3,4-Dihydroxy- and 3,4-methylenedioxy- phenanthrene-type alkaloids with high selectivity for D2 dopamine receptor

Article abstract of DOI:10.1016/j.bmcl.2013.06.078

Dopamine-mediated neurotransmission plays an important role in relevant psychiatric and neurological disorders. Nowadays, there is an enormous interest in the development of new drugs acting at the dopamine receptors (DR) as potential new targets for the

Full text of DOI:10.1016/j.bmcl.2013.06.078

Bioorganic & Medicinal Chemistry Letters 23 (2013) 4824–4827
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
3,4-Dihydroxy- and 3,4-methylenedioxy- phenanthrene-type
alkaloids with high selectivity for D2 dopamine receptor
Laura Moreno a, Nuria Cabedo b, María Dolores Ivorra a, María-Jesús Sanz c,d, Arturo López Castel e,
M. Carmen Álvarez e, Diego Cortes a,
⇑
a Departamento de Farmacología, Laboratorio de Farmacoquímica, Facultad de Farmacia, Universidad de Valencia, Avda. Vicente Andrés Estellés s/n, Burjassot, 46100 Valencia, Spain
b Centro de Ecología Química Agrícola-Instituto Agroforestal Mediterráneo, UPV, Campus de Vera, Edificio 6C, 46022 Valencia, Spain
c Departamento de Farmacología, Facultad de Medicina, Universidad de Valencia, 46013 Valencia, Spain
d Institute of Health Research-INCLIVA, University Clinic Hospital of Valencia, Valencia, Spain
e Valentia BioPharma, Parque Científico de la Universidad de Valencia, Catedrático Jose Beltrán 2, Paterna, 46980 Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Dopamine-mediated neurotransmission plays an important role in relevant psychiatric and neurological
disorders. Nowadays, there is an enormous interest in the development of new drugs acting at the dopa-
mine receptors (DR) as potential new targets for the treatment of schizophrenia or Parkinson’s disease.
Previous studies have revealed that isoquinoline compounds such as tetrahydroisoquinolines (THIQs)
can behave as selective D2 dopaminergic alkaloids. In the present study we have synthesized five apor-
phine compounds and five phenanthrene alkaloids and evaluated their potential dopaminergic activity.
Binding studies on rat striatal membranes were used to evaluate their affinity and selectivity towards
D1 and D2 DR. Phenanthrene type alkaloids, in particular the 3,4-dihydroxy- and 3,4-methylenedioxy
derivatives, displayed high selectivity towards D2 DR. Therefore, they are potential candidates to be used
in the treatment of schizophrenia (antagonists) or Parkinson’s disease (agonists) due to their scarce D1
DR-associated side effects.
Received 30 May 2013
Revised 24 June 2013
Accepted 27 June 2013
Available online 4 July 2013
Keywords:
Aporphines
Phenanthrene alkaloids
Dopamine receptors
Structure–activity relationships
Ó 2013 Elsevier Ltd. All rights reserved.
Dopamine-mediated neurotransmission plays an important role
in several psychiatric and neurological disorders which affect sev-
eral million people worldwide. Researchers have focused on vari-
ous approaches towards modulating dopaminergic activity via
the dopamine receptors (DR) as a potential means of treating
schizophrenia and Parkinson’s diseases. The consequences of an
activation or blockade of DR are wide-ranging, and a perturbation
of dopamine neurotransmission may result in profound neurolog-
ical, psychiatric, or physiological signs and symptoms. For these
reasons, much research has focused on the discovery of novel
dopaminergic ligands as potential drug candidates.1 From a thera-
peutic point of view, drugs acting at D2-like DR have become very
relevant since most used antipsychotics in schizophrenia or bipolar
disease treatment are D2 antagonists and they are also involved in
dopamine’s release.2
with dopamine and can interact with DR.5,6 Aporphine alkaloids
constitute one of the largest groups of isoquinolines,7 and many
of them display pharmacological activities such as antioxidant,
antiplatelet, antitumor, anticonvulsant, antineoplastic, cytotoxic,
and antiparkinsonian.8 In addition, aporphines’ Hofmann degrada-
tion products, phenanthrene alkaloids, have also attracted
researchers’ attention since they can exert varied and powerful
biological activities including
a
1-adrenoceptor antagonism,9 inhi-
bition of acetylcholinesterase10 and intestinal glucose uptake11
,
antioxidant activity12,13 or impairment of leukocyte–endothelial
cells interactions.14 Moreover, since they have been reported to
produce effects associated with DR stimulation,15 interaction with
DR is therefore expected but barely explored.
Our group has long been interested in finding new and potent
dopaminergic ligands. Previous observations reported by us have
demonstrated that somenatural and synthetic1-benzyl-THIQs alka-
loids can bind to DR.16–20 In the present study, we have carried out
the total synthesis of phenanthrene alkaloids, and take advantage
of the diversity generated over the synthetic route of compounds
which might display dopaminergic activity. Phenanthrene alkaloids
are good candidates to bind to DR with high affinities due to
their structural similarity to dopamine. Furthermore, as opposed to
1-benzyl-tetrahydroisoquinolines (BTHIQ) or aporphines where
the amino function is held into a ring system, the flexible disposition
Isoquinoline alkaloids are a large family of natural products
with a variety of powerful biological activities,3 including seroto-
ninergic and dopaminergic.4 Tetrahydroisoquinolines (THIQs), the
most numerous naturally occurring alkaloids, include 1-benzyl-
THIQs and aporphines, both of which share structural similarities
⇑
Corresponding author. Tel.: +34 963544975.
E-mail address: dcortes@uv.es (D. Cortes).
URL: http://www.farmacoquimicavalencia.es (D. Cortes).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.06.078

