Extremely potent orally active benzo[g]quinoline analogue of the dopaminergic prodrug: 6-(N,N-Di-n-propyl)amino-3,4,5,6,7,8-hexahydro-2H- naphthalen-1-one

Article abstract of DOI:10.1021/jm051111h

Enone prodrugs of dopaminergic catecholamines represent a new type of prodrug in the research area of dopamine agonists. Here, we demonstrate the first benzo[g]quinoline-derived enone that induces potent dopamine agonist effects similar to aminotetralin-derived enones. Significant effects of (-)-4 were observed in microdialysis studies after administration of 1 nmol kg ^-1 sc and 3 nmol kg^-1 po. With a potency comparable to that of the most potent apomorphines, (-)-4 could potentially compete with L-DOPA and apomorphine in the treatment of Parkinson's disease.

Full text of DOI:10.1021/jm051111h

1494
J. Med. Chem. 2006, 49, 1494-1498
Brief Articles
Extremely Potent Orally Active Benzo[g]quinoline Analogue of the Dopaminergic Prodrug:
6-(N,N-Di-n-propyl)amino-3,4,5,6,7,8-hexahydro-2H-naphthalen-1-one
Danyang Liu,* Ha˚kan V. Wikstro¨m, Durk Dijkstra, Jan B. de Vries, and Bastiaan J. Venhuis
Department of Medicinal Chemistry, UniVersity Centre for Pharmacy, UniVersity of Groningen, Antonius Deusinglaan 1,
NL-9713 AV Groningen, The Netherlands
ReceiVed NoVember 4, 2005
Enone prodrugs of dopaminergic catecholamines represent a new type of prodrug in the research area of
dopamine agonists. Here, we demonstrate the first benzo[g]quinoline-derived enone that induces potent
dopamine agonist effects similar to aminotetralin-derived enones. Significant effects of (-)-4 were observed
in microdialysis studies after administration of 1 nmol kg-1 sc and 3 nmol kg-1 po. With a potency comparable
to that of the most potent apomorphines, (-)-4 could potentially compete with L-DOPA and apomorphine
in the treatment of Parkinson’s disease.
Introduction
In the treatment of Parkinson’s disease (PD), an important
property of a dopamine (DA) agonist has been suggested to be
a long duration of action, which could counteract the develop-
ment of dyskinesias.1 We previously described that (S)-6-(N,N-
di-n-propylamino)-3,4,5,6,7,8-hexahydro-2H-naphthalen-1-
one [(S)-PD148903, 1] is bioactivated in vivo to its corresponding
catecholamine [(S)-5,6-di-OH-DPAT, 2], which acts as a potent
mixed DA D1/D2 full agonist (Figure 1).2 Furthermore, several
Figure 1. Molecular structures of (S)-PD148903 (1), 5,6-di-OH-DPAT
(2), (-)-N-(n-propyl)-6,7-dihydroxybenzo[g]quinoline (3), and its
analogous potential enone prodrug (4).
analogues of 1 with different N-alkyl substituents induce similar
or more potent dopaminergic effects.3 Evidently, the aminote-
tralin-derived analogues of 1 contain a “template” for bioacti-
vation.
Scheme 1a
It was interesting to extend this prodrug concept to the benzo-
[g]quinolines, examplified with N-(n-propyl)-6,7-di-OH-benzo-
[g]quinoline (6,7-di-OH-PBGQ, 3). Similar to aminotetralin (2),
the 6,7-di-OH-PBGQ (3) is a catecholamine and known potent,
centrally acting DA receptor agonist.4,5 It is not clear if 3
displays both D1 and D2 agonistic effects. However, its N-ethyl
analogue TL333 displays such a mixed profile, as reported by
Itoh et al.6 Therefore, tricyclic enone 1-propyl-2,3,4,4a,5,7,8,9,-
10,10a-decahydro-1H-benzo[g]quinolin-6-one (4) was prepared
and pharmacologically evaluated.
