution of (þ)-3 (5 g, 15.7 mmol, ee 100%) in CH2Cl2 (50 mL) at
0
ꢂC. The mixture was stirred at 0 ꢂC for 30 min and then poured
(75 MHz, CDCl3) d 147.3, 147.2, 130.0, 127.1, 111.7, 108.0, 70.0, 57.0,
into 200 mL of ice-water. The mixture was neutralized with NH4OH
to pH ¼ 8, and extracted with CH2Cl2. The organic layer was washed
with brine, dried over Na2SO4, and concentrated under vacuum to
give a yellow oil (5.2 g). The crude product was purified by column
56.0, 55.9, 54.3, 52.7, 40.0, 39.6, 35.5, 28.4, 25.7, 23.2, 22.4; ESI-MS
m/z 320.2 [M þ H]þ; HRMS calcd for C19H30NO3 [M þ H]þ m/z
320.2226, found 320.2248.
chromatography (SiO2, CH2Cl2:CH3OH ¼ 250:1) to give (þ)-7 as
4.1.11. (8aS,9aR,10aR)-5,8,8a,9a,10,10a-Hexahydro-2,3-dimethoxy-
8a-(2-methylpropyl)-6H-benz[a]oxireno[g]quinolizine ((þ)-8)
To a solution of (þ)-7 (100 mg, 0.33 mmol) in MeOH (1.7 mL)
23
a yellow oil (2.48 g, 57%), [
a
]
¼ þ250.9ꢂ (c 0.53, MeOH). 1H NMR
D
(300 MHz, CDCl3):
d 6.65 (s, 1H), 6.57 (s, 1H), 5.50 (m, 1H), 3.85 (s,
3H), 3.84 (s, 3H), 3.35 (m, 1H), 3.22 (d, 1H, J ¼ 15.6 Hz), 3.14e3.02
(m, 2H), 2.92 (m, 1H), 2.65e2.45 (m, 3H), 2.20e2.12 (m, 1H),
1.94e1.70 (m, 3H), 0.91 (d, 3H, J ¼ 5.3 Hz), 0.89 (d, 3H, J ¼ 5.3 Hz);
was added a solution of 70% perchloric acid (28 mL, 0.33 mmol) in
MeOH (1.7 mL) and, then, 85% mCPBA (103 mg, 0.876 mmol) was
added. The reaction mixture was stirred in the dark at room
temperature for 38 h. Thereafter, the mixture was poured into
saturated aqueous sodium sulfite solution (16 mL), then water
(16 mL) and CH2Cl2 (40 mL) were added. The mixture was basified
with saturated aqueous NaHCO3, and extracted with CH2Cl2. The
organic layer was washed with brine, dried over Na2SO4, and
concentrated under vacuum to give a yellow oil (160 mg). The crude
product was purified by column chromatography (SiO2, petroleum
13C NMR (75 MHz, CDCl3):
d 147.4, 135.5, 130.2, 126.6, 120.2, 111.3,
108.6, 58.8, 58.6, 56.0, 55.8, 51.5, 45.0, 33.4, 29.0, 26.2, 22.9, 22.2;
ESI-MS m/z 302.2 [M þ H]þ; HRMS calcd for C19H28NO2 m/z
302.2120, found 302.2138.
4.1.8. (11bS)-1,6,7,11b-Tetrahydro-9,10-dimethoxy-3-(2-
methylpropyl)-4H-benzo[a]quinolizine ((ꢁ)-7)
Dehydration of (ꢁ)-3 with PCl5 was carried out following the
ether:EtOAc: NH4OH ¼ 70:10:1.4) to give (þ)-8 (49 mg, 46.7%).
