Synthesis of 3H-(-)-galanthamine

Article abstract of DOI:10.1002/jlcr.500

³H-(-)-galanthamine (0.881TBq/mmol) was synthesized via stereoselective reduction of (-)-narwedine with tritiated L-Selectride. This reaction sequence was favored over an exchange of aromatic bromine with lithium aluminium tritide. Copyright

Full text of DOI:10.1002/jlcr.500

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Labelled Cpd Radiopharm 2001; 44: 661–669.
DOI: 10.1002/jlcr.500
3
Synthesis of H-(–)-galanthamine
ElmarLinnemann and GregorFels*
.
Universitat Paderborn, FB 13 – Organische Chemie, Warburger Str.100,
D-33098 Paderborn, Germany
Summary
3H-(–)-galanthamine (0.881 TBq/mmol) was synthesized via stereose-
lective reduction of (–)-narwedine with tritiated L-Selectride1. This
reaction sequence was favored over an exchange of aromatic bromine
with lithium aluminium tritide. Copyright # 2001 John Wiley & Sons
Ltd.
Key Words: (–)-galanthamine; tritiation; radiosynthesis; Alzheimer’s
disease; acetylcholinesterase inhibitor
Introduction
Galanthamine, an alkaloid of the amaryllidaceae family1 is known to
act as a competitive and reversible inhibitor of the enzyme acetylcho-
linesterase2 (AChE). Furthermore, galanthamine is a non-competitive
nicotinic receptor agonist, and an allosterically potentiating ligand for
the nicotinic response induced by acetylcholine3 (ACh). This compound
therefore has the potential to exhibit a dual function at the nicotinic
synapse.4 Based on the cholinergic hypothesis, galanthamine is a
promising drug for the treatment of Alzheimer’s disease (AD).5 The
hydrobromide salt of galanthamine (Nivalin1) has been approved for
the treatment of AD in several European countries. In addition, this
drug has been used for 30 years in the clinical treatment of different
neurological illnesses and in anaesthesia.6
*Correspondence to: Gregor Fels, Universitat Paderborn, FB 13-Organische Chemie, Warburger
Str. 100, D-33098 Paderborn, Germany. E-mail: gf@chemie.uni-paderborn.de
Copyright # 2001 John Wiley & Sons, Ltd.
Received 7 March 2001
Revised 10 May 2001
Accepted 4 June 2001
Published online \ \ \ \

662
E. LINNEMANN AND G. FELS
Forfurtherinvestigation of the AChE inhibition by galanthamine as
well as foran analysis of the interaction between galanthamine and the
nicotinic acetylcholine-receptor,7 tritiated (–)-galanthamine of high
specific activity was needed.
Results and discussion
Two alternative routes for specific labelling of (–)-galanthamine 2a have
been investigated as outlined in Figure 1.
Investigation of route A
Hydrogenolysis of aromatic compounds by exchange of halogen atoms
with LiAlT4 is a widely known and convenient method fortirtiation.
Our first approach towards the synthesis of 3H-(–)-galanthamine
3
Figure 1. Potential routes to H-(–)-galanthamine
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 661–669

SYNTHESIS OF 3H-(–)-GALANTHAMINE
663
therefore was based on the tritiation of readily available 8-bromo-(–)-
galanthamine 1 with LiAlT4. (–)-Galanthamine hydrobromide 4 is
commercially available as Nivalin1 and can be monobrominated using
hydrogen-peroxide in formic acid8 (Figure 2). Reduction of 8-bromo-
(–)-galanthamine 1 with LiAlH4 resulted in a complete reaction within
20 h using four equivalents of hydride reagent with a yield of 79%.9 In
order to investigate isotope effects in this reaction we substituted
LiAlH4 with LiAlD4 and obtained 8-2H-(–)-galanthamine 2b in a similar
yield of 74%; however, a reaction time of 52 h and quenching of the
reaction product with D2O (Figure 2) is required.