Chemistry
The final compound 4 was prepared with the following
strategy. The starting material 3-(4-methoxyphenyl)acrylic acid
(5) was converted to the secondary amine 8 in three steps in
high yields as given in Scheme 1. The heterocyclic ring system
(A) was formed by performing a Birch reduction of the aromatic
ring in 8 using Li/NH3(l).7,8 The combination of Na/NH3(l) was
also tested;9 however, less than 50% of the secondary amine
(8) was reduced. The greater solubility of lithium in liquid
ammonia and a higher reduction potential for lithium enhanced
the reducing power of such a reaction.10 Upon acidic hydrolysis,
cyclization occurred to give 9 (ring B) as a diastereomeric
mixture with a ratio of 80:20 (cis/trans) as determined by GC
analysis in 70% yield. Because of a favorable steric requirement
a Reagents: (a) 3 bar of H2, 10% Pd/C; (b) (i) SOCl2; (ii) n-PrNH2,
87% over two steps; (c) LiAlH4, 91%; (d) (i) Li/NH3(l); (ii) H+/H2O, 70%;
(e) Br(Ph)3P+(CH2)3CO2Et, t-BuOK; (f) PPA, 100 °C, 34% over two steps.
for an approach of the nitrogen atom to the conjugated system,
this isomerization feature can be interpreted by assuming a
predominant cis attack in preference to a trans attack in the
Michael-type addition step.11 It has been described12 that cis-9
could isomerize to the trans form in acidic media through the
retro-Michael process; however, in our experiment no trans-
formation was found. Interestingly, in the presence of 1% KOH
in ethanol solution at room temperature, the cis isomer can be
efficiently transformed into the trans isomer with a ratio of 11:
89 (cis/trans)13 in 78% yield.
* To whom correspondence should be addressed. Phone: +31-50-
3633306. Fax: +31-50-3636908. E-mail: d.liu@rug.nl.
10.1021/jm051111h CCC: $33.50 © 2006 American Chemical Society
Published on Web 01/25/2006

Brief Articles
Journal of Medicinal Chemistry, 2006, Vol. 49, No. 4 1495
Table 1. In Vitro Affinity of (+)-4 and (-)-4 for Several DA Receptor
Subtypesa
DA D1
DA D2
DA D3
DA D4
(SCH23390)b
(spiperone),b %
(spiperone)b
(nemonapride)b
(-)-4
(+)-4
100/-1%
100/7%
100/15%
100/3%
50/23%
50/28%
50/-4%
50/-19%
a Ki (nM), n ) 3. b Tritiated radioligand used.
Figure 2. X-ray structure of enantiopure trans-(-)-4‚HCl salt.
Scheme 2. Suggested Metabolism of 4a
a Only the (-)-enantiomer of 4 gave detectable level of a compound
believed to be 6,7-di-OH-PBGQ (3).
Figure 3. Effect of (-)-4 (1, 3, and 10 nmol kg-1 sc; 9, 2, and 1,
respectively) on striatal DA release in freely moving rats. The results
are the mean ((SEM) of data obtained from four rats: (/) p < 0.05.
The third ring (C) was constructed by the introduction of a
C4 moiety through a Wittig reaction.14 The mixture of E and Z
isomers was used without further purification for the final ring
closure in polyphosphoric acid (PPA) at 100 °C.15 The last
cyclization step was highly regioselective at that temperature.
It was observed that when the reaction was carried out at 115
°C, up to 30% of the benzo[h]quinoline analogue of 4 was
isolated as side product. The racemic mixture was separated
into its enantiomers by preparative chiral HPLC with the eluent
hexane/2-propanol (99/1, containing 0.1% TEA); see Supporting
Information for details.
From the X-ray analysis of enantiopure trans-(-)-4 in Figure
2, it can be seen that the C7-N bond and C11-C12 bond
(torsion angle 176°) and C6-C7 and C10-C11 (torsion angle
175°) are in trans configuration. The absolute configuration was
proved as (R,R); see details in Supporting Information.
Both enantiomers of 4 were given in a dose of 10 µmol/kg
po to male Wistar rats, and the blood and the brain samples
were taken as described.16 After the usual workup procedure
with 60% CH3CN in water for the precipitation of proteins, the
samples were injected into the LC/MS/MS system. The identity
of (-)-4 and (+)-4 was confirmed by U. Jurva16 using high-
performance liquid chromatography/tandem mass spectrometry
(LC/MS/MS). Since peaks 261 and 263 were found in the
samples from (-)-4 administrated rats, compared with a standard
sample of 6,7-di-OH-PBGQ (3 as trans isomer, MW 261)
synthesized in our lab, it is reasonable to assume that the
catecholamine 3 is the active form of (-)-4 in vivo (Scheme
2).
