Synthesis and anti-tumor activity of marine alkaloids

Article abstract of DOI:10.1016/j.bmcl.2021.128009

Marine alkaloids were divided into five categories from the perspective of anti-tumor activity. The optimization process, chemical synthesis, anti-tumor activity evaluation and structure–activity relationship of various compounds were discussed.

Full text of DOI:10.1016/j.bmcl.2021.128009

Bioorg. Med. Chem. Lett. 41 (2021) 128009
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Bioorganic & Medicinal Chemistry Letters
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Synthesis and anti-tumor activity of marine alkaloids
,
,
Shiyang Zhoua b, Gangliang Huanga *, Guangying Chenb
a College of Chemistry, Chongqing Normal University, Chongqing 401331, China
b Key Laboratory of Tropical Medicinal Resource Chemistry of Ministry of Education, Key Laboratory of Tropical Medicinal Plant Chemistry of Hainan Province, College
of Chemistry and Chemical Engineering, Hainan Normal University, Haikou, Hainan 571158, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Marine alkaloids were divided into five categories from the perspective of anti-tumor activity. The optimization
process, chemical synthesis, anti-tumor activity evaluation and structure–activity relationship of various com-
pounds were discussed.
Marine alkaloid
Synthesis
Anti-tumor activity
Introduction
In the treatment of many diseases, many alkaloids are good medicine,
but some alkaloids can be used to make pesticides for agriculture 17–19
.
Alkaloids are natural compounds (natural products) that contain
nitrogen and have significant biological activities 1. They generally have
chemical properties similar to bases. Some nitrogenous compounds
derived from plants that are not alkaline, but still be classified as alka-
loids because of their obvious biological activity 2–4. It can be seen from
the chemical structure of alkaloids that most alkaloids have complex
nitrogen heterocyclic structure and a few alkaloids are organic amine
compounds of not nitrogen heterocyclic structure 5–9. The definition of
alkaloids by the international union of pure and applied chemistry
(IUPAC) is not been strictly and exactly. For example, some vitamins,
amino acids, peptides and other nitrogenous compounds derived from
natural products do not belong to the category of alkaloids 10. Alkaloids
are widely distribute in nature, especially in plants, and also in animals,
microorganisms and marine organisms. There are many known types of
alkaloids, about 10,000 of them, and some structural formulas have not
been fully determined 11,12. As there are many types of alkaloids, each of
which has a different structural formula, their properties will be
different from each other. Although the amount of alkaloids contained in
living organisms is small, but it is closely related to human beings. It has
long been recognized that some plants or extracts containing alkaloids
can cure diseases or be used as poisons 13. Since 1806, the German
pharmacist Serturner isolated morphine from poppy, there have
been>6,000 alkaloids isolated from plants and animals. Through the
joint efforts of medical chemists, pharmacologists and other scholars,
people have found that a variety of alkaloids have anti-tumor, anti-
bacterial, anti-viral and other aspects of biological activities 14–16. So far,
nearly 100 alkaloid compounds have been used or used in clinical trials.
Because of the wide range of biological activities, the alkaloids have
been widely concerned.
The ocean is the birthplace of life and have rich in biological re-
sources, it is a huge natural products screening resources 20. So far, re-
searchers have found>300,000 species of marine organisms in the
ocean, and more than a one million new species are expect to remain
undiscovered 21–24. Marine organisms are secondary metabolites that
have formed and accumulated a large number of special chemical
structures and significant biological activities in the process of long-term
evolution and metabolism in special environments such as high salt,
high pressure, low temperature, hypoxia, and lack of light 25. Its in the
anti-viral, anti-inflammatory and anti-tumor and other aspects of the
role of significant. Marine alkaloids are a secondary metabolite of ma-
rine organisms, mainly derived from marine organisms such as sponges,
algae, coelenterates and tunica 26–29. Marine alkaloids are a kind of
alkaline natural products with important biological activities including
amine nitrogen functional groups and complex carbon skeleton ring
structure 30. The main biological activities of marine alkaloids have anti-
tumor, anti-fungus, anti-virus, anti-malaria, anti-fungal and anti-
osteoporosis. Many marine alkaloids may be used as anti-tumor, anti-
viral and anti-fungal clinical drugs or as the lead compounds for struc-
tural modification, it have good medicinal prospects 31. In recent years,
the marine drugs research has been paid more and more attention by
scholars, especially in the field of marine alkaloid drugs. Many kinds of
marine alkaloids have been found and extracted from marine organisms,
but most of the marine alkaloids extraction quantity is small and the
efficiency is low 32–35. Representative compounds of these marine
* Corresponding author.
E-mail address: huangdoctor226@163.com (G. Huang).
https://doi.org/10.1016/j.bmcl.2021.128009
Received 13 November 2020; Received in revised form 20 March 2021; Accepted 28 March 2021
Available online 6 April 2021
0960-894X/© 2021 Elsevier Ltd. All rights reserved.

S. Zhou et al.
Bioorganic & Medicinal Chemistry Letters 41 (2021) 128009
alkaloids include Viscosaline compound 1, Theonelladin C compound 2,
Polyandrocarpamine A and B compounds 7–8, Leucettamine B com-
pound 9, Granulatimide compound 10, Isogranulatimide compound 11,
Rigidin A-D compounds 12–15, Coscinamide A-C compounds 16–18,
Makaluvamine
A compound 3, Ellipticine compound 4, Neo-
amphimedine compound 5, Dispacamide compound
6
,
Fig. 1. The chemical structure of marine alkaloids.
2

S. Zhou et al.
Bioorganic & Medicinal Chemistry Letters 41 (2021) 128009
Fig. 1. (continued).
Asterriquinone 19, Dragmacidin 20, Dragmacidin A-C 21–23, Dragma-
cidin D 24, Topsentin A, B1 and B2 compounds 25–27, Nortopsentin A-C
compounds 28–30 (Fig. 1). In addition, these marine alkaloids have the
disadvantages of high cytotoxicity, low selectivity index and low activity
36. These problems will limit the clinical research and industrial devel-
opment of marine alkaloid drugs 37–39. Therefore, it has become an ur-
gent task for research and development to find marine alkaloid
analogues and derivatives that are economical and environmentally
friendly. A large number of target compounds can be obtained for ac-
tivity screening by rational drugs design, structural modification of lead
compounds and chemical synthesis 40. In this way, may be to solve the
shortage of natural marine alkaloids, and it provides a good platform for
the development of marine alkaloid derivatives and analogues 41. This
work system reviews the recent development of marine alkaloid de-
rivatives and analogues in the field of medical chemistry over the last 10
years (20010–2019). We divided marine alkaloid derivatives and ana-
logues into five types from the point of view for biological activity,
including the research of anti-tumor, anti-malaria, anti-bacterial, anti-
virus (HCV), and other type, and elaborated on these activities. We also
briefly discussed the optimization process, chemical synthesis, and anti-
tumor activity evaluation of each compounds.
3