¼ þ189.1ꢂ (c 0.4, MeOH). IR (KBr, cmꢁ1) 3416, 2954, 2762,
23
procedure for preparation of (þ)-7 to give (ꢁ)-7 as a yellow oil
[a
]
D
22
(62.5%), [
d
a
]
¼ ꢁ263.3ꢂ (c 0.20, MeOH). 1H NMR (300 MHz, CDCl3)
1609, 1514, 1458, 1385, 1369, 1333, 1260, 1230, 1210, 1142, 1103,
D
6.66 (s, 1H), 6.59 (s, 1H), 5.53 (s, 1H), 3.86 (s, 3H), 3.85 (s, 3H),
1022, 877, 781; 1H NMR (300 MHz, CDCl3)
d 6.57 (s, 1H), 6.54 (s, 1H),
3.43e3.41 (m, 1H), 3.31e3.09 (m, 4H), 2.70e2.58 (m, 3H), 2.27e2.18
(m, 1H), 1.93e1.71 (m, 3H), 0.91 (d, 6H, J ¼ 5.8 Hz), 0.89 (d, 6H,
3.86 (s, 6H), 3.27 (d, 1H, J ¼ 12.9 Hz), 3.16e2.92 (m, 4H), 2.60e2.49
(m, 3H), 2.40 (ddd, 1H, J ¼ 3.3, 11.4, 11.4 Hz), 2.00e1.82 (m, 2H),
1.63e1.41 (m, 2H), 0.99 (d, 3H, J ¼ 6.5 Hz), 0.97 (d, 3H, J ¼ 6.5 Hz);
J ¼ 5.3 Hz); 13C NMR (75 MHz, CDCl3)
d 147.5, 147.4, 135.6, 130.4,
126.8, 120.3, 111.4, 108.7, 59.0, 58.8, 56.1, 55.9, 51.6, 45.2, 33.2, 29.1,
26.3, 23.0, 22.4; ESI-MS m/z 302.2 [M þ H]þ; HRMS calcd for
C19H28NO2 [M þ H]þ m/z 302.2120, found 302.2138.
13C NMR (75 MHz, CDCl3)
d 147.5, 147.3, 129.3, 126.9, 111.3, 108.4,
58.4, 57.8, 57.6, 56.6, 56.0, 55.8, 51.7, 43.8, 32.4, 29.0, 25.2, 23.1, 22.9;
ESI-MS m/z 318.2 [M þ H]þ; HRMS calcd for C19H28NO3 [M þ H]þ m/
z 318.2069, found 318.2088.
4.1.9. (2S,3S,11bR)-3-Isobutyl-9,10-dimethoxy-1,3,4,6,7,11b-
hexahydro-2H-pyrido[2,1-a]isoquinolin-2-ol ((þ)-5)
4.1.12. (8aR,9aS,10aS)-5,8,8a,9a,10,10a-Hexahydro-2,3-dimethoxy-
To a solution of (þ)-7 (100 mg, 0.33 mmol) in dry THF was added
dropwise 2 M boraneeMe2SeTHF (0.73 mL, 1.46 mmol) at 0 ꢂC. The
reaction mixture was stirred at below 15 ꢂC for 48 h, and then
0.2 mL of water was added and the mixture was basified to pH ¼ 12
with 30% aqueous NaOH solution. 30% Hydrogen peroxide solution
(0.28 mL) was added and the mixture was stirred at reflux for 1.5 h.