Figure 4. Effect of (-)-4 (3 and 10 nmol kg-1 po; 9 and 2,
respectively) on striatal DA release in freely moving rats. The results
are the mean ((SEM) of data obtained from four rats: (/) p < 0.05.
pharmacological effects of DA agonists. Stimulation of the
autoreceptors leads to a down-regulation of dopamine synthesis
and release. The results are expressed in relation to basal DA
levels. For (-)-4, a significant decrease of 20-70% was
observed after administration of 1, 3, or 10 nmol kg-1 sc (Figure
3). Administration of 3 or 10 nmol kg-1 po induced a significant
decrease of 45% and 60%, respectively (Figure 4). During the
microdialysis experiments the rats displayed behavior like
yawning, sniffing, penile grooming, and locomotor activity.19-21
Comparison of these results with previously published data
from our lab17 clearly show that (-)-4 induces strong effects
after oral administration. For example, to achieve a 45%
decrease in DA level after oral administration, (-)-4 is >100-
fold more potent than N-propylnorapomorphine and (-)-1. After
subcutaneous administration, (-)-4 is >10 times more potent.2,17
Further comparison of the effects of these compounds shows
that the onset of action of (-)-4 is more gradual and the effect
is more long lasting. This may be an important characteristic
because this could potentially avoid a steep plasma concentration
curve that could lead to adverse effects. Compound (+)-4 was
found to be inactive at a dose of 1 µmol kg-1 sc as shown in
Figure 5.
Oral administration of 10 µmol/kg of (+)-4 did not induce
any biochemical or behavioral effects. Peak 263 was found in
brain and plasma samples; however, 3 was not detected in the
samples collected.
Results and Discussion
In vitro, neither of the enantiomers of 4 displays any affinity
for the DA D1, D2, D3, and D4 (Table 1). However, biochemical
experiments in vivo strongly indicate that (-)-4 is converted
to a DA agonist. The pharmacological effects of both enanti-
omers were studied by measuring their effects on the DA levels
in the corpus striatum, the brain area of interest in Parkinson’s
disease, using microdialysis in freely moving rats.17,18 Microdi-
alysis is a presynaptic model suitable for investigating the

1496 Journal of Medicinal Chemistry, 2006, Vol. 49, No. 4
Brief Articles
to -60 °C, and liquid NH3 (120 mL) was introduced. Li metal
(1.6 g) was gradually added in small portions, and the blue mixture
was stirred at -60 °C for 4 h. The color was discharged by addition
of a MeOH/aqueous NH4Cl (saturated) solution (1/1, 40 mL). After
NH3 was evaporated, the pH of the slurry was adjusted to 1 by the
addition of concentrated HCl and the mixture was stirred overnight
at room temperature. The mixture was basified with 4 N NaOH.
After general workup, a dark-red oil was obtained that was purified
by column chromatography (silica, treated with NH3, DCM/
methanol, gradient) to yield 9 as a mixture of trans and cis isomers
(1:4), a light yellow oil (8.1 g, 41.5 mmol, 70%). Rf: trans 0.72,
cis 0.78 (DCM/MeOH ) 5:1). trans-9: 13C NMR (CDCl3) δ 207.9,
61.6, 58.9, 52.5, 50.1, 43.5, 38.5, 38.0, 28.7, 22.8, 14.8, 9.3 ppm;
1H NMR (CDCl3) δ 2.95 (dt, 1H, J ) 6.4, 13.9 Hz), 1.11-2.60
(m, 17H), 0.72 (t, 3H, J ) 7.3 Hz) ppm; MS (EI) m/z 195 (M+).
cis-9: 13C NMR (CDCl3) δ 209.0, 62.6, 60.0, 53.5, 51.1, 44.5, 39.5,
39.0, 29.7, 23.8, 15.8, 10.3 ppm; 1H NMR (CDCl3) δ 3.54 (t, 1H,
J ) 4.4 Hz), 1.14-2.90 (m, 17H), 0.76 (t, 3H, J ) 7.3 Hz) ppm;
MS (EI) m/z 195 (M+). Anal. (C12H21NO) C, H, N.