S. Zhou et al.
Bioorganic & Medicinal Chemistry Letters 41 (2021) 128009
Synthesis and anti-tumor activity
differentiation and proliferation, uncontrolled growth, invasion and
metastasis 43–45. Intercellular division is the preparation period for
mitosis or meiosis in eukaryotic cells, during which the cell completes
the replication of DNA molecules and the synthesis of related proteins.
The interphase of cell division is much longer than the interphase of cell
In medicine, the cancer is to point to the malignant tumor that
originates from epithelial tissue, it is the most common kind of disease in
malignant tumor 42. The cancer was characterized by abnormal cell
Scheme 1. Synthesis of the marine alkaloids Granulatimide and Isogranulatimid analogues compounds 44–48, 55–58 and 68–70. Reagent and conditions: (a) Br2,
DMF, 2 h; (b) TFA, CH2Cl2, r.t., 4 h; (c) SnCl2, toluene, reflux, 24 h; (d) Pd, micro wave 300 W, 190℃, 15 min; (e) LiHMDS, THF, ꢀ 15℃ then r.t., 12 h; (f) hν, CH3CN,
r.t., 5 h; (g) (COCl)2, Et2O, 25℃, 3 h; (h) EtOH, Et3N, 78℃, 2 h; (i) i t-BuOK, THF, r.t., 12 h; ii HCl; (j) hν, CH3CN, r.t., 5 h.
4

S. Zhou et al.
Bioorganic & Medicinal Chemistry Letters 41 (2021) 128009
division 46. The interphase is usually divided into G1, S and G2 phases.
The mitotic interphase is an important preparatory process for the whole
process of mitosis. Studies have shown that G1 and G2 checkpoints, if
activated during the intermitotic period, can damage the normal repli-
cation of DNA, thereby blocking subsequent cell division 47. In this case,
it can provide valuable time to repair normal DNA. The study found that
in>50% of cancer cells, the G1 checkpoint was damaged because of
mutations in the p53 anti-oncogene. Relevant researchers suggest that
will be able to damage the DNA of the drugs and G2/M checkpoint in-
hibitor together, there may be selectively kill cancer cells into premature
and mitosis, and shall not affect the normal cell mitosis, and think that
this combination was used in the treatment of cancer is a primitive
method and promising method of chemotherapy 48. Under this biolog-
ical background, the marine alkaloids Granulatimide and Iso-
granulatimide were isolated and extracted from marine organisms with
G2/M checkpoint of inhibiting cell cycle 49. The acquisition of these two
marine alkaloids has attracted the attention of researchers, and they are
a class of inhibitors with great research value. From the perspective of
mechanism of action, these marine alkaloids have been identified as
inhibitors that can selectively inhibit checkpoint kinase 1 (Chk1), a key
enzyme in G2/M checkpoint 50,51. Relevant studies have shown that the
maleimide groups of these compounds play an important role in
affecting biological activity, and they can establish two hydrogen bonds
with Glu85 and Cys87 in the ATP binding pocket of the enzyme 52. On this
basis, Sebastien Deslandes et al 53 designed three series of Granulatimide
and Isogranulatimide analogues for in vitro evaluation of anti-tumor
activity, a kinase inhibition and G2/M checkpoint abrogation. It could
be seen from the chemical structure of the designed target compounds
that the newly designed compounds retain the skeleton structure of the
lead compounds, and the main modification was to introduce different
substituents into the benzene ring to obtain the active compounds. In the
process of synthesizing the target compounds Granulatimide and Iso-
granulatimide analogues, they used the operation described in Scheme
1. In the first series of the target compounds 45–49, the substituted in-
doles compounds 31–33 were as starting materials and brominated with
bromine in N, N-dimethylformamide (DMF) to obtain 3-bromine de-
rivatives compounds 34–36. The compound 34–36 were coupled with
pyrrole in methylene dichloride to obtain the intermediate compounds
37–39. The compounds 37–39 had a condensation reaction with mal-
eimide or methylmaleimide. In this step, the toluene was used as the
reaction solvent and SnCl2 as the reaction catalyst, and the compounds
40–43 were obtained by Michael addition. The compounds 40–43 were
cyclized and prepared by microwave irradiation with Pd as catalyst to
obtain the target componds 44–47. Meanwhile, the target compound 46
was demethylated in methylene dichloride with tribromo-based boron
to obtain the target compound 48. In the second series of the target
compounds, the compounds 34–36 were used as the starting material for
reaction, and the intermediate compounds 49–51 were obtained by
coupling reaction with pyrazole in methylene dichloride. The com-
pounds 49–51 condensates with N-t-butyldimethylsilyl (TBDMS)-
dibromo-maleimide in tetrahydrofuran (THF) to obtain low yield in-
termediate compounds 52–54. The intermediate formed in this step does
not form a ring, and the TBDMS protective group does not detach during
the reaction. The compounds 52–54 were cycloformed by acetonitrile
under light conditions to obtain the target compounds 55–57. Mean-
while, the target compound 57 was demethylated in methylene
dichloride with tribromo-based boron to obtain the target compound 58.
In the third series of the target compounds synthesis, they first converted
compounds 31–33 into compounds 62–64 in two steps by referring to
related literatures. Then compounds 62–64 reacted with N-Boc-pyr-
azole-3-acetamide in tetrahydrofuran (THF) to obtain the intermediate
compounds 65–67. Finally, the compounds 65–67 in acetonitrile, under
light conditions to form a ring reaction, and then the protection of the
Boc group (cracking reaction), the target compounds 68–70 were ob-
tained. This synthesis route used cheap reagent, the synthesis operation
was relatively simple, the use of functional group protection was easy to
group protection, and in the subsequent process of deprotection was
easy to leave. In addition, microwave irradiation was used in the syn-
thesis method, which reduced the reaction time and increased the yield.
However, this synthesis route also has some shortcomings, the yield of
some compounds was relatively low (3%), which brings great difficulties
for industrialization in the future. Further optimization of process con-
ditions is needed to improve the reaction yield and reduce the synthesis
cost. In the process of biological activity study, the growth inhibition
activity of the synthetic compounds was evaluated in vitro. The test re-
sults showed that in cancer cells, these compounds showed a degree of
sensitivity and resistance to pro-apoptotic stimuli. Of the three series of
the target compounds, the second series had the highest average growth
inhibition activity on tumor cells, with properties like the target com-
pounds 55 and 58, and the average IC50 growth inhibition concentration
was 11 nM and 16 nM, respectively. In addition, the compounds 40–43
and 52–54 were found to have good anti-tumor activities during the
activity screening of intermediates, with an average IC50 growth inhi-
bition concentration of about 20 nM, among which the compound 43
had the best activity (IC50 = 13 nM). On the basis of in vitro screening,
the ability of the compounds to eliminate the activated G2/M checkpoint
in HCT116p53 cells was further investigated (Table 1). The test data
showed that the target compound 55 had good activity (23.9), while the
activity of other compounds was weak. To gain insight into the kinase
selectivity of the synthesis compounds, they tested the selectivity of
serine/threonine kinases. The results of selectivity test showed that
these compounds had weak inhibitory activity and poor selectivity
against kinases. In addition, the compounds 55, 40 and 43 were quan-
titatively examined by electron microscopy. The results showed that the
compounds 55, 40 and 43 induced cellular inhibition, which reduced
cell proliferation by significantly expanding the size of cancer cells, thus
reflecting the tissue changes in the actin cytoskeleton. In general, ac-
cording to the existing data analysis, the target compound 55 and the
intermediate compounds 40 and 43 have good anti-tumor activities.
These compounds were worthy of further structural modification and
other pharmacological studies, so that can reach the true meaning of
drugs and become the ideal anti-cancer drugs as soon as possible.
The isolation of biological activity natural products from marine
organisms, including the most effective anti-tumor drugs found in them,
has become an important source 54–56. These anti-tumor drugs obtained
from marine organisms have structural diversity and complexity. And
most of these compounds with anti-tumor activity are alkaloids, which
are commonly referred to as marine alkaloids 56–59. Among these marine
alkaloids, a class of marine alkaloids with bis-indole structure is well
known 60. This kind of bis-indole marine alkaloid is usually isolated from
the metabolites of the deep-sea sponge, and has been found to have good
anti-inflammatory, anti-tumor, anti-viral and anti-bacterial biological
activities in the screening process 61. The chemical structure of this type
of bis-indole marine alkaloids was characterized by the fact that they are
bound by two indoles through their 3 positions to intermediate ligands
(including ring structure and chain structure). Related studies found that
Nortopsentins A-C bis-indole marine alkaloids with imidazole as the
five-memenon ring as the connector was 4.5–20.7 μM to the IC50 of P388
Table 1
Evaluation of the G2/M checkpoint abrogation.
Compounds
Mitotic index
44
9.6
55
23.9
13.9
3.9
56
58
69
0
38
5.0
39
8.3
53
5.5
Isogranulatimide
Granulatimide
17.2
19.4
5

