CHIRALITY 9:59-62 (1997)
`
`Absolute Configuration of
`(+)-o -Dihydrotetrabenazine, an Active Metabolite
`of Tetrabenazine
`MICHAEL R KILBOURN, 1. LIHSUEH C. LEE,1 MARY J. HEEG,2 AND DOUGLAS M. JEWEMT 1
`1Division of Nuclear Medicine, Department of Internal Medicine, University of Michigan Medical School,
`Ann Arbor, Michigan
`2Department of Chemistry, Wayne State University, Detroit, Michigan
`
`Chiral column liquid chromatography and enantiospecific enzymatic
`ABSTRACT
`hydrolysis were utilized to separate the enantiomers of a- and 3-dihydrotetrabenazine
`and a-9-O-desmethyldihydrotetrabenazine, three benzo [a] quinolizines derived from the
`amine-depleting drug tetrabenazine. An X-ray crystal structure analysis of (-)-a-9-O-
`desmethyldihydrotetrabenazine gave an absolute structure of that compound as the 2S,
`3S, 11bS isomer. Therefore, (-)-a-dihydrotetrabenazine also has the 2S, 3S, 11bS abso-
`lute configuration. (+-)--Dihydrotetrabenazine, the single biologically active isomer from
`the metabolic reduction of tetrabenazine, thus has the absolute configuration of 2R, 3R,
`11bR. For further in vitro and in vivo studies of the vesicular monoamine transporter, it
`is now possible to use the single enantiomer of radiolabeled (x-dihydrotetrabenazine.
`Chirality 9:59-62, 1997. © 1997 Wiley-Liss, Inc.
`
`KEY WORDS: resolution of enantiomers; chiral column chromatography; enzyme; X-ray
`crystallography
`
`INTRODUCTION
`
`MATERIALS AND METHODS
`
`Tetrabenazine, used clinically for the management of
`movement disorders, functions to deplete brain mono-
`amine levels by inhibition of the vesicular monoamine
`transporter type 2 (VMAT2).1 In rodents and humans, tet-
`rabenazine is rapidly and extensively metabolized by re-
`duction of the 2-keto group, producing a- and p-dihydro-
`tetrabenazine 2 (Fig. 1). These alcohols also have high in
`vitro affinity for the VMAT2, and are likely the pharmaco-
`logically active agents in the mammalian brain. 3 o-Dihy-
`drotetrabenazine and related benzo [a] quinolizines have re-
`cently been labeled with tritium and carbon-11 radioiso-
`topes and used for in vitro and in vivo studies of the
`VMAT2 in animal and human brain.4 ,5
`As part of our study of the structure-activity relationship
`for the binding of benzo [a] quinolizines to the VMAT2 6 we
`determined the relative configurations of a- and 3-dihydro-
`tetrabenazines, demonstrating that these compounds (and,
`indirectly, tetrabenazine) were racemic mixtures of two
`enantiomers. After completing resolution of the enantio-
`mers on a chiral HPLC column, we then reported that the
`in vitro binding of a-dihydrotetrabenazine was stereospe-
`cific, with a high binding affinity (Ki = 0.97 nM) only for the
`(+)-isomer.7 At that time, we were unable to determine the
`absolute configuration, and thus report here syntheses,
`resolution and X-ray crystallographic studies which allow
`the assignment of the absolute configuration of (+)-a-
`dihydrotetrabenazine.
`
`© 1997 Wiley-Liss, Inc.
`
`Chemicals were purchased from Aldrich Chemical Co.
`and are reagent grade unless otherwise noted. Procine pan-
`creatic lipase (Altus 3) and ChiroCLEC TM-PC dry (Altus 20)
`were purchased from Altus Biologic Inc. (Boston, MA).
`Silica gel for column chromatography (70-230 mesh
`ASTM) and silica gel thin layer chromatography plates
`were purchased from Merck Co. (±)-a-Dihydrotetrabena-
`zine was prepared by hydride reduction of tetrabenazine
`(Fluka Chem. Co.) as previously described. 6 Melting
`points were determined on a Mel-Temp apparatus and are
`uncorrected. Optical rotations were obtained using a Per-
`kin-Elmer PE-241 polarimeter, using the sodium line.
