Enantioselective synthesis of tetrahydrocarbazoles via trienamine catalysis and their anxiolytic-like activity

Article abstract of DOI:10.1016/j.bmcl.2020.127063

The first study about the anxiolytic activity of two chiral tetrahydrocarbazoles is presented. This new chiral compounds were prepared through an organocatalytic strategy via trienamine activation. The in situ ortho-quinodimethane species, formed by the c

Full text of DOI:10.1016/j.bmcl.2020.127063

Journal Pre-proofs
Enantioselective synthesis of tetrahydrocarbazoles via trienamine catalysis
and their anxiolytic-like activity
Tushar Janardan Pawar, Edson E. Maqueda-Cabrera, Angel Josabad Alonso-
Castro, José Luis Olivares-Romero, David Cruz Cruz, Clarisa Villegas Gómez
PII:
S0960-894X(20)30141-4
DOI:
Reference:
https://doi.org/10.1016/j.bmcl.2020.127063
BMCL 127063
To appear in:
Bioorganic & Medicinal Chemistry Letters
Received Date:
Revised Date:
Accepted Date:
5 December 2019
8 February 2020
24 February 2020
Please cite this article as: Janardan Pawar, T., Maqueda-Cabrera, E.E., Josabad Alonso-Castro, A., Luis Olivares-
Romero, J., Cruz Cruz, D., Villegas Gómez, C., Enantioselective synthesis of tetrahydrocarbazoles via trienamine
catalysis and their anxiolytic-like activity, Bioorganic & Medicinal Chemistry Letters (2020), doi: https://doi.org/
10.1016/j.bmcl.2020.127063
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Enantioselective synthesis of
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tetrahydrocarbazoles via trienamine
catalysis and their anxiolytic-like activity
Tushar Janardan Pawar, Edson E. Maqueda-Cabrera, Angel Josabad Alonso-Castro, José Luis Olivares-Romero,
David Cruz Cruz* and Clarisa Villegas Gómez
O
O
N
NC
Ph
COOEt
N
CN
COOEt
Ph
H
N
PG
N
PG
N
PG
THC's with Anxiolytic activity

Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com
Enantioselective synthesis of tetrahydrocarbazoles via trienamine catalysis and
their anxiolytic-like activity.
Tushar Janardan Pawara, Edson E. Maqueda-Cabreraa, Angel Josabad Alonso-Castrob, José Luis Olivares-
Romeroc, David Cruz Cruza* and Clarisa Villegas Gómeza*
aDepartamento de Química, División de Ciencias Naturales y Exactas, Universidad de Guanajuato, Noria Alta S/N Guanajuato, Gto. 36050, México.
bDepartamento de Farmacia, División de Ciencias Naturales y Exactas, Universidad de Guanajuato, Noria Alta S/N Guanajuato, Gto. 36050, México.
cRed de Estudios Moleculares Avanzados, Instituto de Ecología AC, Carretera Antigua a Coatepec 351, El Haya, Xalapa, Ver. 91070, México.
ARTICLE INFO
ABSTRACT
Article history:
Received
Revised
Accepted
Available online
The first study about the anxiolytic activity of two chiral tetrahydrocarbazoles is presented. This
new chiral compounds were prepared through an organocatalytic strategy via trienamine
activation. The in situ ortho-quinodimethane species, formed by the condensation of the N-
protected 2-methylindole acrylaldehyde with a sterically hindred diarylsilylprolinol ether
derivative as catalyst, easily participate in a Diels–Alder reaction with the ethyl cyanophenyl
acrylate as dienophile, in good yields and excellent stereoselectivity. These compounds showed
activity against anxiety and mood disorders that can possibly contribute in the discovery of new
drugs. In addition, the use of N-protected 2-methylindole acrylaldehyde will set a new base for
the synthesis of medically and pharmacologically important tetrahydrocarbazoles via trienamine
catalysis.
Keywords:
Tetrahydrocarbazoles
Organocatalysis
Trienamine catalysis
Anxiolytic activity
2009 Elsevier Ltd. All rights reserved.
Chiral tetrahydrocarbazoles (THC’s) are the most widely
recognized class of natural indole alkaloids.1 In the interest of its
biological and pharmaceutical development which is consistently
encouraging the discovery of new efficient synthetic pathways to
establish novel chiral THC framework.2 It has been observed
over the century that the discovery of novel chiral THC’s always
appears with new extraordinary bioactivities as well as bunch of
naturally occurring THC alkaloids are found in ancient
medicines.3 For instance, Dasycarpidone I and Uleine II
exhibited antiplasmodial activity in vitro against P. falciparum
and T. cruzi.3a-b Also, a novel synthetic drug with a THC core, the
(R)-Ramatroban III, used in the treatment of allergic rhinitis,
asthma and coronary artery disease.3c-d In the same way,
Fischambiguine B IV showed inhibitory activity against M.
tuberculosis.3e Nevertheless, some great examples of structurally
simple synthetic drugs can explain a potential use of THC core,
for example, the 1-ethyl-8-n-propyl-1,2,3,4-tetrahydrocarbazole-
1-acetic acid V was found to be a novel anti-inflammatory agent
as well as 6-Chloro-1,2,3,4-tetrahydrocarbazole-2-carboxylic
acid VI was discovered as clinically active in the treatment of
acute gout (Figure 1). 3f
whereas chiral THC alkaloids are pharmacologically important
compounds that make worthy to synthesize it and perform
biological tests to disclose new activities of these structures.
Consequently, over the past decades, chemists are focusing
more towards new methodologies to synthesize known or novel
chiral THC frameworks. At the same time, the newly developed
trienamine catalysis strategy has attracted considerable attention
for the synthesis of chiral privileged structures.5 In fact, during its
conceptualization,
a new methodology was developed to
synthesize chiral THC´s by using 2-methylindole acrylaldehyde 1
as a masked 2,4-dienal.6 In this strategy, the condensation of a
chiral secondary amine with the acrylaldehyde 1 lead to the
formation of a ortho-quinodimethane intermediate I, which is an
active trienamine species with the ability to react with a variety
O
H3C
H
N
H3C
S
H
N
H
N
O
F
N
H
H
N
N
CO2H
H
H
O
I
II
III
Me
Cl
Me
H
Me
Cl
From the past 20 years, it has been observed that depression,
anxiety and mood disorder are the cause of morbidity in the
developed nations. Hence, the circumstance call for research for
the discovery of new drugs or medicines that possess anxiolytic
activity with no toxicity and withdrawal effect.4 In this era, a
number of drugs has been reported with this type of activity,
HO
CN
CO2H
N
H
N
H
CO2H
O
N
H
Me
Me
IV
V
VI
Figure 1. Some examples of THC’s that possess potent biological activities