Figure 5. Effect of (+)-4 (1 µmol kg-1 sc) on striatal DA release in
freely moving rats. The results are the mean ((SEM) of data obtained
from four rats: (/) p < 0.05.
Conclusion
trans-N-n-Propyl-7-keto-1,2,3,4,4a,5,8,8a-octahydro[6H]-
quinoline (9). cis-9 (4.4 g, 22.6 mmol) was dissolved in 1% KOH
ethanol solution (440 mL) and stirred at room temperature under
N2 for 3 days. The solvent was evaporated and worked up. After
evaporation of the solvent, the residue was purified by column
chromatography (silica, treated with NH3, DCM/MeOH, gradient)
to yield 3.4 g of mainly trans-9 (17.4 mmol, 78%; according to
GC, 10% cis left; the mixture was used for further reaction).
N-n-Propyl-2,3,4,4a,5,6,7,8,9,10,10a-decahydrobenzo[g]quino-
line-6-one (4). To a suspension of t-BuOK (3.8 g, 34 mmol) in
dry DMF (6 mL) under N2 at 0 °C was added dropwise a solution
of (3-ethoxycarbonylpropyl)triphenylphosphonium bromide (14.1
g, 37.4 mmol) in dry DMF (50 mL). After the addition, the mixture
was stirred at 0 °C for 30 min. A solution of 9 (3.3 g, 17.0 mmol)
in dry DMF (6 mL) was added at 0 °C under N2. After the mixture
was stirred at 0 °C for 4 h, the temperature was allowed to rise to
room temperature and the stirring was continued overnight. After
general workup, a brown oil 10 was obtained and was dissolved in
DCM (25 mL). The solution was added to PPA (45 g) at 100 °C
while stirring for 4 h. After cooling, ice (100 g) was slowly
introduced and extracted with DCM to remove the byproduct from
the last step. Ammonia (25% water solution) was added until pH
> 8 was attained. After general workup, the residue was purified
by column chromatography (silica, treated with NH3, DCM/
methanol, gradient) to result in trans-4 (1.4 g, 5.67 mmol, 33%
over two steps) as a light-yellow oil. 13C NMR (CDCl3) δ 197.2,
153.2, 129.9, 59.3, 54, 51.3, 36.2, 36.0, 35.5,30.0, 29.5, 28.2, 23.8,
The lack of affinity of (-)-4 observed in vitro and the highly
potent dopaminergic effects in vivo indicate a bioactivation
mechanism similar to that of (-)-1 and its close analogues. The
“template for bioactivation” of enones thus may very well extend
from bicyclic systems to a tricyclic system.
In microdialysis, (-)-4 is active at extremely low doses sc
and po. The effects observed in this model are stronger than
those induced by N-propylnorapomorphine and (-)-1. Further-
more, the effects at the lower doses tested last considerably
longer.
These properties taken together make (-)-4 an extremely
interesting candidate for development into a drug to improve
the drug therapy of PD.
Experimental Section
3-(4-Methoxyphenyl)propionic Acid N-Propylamide (7). 3-(4-
Methoxyphenyl)acrylic acid (10 g, 55.7 mmol) was dissolved in
ethanol (200 mL), and a catalytic amount of 10% Pd/C (120 mg)
was added. After shaking for 3 h under an H2 atmosphere (3 bar)
at room temperature, the mixture was filtered over Celite and
evaporated. The residue (10.2 g, white solid) obtained was refluxed
in DCM (220 mL) with thionyl chloride (10 mL, 137 mmol) for
1.5 h. The volatiles were evaporated, and the resulting oil was
dissolved in DCM (100 mL). This solution was added to a
vigorously stirred mixture of 5% aqueous NaOH (230 mL), DCM
(120 mL), and n-propylamine (7 mL, 85 mmol). After the mixture
was stirred for 1 h, the aqueous layer was extracted with DCM (3
× 100 mL). The general workup procedure yielded the amide 7 as
a white solid (10.7 g, 48.4 mmol, 87%), mp 90-91 °C. IR (neat)
cm-1 3303, 2960, 1641, 1542; 13C NMR (CDCl3) δ 170.7, 156.5,
131.4, 127.8, 112.4, 53.7, 39.7, 37.3, 29.4, 21.3, 9.8 ppm; 1H NMR
(CDCl3) δ 7.11 (d, 2H, J ) 4.4 Hz), 6.81 (d, 2H, J ) 4.4, 2.2 Hz),
5.52 (br s, 1H), 3.77 (s, 3H), 3.14 (m, 2H), 2.91 (m, 2H), 2.43 (m,
2H), 1.45 (m, 2H), 0.85 (t, 3H, J ) 7.3 Hz) ppm; MS (EI) m/z 221
(M+).