`
`Synthesis of (±)-e -9-O-Desmethyldihydrotetrabenazine
`(2, 9-Dihydroxy-3-Isobutyl-1 O-Methoxy- 1,2,3,4,6,7-
`Hexahydro- 11 bH-Benzo[ajquinolizine)
`
`oc-Dihydrotrabenazine was selectively demethylated at
`the 9-methoxy group using sodium hydride/N-methyl ani-
`line/HMPA12. N-Methylaniline
`(7.89 g, 74 mmol) was
`added dropwise at 65 0 C to a stirred suspension of sodium
`
`Contract Grant sponsor: National Institutes of Health; Contract Grant num-
`bers MH 47611, NS 15655, and T32-CA09015; Contract Grant sponsor:
`Department of Energy; Contract Grant number DOE-DE-FG021-
`87ER60651.
`*Correspondence to: Dr. Michael IR Kilbourn, Cyclotron/PET Facility,
`3480 Kresge III, University of Michigan, Ann Arbor, MI 48109.
`Received 23 July 1996; Accepted 12 September 1996
`
`Apotex Ex. 1018
`
`Apotex v. Auspex
`IPR2021-01507
`
`

`

`KILBOURN ET AL.
`
`CH3O
`
`C 3 O
`
`C11
`
`CH 3O
`
`N 4
`
`j
`
`lb
`
`0
`
`Tetrabenazine
`
`OH
`
`Dihydrolet rabenazine
`
`Fig. 1. Structures of tetrabenazine and dihydrotetrabenazine. For sim-
`plicity, a single enantiomer of each (3R, llbR-tetrabenazine and 2R, 3R,
`1lbR-dihydrotetrabenazine) are shown.
`
`hydride (2.677 g; 111 mmol) in dry xylene (20 ml) and
`hexamethylphosphoramide (13.20 g; 74 mmol). After 15
`min (±)-c-dihydrotetrabenazine (11.900 g, 37.3 mmol) sus-
`pended in 10 ml xylene was added dropwise with stirring.
`The suspension was stirred 48 h at 650C. The reaction
`mixture was hydrolyzed with 5% HC1 (50 ml) and extracted
`several times with ether to remove unreacted starting ma-
`terial. The aqueous phase was filtered through a glass frit,
`and to the clear brown solution was added HCI (conc.)
`dropwise to precipitate the product. The solid was then
`dissolved in methanol/NH 4OH to obtain the free base of
`the crude product. The resulting solid was recovered by
`filtration and recrystallized several times from small vol-
`umes of methanol. Yield of (±)-ca-9-O-desmethyldihydrotet-
`rabenazine was 3.444 g (30%). The product was >98% pure
`as determined by HPLC (Inertsil C8; UV 240 nm; 1:5.5
`CH 3 CN:10 mM ammonium acetate, pH 4.5; R, 12 min) and
`was identical by HPLC and 'H-NMR to a sample of (±)-9-
`desmethyl-o-dihydrotetrabenazine synthesized stepwise
`from 3-benzyloxy-4-methoxybenzaldehyde. 2 m.p. 187-190'
`(lit.2 192.5-193-). 1H nmr (8 ppm, CD 3OD): 6.84 (1H,s,H-
`8), 6.67 (1H,s,H-11), 4.43 (1H,d,12Hz,H-11b), 3.87
`(3H,s,CH3-10), 3.71 (1H,m,H-6), 3.63 (2H,m,H-4, 2), 3.33
`(1H,m,H-7), 3.26 (1H,m,H-6), 3.00 (2H,m,H-4, 7), 2.89
`(1H,m,H-1), 1.99 (1H,m,H-3), 1.77 (3H,m,H-1, 1', 2'), 1.15
`(1H,ddd, 16Hz,8Hz,8Hz,H-I'), 1.00 (3H,d,7.5Hz, CH 3-3'),
`0.97 (3H,d,7.5Hz, CH 3-3'). O3 C nmr (8 ppm, CD 3OD): 148.4
`(C-9), 147.7 (C-10), 125.2 (C-11a), 124.5 (C-7a), 116.1 (C-8),
`109.4 (C-11), 72.0 (C-2), 62.6 (C-11b), 58.6 (C-4), 56.6
`(OCH3 ), 51.9 (C-6), 40.4 (C-3), 39.9 (C-i), 39.2 (C-I'), 27.1
`(C-7), 26.2 (C-2'), 24.9 (C-3'), 21.9 (C-3'). Anal. (HRMS)
`Calcd. 305.1991; Found 305.1977. Specific rotation [o] = 0.