An isotope effect presumably is the cause for more than a doubling of
the reaction time and probably a further increase of the reaction time
can be expected if LiAlT4 is used instead of LiAlD4. In addition, in
order to obtain high incorporation of deuterium (e.g. >70%) it was
necessary to employ deuterium oxide as quenching reagent. If H2O is
used rather than D2O, a deuteration rate of only 20% was observed.
This finding parallels the results of a deuteration of anthracene
derivatives described in the literature.10 In addition, that study reports
an even higher incorporation rate when the system LiAlH4/D2O is used
rather than LiAlD4/H2O.
Investigation of route B
In the large-scale synthesis of (–)-galanthamine as described by Jordis
et al. 11, the final step consists of a diastereoselective reduction of (–)-
narwedine 3 with L-Selectride1. This reaction gives high yields with
virtually no by-product formation. In order to make use of this
procedure for the tritiation of 3, tritiated L-Selectride1 7 is needed.
The synthesis of this compound has been described in the literature12
(Figure 3).
Figure 2. Synthesis of 8-2H-(–)-galanthamine via route A
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E. LINNEMANN AND G. FELS
Figure 3. Synthesis of tritiated L-Selectride1
7
2
Figure 4. Reduction of (–)-narwedine 3 with H-L-Selectride1
8
In order to investigate the applicability of this reaction to the
synthesis of tritiated (–)-galanthamine we have synthesized deuterated
L-Selectride1 (Li(sec-Bu)3BD) 8 based on the procedure described for
the tritiated derivative. Compound 8 was used without purification in
the reduction of (–)-narwedine 3. The reaction proceeds with 84% yield
at ꢀ208C within 1 h and gives the expected deuterated (–)-galanthamine
2d (Figure 4).
Virtually, no difference in the reaction time was observed with respect
to the corresponding hydrogen reaction and the yield, purity and
stereochemistry are as in the corresponding synthesis of (–)-galantha-
mine 2a. NMR-analysis of the reaction product proved that the
incorporation of deuterium took place exclusively in the required
position. The incorporation of deuterium in compound 2d was
determined to be >95%, and the coupling constant for the 13C–2H
coupling was found to be 21 Hz. Due to these results, route B was
selected forthe synthesis of specifically tritiated (–)-galanthamine 2e.
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 661–669

SYNTHESIS OF 3H-(–)-GALANTHAMINE
665
Synthesis of 3-3H-(–)-galanthamine 2e via route B
(–)-Narwedine 3 was reduced with tritiated L-Selectride1 7 using the
conditions and work up procedure described for the corresponding
deuteration reaction which gave a 61% yield and a total radioactivity of
6.4 Ci. (0.237 TBq) of 3-3H-(–)-galanthamine 2e. The specific activity
was determined via MS to 23.8 Ci/mmol (0.881 TBq/mmol).
Experimental
[4aS,6R,8aS)-1-bromo-3-methoxy-4a,5,9,10,11,12-hexahydro-11-
methyl-6H-benzofuro[3a,3,2-ef][2]benzazepine-6-ol
(8-bromo-(–)-galanthamine) 1
To a solution of galanthamine hydrobromide 4 (4 g, 10.8 mmol)
in formic acid (40 ml) was added hydrogen peroxide solution (40 ml,
35%, v/v). The reaction mixture was heated for 20 min at 1008C. The
reaction flask was cooled quickly to room temperature and the pH value
was adjusted with concentrated ammonia solution (25%, v/v) to pH 10.
The mixture was extracted 3 times with ethyl acetate (50 ml). The
organic layer was washed once with brine (50 ml) and dried over
MgSO4. The solvent was evaporated and the remaining solid was
purified via column chromatography (silica gel 150 g, CHCl3 92%:
MeOH 8%) to yield 8-bromo-(–)-galanthamine 1 (2.22 g, 6.08 mmol,
56.3% of theory, Lit8 64%) as a white foam.