N-(3-(4-Methoxyphenyl)propyl)-N-propylamine (8). To a stirred
mixture of LiAlH4 (3.6 g, 94.6 mmol) in THF (80 mL) was added
dropwise a solution of amide 7 (10.4 g, 47.1 mmol) in THF (80
mL). After refluxing for 3.5 h, the mixture was cooled to 50 °C
and excess hydride was destroyed by usual workup procedure. The
amine 8 was obtained as an oil (8.9 g, 43 mmol, 91%). IR (neat)
cm-1 2931, 2832, 1612, 1513; 13C NMR (CDCl3) δ 156.2, 132.6,
127.7, 112.2, 53.7, 50.3, 47.9, 31.2, 30.3, 21.5, 10.3 ppm; 1H NMR
(CDCl3) δ 7.11 (d, 2H, J ) 8.8 Hz), 6.81 (d, 2H, J ) 8.6 Hz),
3.77 (br, s, 3H), 2.51-2.66 (m, 6H), 1.98 (br, s, 1H), 1.42-1.83
(m, 4H), 0.90 (t, 3H, J ) 7.6 Hz) ppm; MS (EI) m/z 207 (M+).
N-n-Propyl-7-keto-1,2,3,4,4a,5,8,8a-octahydro[6H]quinoline
(9). Amine 8 (12.3 g, 59.4 mmol) was dissolved in dry THF (120
mL) and t-BuOH (11.9 mL, 130 mmol). The mixture was cooled
1
20.8, 16.3,16.6 ppm; H NMR (CDCl3) δ 3.51 (d, 1H, J ) 11.0
Hz), 2.65 (m, 2H), 2.21-2.47 (m, 8H), 1.87-2.18 (m, 4H), 1.60-
1.79 (m, 3H), 1.37-1.59 (m, 2H), 1.01-1.31 (m, 2H), 0.86 (t, 3H,
J ) 7.3 Hz) ppm. MS (EI) m/z 247 (M+). Anal. (C16H25NO) C, H,
N. cis-4 (0.13 g, 0.53 mmol, 3% over two steps) was obtained as
a light-yellow oil. 13C NMR (CDCl3) δ 197.6, 152.5, 128.8, 55.2,
53.3, 45.2, 36.4, 31.9, 29.9, 26.4, 24.0, 23.5, 21.0, 18.8, 10.5 ppm;
1H NMR (CDCl3) δ 3.03 (t, 1H, J ) 6.1 Hz), 2.52-2.61 (m, 1H),
2.34-2.49 (m, 6H), 2.21-2.30 (m, 4H), 2.02 (m, 3H), 1.18-1.68
(m, 7H), 0.87 (t, 3H, J ) 7.3 Hz) ppm; MS (EI) m/z 247 (M+).
The products were subsequently converted to the hydrochloric salt
and recrystallized from ethanol/diethyl ether: cis-4‚HCl, mp 186
°C; trans-4‚HCl, mp 237 °C.
Pharmacology. All enones were tested as their hydrochloride
salts unless noted otherwise. The drugs were dissolved in physi-
ological (0.9%) saline immediately before use. All in vivo experi-
ments were performed at the laboratory animal unit of the
Rijksuniversiteit Groningen, The Netherlands. Microdialysis pro-
cedure was performed following the literature procedure.17
Receptor Binding. The in vitro binding affinity experiments
were performed at Lundbeck AC/S, Copenhagen, Denmark.
