(±)-Nantenine analogs as antagonists at human 5-HT2A receptors: C1 and flexible congeners

Article abstract of DOI:10.1016/j.bmcl.2009.03.048

C1 and flexible analogs of (±)-nantenine were synthesized and evaluated for antagonist activity at human 5-HT_2A receptors in a calcium mobilization assay. This work has resulted in the identification of the most potent 5-HT_2A antagon

Full text of DOI:10.1016/j.bmcl.2009.03.048

Bioorganic & Medicinal Chemistry Letters 19 (2009) 2530–2532
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Bioorganic & Medicinal Chemistry Letters
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( )-Nantenine analogs as antagonists at human 5-HT2A receptors: C1 and flexible
congeners
Sandeep Chaudhary a, Stevan Pecic a, Onica LeGendre a, Hérnan A. Navarro b, Wayne W. Harding a,
*
a Department of Chemistry, Hunter College, and the Graduate Center of the City University of New York, 695 Park Avenue, New York, NY 10065, USA
b Center for Organic and Medicinal Chemistry, Research Triangle Institute, Research Triangle Park, NC 27709, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
C1 and flexible analogs of ( )-nantenine were synthesized and evaluated for antagonist activity at human
5-HT2A receptors in a calcium mobilization assay. This work has resulted in the identification of the most
potent 5-HT2A antagonist known based on an aporphine. Our results also suggest that the C1 position
may be a key site for increasing 5-HT2A antagonist activity in this compound series. In addition, the struc-
tural rigidity of the aporphine core appears to be required for nantenine to function as a 5-HT2A
antagonist.
Received 20 February 2009
Revised 5 March 2009
Accepted 9 March 2009
Available online 16 March 2009
Keywords:
Nantenine
5-HT2A antagonist
SAR
Published by Elsevier Ltd.
MDMA
The monoamine neurotransmitter, serotonin (5-hydroxytrypta-
mine, 5-HT, 1) is known to play a significant role in the central
nervous system (CNS) modulation of appetite, mood, body temper-
ature and sleep in humans.1 There are 14 serotonin receptors pres-
ently known, of which all except one (5-HT3) are G-protein coupled
receptors.2 Ligands for the 5-HT2A receptor are constantly being
developed as chemical tools to study the functional role of this
receptor in hallucinations, depression, anxiety and psychosis.3–7
The role of 5-HT2A receptor blockade in the neuropharmacological
processes of addiction is also of growing interest.8–13
Aporphines are a diverse group of tetracyclic alkaloids found in
several plant species and have been found to show a range of
interesting biological activities such as antiplasmoidal, anti-
helminthic and antitumour activities.14–18 As a result of their bio-
logical activities, new and facile synthetic methodologies to
prepare these compounds are always being explored.19,20
understanding the importance of selective receptor blockade as
well as multi-potent antagonism involving 5-HT2A receptors in
the antagonism of MDMA-induced effects. Aporphines may be
a valuable structural template for such a study, given the appar-
ent promiscuity of these compounds across various CNS targets
including 5-HT subtypes. As such, we have embarked on a study
to evaluate the 5-HT2A antagonist activity of aporphines using 2
as a lead molecule. In this Letter, we report results on the
synthesis and evaluation of C1 and less rigid nantenine analogs
(Fig. 1).
Preparation of C1 analogs commenced with readily available29
4-benzyloxy-3-methoxy phenethylamine (3) which was con-
densed with bromoacid (4) under standard peptide coupling condi-
tions (Scheme 1). The amide (5) thus produced was cyclized under
Bischler–Napieralski30 conditions to afford imine 6, which was
Others have reported the 5-HT2A antagonist properties of the
aporphine alkaloid nantenine (2).21,22 This pharmacodynamic
property seems to be relevant to it’s in vivo activity as an antag-
onist of the designer drug MDMA (methylenedioxymethamphet-
amine, ‘Ecstasy’).23 Although aporphines have been evaluated
as 5-HT1A, a-adrenergic, and dopaminergic D1 and D2 ligands,
very little SAR work has been performed on aporphines as
5-HT2A antagonists.21,24–28 Part of our program is geared towards
* Corresponding author. Tel.: +1 212 772 5359; fax: +1 212 772 5332.
E-mail address: whardi@hunter.cuny.edu (W.W. Harding).
Figure 1. Structures of serotonin (1) and nantenine (2).
0960-894X/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2009.03.048

