Droplet-based continuous flow synthesis of biologically active Bis(indolyl)methanes and Tris(indolyl)methanes

Article abstract of DOI:10.1016/j.tetlet.2020.152178

Bis(indolyl)methanes and Tris(indolyl)methanes are known as an important class of heterocyclic compounds because they could be employed as anticancer drugs, blood sugar stabilizers, and anti-viral agents. Herein, they were prepared using single-phase and droplet-based continuous flow methods. Reactions were optimized under mixed solvent conditions of H₂O and dimethylformamide, and process stability and yields were examined. Excellent process stability and yields were achieved in considerably shortened reaction times when employing droplet-based continuous flow synthesis.

Full text of DOI:10.1016/j.tetlet.2020.152178

Journal Pre-proofs
Droplet-Based Continuous Flow Synthesis of Biologically Active Bis(indol‐
yl)methanes and Tris(indolyl)methanes
Su Min Nam, Yea Seul Jang, Go Eun Son, Chan Ho Song, Insik In, Chan Pil
Park
PII:
S0040-4039(20)30638-9
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.152178
TETL 152178
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
2 June 2020
20 June 2020
25 June 2020
Please cite this article as: Min Nam, S., Seul Jang, Y., Eun Son, G., Ho Song, C., In, I., Pil Park, C., Droplet-
Based Continuous Flow Synthesis of Biologically Active Bis(indolyl)methanes and Tris(indolyl)methanes,
Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.2020.152178
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Droplet-Based Continuous Flow Synthesis of
Biologically Active Bis(indolyl)methanes and
Tris(indolyl)methanes
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a
a
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b,
a,
Su Min Nam , Yea Seul Jang , Go Eun Son , Chan Ho Song , Insik In  and Chan Pil Park 

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Droplet-Based Continuous Flow Synthesis of Biologically Active
Bis(indolyl)methanes and Tris(indolyl)methanes
a
a
a
a
b,
a,
Su Min Nam , Yea Seul Jang , Go Eun Son , Chan Ho Song , Insik In  and Chan Pil Park 
a
Graduate School of Analytical Science and Technology (GRAST), Chungnam National University, Daejeon 34134, South Korea
Department of Polymer Science and Engineering, Korea National University of Transportation, Chungju 27469, South Korea
b
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Bis(indolyl)methanes and Tris(indolyl)methanes are known as an important class of heterocyclic
compounds because they could be employed as anti-cancer drugs, blood sugar stabilizers, and
anti-viral agents. Herein, they were prepared using single-phase and droplet-based continuous
2
flow methods. Reactions were optimized under mixed solvent conditions of H O and
Available online
dimethylformamide, and process stability and yields were examined. Excellent process stability
and yields were achieved in considerably shortened reaction times when employing droplet-based
continuous flow synthesis.
Keywords:
Droplet-based continuous flow synthesis
Bis(indolyl)methanes
Tris(indolyl)methanes
Solid clogging
2
020 Elsevier Ltd. All rights reserved.
—
——


Corresponding author. e-mail: In1@ut.ac.kr
Corresponding author. e-mail: Chan@cnu.ac.kr