S. Zhou et al.
Bioorganic & Medicinal Chemistry Letters 41 (2021) 128009
cells in vitro during the anti-tumor activity test, while the IC50 of P388
the biological activity of these synthesized target compounds 81–85,
they used human tumor cell lines for monolayer cell survival and pro-
liferation tests to detect the anti-tumor activity of the target compounds
in vitro. Using panels of human tumor cell lines of different source/tissue
types, they analyzed the anti-tumor activity and selectivity of the tested
compounds, and evaluated the eligible target compounds for entry into
preclinical studies. In vitro anti-tumor activity of the target compounds
81–85 against 12 human tumor cell lines was screened by monolayer
cell survival and proliferation assay (Table 2). The results of the
screening experiments showed that all the target compounds produced
cells was 0.8–2.1
μ
M of N-methylated derivatives 62–64. The results of in
vitro cytotoxicity studies indicated that the methylated derivatives had
significantly increased the activity of P388. Nortopsentins, a biological
activity compound, was separation from natural products and its weight
was small 65. In order to obtain a large number of compound, several
complete synthesis methods of Nortopsentins and analogues have been
reported 66–68. Because of the abundant biological activity of marine
alkaloids, people often regarded as new drugs or the lead compounds
with biological activity. Therefore, on this basis, the Nortopsentin was
structurally modified, and Dragmacidin analogues were synthesized by
taking six-element cyclopyridine, pyrimidine, pyrazine and pyrazinone
as intermediate ligands. These analogues have been found to have a
broad spectrum of anti-tumor activity in human tumor cells. However,
the Nortopsentin analogues were synthesized by using five-memorial-
heterocyclic imidazole ring as a ligand 69. In the activity test, it was
found that these compounds had significant anti-proliferative activity,
and the IC50 value was at the sub-micromole level. In the process of
modifying the structure of the lead compound, one or two indoles were
modified in addition to the bindoles 70,71. In the activity test results, it
was found that 3-indolyl-5-phenylpyridine exhibited anti-proliferative
cytotoxicity activity at the maximum test concentration of 100
μg/mL,
with an average IC50 value ranging from 4.4 g/mL to 0.37 g/mL. The
μ
μ
substituents on the indoles ring in the target compounds 81–85 have
electron-withdrawing and electron-donating groups. The analysis of the
drug structure–activity relationship (SAR) shows that the anti-tumor
activity of the electron-donating group on the indoles ring in the
target compounds were significantly higher than that of the electron-
withdrawing group. At the same time, there was also a SAR that the
stronger the ability of electron absorption or electron donation, the
better the anti-tumor activity. In addition, the solid space of these sub-
stituents will also affect the anti-tumor activity of the target compounds.
These target compounds showed good in vitro anti-tumor activity, with
characteristics like the target compounds 81 and 82, so the target
compounds 81 and 82 were selected for further anti-tumor activity
study. The target compounds 81 and 82 were further analyzed in
monolayer cultures of 42 human tumor cell lines, including 15 different
types of solid tumors. Relevant test results showed that the target
compounds 81 and 82 had a concentration-dependent inhibitory effect
on tumor cell growth and showed obvious cytotoxicity, with average
activity in the range of 5–15
μM and effectively inhibited CDK1 at the
level of 0.3 ~ 0.7 M. On the basis of previous work, Anna Carbon et al
μ
72 took Notopentine as the lead compound, modified its structure with
pyrrole as the intermediate connector, and modified the substituent on
two indoles, and designed Notopentine analogues with 2, 5-bis (3′-
indolyl) pyrroles structure. In the process of synthesized the target
compounds 81–85, they selected the specific synthesis operation
described in Scheme 2. The N-methylindole (compounds 71–75) used as
starting material, and the compounds were synthesized by two-steps
reaction. In the first step reaction, the Vilsmeier-Haack reaction was
used to treatment the compounds 71–75 with phosphorus oxychloride
and tetramethylsuccinamide, and the symmetrical intermediate com-
pounds 76–80 was obtained. In the second step, the compounds 76–80
was reflux reacted with ammonium acetate and acetic anhydride in
acetic acid, and the corresponding of 2, 5-bis (3′-indolyl) pyrroles the
target compounds 81–85 were obtained after treatment. The synthesis
route only used two steps to obtain the target compounds, has the simple
operation, the reaction step was few, the total yield was medium to
good, the reagent used were cheap and easy to obtain, has laid a solid
foundation for the future industrialization. However, these synthesis
methods were all classical chemical reactions, and there was no inno-
vation in the synthesis method. In the subsequent process optimization,
can be consider other new synthesis methods. In the process of screening
IC50 values of 1.54 μM and 0.67 μM, respectively. Meanwhile, in vitro
cloning experiments, the target compounds 1a-b showed selective anti-
tumor activity for human tumor transplantation, and the sensitive tumor
models were dispersed among different tumor tissue types. Therefore,
Table 2
In vitro activity of compounds 81–85.
Compounds
IC50 (μg/mL)
81
0.37
0.37
3.4
82
83
84
3.4
85
4.4
Adriamycin
0.007
Scheme 2. Synthesis of 2, 5-bis (3′-Indolyl) pyrroles analogues compounds 81–85. Reagent and conditions: (a) POCl3, N, N, N′, N′-tetramethylsuccinamide, then 55-
60℃, 8 h or r.t., 20 h; (b) NH4OAc, (CH3CO)2O, CH3COOH, reflux, 4 h.
6