1H-NMR (CDCl3, 200 MHz): d [ppm]=1.62 (d, 1H, H-9, J 13.87 Hz),
1.97–2.15 (m, 2H, H-9, H-5), 2.45 (s, 3H, NCH3), 2.67 (d, 1H, H-50),
2.98 (d, 1H, H-10), 3.23 (d, 1H, H-100), 3.84 (s, 3H, OCH3), 3.96 (d, 1H,
H-12, J 15.7 Hz), 4.16 (s, 1H, H-6), 4.32 (d, 1H, H-120, J 15.7 Hz), 4.63
(s, 1H, H-4a), 5.99–6.13 (m, 2H, H-8, H-7), 6.93 (s, 1H, H-1).–
13C-NMR (CDCl3, 50 MHz) d [ppm]=30.21 (t, C-5), 34.03 (t, C-9),
42.52 (q, NCH3), 49.17 (s, C-8a), 53.88 (t, C-10), 56.55 (q, OCH3), 62.28
(t, C-12), 89.23 (d, C-4a), 114.94 (d, C-1), 116.19 (d, C-8), 126.68 (t, C-2),
127.89 (s, C-12a), 128.54 (t, C-7), 134.55 (s, C-12b), 144.66 (s, C-3a),
145.88 (s, C-3).–
[4aS,6R,8aS)-3-methoxy-4a,5,9,10,11,12-hexahydro-11-methyl-6H-
benzofuro[3a,3,2-ef][2]benzazepine-6-ol (1H-(–)-galanthamine) 2a
Method A
LiAlH4 (0.030 g, 0.76 mmol) was dispersed in dry THF (15 ml). The
solution was cooled to 08C and 8-bromo-galanthamine 3 (0.07 g,
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E. LINNEMANN AND G. FELS
0.19 mmol) dissolved in dry THF (5 ml) was added slowly. After 20 h
complete conversion of 3 was achieved (determined by GC). The
mixture was quenched with H2O, the solvent evaporated and the
residual solid purified via column chromatography (silica gel 15 g,
1
CHCl3 96%, MeOH 4%) to yield H-(–)-galanthamine 2a (0.522 g,
0.15 mmol, 79% of theory, Lit.7: 49%) as a white foam.
Method B
A solution of n-buthyllithium 5 (1.4 M13 in hexane, 0.63 ml, 0.884 mmol)
and N,N,N0N0-tetramethyl-ethylendiamine (0.16 ml, 1.061 mmol) was
stirred for 2 h at room temperature in the presence of a hydrogen
atmosphere. After addition of the amine a white precipitate was rapidly
formed. The hydrogen gas was displaced by argon and tri-sec-
butylborane solution (1.0 M (THF), 0.89 ml, 0.884 mmol) was added
to the reaction mixture thereby re-dissolving the precipitate. The
solution was cooled to ꢀ308C and a suspension of (–)-narwedine 5
(0.126 g, 0.442 mmol) in THF (0.5 ml) was slowly added. The reaction
mixture was stirred for 0.5 h at ꢀ208C and 0.5 h at room temperature.
Excess of reducing reagent was quenched with methanol at 08C. The
solvent was evaporated off and the remaining solid dissolved in EtOH
(4 ml). HBrsolution (48%, w/v) was added to adjust the pH to 1 and the
solution stored overnight at 08C. The precipitate was filtered, washed
with EtOH (1 ml) and dried under vacuum. The resulting solid was
dissolved in water(4 ml), brought to pH 10 by ammonia solution (25%,
v/v) and extracted 4 times with EtOAc (2.5 ml). The combined organic
layers were washed with brine (2 ml) and dried over MgSO4. The
solvent was evaporated off and the remaining solid dried under vacuum
to yield 1H-(–)-galanthamine 2a (0.103 g, 0.358 mmol, 81.1% of
theory) as a white foam. The spectroscopic data were identical with
literature date.7
1H-NMR (CDCl3, 200 MHz): d [ppm]=1.62 (dd, 1H, 9-H), 1.97–2.18
(m, 2H, 5-H, 9-H0), 2.42 (s, 3H, NCH3), 2.71 (dd, 1H, 5-H0), 3.07 (t, 1H,
10-H), 3.30 (t, 1H, 10-H0), 3.70 (d, 1H, 12-H), 3.85 (s, 3H, OCH3), 4.08–
4.18 (m, 2H, 6-H, 12-H0), 4.63 (s, 1H, 4a-H), 5.97–6.11 (m, 2H, 7-H, 8-
H), 6.62–6.71 (d, 2H, 1-H, 2-H).–
13C-NMR (CDCl3, 50 MHz): d [ppm]=30.37 (t, C-5), 34.21 (t, C-9),
42.52(q, NCH3), 48.63 (s, C-8a), 54.25 (t, C-10), 56.30 (q, OCH3), 61.03
(t, C-12), 62.48 (d, C-6), 89.13 (d, C-4a), 111.55 (d, C-2), 122.48 (d, C-1),
127.27 (d, C-8), 128.04 (d, C-7), 129.69 (s, C-12a), 133.43 (s, C-12b),
144.51 (s, C-3a), 146.21 (s, C-3).–
Copyright # 2001 John Wiley & Sons, Ltd.