2
novel, highly efficient synthetic strategies that minimize
environmental hazards remain highly desirable.
Table 1. Synthesis of TIMs in a batch system and a single-
a
phase continuous flow system
Herein, we report on a continuous flow synthesis for the
efficient preparation of TIMs and BIMs. Continuous flow
synthesis has the advantage of reducing process time and
minimizing process risk, due to excellent heat transfer efficiency
2
3-31
and rapid mixing of reactants.
However, handling insoluble
reactants and products often leads to clogging problem in
microchannel. Although solid reactants and products are often
preferred in the pharmaceutical industry due to relatively
straightforward handling and purification procedures, it would be
realistic barriers in converting traditional batch processes into
continuous flow processes. In general, there have been attempts to
use only soluble reagents or change solvent condition, but the
methods were sometimes inefficient.
Entry
Solvent
DMF
Time (h)
Batch (%)b
CFSc
O
1
24
24
2
2d
THF
22
O
3e
4
THF
10
1
57
80
80
72
73
61
45
70
63
71
O
Ⅹh
2
H O
5f
6
MeOH
4
Ⅹi
Ⅹh
First, in order to investigate the applicability of single-phase
continuous flow processing and establish the conditions, we tested
H
2
O:DMF (4:1)
1
7
H
2
O:DMF (3:2)
O:DMF (1:1)
EA
10
12
6
Ⅹh
1
3, 32-35
previously reported batch reactions.
The use of highly water-
j
8
H
2
△
soluble oxalic acid dihydrate has an advantage, as it can be easily
removed by a simple filtration. In the batch system, a good yield
was obtained under water solvent conditions. However, under the
same conditions, continuous flow synthesis suffered difficulties in
dissolving the reactants in water. Because indole-3-
k
9
△
0f
Ⅹi
1
EtOH
3
j
1
1
CH
3
CN
6
△
2g
H
O:DMF (1:1)
9
△
j
1
2
2
carboxaldehyde was not soluble in H O, clogging by solids during
a
injection significantly reduced the stability of the process (Table
1). Therefore, achieving stable injection of reactants necessitated
the optimization of solvent conditions.
Conditions: indole (2 mmol), indole-3-carboxaldehyde (1 mmol), oxalic
b
acid dihydrate (1 mmol), solvent (5 mL), temperature (80 ℃). NMR
yield. Only applicability to single-phase continuous flow system was
denoted, because reliable yields could not be measured. PTFE tubing (OD:
c
1
.6 mm, ID: 1.0 mm, length: 16 m) and T-junction (1.25 mm thru-hole)
When dimethylformamide (DMF) and tetrahydrofuran (THF)
was tested, the reagents were completely dissolved and clogging
was not detected. However, the synthesis of TIMs was more
d
e
were used. Temperature (60 ℃). Scandium triflate (Sc(OTf)
instead of oxalic acid dehydrate, temperature (60 ℃). Temperature (25
℃
dissolved. Solid precipitated out after 10 min running. Reagents were
completely dissolved and easily injected, but solid precipitated out after 30
min running. Solid precipitated out after 60 min running.
3
, 5 mol%)
f
g
h
). Oxalic acid dihydrate (3 mmol). Reagents were not completely
i
j
2
efficient in poor solvents such as H O and MeOH, unlike BIMs
3
6
synthesis. Since the synthesized TIMs were precipitated
immediately, they could avoid being degraded. The homogeneous
synthesis was less efficient due to acid catalysed decomposition of
k
3
7
Indoles are privileged scaffolds in medicinal chemistry and
their numerous derivatives have been efficacious in the treatment
of a wide range of diseases, including infectious diseases and
TIMs under the reaction conditions. (Table S1 in ESI) To
overcome the TIMS decomposition and reagents clogging, we
employed a solvent mixture of H O and DMF in a ratio of 1:1 (See
2
cancer; thus they are promising starting points for the development
ESI, Figure S1); although this ameliorated the process to an
of novel drugs.1,
2
Bis(indolyl)methanes (BIMs) and
extent, high yields and process stability could not be
simultaneously achieved, with an additional complication of
channel blockage by insoluble products as reaction yields
increased. Likewise, solvents such as ethyl acetate, ethyl alcohol,
acetonitrile, and methyl alcohol, suffered the same
aforementioned setbacks when applied to flow synthesis.
Although sonication was generally effective for eliminating
Tris(indolyl)methanes (TIMs) reportedly induce apoptosis of
human cancer cells, providing the rationale that they could be
employed as anti-cancer drugs. Furthermore, studies have shown
that they are effective as blood sugar stabilizers and anti-viral
3
-10
agents.
BIMs and TIMs found in cruciferous vegetables,
stimulate hormonal metabolism involving estrogen. TIMs possess
antibiotic activity, and BIMs exhibit an array of biological
activities, including antimicrobial, anti-inflammatory and
selective COX-2 inhibitory activities.7, 8, 10 In addition, they can be
utilized as receptors for selective colorimetry and fluorescence
3
8
clogging, applying it over lengthy reaction times and at elevated
temperatures was impractical.
1
1, 12
measurements.
To date, various methods have been reported for the synthesis
of TIMs and BIMs. A common synthetic route is via the formation
of azafulvenium salts generated by an acid-catalysed dehydration
reaction between indoles and aldehydes; the salts are then
transformed into TIMs and BIMs through further reaction with a
1
3
second indole compound. However, the electrophilic
substitution reaction catalysed by Lewis acids (AlCl , BF ), and
protic acids (HCl) requires harsh reaction conditions, and lengthy
3
3
1
0, 14, 15
16, 17
reaction times.
acids,
Catalysts such as heteropolyacids,
solid
1
8, 19
ionic liquids,20 and transition metals
21, 22
are readily
available, however these have the drawback of requiring the use of
expensive and toxic metal ions, or additional synthetic processes
for catalyst preparation. Thus, although numerous synthetic
methodologies for these indole derivatives have been reported,
Figure 1. Comparative illustration of a) single-phase and
b) droplet-based continuous flow synthesis