`
`Resolution of the Enantiomers of
`(±)-at-Dihydrotetrabenazine,
`(±)-P-Dihydrotetrabenazine, and
`(±)-9-O-Desmethyl-ox-dihydrotetrabenazine by
`Chiral Chromatography
`
`High performance liquid chromatography (HPLC) reso-
`lutions of the isomers of (±)-a-dihydrotetrabenazine, (±)-[3-
`dihydrotetrabenazine, and (±)-9-0-desmethyl-o_-dihydro-
`tetrabenazine were done using a preparative HPLC column
`[Chirex 3014: ((S)-val-(R)-l-(o-naphthyl) ethylamine), 20
`x 250 mm: Phenomenex], eluted with 60:30:9.5:0.5 hexane:
`1,2-dichloroethane:ethanol:trifluoroacetic acid at a flow
`rate of 7 ml/min. Isolated products were re-injected until
`pure by analytical HPLC analysis, determined using an ana-
`lytical (4.6 x 250 mm) Chirex 3014 column and the above
`
`solvent mixture at a flow rate of 1 ml/min. To obtain the
`best separation each injection contained no more than 30
`mg, and fractions were pooled to obtain sufficient quanti-
`ties (>300 mg) of both the (-)- and (+)-isomers for further
`chemical or biological characterization.
`For resolution of (±) -- 9-O-desmethyldihydrotetrabena-
`zine, the first enantiomer eluting was the (-)-isomer, >99%
`pure, molecular rotation MXt = -152.7. This material was
`recrystallized from ethanol, and was used to grow the crys-
`tals which were suitable for X-ray structure determination
`(see following section). The (+)-isomer eluted second, and
`was obtained in >98% enantiomeric purity.
`For separation of the (±)--dihydrotetrabenazine, the en-
`antiomers eluted in the same order as for a-9-O-
`desmethyldihydrotetrabenazine. The molecular rotation
`for the (+)-isomer was M t = 84.6.
`Finally, the HPLC column was used for the resolution of
`the enantiomers of (±)-p-dihydrotetrabenazine, where the
`(+)-isomer eluted first the molecular rotation for the (-)-
`isomer was MXt = -174.5.
`
`Synthesis of (±)-c-Dihydrotetrabenazine Acetate Ester
`(2-Acetoxy-3-Isobutyl- 9,1 0-Dimethoxy- 1,2,3,4,6,7-
`Hexahydro- 11 bH-Benzo[alquinolizine)
`A solution of (±)-c-dihydrotetrabenazine (1 g, 3.13
`mmol) in acetic anhydride (20 ml) was heated to reflux for
`5 h. The solvent was then removed by a stream of N2 to
`provide a dark oily residue. The oil was then purified by
`silica gel column chromatography (EtOAc) to provide
`crude ester, which was crystallized from EtOAc. The reac-
`tion yield was almost quantitative. HRMS: calcd 361.2253;
`found 361.2255. The product was used for the enzymatic
`resolution (below) without further purification.
`
`Enzymatic Resolution of (+)-t-Dihydrotetrabenazine
`(26.684 g) was poured into 750 ml of so-
`The Altus #3
`dium phosphate buffer solution (0.12 M, pH = 7) and
`stirred at room temperature for 20 min. The substrate ester
`((±)-a-dihydrotetrabenazine acetate, 738 mg, 2.043 mmol)
`dissolved in acetone (85 ml) was then slowly added. The
`reaction mixture was stirred at room temperature for one
`hour, then transferred to cold room (40C) for 17 d. The
`enzyme was then filtered out (celite). The clear aqueous
`solution was adjusted to pH = 8 and extracted with CH 2C12
`(3 x 150 ml). The combined organic extracts were concen-
`trated and separated by silica gel column chromatography
`(EtOAc eluent) to provide (-)-ester (250 mg, containing a
`small amount of the (+)-ester) and (+)-a-dihydro-
`tetrabenazine (143 mg, 0.448 mmol) which was >99% pure
`by analytical chiral HPLC analysis.