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SYNTHESIS OF 3H-(–)-GALANTHAMINE
667
[4aS,6R,8aS)-1-[2H]-3-methoxy-4a,5,9,10,11,12-hexahydro-
11-methyl-6H-benzofuro[3a,3,2-ef][2]benzazepine-6-ol
(8-2H-(–)-galanthamine) 2b
LiAlD4 (0.032 g, 0.76 mmol) was dispersed in dry THF (15 ml). The
solution was cooled to 08C and a solution of 8-bromo-galanthamine 3
(0.07 g, 0.19 mmol) in dry THF (5 ml) was added slowly. After 52 h
complete conversion of 3 was achieved (determined by GC). The
mixture was quenched with D2O. The solvent was evaporated off and
the residual solid purified via column chromatography (silica gel 15 g,
CHCl3 96%, MeOH 4%) to yield 8-[2H]-(–)-galanthamine 2b (0.407 g,
0.141 mmol, 74% of theory) as a white foam.
1H-NMR (CDCl3, 200 MHz): d [ppm]=1.60 (dd, 1H, 9-H), 1.96–2.17
(m, 2H, 5-H, 9-H0), 2.34 (s, 3H, NCH3), 2.69 (dd, 1H, 5-H0), 3.06 (d, 1H,
10-H), 3.28 (t, 1H, 10-H0), 3.70 (d, 1H, 12-H), 3.84 (s, 3H, OCH3), 4.06–
4.14 (m, 2H, 6-H, 12-H0), 4.62 (s, 1H, 4a-H), 5.96–6.10 (m, 2H, 7-H,
8-H), 6.61–6.73 (d, 1.48H, 1-H, 2-H).–
13C-NMR (CDCL3, 50 MHz): d [ppm]=30.37 (t, C-5), 34.16 (t, C-9),
42.46 (q, NCH3), 48.59 (s, C-8a), 54.22 (t, C-10), 56.30 (q, OCH3), 60.92
(t, C-12), 62.45 (d, C-6), 89.10 (d, C-4a), 111.57 (d, C-2), 122.51 (d, C-1),
127.23 (d, C-8), 128.03 (d, C-7), 129.44 (s, C-12a), 133.42 (s, C-12b),
144.51 (s, C-3a), 146.20 (s, C-3).–
[4aS,6R,8aS)-3-methoxy-4a,5,9,10,11,12-hexahydro-11-methyl-6-[2H]-
benzofuro[3a,3,2-ef][2]benzazepine-6-ol (3-2H-(–)-galanthamine) 2d
Quantities, reaction conditions and workup were identical to the
1
procedure described for the preparation of H-(–)-galanthamine 2a via
method B. Hydrogen gas was replaced by deuterium gas. The reaction
yielded 3-2H-(–)-galanthamine 2d (0.108 g, 0.375 mmol, 85% of theory)
as a white foam.