S. Chaudhary et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2530–2532
2531
Scheme 1. Reagents and conditions: (a) CDI, THF, rt, 20 h, 90%; (b) PCl5, DCM, 0 °C–rt, 96%; (c) NaBH4, MeOH, 0 °C, 4 h, 99%; (d) ethyl chloroformate, K2CO3, DCM, rt, 12 h, 76%;
(e) Pd(OAc)2, ligand A, K2CO3, (CH3)3CCOOH, DMA, 130 °C, 17 h, 76%; (f) H2/Pd–C, MeOH–THF, rt, 38 h, 75%; (g) alkyl bromide, K2CO3, acetone, reflux 12 h; (h) LiAlH4, THF, 0 °C,
5 h, 91%; (i) aq HCHO, NaBH(OAc)3, DCM, rt, 24 h, 96%; (j) LiAlH4, Et2O, 0 °C, 5 h, 91%.
immediately reduced to give 7 without purification (due to appar-
ent instability of the imine).
Compounds 2, 12a–12e, 13, 16 and 18 were evaluated for func-
tional activity at human 5-HT2A receptors using a calcium mobiliza-
tion assay.32 Results are presented in Table 1. The less rigid analogs,
that is, compounds 16 and 18 had significantly reduced antagonist
activities as compared to nantenine, suggesting that the structural
rigidity of the aporphine nucleus is required for 5-HT2A activity.
Increasing the C1 alkyl chain length by one carbon (compound
12a) had little effect on 5-HT2A antagonist activity. However, incre-
mental additions of 2, 3 and 4 carbons gave a progressive increase in
5-HT2A antagonist activity (12b, 12c, 12d). Replacement of the C1
methyl group of nantenine with a methylenecyclopropyl moiety
(compound 12e) resulted in a 12-fold enhancement in activity as
compared to nantenine. Interestingly, the C1 benzyl analog was
found to be a negative allosteric modulator (IC50 = 4600 nM). In
keeping with attributes of successful CNS agents,33 the ClogP values
of the C1 analogs represent reasonable starting points for simulta-
neous optimization of pharmacodynamic properties and blood-
brain barrier penetrability in this compound series.
In conclusion, our biological results indicate that increasing the
length of the C1 alkyl chain beyond two carbon atoms, results in an
increase in 5-HT2A antagonist activity in this series of aporphines.
The C1 site may thus be a key position for further structural mod-
ifications to increase 5-HT2A antagonist activity. Compound 12e34
was the most active compound identified and showed a 12-fold in-
crease in activity as compared to nantenine. This compound is the
most potent 5-HT2A antagonist known with an aporphine skeleton.
Our work has also identified a low activity negative allosteric mod-
ulator, 13. We are continuing to explore the SAR of other nante-
nine-derived aporphines at human 5-HT2A receptors and will
report our findings in due course.
Following protection of amine 7 as the ethyl carbamate, we
were now in a position to attempt the direct biaryl cyclization31
of 8 to give the aporphine core—a key step in this synthetic route.
Following similar methodology to that previously described31 with
some optimization of reaction conditions, we obtained 9 in 76%
yield. Hydrogenolysis of the C1 benzyl group gave key intermedi-
ate 10 which served as a precursor for the C1 alkyl intermediates
11a–11f.
Our plan at this juncture was to reduce the carbamate function-
alities of 11a–11f with LAH to provide the C1 target analogs 12a–
12e and nantenine (2).
However, when LAH reduction was attempted, cleavage of the
carbamate group occurred, giving the corresponding secondary
amines as major products. This result was surprising and is in need
of further investigation. Nevertheless, subsequent reductive ami-
nation of the derived secondary amines with formaldehyde affor-
ded the target analogs 12a–12e and 2. The C1 benzyl derivative
13 was accessed via LAH reduction of 9.
To begin to evaluate the role of molecular rigidity on the 5-HT2A
activity of nantenine, we prepared the benzyltetrahydroisoquino-
line 16 and tertiary amine 18 (Scheme 2). Thus, following similar
procedures as in Scheme 1, the readily available amide 14 was cy-
clized under Bischler–Napieralski conditions. Subsequently, the
imine prepared was reduced and then subjected to reductive ami-
nation conditions providing the seco-ring C derivative 16. Borane
reduction of amide 14 followed by N-alkylation gave compound
18. All compounds were characterized with routine spectroscopic
techniques including 1H NMR, 13C NMR and HRMS.