of dienophiles 2 and construct different chiral THC derivatives 3
(Scheme 1). Recently, many asymmetric transformations have
been reported using this strategy along with the modification of
previous methodologies but none of the chiral THC was studied
before.7
previous report. Although, best conditions were obtained,
subsequently same conditions were tried with N-protected 2-
methyl indole acrylaldehyde 1 along with individual optimization
for compound 1 were performed consequently, earlier condition
was found to be the best as well (Scheme 2).
The purpose of this study is to synthesize novel THC’s, which
can be biologically and pharmacologically important and can be
considered as a member of alkaloids. Herein, we report the
asymmetric synthesis of two novel THC’s efficiently via Diels–
Alder cycloaddition reaction. In addition, the resulting THC’s
turn up with outstanding biological activities, that contribute to
the medicinal field.
O
O
Ph
Ph
N
H
OSiPh3
(20 mol%)
toluene, 70 oC, 60h
CN
CO2Et
NC
Ph
CO2Et
6d
R'
+
R'
N
Ph
N
R
R
1
5
8
O
O
CN
CO2Et
CN
CO2Et
O
O
N
EWG
N
Ph
N
Ph
N
Bn
8b
EWG
EWG
Boc
8a
*
H
EWG
2
*
, 79% yield
98% ee, 92:8 dr
, 90% yield
93% ee, 62:38 dr
N
*
N
PG
PG
N
PG
1
I
3
Scheme 2. Synthesis novel THC’s via trienamine catalysis.
ortho-quinodimethane
Scheme 1. The concept of THC synthesis via trienamine catalysis
Under the optimized conditions, we synthesized two different
THC’s 8a and 8b with different N-protecting groups, Boc and
benzyl respectively. On the basis of this result as well as by
considering the THC motif, we started performing different
biological activity tests on 8a and 8b.
We started this work by analysing the first report of
trienamine strategy by Jørgensen.8 In this study, the 2,4-dienal 4
condense with chiral amine 6a-b to form the corresponding
trienamine intermediate, which through
a
Diels–Alder
cycloaddition with ethyl cyanophenyl acrylate 5 lead to the
expected cyclohexene 7 (Table 1, entries 1 and 2). We initiate our
work by improving the previous conditions. Initially, we check
the reaction with different solvents, with or without additives and
by varying the temperature (r.t. to 70 ºC). By using toluene as
solvent and without additive, the reaction response was satisfied
at 70 ºC. In the trial of use Jørgensen-Hayashi catalyst 6c, the
stereoselectivity was improved (entry 3). However, to our glad
the best results (98% ee and 96:4 dr) were obtained in presence
of more sterically hindered catalyst 6d (entry 4). Thus, we
improved the stereoselectivity of the reaction far better than the
The findings showed that these THC molecules, orally
administered 1h before the experiment, reduced the anxiety-like
behaviour in a dose-dependent fashion in Balb/c mice in the
cylinder exploratory test (Fig. 2 A and B), which is commonly
used for the screening of drugs with anxiogenic or anxiolytic
actions in rodents. The maximal effect shown by these
compounds were: 72% (25 mg/kg, 8a) and 83.4% (25 mg/kg,
8b). These effects were almost comparable to those found with
1.5 mg/kg CNZ. (Fig. 2 A and B). The values for the effective
dose 50 were: 3.3 mg/kg (8a) and 7.7 mg/kg (8b). Molecules 8a
and 8b slightly reduced the time on the rotarod in a dose-
dependent impairment. However, this effect was not significant
compared to the vehicle group (Fig. 2C). Only molecule 8b (100
mg/kg) significantly (p<0.05) reduced the time on rotarod at 60
min-post-treatment. However, this was a partial effect since mice
recover their motor coordination 60 min later (Fig. 2D).
Table 1. Optimization table
O
O
NC
Ph
CO2Et
6
(20 mol%)
CN
CO2Et
addit. (20 mol%)
+
solvent, temp., 60h
Ph
Although the anxiolytic-like actions of these two molecules
were not comparable to those found with CNZ, these two
molecules did not significantly affect motor coordination in mice
at doses lower than 25 mg/kg (Fig.2). Pre-treatment with 0.7
bicuculline (an antagonist of GABAA receptor) partially
abolished the anxiolytic-like effects of molecules 8a and 8b,
whereas pre-treatment with ketanserin (a blocker of 5-
hydroxytryptamine 2 receptor) or yohimbine (an α2-adrenoceptor
blocker) did not change the anxiolytic-like effects of molecules
8a and 8b (Fig. 3). These findings suggest the partial
participation of the GABAergic system in the anxiolytic-like
actions of molecules 8a and 8b. However, other studies will
confirm this hypothesis.
5
4
7
6a
6b
Ar = Ph, R = TES
Ar
Ar = 4-OMe-3,5-(di-tBu)C6H2,
R = TES
Ar
N
H
6c
6d
Ar = Ph, R = TMS
Ar = Ph, R = SiPh3
OR
6
Entry
Cat.
T
Solvent Addit
Yield
(%)d
81
dr
ee
(oC)
50
(%)
89
86
1a
2a
3b
4b
6a
6b
6c
6d
CHCl3
oFBA
86:14
80:20
Our results showed that these THC molecules possess
anxiolytic-like activity and can be used as lead compounds in the
drug discovery area. In further studies, the anxiolytic-like activity
of molecules 8a and 8b will be corroborated in other models of
anxiety-induced in rodents. Also, the trienamine methodology for
the synthesis of THC will set a new base for the organic chemists
to contribute to the medicinal field as it is necessary to uncover
this kind of chiral THC compounds and their biological activities.
50
CHCl3
toluene
toluene
oFBA
87
70
-
-
75
82:18c 93d
96:4c 98d
70
73
aConditions of previous report, breactions were carried out in the scale of 0.1 mmol
of dienophile, 0.2 mmol of aldehyde, and 0.02 mmol of catalyst for 60 h. call
values were calculated by 1H NMR analysis, ddetrminated after isolation of the
product, by HPLC on a chiral stationary phase.