3
carboxaldehyde (entry 1) and 5- cyanoindole-3-carboxaldehyde
Table 3. Synthesis of TIMs and BIMs in the droplet-based
continuous flow system
(
entry 2) to form the corresponding products 1a (70%) and 2a
a
(68%), respectively. Similarly, 2-methyl indole reacted with
indole-3-carboxaldehyde (entry 3) to form the product 3a (65%),
and the reaction of 5-cyanoindole-3-carboxaldehyde with 2-
methyl indole (entry 4) resulted in modest yields of 4a (74%). We
continued to evaluate the scope of the reaction by synthesizing
various BIMs. When benzaldehyde reacted with indole (entry 5),
2
-methyl indole (entry 6), and 5-chloroindole (entry 7), products
5
a (75%), 6a (80%), and 7a (68%) were generated, respectively.
Additionally, it was found that substituted benzaldehydes such as
-bromo- (entry 8), 4-bromo- (entry 9), and 4-chloro- (entry 10)
reacted with indole to form the corresponding products 8a, 9a, and
0a in good yields ranging from 75 to 87%. 4-Chloro-
benzaldehyde reacted with 2-methyl indole to form 11a in a yield
of 83% (entry 11). A residence time of 20 min was sufficient for
reaction completion in all cases. As proof of scalability, the
synthesis of 6a was performed continuously for 9 h, and process
stability was confirmed. The droplet-based continuous flow
synthesis offered high yields and good process stability, even
when solids were formed abruptly.
3
1
Entry
R
H
R′
H
H
H
H
H
H
Cl
H
H
H
H
R″
H
Yield (%)b
70 (1a)
68 (2a)
65 (3a)
74 (4a)
75 (5a)
80 (6a)
68 (7a)
80 (8a)
75 (9a)
87 (10a)
83 (11a)
1
2
H
CN
H
3c
CH
CH
H
3
4c
3
CN
H
5d
6e
In conclusion, we have designed the continuous flow synthetic
protocol for the synthesis of TIMs and BIMs, concurrently
optimizing for process stability and yield. The single-phase
continuous flow synthesis suffered from clogging in the channel,
while droplet-based continuous flow synthesis ensured good
process stability and high yields, within significantly shortened
reaction times. Various TIMs and BIMs were prepared using
inexpensive and non-toxic oxalic acid dihydrate under mixed
CH
3
H
7c
H
H
8d
H
3-Br
4-Br
4-Cl
4-Cl
9d
H
1
0d
H
1f
1
CH
3
2
solvent conditions of H O and DMF. The highest attained yield of
a
Conditions: indole (2 mmol), aldehyde (1 mmol), oxalic acid dihydrate
3 mmol), continuous phase (perfluoro-(2-n-butyltetrahydrofuran), DMF
2.5 mL), H O (2.5 mL), reaction time (20 min), flow rate (See ESI,
2
8
7% was obtained after 20 min at 95 ℃, using perfluoro-(2-n-butyl
(
(
tetrahydrofuran) as the continuous phase, which had the added
b
c
2
Table S1), temperature (95 ℃). Isolated yield. DMF (3 mL), H O (2
Table 2. Reaction optimization for the synthesis of TIMs in
d
e
f
mL). DMF (3 mL), H
4 mL), H O (2 mL).
2 2
O (2.5 mL). DMF (3.5 mL), H O (2 mL). DMF
a
the droplet-based continuous flow system
(
2
We then introduced a droplet-based continuous flow system in
which two immiscible phases, referred to as the continuous phase
and the droplet phase, flow through the system. Because high-
speed mixing is carried out in each droplet, this system has often
been applied to the synthesis of nanoparticles.39-42 The hydrophilic
solution injected into the hydrophobic tubing was expected to form