`
`X-Ray Crystal Structure Analysis
`A sample of crystalline (-)-ox-9-O-desmethyldihydrotetra-
`(C18H 280 3N + CF 3CO 2-CH 3CH 2OH) was
`benazine
`mounted in a thin-walled capillary to preserve the lattice
`solvent. Crystallographic data was collected at room tem-
`perature on a S/N/S automated R3 diffractometer with
`monochromated Mo radiation. Absorption corrections
`were applied in the basis of psi scans. The structure was
`solved using the programs of SHELXL-93. 8 To determine
`
`Apotex Ex. 1018
`
`

`

`ABSOLUTE CONFIGURATION OF (+)-a-DIHYDROTETRABENAZINE
`
`Fig. 2. Diagram of (-)-a-9-.-desmethyldihydrotetrabenazine with thermal ellipsoids representing 50% probability. The absolute configurations at
`atoms C9, Cl, and C12 (corresponding to the lb, 2, and 3-positions of the tetrabenazine structure shown in Fig. 1) are all S.
`
`absolute configuration, the entire sphere of hkl data was
`collected and compared. In SHELXL-93, the absolute con-
`figuration parameter is near zero when the configuration
`assignment is correct, and near +1 when the configuration
`is wrong. For this data set, the absolute structure param-
`eter refined to -0.4 ± 0.3 for the assignment shown in Fig-
`ure 2. When the opposite configuration was tested, the
`parameter was +1.3 t 0.3.
`Crystal data are presented in Table 1, and the final
`atomic coordinates are given in Table 2. The molecular
`geometry and crystallographic labeling are shown in Fig-
`ure 2.
`
`RESULTS AND DISCUSSION
`
`Starting from commercially available tetrabenazine, it is
`possible to synthesize a large number of derivatives
`through reduction, demethylation, and organometallic ad-
`dition reactions. 1,6 Cleavage of the methoxy groups of such
`compounds, either using the basic conditions reported
`here or using acidic conditions such as boron trihalides,9
`are low to moderate yield reactions requiring separation of
`the desired product from numerous impurities. Assign-
`ment of the isolated product as the 9-0-desmethyldihydro-
`tetrabenazine, rather than the 10-0-desmethyl compound,
`was done on the basis of an X-ray crystal structure of the
`racemic mixture (data not shown), and verified by com-
`parison with a sample of authentic 9-0-desmethyldihydro-
`tetrabenazine synthesized from 3-benzyloxy-4-
`methoxybenzaldehyde. 2 Demethylation of tetrabenazine
`using boron tribromide yields primarily the 10-desmethyl
`isomer as determined by NMR and X-ray crystallographic
`analysis (data not shown), rather than the 9-desmethyl iso-
`mer as originally reported. 9 Although a moderate yield re-
`action, the one-step demethylation of a-dihydrotetrabena-
`zine using the sodium hydride/N-methylaniline/HMPA
`method was considerably easier than the multi-step total
`synthesis.
`Through the use of 2-D NMR techniques it has been
`determined that all of these compounds, and tetrabenazine
`itself, are a mixture of enantiomers with the identical rela-
`
`TABLE 1. Crystallographic data for
`[(C18H2 8 0 3N)+(CF 3C0 2 ) - " (CH 3CH 2 0H)]
`465.50 amu
`
`Formula weight
`
`Formula
`
`C22H34F 3NO 6
`9.438 (2) A
`space group
`a
`T
`12.953(3) A
`b
`X
`c
`10.070(2) A
`3
`Goodness of fit
`92.40(2) 0
`1230.0(5) A3
`p(calc)
`V
`!1
`2
`Z
`transmission coefficients 0.940-0.592
`0.045 for I > 2r(l); 0.049 for all 2486 data
`R
`0.120 for I > 2u(I); 0.123 for all 2486 data
`wR
`refinement method: full-matrix least-squares on F 2
`
`monoclinic P21
`22 0 C
`1.54178 A
`1.039
`1.257 g cm - 3
`8.93 cm - '
`
`tionship of configurations at C-3 and C-11b. 6,10 The biologi-
`cal activity of a-dihydrotetrabenazine resides in the (+)-
`isomer (K = 0.97 nM), with a very low affinity in vitro for
`the other enantiomer (2200 nM). 7 Two methods were de-
`veloped for the resolution of enantiomers of these benzo [a-
`] quinolizines. Application of chiral column HPLC provides,
`using repetitive injections, a suitable method for resolution
`and purification of sufficient amounts of resolved isomer
`for further use: the resolved (+)-o-9-O-desmethyldihydro-
`tetrabenazine, for example, serves as the precursor for the
`(via alkyla-
`synthesis of (+)--c-["CIdihydrotetrabenazine
`tion with ["Clmethyl iodide) which is used in imaging of
`human brain monoamine vesicular transporters using pos-
`itron emission tomography." As an alternative method of
`synthesis of the resolved isomer, we also examined the
`enantiospecific hydrolysis of an acetate ester of (+)-O--
`dihydrotetrabenazine using porcine pancreatic lipase (Al-
`tus 3). This enzyme selectively deacetylated the (+)-ester
`to provide a mixture of (+)-c-dihydrotetrabenazine and un-
`hydrolyzed ester (enriched in the (-)-isomer). After sepa-
`ration of alcohol and ester on a silica gel column, pure
`(+)-a-dihydrotetrabenazine could be obtained. Although
`this was a suitable method for also obtaining useful quan-
`tities of pure resolved isomer, attempts to scale the enzyme
`synthesis 10-fold were less successful.