(A)
(B)
30
25
20
15
10
5
30
25
20
15
10
5
**
**
**
**
**
**
**
**
**
**
0
0
CNZ
1.5 mg/kg
CNZ
1.5 mg/kg
Vehicle
0.1
1
10
25
Vehicle
0.1
1
10
25
8a (mg/kg)
8b (mg/kg)
Treatments
Treatments
(C)
250 (D)
250
200
150
100
50
200
150
100
50
**
**
Vehicle
vehicle
**
CNZ 1.5 mg/kg
8a (10 mg/kg)
8a (25 mg/kg)
8a (100 mg/kg)
CNZ (1.5 mg/kg)
8b (10 mg/kg)
8b (25 mg/kg)
8b (100 mg/kg)
**
**
0
0
0
20
40
60
80
100
120
20
40
60
80
100
120
Time (min)
Time (min)
Figure 2. Anxiolytic-like and locomotor effects of molecules 8a and 8b. The anxiolytic effects of 8a and 8b (0.1–25 mg/kg p.o.) were evaluated using the
cylinder exploratory test, recording the number of readings (A and B). The effects of 8a and 8b (10-100 mg/kg) on locomotion in mice were evaluated
with the rotarod test (C and D). Additional groups were administered with 1.5 mg/kg clonazepam (CNZ) as the positive control or the vehicle (saline
solution). Data are demonstrative of two parallel experiments (n = 8). Results represent the mean ± standard error media (SEM). ** represents p ≤ 0.05 in
comparison to the vehicle group, using ANOVA and Dunnett´s post hoc test.
(A)
(B)
25
20
15
10
5
25
20
15
10
5
**
**
**
**
**
**
**
**
0
0
8a (3.3 mg/kg)
-
-
-
-
+
-
+
+
-
+
-
+
-
8b (7.7 mg/kg)
Bicuculline (0.7 mg/kg)
Ketanserin (1 mg/kg)
Yohimbine 1 mg/kg
-
-
-
-
+
-
+
+
-
+
-
+
-
Bicuculline (0.7 mg/kg)
Ketanserin (1 mg/kg)
Yohimbine 1 mg/kg
-
+
-
-
-
+
-
-
-
-
+
-
-
+
Treatments
Figure 3. Possible mechanism of action of the Anxiolytic-like effects of molecules 8a (A) and 8b (B) in the cylinder exploratory test during 5-min
exposure. Bars represent mean values (± SEM) for the experimental group. ** P<0.05, compared to the vehicle group.

Acknowledgments
This work was made possible by grants from CONACYT
(Project supported by the Fondo Sectorial de Investigación para
la Educación, project: 253621) and DAIP-UG (project:
111/2019). T.J.P. thanks CONACYT for the Ph.D. scholarship.
National Laboratory of Molecular Spectroscopy of the
Universidad de Guanajuato is also gratefully acknowledged.
Thanks to Jessica R. Gutiérrez Cano for perform the first
organocatalytic reactions.
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