The compounds trans-(-)-4 and (+)-4 were tested for in vitro
binding at the D1, D2, D3, and D4 receptors. The displacement of
the radioligand was measured at a fixed concentration of the test
compound (50 or 100 nM). The estimate was based on this

Brief Articles
Journal of Medicinal Chemistry, 2006, Vol. 49, No. 4 1497
(3) Venhuis, B. J.; Dijkstra, D.; Wustrow, D. J.; Meltzer, L. T.; Wise,
L. D.; Johnson, S. J.; Heffner, T. G.; Wikstro¨m, H. V. Orally active
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below the IC50 of the radioligand. When the displacement of the
radioligand is below the IC50 at the tested concentration, the test
compound is considered to be inactive at the receptor.
D1 Binding. The method was described by Hyttel and Larsen.22,23
By this method, the inhibition by drugs of the binding of [3H]SCH
23390 (0.20 nM, Kd ) 0.45 nM) to dopamine D1 receptors in
membranes from rat corpus striatum was determined in vitro.
D2 Binding. The method was described by Hyttel and Larsen.24,25
By this method, the inhibition by drugs of the binding of
[3H]spiperone (0.50 nM, Kd ) 0.20 nM) to dopamine receptors in
membranes from rat corpus striatum is determined in vitro.
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1965, 48, 808-816.
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7(1H)-quinolones. III. Cis-trans isomerization of octahydro-7(1H)-
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D3 Binding. A modified method from R. G. Mackenzie et al.26
was used. By this method, the inhibition by drugs of the binding
of [3H]spiperone (0.3 nM, Kd ) 0.45 nM) to membranes of human
cloned dopamine D3 receptors expressed in Chinese hamster ovary
(CHO) cells is determined in vitro. CHO cells expressing the human
cloned D3 dopamine receptor are harvested, and the cell suspension
was centrifuged at 1000 rpm for 7 min at 4 °C. The supernatant is
frozen. On the day of the experiment, the cell pellet is thawed at
room temperature and diluted in assay buffer (25 mM Tris-HCl
(pH 7.4) + 6.0 mM MgCl2 + 1.0 mM EDTA) to the desired
concentration. An amount of 50 µL of displacer (10 µM haloperidol,
text compound or assay buffer) and an amount of 230 L of buffer
are added to a 96-well deep plate. Then 50 µL of 0.3 nM [3H]-
spiperone is added. The reaction is initiated by addition of 670 µL
of membrane suspension (test concentration of 26 µg of protein/
670 µL). Packard GF/C unifilter (96 well) is pretreated with 0.1%
PEI solution 10-15 min before filtration. After 60 min of incubation
at 25 °C the reaction is terminated by filtration at the Tomtec
unifilter. The filters are washed twice with ice-cold assay buffer.
The filters are dried for 1.5 h at 50 °C, 35 µL of scintillation liquid
is added, and bound radioactivity is counted in Wallac Tri-Lux
scintillation counters.
D4 Binding. The method was described by Meier et al.27 By
this method, the inhibition by drugs of the binding of [3H]-
nemonapride (0.50 nM, Kd ) 0.20 nM) to cloned human dopamine
D4 receptors is determined in vitro.
X-ray Crystallographic Data of Compound (-)-4. The HCl
salt of (-)-4 was recrystallized from absolute EtOH and ether as a
colorless cubic crystal. The crystallographic data for (-)-4 in this
paper has been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication no. CCDC-273828. A copy
of the data can be obtained free of charge at www.ccdc.cam.ac.uk/
conts/retrieving.html or form the Cambridge Crystallographic Data
Centre (CCDC) at 12 Union Road, Cambridge CB2 1EZ, U.K. (fax
+44-1223/336-033, e-mail deposit@ccdc.cam.ac.uk).
Acknowledgment. We gratefully acknowledge Dr. Auke
Meetsma (Crystal Structure Center, University of Groningen)
for X-ray analysis and Lundbeck A/S (Denmark) for the in vitro
binding study and financial support.
Supporting Information Available: General experimental
procedures including results from chiral HPLC separation of trans-
4, elemental analysis data, and selected X-ray data. This material
is available free of charge via the Internet at http://pubs.acs.org.
(22) Hettel, J. Preferential Labeling of Adenylate Cyclase Coupled
Dopamine Receptors with Thioxanthene Neuroleptics. In AdVances
in Dopamine Research: AdVances in the Biosciences; Kohasaka, M.,
Shohmori, T., Woodruff, G. N., Eds.; Pergamon Press Ltd.: Oxford,
U.K., 1982; pp 147-152.
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