`Of the pure resolved compounds which were prepared,
`
`Apotex Ex. 1018
`
`

`

`TABLE 2. Atomic coordinates (x 10^4) and equivalent
`isotropic displacement parameters (A^2 x 10^3)a
`
`x
`
`y
`
`z
`
`U (eq)
`
`KILBOURN ET AL
`11 labeled forms,7 providing the optimal radioligands for in
`vitro and in vivo studies of the vesicular monoamine trans-
`porter type 2.
`Finally, as o-dihydrotetrabenazine is obtained from tet-
`rabenazine by a simple hydride reaction which produces
`the chiral center at C-2 but does not racemize the carbon
`centers at C-3 and C-11b, these studies would suggest that
`tetrabenazine itself may consist of active (3R, llbR) and
`inactive (3S, llbS) enantiomers. The relative biological ac-
`tivities of the enantiomers of tetrabenazine do, however,
`remain to be verified.
`
`ACKNOWLEDGMENTS
`This work was supported by National Institutes of Health
`Grants MH 47611, NS 15655, and T32-CA09015 (to L.C.L.),
`and Department of Energy Grant DOE-DE-FG021-
`87ER60651.
`
`LITERATURE CITED
`1. Pletscher, A, Brossi, A, Gey, K.F. Benzoquinoline derivatives: A new
`class of monoamine decreasing drugs with psychotropic action. Int.
`Rev. Neurobiol. 4:275-306, 1962.
`2. Schwarz, D.E., Bruderer, H., Rieder, J. Brossi, A. Metabolic studies of
`tetrabenazine, a psychotropic drug in animals and man. Biochem.
`Pharmacol. 15:645-655, 1966.
`3. Mehvar, R., Jamali, F. Concentration-effect relationships of tetrabena-
`zine and dihydrotetrabenazine in the rat. J. Pharm. Sci. 76:461-465,
`1987.
`4. Henry, J.-P., Scherman, D. Radioligands of the vesicular monoamine
`transporter and their use as markers of monoamine storage vesicles.
`Biochem. Pharmacol. 38:2395-2404, 1989.
`5. Kilbourn, M.R. PET radioligands for vesicular neurotransmitter trans-
`porters. Med. Chem. Res. 5:113-126, 1994.
`6. Lee, L.C., Vander Borght, T., Sherman, P.S., Frey, K.A, Kilbourn, M.R.
`In vitro and in vivo structure-activity relationships of benzoisoquinoline
`ligands for the brain synaptic vesicle monoamine transporter. J. Med.
`Chem. 39:191-196, 1996.
`7. Kilbourn, M., Lee, L., Vander Borght, T., Jewett, D., Frey, K. Binding
`of x-dihydrotetrabenazine to the vesicular monoamine transporter is
`stereospecific. Eur. J. Pharmacol. 278:249-252, 1995.
`8. Sheldrick, G.M. SHELXL-93, University of Gottingen, Gottingen, Ger-
`many, 1993.
`9. DaSilva, J.N., Kilbourn, M.R., Mangner, T.J. Synthesis of [nCitetra-
`benazine, a vesicular monoamine uptake inhibitor, for PET imaging
`studies. Appl. Radiat. Isot. 44:673-676, 1993.
`10. Rubiralta, M., Diez, A, Bosch, J., Feliz, M., Solans, X. NMR spectros-
`copy and X-ray crystallography of benzo[a]quinolizidines. Hetero-
`cycles 27:1653-1664, 1988.
`11. Koeppe, RA., Frey, K.A., Vander Borght, T.M., Karlamangla, A., Jew-
`ett D.M., Lee, L.C., Kilbourn, M.R, Kuhl, D.E. Kinetic evaluation of
`[' 1C]dihydrotetrabenazine by dynamic PET: Measurement of the ve-
`sicular monoamine transporter. J Cerebral Blood Flow Metab. 1996
`16:1288-1299, 1996.