L. Moreno et al. / Bioorg. Med. Chem. Lett. 23 (2013) 4824–4827
4825
H3CO
H3CO
H3CO
H3CO
H3CO
H3CO
(a)
(b)
NH2
NH
N
O
3,4-dimethoxy-
phenyl-ethylamine
1
2a
(77%)
Br
(77 %)
Br
(c)
2
1
H3CO
H3CO
H3CO
H3CO
H3CO
H3CO
N6
NH
N
(d)
(e)
Boc
Boc
11
2b
(50%)
8
2c
(97%)
3a
(84%)
Br
Br
Scheme 1. Synthesis of aporphine 3a. Reagents and conditions: (a) 2-bromophenylacetyl chloride, CH2Cl2, 5% aq NaOH, rt, 16 h; (b) POCl3, CH3CN, N2, reflux, 1 h; (c) NaBH4,
CH3OH, N2, rt, 2 h; (d) di-tert-butyl dicarbonate, CH2Cl2, rt, 2 h; (e) 2-diphenylphosphino-20-(N,N-dimethylamino)biphenyl, Pd(OAc)2, K2CO3, DMA, 130 °C, 4 h.
Scheme 2. Synthesis of phenathrenes 4a–4c. Reagents and conditions: (a) LiAlH4, THF, reflux, 4 h; (b) CH3I, CH3OH, Et2O, rt, 3 h; (c) KOH 3 N, CH3OH, 45 °C, 6 h; (d) BBr3,
CH2Cl2, N2, rt, 2 h; (e) CH3I, CH3OH, Et2O, rt, 1 h.
HO
HO
H3CO
H3CO
O
O
of the aminoethyl chain in phenanthrene alkaloids might reach the
required conformation for an optimal DR interaction.
N
N
N
(a)
(b)
The general synthetic plan for these compounds consisted in
preparing the appropriate b-phenyl acetamide 1 under Shotten–
Baumann conditions to be then cyclized by Bischler–Napieralski
cyclodehydration followed by imine reduction.18,19 This sequence
led us to obtain expected 1-(20-bromobenzyl)-tetrahydroisoquino-
line 2b. The BTHIQ derivatives with a bromine atom placed over
the phenyl ring (after protection of amino function with a BOC
group) were then subjected to direct arylation to generate apor-
phine 3a (Scheme 1).21 After reduction of the carbamate protecting
group, aporphine 3b was N-methylated to obtain ammonium salt
3c which, under basic Hofmann’s degradation conditions, gave
the corresponding phenanthrene derivative 4a (Scheme 2).22
Thereafter, these phenanthrene alkaloids were again N-methylated
in order to further explore the influence of a quaternary ammo-
nium group over dopaminergic activity (4c). The deprotection of
dimethoxy groups from aporphine and phenanthene compounds
was performed using BBr3 to afford catechols 3d and 4b. Methyl-
enedioxy derivative 3e was formed by the reaction of 3d with
CsF and dichloromethane (Scheme 3).17–19 Using this sequence,
several known alkaloids were obtained, aporphines ( )-nuciferine
(3b), ( )-roemerine (3e), ( )-roemrefidine (3f), and phenanthrenes
atherosperminine (4a) and stephenanthine (4d).8
3b
3e
(76%)
3d
(100%)
(c)
N+
N
O
O
O
O
O
O
N+
(e)
(d)
4e
(99%)
3f
(100%)
4d
(84%)
Scheme 3. Synthesis of phenanthrenes 4d, 4e. Reagents and conditions: (a) BBr3,
CH2Cl2, N2, rt, 2 h; (b) CH2Cl2, CsF, DMF, reflux, 2 h; (c) CH3I, CH3OH, Et2O, rt, 3 h; (d)
KOH 3 N, CH3OH, 45 °C, 6 h; (e) CH3I, CH3OH, Et2O, rt, 1 h.
(Table 1).17–19 Many of these compounds were able to displace
both 3H-SCH 23390 and 3H-raclopride from their specific binding
sites in rat striatum at micromolar or nanomolar concentrations.
In general, all the tested compounds followed the same ten-
dency: (a) The nonquaternary aporphine compounds displayed
similar potency against D1 receptor (in the low micromolar range)
regardless whether the oxygenated functions over the A-ring were
protected or not. However, the affinity was substantially increased
All the aporphines and phenanthrenes were assayed in vitro for
their ability to displace selective D1 and D2ꢀDR ligands from their
respective specific binding sites in the striatal membranes