S. Zhou et al.
Bioorganic & Medicinal Chemistry Letters 41 (2021) 128009
combined with these relevant data, the results indicate that the target
compounds 81 and 82 were likely to become a new anti-tumor drug,
which needs further studies, including in vivo anti-tumor activity eval-
uation and pharmacokinetic properties.
dichloromethane with methyl sulfonyl chloride to obtain intermediates
compounds 94–97. Next, the compounds 94–97 were etherized with 3-
(pyrid-3-yl) propan-1-ol by a phase transfer catalyst tetrabutyl ammo-
nium bromide, and the compounds 98-101with protective group were
obtained. Finally, the protective group of the compounds 98–101 were
removed with hydrochloric acid, and the target compounds 102–105
were obtained with the yield of 71–100%. The synthesis route has the
advantages of simple operation, low cost and easy availability of re-
agents. At the same time, the yield of each step of the synthesis route was
relatively high, and the reaction conditions were relatively mild, which
was conducive to industrial production in the future. In the course of the
anti-tumor biological activity study, they evaluated the anti-tumor ac-
tivity of all the synthesized target compounds 102–105 and in-
termediates 98–101 in vitro for colon cancer RKO-AS-45–1 and uterine
cancer HeLa (Table 3). Meanwhile, the target compounds 102–105 and
intermediates 98–101 were tested in non-cancerous human lung fibro-
blasts WI-26VA4 to evaluate the selectivity index (SI) of the compounds.
In vitro the MTT assay showed that the compounds 100 and 101, 104
had good anti-tumor activity. In RKO-AS-45–1 cells, the IC50 values were
Cancer is one of the most important health problems in the world,
and the number of deaths from it remains high, especially in developing
countries 73. In order to better treat cancer, it is very urgent to find new
drugs with small toxicity and side effects and good anti-tumor activity,
as well as ways to treatment cancer 74–76. In the search for new anti-
cancer drugs, people have set their sights on natural products. In
recent years, many new compounds have been found in the secondary
metabolites of marine organisms 77. These marine organisms have been
shown to be an important source of new compounds. Many new com-
pounds obtained from marine organisms have been found to have a good
inhibitory effect on tumor cell lines during the activity screening pro-
cess. The secondary metabolites of sponges are abundant in marine or-
ganisms, and the discovery of these metabolites has been widely
reported by researchers 78–80. Some compounds were found to have anti-
tumor activity during in vitro activity screening, and on this basis, ana-
logues of these metabolites were synthesized 81–83. Many compounds
were also found to have anti-tumor activity in the activity evaluation.
Rich structural diversity and bioactive peptides were separation from
marine sponge, and these compounds include the cyclopeptide pertha-
mides G–K, glycopeptide theonellamides A-G and 3-alkylpyridine alka-
loids (3-APAs) and Theonelladines A-D 84–86. Different kinds of
metabolites have been separation from marine sponges, and these
compounds and synthetic analogues have been widely concerned for
their cytotoxicity activity on human tumor cell lines. Theonelladine C is
an analogue of 3-alkylpyridine alkaloid, which was found to have anti-
protozoa activity and pro-apoptotic effect on human colon cancer (RKO-
AS-45-1) cell line in the activity screening. Therefore, on this basis,
Alessandra Mirtes Marques et al 87 was further modified the structure of
Theonelladine C to obtain target compounds with anti-tumor activity.
During structural design, they introduced oxygen-containing carbon
chains into the lead compound 3-alkyl chain and changed the length of
the chain to alter its anti-tumor activity 88,89. They designed a marine
alkaloid analogues of 3-alkylpyridine with alkyl chains ranging in length
from 6 to 12 carbon atoms. At the same time, a hydroxyl group was
attached to the end of the alkyl chain to improve the water solubility of
the analogues and to provide the necessary preparation for further
structural modification in the future. In the synthesis route selection of
the new 3-APA analogues compounds 102–105, they used Scheme 3
description for the synthesis operation. From the synthesis route, it
could be seen that the key step in the synthesis of these analogues was
the Williamson etherification reaction under mild conditions using
phase transfer catalysis (PTC). The first step of this synthesis route was
to selectively protect different diols (compounds 86–89) to produce
corresponding tetrahydropyronic acetal (compounds 90–93) in a yield
of 74%-89%. The compounds 90–93 were further treated in
5.1, 3.2 and 19.1
μM, respectively. In anti HeLa cells, the IC50 values
ranged from 4.0 to 9.4
μM. The drug structure activity relationship
(SAR) analysis showed that the relationship between the IC50 value and
the length of the alkyl chain of the compounds could be established.
With the increasing length of the carbon chain of the alkyl chain, the
IC50 value of the two tumor cell lines decreased continuously, among
which the compounds with 10 carbon atoms had the best anti-tumor
activity (compounds 100 and 104). However, as the carbon chain
continued to lengthen, its anti-tumor activity decreased when the car-
bon chain exceeded 10 carbon atoms. In addition, the selectivity of
compound 104 in the in vitro activity evaluation was found to be the
highest among the compounds tested, with the SI of 5.18 for RKO-AS-
45–1 and 11.65 for HeLa cells. On this basis, the compounds 100 and
Table 3
In vitro cytotoxic activity.
Compounds
IC50
(μ
M) ± SD
SI
PKO AS-
HeLa
WL-26VA4
PKO AS-
HeLa
45–1
45–1
98
>300
>300
>300
nd
nd
99
23.7 ± 1.7
5.1 ± 1.1
3.2 ± 1.7
>400
8.1 ± 2.7
4.0 ± 0.8
9.4 ± 0.7
>400
37.8 ± 5.0
6.4 ± 0.7
11.3 ± 1.4
>400
1.59
1.25
3.53
nd
4.66
1.60
1.20
nd
100
101
102
103
>300
191.8 ±
10.1
167.0 ±
10.5
nd
0.87
104
105
19.1 ± 4.4
131.9 ±
16.8
8.5 ± 2.4
8.8 ± 1.9
99.1 ± 11.2
34.1 ± 6.5
5.18
0.25
11.65
3.87
Etoposide
1.4 ± 0.6
2.7 ± 0.4
nd
-
-
Scheme 3. Synthesis of 3-alkylpyridine marine alkaloid analogues compounds 102–105. Reagent and conditions: (a) NaHSO4, DHP, DMSO, hexane, 40 ◦C, 16 h; (b)
MsCl, Et3N, CH2Cl2, r.t., 10 h; (c) 3-(pyrid-3-yl) propan-1-ol, NaOH/H2O, Bu4N+Brꢀ , Et2O, r.t., 72 h; (d) MeOH, HCl, r.t., 12 h.
7