The reaction mixture was then cooled and 10 mL of water was
added, and extracted with EtOAc. The organic layer was washed
with brine, dried over Na2SO4, and concentrated under vacuum to
give a yellow oil (110 mg). The crude product was purified by
preparative TLC to give (þ)-5 (43 mg, 41%) as a yellow oil, ee 99.0%
(Chiral HPLC analytical conditions: Chiralpak IC column,
4.6 mm ꢃ 250 mm, eluting with 100% i-PrOH þ 0.1% Et2NH, flow
8a-(2-methylpropyl)-6H-benz[a]oxireno[g]quinolizine ((ꢁ)-8)
Epoxidation of (ꢁ)-7 was carried out following the procedure for
22
preparation of (þ)-8 to afford (ꢁ)-8, [
a
]
D
¼ ꢁ190.5ꢂ (c 0.21,
MeOH). 1H NMR (300 MHz, CDCl3)
d 6.57 (s,1H), 6.55 (s, 1H), 3.84 (s,
6H), 3.29 (d, 1H, J ¼ 13.1 Hz), 3.17e2.98 (m, 4H), 2.60e2.40 (m, 4H),
2.00e1.81 (m, 2H), 1.64e1.42 (m, 3H), 0.99 (d, 6H, J ¼ 7.4 Hz); 13C
NMR (75 MHz, CDCl3)
d 147.5, 147.3, 129.3, 126.9, 111.2, 108.4, 58.3,
57.8, 57.6, 56.6, 56.0, 55.8, 51.7, 43.8, 32.3, 28.9, 25.1, 23.1, 22.9; ESI-
MS m/z 318.2 [M þ H]þ; HRMS calcd for C19H28NO3 [M þ H]þ m/z
318.2069, found 318.2076.
4.1.13. (2R,3S,11bR)-3-Isobutyl-9,10-dimethoxy-1,3,4,6,7,11b-
hexahydro-2H-pyrido[2,1-a]isoquinolin-2-ol ((þ)-4)
rate 0.5 mL/min, oven temperature 35 ꢂC, detection UV 220 nm).
To a solution of (þ)-8 (100 mg, 0.33 mmol) in dry THF was added
dropwise 2 M boraneeMe2SeTHF (0.73 mL, 1.46 mmol) at 0 ꢂC. The
reaction mixture was stirred at below 15 ꢂC for 48 h, and then
0.5 mL of water was added. The mixture was heated at reflux for
30 min. Then 30% aqueous NaOH solution (0.67 mL) and aqueous
30% hydrogen peroxide solution (0.28 mL) were added. The
resulting mixture was heated at reflux for an additional 1.5 h. After
cooling, 10 mL of water was added, and the mixture was extracted
with EtOAc. The organic layer was washed with brine, dried over
Na2SO4, and concentrated under vacuum to give a yellow oil. The
crude product was purified by preparative TLC to afford (þ)-4 as
a white solid (32 mg, 31%), ee 100% (Chiral HPLC analytical condi-
tions: Chiralpak IC column, 4.6 mm ꢃ 250 mm, eluting with 100% i-
PrOH þ 0.1% Et2NH, flow rate 0.5 mL/min, oven temperature 35 ꢂC,
23
[
a]
¼ þ120.6ꢂ (c 0.11, MeOH). 1H NMR (300 MHz, CDCl3):
d 6.68
D
(s, 1H), 6.57 (s, 1H), 3.84 (s, 6H), 3.54 (m, 1H), 3.08 (m, 1H), 2.90 (m,
1H), 2.76 (dd, 1H, J ¼ 3.4, 11.6 Hz), 2.62e2.53 (m, 3H), 2.20e2.15 (m,
1H), 1.92e1.45 (m, 5H), 1.27e1.18 (m, 1H), 0.90 (d, 3H, J ¼ 6.7 Hz),
0.88 (d, 3H, J ¼ 6.7 Hz); 13C NMR (75 MHz, CDCl3)
d: 147.2, 130.1,
127.2, 111.7, 108.1, 70.0, 57.0, 56.0, 55.9, 54.3, 52.7, 40.0, 39.6, 35.5,
28.4, 25.7, 23.2, 22.4; ESI-MS m/z 320.2 [M þ H]þ; HRMS calcd for
C19H30NO3 m/z 320.2226, found 320.2248.