M.p.: 1288C (Lit.8 1H-(–)-galanthamine: 128–1298C)
[a]25=ꢀ96.3 (c=1.5; CHCl3) (Lit.14 1H-(–)-galanthamine): [a]25=ꢀ93.4
(c=1.0; CHCl3)
TLC: Rf=0.46 (THF 93%:EtOH 4%:Et3N 3%)
1H-NMR (CDCl3, 200 MHz): d [ppm]=1.55 (dd, 1H, 9-H), 1.94–2.07
(m, 2H, 5-H, 9-H0), 2.39 (s, 3H, NCH3), 2.68 (dd, 1H, 5-H0), 3.03 (d, 1H,
10-H), 3.25 (t, 1H, 10-H0), 3.66 (d, 1H, 12-H), 3.81 (s, 3H, OCH3), 4.08
(d, 1H, 12-H0), 4.59 (s, 1H, 4a-H), 5.89–6.12 (m, 2H, 7-H, 8-H), 6.58–
6.68 (d, 2H, 1-H, 2-H).–
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668
E. LINNEMANN AND G. FELS
13C-NMR (CDCl3, 50 MHz): d [ppm]=30.28 (t, C-5), 34.20 (t, C-9),
42.48 (q, NCH3), 48.59 (s, C-8a), 54.43 (t, C-10), 56.26 (q, OCH3), 60.99
(t, C-12), 61.99 (D-triplett, C-6), 89.06 (d, C-4a), 111.50 (d, C-2), 122.43
(d, C-1), 127.31 (d, C-8), 127.94 (d, C-7), 129.71 (s, C-12a), 133.41 (s, C-
12b), 144.52 (s, C-3a), 146.20 (s, C-3).–
Analytical calculation forC 17H20DNO3 [288.36]
% C
% H+D
% N
Calculated
Found
70.81
70.30
7.06
7.46
4.86
4.86
[4aS,6R,8aS)-3-methoxy-4a,5,9,10,11,12-hexahydro-11-methyl-6-[3H]-
benzofuro[3a,3,2-ef][2]benzazepine-6-ol (3-3H-(–)-galanthamine) 2e
The reaction flask was evaporated and tritium gas introduced. A
solution of n-buthyllithium (1.6 M (hexane), 0.63 ml, 1.0 mmol) and
N,N,N0,N0-tetramethyl-ethylendiamin (0.16 ml, 1.061 mmol) was added
and stirred until the pressure remained constant for 30 min. The
reaction mixture was degassed and the solvent removed by lyophiliza-
tion. Nitrogen gas was admitted to 784 mbar and tri-sec-butylborane
solution (1.0 M (THF), 0.89 ml, 0.884 mmol) was added to the reaction
mixture thereby re-dissolving the precipitate. The solution was cooled to
ꢀ308C and a suspension of (–)-narwedine 3 (0.126 g, 0.442 mmol) in
THF (1.0 ml) was added slowly. The reaction was stirred for 0.5 h at
ꢀ208C and 1 h at room temperature. Excess of reducing reagent was
quenched with methanol at 08C. The reaction mixture was degassed and
the solvent removed by lypohilization. Labile tritium was removed by
dissolving the solid in methanol and removing the solvent by
lyophilization. The remaining solid was dissolved in EtOH (4 ml).
HBrsolution (48%, w/v) was added until pH 1 was erached and the
solution was stored overnight at 08C. The precipitate was filtered,
washed with EtOH (1 ml) and dried under vacuum. The derived solid
was dissolved in water(4 ml), brought to pH 10 with ammonia solution
(25%, v/v) and extracted 4 times with EtOAc (2.5 ml). The combined
organic layers were washed with brine (2 ml) and dried over MgSO4.
The solvent was evaporated off and the remaining solid dried under
vacuum to yield 3-3H-(–)-galanthamine 2e (77.8 mg, 0.269 mmol, 60.8%
of theory) as a white foam.
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 661–669

SYNTHESIS OF 3H-(–)-GALANTHAMINE
669
Specific activity (MS): 23.8 Ci/mmol (0.881 TBq/mmol)
Total radioactivity (LSC): 6.4 Ci (0.237 TBq).
TLC: Rf=0.46 (THF 93%: EtOH 4%: Et3N 3%)
Acknowledgements
We thank Dr. U. Pleiss, BAYER AG Wuppertal, Germany for
performing the MS experiments on the 3-3H-(–)-galanthamine 2e.
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