S. Zhou et al.
Bioorganic & Medicinal Chemistry Letters 41 (2021) 128009
104 were selected for further related activities. Micronucleus and
TUNEL tests showed that the compounds 100 and 104 had mutational
effects and induced apoptosis. At the same time, the compound 104 also
altered the myogenic RKO-AS-45–1 cells in the cytoskeleton. In general,
according to the analysis of the existing test results, the compounds 100
and 104 could be used as potential anti-tumor drug candidates, or as
lead compounds for structural modification, which needs further
research and development.
β-tubulin. On the basis of previous work, Robert Scott et al 98 was further
modified the structure of the lead compound and introduced different
substituents on the C2 position to change the biological activity of the
target compounds. In structural design, two series of the target com-
pounds were designed. During the synthesis of these target compounds
108–128, they selected Scheme 4 detailed description for the synthesis
operation. In the first series, the target compounds were obtained by
two-steps reaction. The N-methylsulfonamidoacetophenone (compound
106) was used as the starting material of the reaction. The reaction was
stirred with benzaldehyde and cyanoacetamide in ethanol, the potas-
sium carbonate as acid dressing agent, and the intermediate compound
107 was obtained. In the second step, the ring formation reaction was
carried out. The sodium ethanol was prepared by reacting Na with
ethanol, and the compound 107 was reacted with the corresponding
ester to obtain the target compounds 108–122. In the second series, the
target compounds 123–128 were synthesized in one step. Also, the
compound 106 was used as the starting material of the reaction, and the
corresponding aldehydes, cyanoacetamide and MeC(OEt)3 were syn-
thesized in ethanol by pot method. In this step, potassium carbonate was
used as an acid dressing agent. The reaction temperature of 90℃ was
used for 24 h, followed by the reaction temperature of 150℃ for 3–6 h,
and the yield of 40–65% could be obtained. The synthesis route has the
advantages of simple operation, low cost and easy availability of re-
agents, and the desired target compounds could be obtained by one-step
or two-steps reaction. The one-pot method was applied to the synthesis
of target compounds or intermediates, which greatly improves the re-
action efficiency and reduces the synthesis cost. However, this synthesis
route also has some shortcomings. The yield of the reaction was
generally low, especially the yield of the target compound 111 was only
18%, which brings great difficulties for future industrialization. There-
fore, it was necessary to further optimize the reaction conditions in the
subsequent process optimization to increase the yield of the reaction.
During the activity study, HeLa cell line was used as the model of human
cervical adenocarcinoma, and MCF-7 cells were used as the model of
breast adenocarcinoma to evaluate the anti-tumor proliferation activity
of the first series of target compounds in vitro. In vitro screening showed
that except for the target compound 108, most of the other target
compounds introduced at C2 had anti-tumor cell proliferation activity.
Activity of this series of compounds for drug structure–activity rela-
tionship (SAR) analysis found that C2-aryl (compounds 111 and 112),
C2-OEt (compound 114) and branched C2-alkyl (compound 115) ac-
tivity was only two digits to a single-digit micromolar, linear C2-alkyl
Anti-tumor drugs are a class of drugs used to treat cancer, including
chemotherapy drugs and biological agents 90. In recent years, the
development of molecular oncology and molecular pharmacology has
gradually clarified the nature of tumors 91–93. The invention and appli-
cation of advanced technologies such as large-scale rapid screening,
combinatorial chemistry and genetic engineering have accelerated the
process of drug development, and the research and development of anti-
tumor drugs have entered a new era 94. After years of development,
many important advances have been made in the development of anti-
tumor drugs. However, in the face of the most serious threat to human
life and health, which accounts for>90% of malignant tumors, there is
still a lack of effective and highly specific drugs 95. On the one hand
reflects the difficulty in the development of anti-tumor drugs, on the
other hand, it also means that the development of anti-tumor drugs still
needs the application of new ideas, new technologies and new methods.
At present, drug treatment has become one of the important methods for
the clinical treatment of cancer, and the sales of anti-tumor drugs have
been increasing year by year due to the high incidence and mortality of
cancer 96. The marine alkaloids with pyrrole structure obtained from
marine organisms have abundant biological activities found in the ac-
tivity screening process. Among these marine alkaloids, Rigidins A, B, C,
and D are one kind of them. Related studies have shown that these
compounds have some potential for biological activity, but few re-
searchers have conducted in-depth studies to explore more biological
activity. Liliya V Frolova et al 97 have completed the total synthesis of
Rigidins A, B, C and D and carried out a comprehensive study on their
biological activities. It was found that L1210 cells of mouse leukemia
had good anti-proliferation activity, but the activity of cultured human
cancer cells was very low. On this basis, the structure of 7-deazahypox-
anthine, 7-deazaadenine and 7-deazapurine were synthesized by
modifying 7-azathathine skeleton of Rigidins. In the study of the activity
of its compounds, the anti-proliferation activity of tumor cells has been
greatly improved. These compounds have also been found to damage the
microtubule tissue in cancer cells by binding to the colchicine site of
Scheme 4. Synthesis of marine alkaloid Rigidins analogues compounds 108–128. Reagent and conditions: (a) benzaldehyde, cyanoacetamide, K2CO3, EtOH, reflux,
14 h; (b) corresponding ester, Na, EtOH, reflux, 10 h; (c) corresponding aldehydes, cyanoacetamide, MeC(OEt)3, K2CO3, EtOH, 90 ◦C, 24 h, then 150℃, 3–6 h.
8