4826
L. Moreno et al. / Bioorg. Med. Chem. Lett. 23 (2013) 4824–4827
Table 1
Affinity values (Ki, pKi) and selectivity (ratio KiꢀD1/KiꢀD2) determined in binding experiments to D1 and D2ꢀDR of compounds 3b, 3d, 3e, and 4a, 4b, 4d
Compounds
Specific ligand D1 [3H]-SCH 23390
Specific ligand D2 [3H]-raclopride
Ki D1/D2
Ki (l
M)
pKi
Ki (
lM)
pKi
Aporphines
3b
3d
3e
0.630 0.149
0.401 0.044
0.377 0.148
6.231 0.123f
6.401 0.047g
6.489 0.167h
0.209 0.086
0.045 0.019
0.084 0.012
6.761 0.195
7.413 0.174a,d
7.080 0.061b,d
8.9
4.4
Phenanthrenes
4a
4b
4d
2.944 0.304
7.699 0.330
6.347 0.415
5.535 0.04
5.208 0.212
5.383 0.274
0.323 0.026
0.049 0.018
0.066 0.009
6.494 0.037b
7.414 0.243c,f
7.189 0.063c,e
9.1
157
96
Data were displayed as mean SEM for 3–5 experiments.
ANOVA, post Newmann–Keuls multiple comparison tests.
a
p < 0.05.
p < 0.01.
p < 0.001, versus D1-like DR.
p < 0.05 versus 3b.
p < 0.05 versus 4a.
p < 0.01 versus 4a.
p < 0.001 versus 4b.
p < 0.001 versus 4d.
b
c
d
e
f
g
h
Figure 1. Selectivity of compounds 3d and 4b at the D2-like dopaminergic receptors ([3H]-raclopride binding). Data were displayed as mean SEM for 3–5 experiments.
towards D2 receptor, with Ki values reaching the nanomolar range,
when the oxygenated functions were in the catechol or the meth-
ylenedioxy form. The highest affinity of compounds with free phe-
nol groups has been previously described in several
isoquinolines.17–20 (b) A similar behavior was observed in the non-
quaternary phenanthrene alkaloids. Both, the methylenedioxy and
the catechol groups improved the affinity towards D2ꢀDR (showing
potent affinities within the nanomolar range). In contrast, their
interaction with the D1ꢀDR was clearly undermined. (c) Regarding
the quaternary ammonium compounds, both aporphines (3c, 3f)
and phenanthrenes (4c, 4e) were inactive, probably due to the
nitrogen atom in the quaternary form hindering its role as anchor-
ing group with Asp86 which is essential for DA binding to D2ꢀDR.23
When both series were compared, they displayed similar D2
affinities, but the conformationally restricted aporphines bound
better to D1 receptor than phenanthrenes (Table 1, Fig. 1). There-
fore, it seems that the disposition of all carbon and hydrogen atoms
lying in one plane (the phenanthrene ring) was slightly unfavor-
able towards D1ꢀDR interaction but not to D2ꢀDR. Indeed, phenan-
threnes showed increased selectivity towards D2ꢀDR (compound
4b D1/D2 ratio: 157) which may be of relevance from a pharmaco-
logical point of view. As previously mentioned, drugs acting at
D2-like DR are of special therapeutic potential since while the
antagonists are used in the treatment of schizophrenia (antipsy-
chotics), the agonists are employed in the treatment of Parkinson’s
disease.24,25 Of note, other studies have even revealed the potential
antidepressant effect of D2 agonist.3,18 Therefore, the therapeutic
potential of these compounds is noticeable given their selectivity
on D2-related activities and their minimal D1-associated collateral
effects.
In summary, we have carried out the total synthesis of five
aporphine compounds and five phenanthrene alkaloids and evalu-
ated their potential dopaminergic activity towards D1 and D2 DR.
Our results have demonstrated that phenanthrene alkaloids, in
particular the 3,4-dihydroxy- and 3,4-methylenedioxy derivatives,
display high selectivity towards D2ꢀDR. Taking into account that
D1ꢀDR interaction may lead to undesirable side effects, including
dyskinesia and cardiovascular disturbances, the D2 selective com-
pounds synthesized may constitute new and useful candidates
for the treatment of schizophrenia (antagonists) or Parkinson’s dis-
ease (agonists).
Acknowledgments
This study was supported by grants SAF2011-23777, Spanish
Ministry of Economy and Competitiveness, RIER RD08/0075/

L. Moreno et al. / Bioorg. Med. Chem. Lett. 23 (2013) 4824–4827
4827
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