spherical droplets to minimize contact with the surface. More
importantly, it was expected that the continuous phase, immiscible
with the droplet phase and the solids, would expel them
simultaneously (Figure 1).
Yield (%)b
Entry
1c
Indole (mmol)
Temp (℃)
Time (min)
2
80
30
23
25
53
52
63
67
70
35
72
-
2c
3d
4
3
4
4
5
6
6
2
2
2
80
80
95
95
95
95
95
95
80
30
40
20
30
20
50
30
30
30
Initially, we tested a mineral oil as the continuous phase (See
ESI, Table S3). All reactions proceeded smoothly without
clogging, and the reaction was stable over extended periods of time
5
6
(Table 2). However, approximately 30% of the indole was
7
extracted into the mineral oil layer, necessitating the addition of
excess indole (entries 1-8). When perfluoro-(2-n-butyl
tetrahydrofuran) was used for the continuous phase, a 72% yield
was obtained, without the need for excess indole (entry 9). The
product precipitated as a solid and was not decomposed under
acidic reaction condition, but the indole-3-carboxaldehyde and
indole dissolved in the solution were decomposed to some extent.
The perfluoro-(2-n-butyl tetrahydrofuran) could be reused without
8
9e
10f
a
Conditions: indole-3-carboxaldehyde (1 mmol), oxalic acid dihydrate (3
mmol), DMF (2.5 mL), H O (2.5 mL), continuous phase (mineral oil), flow
rate (see ESI, Table S1), PTFE tubing (OD: 2.1 mm, ID: 1.5 mm, length:
4 m). NMR yield. Oxalic acid dihydrate (1 mmol). Oxalic acid
2
b
c
d
2
e
dihydrate (2 mmol). Continuous phase (perfluoro(2-n-butyltetrahydro
2
performing any purification process. N gas was also tested as the
continuous phase; however, clogging had recurred (entry 10).
f
2
furan)). Continuous phase (N gas).
advantage of being reusable without the need for purification.
Unlike sonication, the droplet-based method is tolerant of wide
temperature ranges, and is expected to enable large-scale
production of insoluble pharmaceuticals.
Under optimized reaction conditions shown in Table 2, the
scope of the reaction was evaluated with a variety of aldehydes
and indoles. (Table 3) Solvent ratio was varied slightly depending
on the solubility of the reagents, in order to achieve stable reactant
injection and a steady droplet flow. Indole reacted with indole-3-
Acknowledgments

4
We gratefully acknowledge financial support from the National
Research Foundation (NRF-2016M3A9E1918329, NRF-
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The bioactive substances, Bis(indolyl)methanes and
Tris(indolyl)methanes, were prepared using single-
phase and droplet-based continuous flow methods.
The droplet-based continuous flow synthesis
offered excellent process stability and yields.
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5
Unlike sonication, the droplet-based method is
tolerant of wide temperature ranges in production
of insoluble pharmaceuticals.
Graphical Abstract
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template box below.
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Droplet-Based Continuous Flow Synthesis of
Biologically Active Bis(indolyl)methanes and
Tris(indolyl)methanes
Leave this area blank for abstract
info.
a
a
a
a
b,
a,
Su Min Nam , Yea Seul Jang , Go Eun Son , Chan Ho Song , Insik In  and Chan Pil Park 