S. Zhou et al.
Bioorganic & Medicinal Chemistry Letters 41 (2021) 128009
derivatives compounds116-118 in sub-micromolar concentrations
showed inhibitory effect, and C2-methyl compound (compound 119)
under the nanomolar could also show activity of inhibiting tumor cell
proliferation. When fluorine-containing groups were introduced on C2-
Me, the activity gradually decreased with the increase of the space of C2
position, that were CH3 (compound 119) > CHC2F (compound 120) >
CHF2 (compound 121) > CF3 (compound 123). It could be seen from the
activity results of these target compounds that the anti-proliferative
activity of this series of compounds was determined by the spatial
properties of the C2 substituents rather than by the electronic properties.
In vitro screening of the second series of compounds, the anti-
proliferative activity of the target compounds against U87 and A549
were added. The results showed that the anti-proliferation activity of
this series of compounds was lower than that of the target compound
119 (Table 4), and most of the target compounds exhibited inhibitory
effect at the sub-micromolarity concentration, except for the target
compound 126. The second series of target compounds introduce
different substituents on the C7 site on the basis of target compound
119. The result of such activity was that the polarity of the target
compound was reduced, which damages the permeability of the cells
and thus reduces the anti-tumor proliferation activity of the target
compounds. On the basic basis, the target compound 119 with the best
activity was selected to study the effect on microtubules in cells. The
results showed that the target compound 119 could achieve the anti-
proliferation activity of tumor cells by acting on microtubule-targeted
proteins. According to the existing data, C2-aryl and C2-alkyl-deni-
troxanthines marine alkaloid analogues have good anti-tumor activities.
In particular, the target compound 119 could be used as a candidate for
anti-tumor drugs and has the potential to develop into a new anti-tumor
drug, which needs further research and development in the future.
There are many ways to discovery drugs, and natural products are an
important way to get new drugs or lead compounds 99. Sources of nat-
ural products include compounds with certain biological activity ob-
tained from plant, animal, and microbial metabolites by modern
separation techniques. The isolation of biological activity drugs from
naturally occurring secondary metabolites has become an important
field in the development of new drugs 100–102. These natural products
have unique chemical structure, play an important role in drug discov-
ery and innovation, and have irreplaceable significance in the treatment
and prevention of human diseases. In the natural products separation
and extracted by people, the alkaline compounds and alkaloids are
activity against topoisomerase II .Based on the previous work, Sebastien
107
Boucle et al
designed compounds with praziquantel structure for
anti-tumor activity. As could be see from the chemical structure, the new
compounds were Makaluvamine A analogues compounds 135 and 136.
Meanwhile, the target compounds of pentacyclic structure were inves-
tigated as potential anti-topoisomerase II inhibitors. In the synthesis
process of the target compounds, they chose the specific operation
described in Scheme 5 to synthesize the target compounds. To obtain the
target compounds 135 and 136, they used 1, 2-phenylenediamine
(compound 129) as the starting material to reaction with bromoace-
tylbromide in tetrahydrofuran (THF) to produce dihydroquinoxalinone
compound 130. The condensation reaction of compound 130 and
dimethylacryloylchloridel in pyridine produces the compound 131. The
compound 131 was cyclized in methylene dichloride using anhydrous
aluminum chloride as catalyst, and the tricyclic intermediate compound
132 (yield 83%) was obtained. The intermediate compound 133 con-
taining amino group was obtained by two-steps reaction of nitrification
and catalytic hydrogenation of compound 132. Nitrification was the
compound 132 with nitric acid in methylene dichloride. Then the
product was reduced by catalytic hydrogenation without separation,
and the compound 133 was obtained. The compound 133 reacts with 2-
bromoiodobenzene in anhydrous dioxane to obtain the compound 134,
with a yield of 71%. Finally, the compound 134 underwent cycliniza-
tion, and through the Heck reaction generated pentacyclic compound
135. In the reaction process, anhydrous dioxane was used as the solvent,
palladium acetate as the catalyst and potassium carbonate as the acid
dressing agent, and the yield was up to 40%. At the same time, the
compound 132 reduced with boron/tetrahydrofuran, and the target
compound 136 was obtained with the yield of 81%. The synthesis route
was characterized by simple operation, inexpensive and readily avail-
able raw materials, and moderate to good yield per step. However, this
synthesis route also has some shortcomings, and used the precious metal
Pd as the reaction catalyst, which brings certain difficulties for the future
industrialization. At the same time, in the process of the compound 133
synthesis, the yield of this step was only 29%, the low yield will increase
the synthesis cost, which needs to be improved in the subsequent process
optimization. In the process of studying the biological activity of the
synthesized compounds, they evaluated the anti-tumor activity of these
compounds and the inhibition efficiency of topoisomerase II. First, they
tested the compounds against different cancer cell lines to assess its anti-
tumor effects (Table 5). They then investigated the inhibitory efficiency
of the synthesized compounds against topoisomerase II in order to
determine the mechanism of action of these newly synthesized marine
alkaloid analogues. They selected different human cancer cell lines
CACO-2, HCT-116, HUH-7, MDA-MB-231, PC-3, and NCI for in vitro
cytotoxicity activity evaluation. In vitro anti-tumor activity evaluation
test results showed that the compounds 134–136 had good anti-tumor
activity, among which the compound 136 had cytotoxicity to CACO-2,
widely concerned by researchers. Marine organism has species diversity
103
and biological activity diversity
. So far, researchers have
found>300,000 species in the ocean, and it was estimated that>1
million new species of ocean have not been found. With this quantitative
advantage, obtaining new biological activity compounds from marine
organisms has become an important pathway. At present, the research
direction of people has begun to turn to marine organisms, many bio-
logical activity marine alkaloids have been separation from marine or-
ganisms 104. So far, the anti-tumor alkaloids separation from marine
sponges include Ellipticine, Neoamphimedine and Makaluvamine A.
Between 2001 and 2010, most of the compounds separation from marine
sponges were alkaloids (marine alkaloids) and macrocyclic compounds.
During this period, a marine alkaloids with pyrrolidone structure was
separation from the marine sponge 105,106. This substance was found to
have significant cytotoxicity effects on a variety of tumor cells in vitro in
the course of biological activity studies, as well as potential biological
HCT-116, PC-3 and NCI cell lines of 15, 15, 15 and 10 μM, respec-
tively, but had no obvious toxicity to human fibroblasts. However, the
compounds 134 and 135 also have certain inhibitory effects on human
fibroblasts cell, so there was no obvious selectivity for cell inhibition. In
the process of screening the anti-topoisomerase II activity of the syn-
thesized compounds, the concentration of the compounds used for
electrophoretic analysis was 100 μM. The results of anti-topoisomerase
II activity test of the compounds showed that the compound 135
inhibited the activity of human DNA topoisomerase II better than the
positive control doxorubicin at 100 μM. Other synthetic compounds
showed no inhibition of topoisomerase II. By in vitro anti-tumor activity
of synthetic compounds screening and test of topoisomerase II activity,
could be on the basis of compounds 135 or 136 further design create
more analogue, the structure activity relationship (SAR) and drug
research, in order to choose the has stronger anti-tumor activity and
higher selectivity of target compounds.
Table 4
Anti-proliferative activities of synthesized compounds.
Compounds
IC50
(μM)
HeLa
MCF-7
U-87
A549
119
123
0.029 ± 0.001
0.27 ± 0.01
0.035 ± 0.003
0.23 ± 0.00
0.077 ± 0.002
0.90 ± 0.16
0.25 ± 0.01
0.60 ± 0.23
The natural products are rich in biological activity compounds which
have the potential to be developed into drugs 108–111. With the research
9

S. Zhou et al.
Bioorganic & Medicinal Chemistry Letters 41 (2021) 128009
Scheme 5. Synthesis of compounds 135–136. Reagent and conditions: (a) bromoacetylbromide, THF, Et3N, reflux, 2 days; (b) dimethylacryloylchloride, pyridine,
reflux, 2 h; (c) AlCl3, DCM, 24 h; (d) i HNO3, DCM/DMF, reflux, 24 h; ii H2, Pd/C, DMF/DMSO, reflux, 18 h; (e) 2-bromoiodobenzene, Pd2dba3, Xantphos, K2CO3,
dioxane, schlenk, 18 h; (f) Pd(OAc)2, Pcy3HBF4, K2CO3, DMA, reflux, 24 h; (g) BH3/THF, THF, reflux, 2 h.
activity) has been widely studied and concerned 122–126. Previous
Table 5
studies have shown that the synthesized 3-APA analogues exhibited two
Cytotoxicity of the synthesized compounds.
aspects of anti-malarial or anti-tumor biological activity in the evalua-
tion process of biological activity 127. And in these two aspects of activity
showed a certain inhibition potential, and even some of the compounds
may become ideal new drugs 128–132. Tumor is one of the major health
problems in the world, which has been concerned by people all over the
world 133–135. In the previous work, 3-APA analogues were found to
promote DNA damage, induce apoptosis, and alter two human tumor
cell lines: RKO-AS-45–1 and HeLa actin cytoskeleton during active
screening 135. The naturally occurring molecules containing thiocyanate
structures are rare, and these functional groups are mainly present in the
formation of cruciferae glucosides during the desaccharification of some
natural products with anti-tumor activity. It has also been shown that
compounds containing thiocyanate groups also have anti-parasite ef-
fects, showing high cytotoxicity in anti-parasite studies. In view of these
data, Maria Cristina S Barbosa et al 137 designed two new 3-APA ana-
logues to study their anti-tumor activity by using Theoneladin C as the
lead compound and by introducing thiocyanate groups into its chemical
structure. During the synthesis of the target compounds 141 and 142,
they chose the route described in Scheme 6 to conduct the synthesis
operation. They selected the previously reported synthetic product
compounds 1a-b as the starting material and synthesized the designed
target compounds 141 and 142 in two steps. In the first step, the hy-
droxyl compounds 137 and 138 were reacted with methyl sulfonyl
Compounds
IC50
(
μM)
HUH-
7
CaCo-
2
MDA-
HCT-
116
Pc-
NCI
Fib.
MB-231
3
Hum
134
135
136
20
20
20
20
20
15
>25
20
25
10
15
25
6
>2
5
25
6
20
15
1010
>25
and development of modern drugs, obtaining new drugs or lead com-
pounds from marine organisms has become an important source. Marine
sponges have been well studied and many biological activity compounds
have been separation from these relatively simple organisms in recent
years, including some new structural compounds 112–116. In the process
of screening these compounds for biological activity in vitro, the results
show that many of them have some specific biological activity and play
an important role in the development of new drugs 117 Lithistida is one
of the sponge, people separation get rich diversity, structure diversity of
biological activity of 3-alkylpyridine alkaloids (3-APAs), such as Visco-
saline and Theonelladin A-D 118–121. From the analysis of the structure of
these obtained marine alkaloids, it is found that 3-APAs has a pyridine
structure and an alkyl chain with variable length on the side chain, and
the position of this alkyl chain is usually in the 1 or 3 position. The
discovery that most marine alkaloids have cytotoxicity (anti-tumor
10