`12. Loubinoux, B., Coudert, G., and Guillaumet, G. Selective demethyl-
`ation of aryl methyl ethers. Synthesis 638-640, 1983.
`
`1435 (3)
`122 (5)
`C (1)
`1764 (3)
`-940 (4)
`C (2)
`o (1)
`1313 (2)
`-2234 (3)
`2588 (3)
`-667 (4)
`C (3)
`2844 (2)
`(3)
`-1801
`0 (2)
`3051 (3)
`636 (4)
`C (4)
`2723 (3)
`1709 (4)
`C (5)
`1886 (3)
`1468 (4)
`C (6)
`1490 (4)
`2618 (5)
`C (7)
`1854 (3)
`4067 (5)
`C (8)
`3003 (2)
`4008 (3)
`N (1)
`3298 (3)
`3112 (4)
`C (9)
`4461 (3)
`2971 (4)
`C (10)
`5007 (3)
`4398 (4)
`C (11)
`O (3)
`6097 (2)
`4213 (3)
`4658 (3)
`5363 (4)
`C (12)
`3477 (3)
`5417 (5)
`C (13)
`5140 (4)
`6837 (5)
`C (14)
`4861 (4)
`7918 (5)
`C (15)
`5390 (7)
`9322 (7)
`C (16)
`5086 (7)
`7371 (8)
`C (17)
`3619 (4)
`-1586 (7)
`C (18)
`O (4)
`2861 (4)
`4545 (5)
`O (5)
`3767 (2)
`2845 (4)
`3455 (3)
`3561 (6)
`C (19)
`3828 (6)
`3157 (10)
`C (20)
`3173 (6)
`3293 (11)
`F (1)
`4274 (5)
`1984 (6)
`F (2)
`4557 (7)
`4032 (9)
`F (3)
`1868 (3)
`6317 (5)
`0 (6)
`1477 (7)
`7318 (11)
`C (21)
`2111 (8)
`8077 (10)
`C (22)
`aU(eq) is defined as one-third of the trace of the orthogonalized Uij tensor.
`
`8178 (4)
`8932 (4)
`8819 (3)
`9805 (4)
`10555 (3)
`9857 (4)
`9052 (3)
`8226 (3)
`7365 (4)
`7817 (5)
`7992 (3)
`9139 (4)
`9133 (4)
`9241 (4)
`9252 (3)
`8151 (4)
`8145 (6)
`8309 (6)
`7321 (5)
`7700 (9)
`5890 (6)
`11551 (6)
`4869 (3)
`5673 (3)
`4771 (4)
`3387 (6)
`2497 (4)
`3212 (5)
`3070 (7)
`3215 (4)
`4081 (11)
`4932 (8)
`
`49 (1)
`49 (1)
`60 (1)
`45 (1)
`62 (1)
`48 (1)
`42 (1)
`43 (1)
`55 (1)
`54 (1)
`40 (1)
`45 (1)
`50 (1)
`45 (1)
`52 (1)
`46 (1)
`50 (1)
`62 (1)
`74 (1)
`114 (2)
`108 (2)
`71 (1)
`105 (1)
`73 (1)
`63 (1)
`112 (2)
`264 (5)
`178 (2)
`253 (4)
`93 (1)
`171 (4)
`147 (3)
`
`we were successful at growing good crystals from the (-)-
`o-9-O-desmethyldihydrotetrabenazine, and thus the deter-
`mination of absolute configuration was done on that com-
`pound. The crystal structure determined is shown in Fig-
`ure 2, and can be assigned as the 2S, 3S, 11bS isomer. As
`(-)-(x-9-O-desmethyldihydrotetrabenazine can be con-
`verted to the corresponding (-)-ca-dihydrotetrabenazine by
`simple 0-methylation of the phenol group, the absolute
`configuration of (-)-c-dihydrotetrabenazine (the inactive
`isomer) is also 2S, 3S, 11bS, and thus the configuration of
`the high affinity (+)-a-dihydrotetrabenazine is 2R, 3R, llbR
`(as depicted in Fig. 1). The resolution of the desmethyl
`compound, o-9-O-desmethyldihydrotetrabenazine, has also
`allowed us to prepare the single active enantiomer (+)-2R,
`3R, llbR-dihydrotetrabenazine in both tritium and carbon-
`
`Apotex Ex. 1018
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`