4.1.10. (2R,3R,11bS)-3-Isobutyl-9,10-dimethoxy-1,3,4,6,7,11b-
hexahydro-2H-pyrido[2,1-a]isoquinolin-2-ol ((ꢁ)-5)
Hydroboration-oxidation of (ꢁ)-7 was carried out following the
procedure for preparation of (þ)-5 to afford (ꢁ)-5 as a yellow oil, ee

1848
Z. Yao et al. / European Journal of Medicinal Chemistry 46 (2011) 1841e1848
21
detection UV 220 nm). [
a
]
¼ þ148.2ꢂ (c 0.16, MeOH). 1H NMR
a Beckman 6000 liquid scintillation counter (Beckman Coulter
Instruments, Fullerton, CA). Data were analyzed by using GraphPad
Prism software (San Diego, CA) from a 13 point dose-response curve
that spanned 1 ꢃ 10(ꢁ10) to 1 ꢃ 10(ꢁ4) M.
D
(300 MHz, CDCl3) d 6.68 (s, 1H), 6.58 (s, 1H), 3.95e3.92 (m, 1H), 3.84
(s, 6H), 3.11e3.07 (m, 2H), 2.86e2.83 (m, 2H), 2.60e2.55 (m, 1H),
2.44e2.29 (m, 3H), 1.92 (brs, 1H), 1.64e1.61 (m, 4H), 1.26e1.22 (m,
1H), 0.93 (d, 3H, J ¼ 6.3 Hz), 0.89 (d, 3H, J ¼ 6.4 Hz); 13C NMR
(75 MHz, CDCl3)
d 147.4, 147.3, 129.9, 126.7, 111.6, 108.0, 71.8, 60.6,
Acknowledgment
57.8, 56.0, 55.9, 52.2, 38.1, 36.2, 34.0, 29.2, 26.1, 23.8, 22.0; ESI-MS
m/z 320.2 [M þ H]þ; HRMS calcd for C19H30NO3 [M þ H]þ m/z
320.2226, found 320.2248.
This work was supported in part by the “111 Project” from the
Ministry of Education of China and the State Administration of
Foreign Expert Affairs of China (No. 111-2-07). It was additionally
supported in part by the Intramural Research Programs of the
National Institute on Aging and National Institute on Drug Abuse,
NIH.
4.1.14. (2S,3R,11bS)-3-Isobutyl-9,10-dimethoxy-1,3,4,6,7,11b-
hexahydro-2H-pyrido[2,1-a]isoquinolin-2-ol ((ꢁ)-4)
Reductive ring-opening of (ꢁ)-8 was carried out following the
procedure for preparation of (þ)-4 to give (ꢁ)-4 as a white solid, ee
100% (Chiral HPLC analytical conditions: Chiralpak IC column,
4.6 mm ꢃ 250 mm, eluting with 100% i-PrOH þ 0.1% Et2NH, flow
Appendix. Supporting information
rate 0.5 mL/min, oven temperature 35 ꢂC, detection UV 220 nm).
22
¼ ꢁ143.6ꢂ (c 0.23, MeOH). 1H NMR (300 MHz, CDCl3)
d 6.68
Supplementary data related to this article can be found online at
doi:10.1016/j.ejmech.2011.02.046.
[
a]
D
(s, 1 H), 6.58 (s, 1H), 3.95e3.92 (m, 1H), 3.84 (s, 6H), 3.13e3.02 (m,
2H), 2.91e2.82 (m, 2H), 2.60e2.55 (m, 1H), 2.48e2.29 (m, 3H), 1.92
(brs, 1H), 1.67e1.58 (m, 4H), 1.26e1.19 (m, 1H), 0.94 (d, 3H,
References
J ¼ 6.4 Hz), 0.90 (d, 3H, J ¼ 6.4 Hz); 13C NMR (75 MHz, CDCl3)
d 147.4,
147.3, 129.9, 126.7, 111.6, 108.0, 71.9, 60.7, 57.8, 56.0, 55.9, 52.2, 38.1,
36.3, 34.0, 29.2, 26.1, 23.8, 22.0; ESI-MS m/z 320.2 [M þ H]þ; HRMS
calcd for C19H30NO3 [M þ H]þ m/z 320.2226, found 320.2236.
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