S. Zhou et al.
Bioorganic & Medicinal Chemistry Letters 41 (2021) 128009
Scheme 6. Synthesis of compounds 141 and 142. Reagent and conditions: (a) MsCl, Et3N, CH2Cl2, r.t., 10 h; (b) TBAB, KSCN, THF, reflux, 3 h.
chloride in methylene dichloromethane to obtain the intermediate
compounds 139 and 140, which could reach the yield of 52%-83% after
a simple treatment. The second step was accomplished by the occur-
rence of SN2 nucleophilic substitution, in which the intermediate com-
pounds 139 and 140 were converted to the corresponding target
compounds 141 and 142 containing thiocyanate groups. In this step, the
compounds 139 and 140 was substituted with potassium thiocyanate in
tetrahydrofuran (THF) and tetrabutyl ammonium bromide was used as
catalyst. The desired target compounds 141 and 142 were obtained by
nucleophilic substitution, and the yield was 55–78%. The synthesis
route has the advantages of short reaction steps, high yield of each step,
easy to control the total yield of the reaction. At the same time, the re-
agent used were cheap and easy to get, the reaction conditions was mild,
which lays a solid experimental foundation for future industrialization.
In the activity screening process of the target compounds 141 and 142,
they first evaluated the anti-malarial activity of the target compounds
(Table 6). In vitro test results showed that target compounds 141 and
142 inhibited the growth of parasites with IC50 of 5.5 and 2.3 µM,
respectively. These results indicate that these compounds have anti-
malarial activity, but the selectivity was not high, which reflects that
these compounds have high cytotoxicity to the control cell line and not
conducive to proprietary medicine. Because these compounds have good
cytotoxicity, they have certain advantages in anti-tumor. Next, they
focused on the anti-tumor activity of target compounds 141 and 142. In
vitro activity evaluation, they evaluated the cytotoxicity of the com-
pounds to human cancer lines colon cancer (RKO-AS-45–1) and cervical
cancer (HeLa) to evaluate the anti-tumor activity of target compounds
141 and 142. Meanwhile, non-tumor human cell line (lung fibroblast
WI-26VA4) was used as a reference to determine the selectivity index of
the target compounds. Because the selectivity index was one of the
important indexes to evaluate the activity of taeget compounds. The
anti-tumor activity of target compounds 141 and 142 were investigated
in vitro. The IC50 values of target compound 141 on RKO-AS-45–1 cells
high and effective (active) compounds, in the process of killing tumor
cells, should be almost impossible to kill normal cells. In determining the
selectivity index of target compounds 141 and 142, it was found that the
target compound 141 was selective for tumor cell line RKO-AS-45–1 (SI
= 3.59). In order to further investigate the cytotoxicity mechanism of
target compounds 141 and 142, they conducted a variety of gene
toxicity tests in vitro, including micronucleus assay, comet assay, Ames
assay and annexin-V/propidium iodide staining. The results of relevant
experiments showed that the synthetic alkaloids could induce the wrong
separation of chromosomes and damage DNA during the cell division of
human tumor cells, resulting in cell death. These results preliminarily
indicate that the target compound 141 may be a promising candidate for
anti-tumor chemotherapy drugs, which needs to be further studied and
developed in the future, so as to be an anti-tumor drug for clinical use as
soon as possible to treat more cancer patients.
Cancer is one of the major diseases that seriously endanger human
health in today’s society 138–142. In the treatment of cancer, there are
mainly surgical treatment, chemotherapy, radiotherapy and other
means, among which chemotherapy is the most important treatment
method currently used 143–146. Research and development of novel
compounds with unique chemical structure has become an important
research direction as anti-cancer drugs. The discovery of new drugs or
lead compounds from natural products provides an important source
147. On the basis of natural products, many derivatives or analogues with
biological diversity have been synthesized, especially in the aspect of
anti-tumor activity. Alkaloids are a kind of natural products widely
distributed in plants 148–152. They have a variety of chemical structures
and play an important role in medicine. Among these natural alkaloids,
the β-carboline alkaloids are a kind of widely distributed natural prod-
ucts with biological activities, which belong to the indoles alkaloids and
have a unique structure of tricyclic pyrido [3, 4-b] indoles ring 153–157
.
The Pityriacitrin is a marine alkaloid separation from marine organisms.
From the chemical structure analysis, it can be found that Pityriacitrin
tricyclic pyrido [3, 4-b] indoles ring structure is connected to the indoles
ring structure at the C-1 position 158–161. The Pityriacitrin has been
widely studied because of its unique chemical structure. In recent years,
some derivatives and analogues of Pityriacitrin have been separation or
synthesized by researchers and their biological activities have been
studied 162. It has been found that these compounds have a wide range of
biological activities, especially good anti-tumor activity and good
cytotoxicity. Natural products are always present in small amounts in
nature, and in order to obtain large amounts of compounds it is
impossible to extract them by the method of extraction 163–165. The
application of synthetic methods will change this situation, and a large
number of desired target compounds can be obtained through synthesis,
and HeLa cells were 0.8 and 12.4 μM, respectively, and the IC50 values of
target compound 142 on RKO-AS-45–1 cells and HeLa cells were 6.36
and 3.8 μM, respectively. Anti-tumor drugs need to be able to choose
Table 6
In vitro inhibitory concentrations (IC50) and selectivity index (SI) obtained for
the human cancer cell lines RKO-AS45-1 and HeLa cells after exposure to
different concentrations of the target compounds 141 and 142.
Compounds
IC50
(
μM) ± SD
SI
PKO-AS45-
1
HeLa
WI-26VA4
PKO-AS45-
1
HeLa
141
142
0.80 ± 0.11
12.40 ±
2.25
2.87 ±
0.85
3.59
0.23
1.27
which can be applied to relevant research or used directly as drugs.
168
Tingting Xu et al
used the Pityriacitrin as the lead compound to
6.36 ± 1.30
3.80 ± 0.58
4.83 ±
1.14
0.76
modify its structure to obtain a large number of cytotoxic compounds.
11