2532
S. Chaudhary et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2530–2532
Scheme 2. Reagents and conditions: (a) PCl5, DCM, 0 °C–rt; (b) NaBH4, MeOH, rt; (c) aq HCHO, NaBH(OAc)3, DCM, rt, 62% over three steps; (d) BF3ÁOEt2, BH3ÁTHF, THF, 88%.
14. Rasoanaivo, P.; Ratsimamanga-Urverg, S.; Rafatro, H.; Ramanitrahasimbola, D.;
Palazzino, G.; Galeffi, C.; Nicoletti, M. Planta Med. 1998, 64, 58.
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Table 1
Apparent affinity of nantenine analogs at human 5-HT2A receptors and ClogP values32
CLogPb
a
Compd
R
Ke (nM)
2
CH3
850 ( 5.8)
890 ( 430)
297 ( 130)
274 ( 80)
171 ( 50)
68 ( 8)
3.60
4.13
4.66
5.19
5.72
4.58
5.37
3.82
3.69
—
12a
12b
12c
12d
12e
13
–CH2CH3
–(CH2)2CH3
–(CH2)3CH3
–(CH2)4CH3
–CH2–cyclopropyl
–CH2Ph
—
4600c
16
18
>10,000
—
—
5180d
Ketanserin
32c,d
a
Ke (apparent affinity) values are means of at least two experiments carried out
in duplicate; standard deviation is given in parentheses.
b
Determined with ChemBioDraw Ultra 11.0.
IC50 determined in the presence of 5-HT EC80
Experiment run once.
c
.
d
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Med. Chem. Lett. 2007, 17, 4128.
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K.; Sobarzo, E. M.; D’Ocon, P. Planta Med. 2005, 71, 897.
Acknowledgements
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O.L. acknowledges the RISE program at Hunter College for
financial support. This publication was made possible by Grant
Number RR03037 from the National Center for Research Resources
(NCRR), a component of the National Institutes of Health.
30. Wang, Y. C.; Georghiou, P. E. Org. Lett. 2002, 4, 2675.
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32. Calcium mobilization human 5-HT2A receptor functional assays. Stably expressed
human 5-HT2A receptor in CHO-K1 cells (ATCC), were used for the calcium
mobilization functional assay. The calcium 4 dye assays (Molecular Devices,
Sunnyvale, CA) were run according to manufacturer’s specifications. Briefly,
wells of black clear-bottom 96-well tissue culture-treated plates were seeded
with 20,000 cells the afternoon before assay. On the day of assay, the cells were
incubated with the calcium indicator dye for 1 h @ 37 °C. The test compounds
were preincubated with the cells during the last 15 min of the dye incubation.
The plate was then placed into a FlexStation pre-warmed to 37 °C. Basal or
unstimulated fluorescence intensity was recorded for 13 s followed by the
addition of 5-HT (antagonist and Ke assays). Fluorescence intensity was
recorded for an additional 47 s. The effect of test compound was determined by
subtracting the minimum from the maximum fluorescence recorded for each
well during the 47 s recording period. Thus, each well served as its own
control. All samples were run in duplicate.
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