S. Zhou et al.
Bioorganic & Medicinal Chemistry Letters 41 (2021) 128009
From the structure of the designed target compounds, it can be seen that
the target compounds retains the chemical structure of Pityriacitrin, and
different amide groups were introduced in the C-6 position of the tri-
cyclic pyrido [3, 4-b] indoles ring, and the design results in the marine
alkaloid β-carboline analogues. In this work, they combined the phar-
macophones of β-carboline with the biologically functional amide
groups, thus synthesizing target compounds 147–162 of the structure of
β-carboline analogues. During the synthesis route, they selected Scheme
7 detailed description for the synthesis operation. The synthesis of the
target compounds 147–162 were prepared by four-steps reaction with
tryptophan (compound 143) as the starting material. In the first step, the
compound 143 was added to anhydrous methanol and then reacted with
sulfoxide chloride to obtain the crude product methyl tryptophan hy-
drochloride (compound 144) without separation. In the second step,
iodine and 1-(1H-indol-3-yl) ethanone were added to dimethylsulfoxide
(DMSO) for the reaction, then compound 144 was added to continue the
reaction, and 1-(1H-indole-3-carbonyl)-9H-pyrido [3, 4-b] indole-3-
carboxylate (compound 145) was generated by simple condensation
ring reaction. In the direct nucleophilic substitution reaction of the
compound 145, it was found that the reaction could not be carried out,
so the reaction pathway was changed. The third step was to put com-
pound 145 in the solution of sodium hydroxide. The hydrolysis reaction
easily occurs, and the intermediate 146 could be obtained. In the last
step, the compound 146 was reacted with different substituted amines.
The dimethylformamide (DMF) was used as the reaction solvent and
HOBt/EDCI/Et3N as the reaction catalyst to obtain the target com-
pounds 147–162. The synthesis uses inexpensive and readily available
reagents, and the yield of the target compounds were moderate to good
through a simple synthesis operation. This synthetic route has laid a
certain experimental foundation for future industrialization. However,
there was some problems in the green environmental protection of this
synthesis route. Reagents such as chlorinated sulfoxide were used in the
experiment, and some polluting gases were generated in the reaction
process, which cannot meet the requirements of green chemistry, and
the subsequent process conditions need to be optimized. In the course of
the biological activity study, they tested the cytotoxic effect of the target
compounds marine alkaloid β-carboline analogues compounds 147–162
in vitro. The SGC-790, A875, HepG2 and MARC145 cell lines were used
as test subjects. In vitro cytotoxicity tests result showed that the target
compounds exhibited moderate to good inhibitory activity against all
four cell lines. The activity of target compounds (159 and 160) with
direct aromatic groups on amide was lower than that of benzylamine
substituted compounds (153–158) and phenylethylamine substituted
compounds (147–152). The compounds containing 3-hydroxyl group
(compound 151) was significantly more biological activity than con-
taining 3-methoxy substituents compound (compound 148). In order to
further study the biological activity of these target compounds and to
screen out the more active compounds, the IC50 value of the target
compounds was also tested in vitro. The results showed that the target
compounds 151,158, 161 and 162 showed good inhibitory activity
(Table 7). The IC50 values of the target compound 161 were 6.82 ± 0.98,
8.43 ± 1.93, 7.69 ± 2.17 and 7.19 ± 1.43 μM, respectively. The results
of drug structure–activity relationship (SAR) showed that benzylamine
substituted compounds usually had better activity than phenylethyl-
amine substituted compounds, and the activity of these two target
compounds were obviously better than that of aromatic amine
substituted compounds, which demonstrated the importance of the link
chain between aromatic rings and amides. In addition, the biological
activity of most of the target compounds was better than that of the
β-carboline and β-carboline B, which indicated that the presence of
amide groups in the compounds was beneficial to the anti-tumor activity
of the target compounds. In addition, hydroxyl phenyl substituted
compounds (158 and 151) have significantly higher activity than other
corresponding compounds, possibly because the polarity of the hydroxyl
phenyl was favorable for binding to the target protein. The target
compound 161 was a compound containing a special sulfonyl group and
has the highest inhibitory activity. The special properties of hydrox-
yphenyl and sulfur groups will help to improve the biological activity of
the target compounds. On the basis of the dose response analysis of the
target compounds 151,158, 161 and 162 the results showed that these
target compounds had a concentration-dependent inhibitory effect on
cell lines. In general, these target compounds have good cytotoxic ac-
tivity and can be used as potential cell agents.
Table 7
Cytotoxic activities of the target compounds.
Compounds
IC50 (μM)
SGC-7901
A875
HepG2
MARC145
9e
17.65 ± 5.84
16.18 ± 2.31
6.82 ± 0.98
14.30 ± 2.57
53.58 ± 1.99
11.91 ± 0.80
8.47 ± 2.96
8.43 ± 1.93
10.46 ± 1.34
62.12 ± 17.83
8.63 ± 3.31
11.08 ± 3.33
7.69 ± 2.17
9.99 ± 1.82
66.42 ± 12.99
13.77 ± 3.75
21.77 ± 6.94
7.19 ± 1.43
9 l
9o
9p
16.27 ± 0.07
115.54 ± 8.30
5-FU
Scheme 7. Synthesis of marine alkaloid oriented β-carboline analogues compounds 147–162. Reagent and conditions: (a) SOCl2, MeOH; (b) 1-(1H-indol-3-yl)
ethanone, I2, DMSO; (c) NaOH, MeOH, r.t.; (d) NH2(CH2)nR, HOBt, EDCI, Et3N, DMF, r.t.
12

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Bioorganic & Medicinal Chemistry Letters 41 (2021) 128009
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We discussed the optimization process, chemical synthesis, anti-
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each type of compounds. On this basis, we evaluated the synthesis route
of each type of compounds, so as to provide necessary reference for
future to process optimization. The abundant SAR may be provides a
reasonable way for the design and develop of novel marine alkaloid
derivatives or analogues. It was an important way to obtain the lead
compounds from marine alkaloids, and it is very prospect to obtain a
large number of derivatives and analogues by structural modification.
These derivatives and analogues have anti-tumor activity diversity,
which combined with the SAR, the provide a good research platform for
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
32 Guzman EA, Johnson JD, Carrier MK, et al. Selective cytotoxic activity of the
marine-derived batzelline compounds against pancreatic cancer cell lines. Anti.
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34 Dasari R, Kornienko A. Multicomponent synthesis of the medicinally important
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The project was supported by key project of science and technology
research program of Chongqing Education Commission of China (No.
KJZD-K202000503). The work was also supported by the Scientific
Research Foundation for the Returned Overseas Chinese Scholars, State
Education Ministry (No. 2015-1098), Chongqing Key Research Project
of Basic Science & Frontier Technology (No. cstc2017jcyjBX0012),
China.
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