π-Delocalized β-carbolinium cations as potential antimalarials

Article abstract of DOI:10.1016/j.bmcl.2004.01.055

Several β-carboline compounds including natural products and their corresponding salts were synthesized and evaluated for antimalarial activity and cytotoxicity levels. Quaternary carbolinium cations showed much higher potencies than neutral β-carbolines

Full text of DOI:10.1016/j.bmcl.2004.01.055

Bioorganic & Medicinal Chemistry Letters 14 (2004) 1689–1692
ꢀ-Delocalized ꢁ-carbolinium cations as potential antimalarials
Kiyosei Takasu,a,* Tsubasa Shimogama,a Chalerm Saiin,a Hye-Sook Kim,b
Yusuke Watayab and Masataka Iharaa,*
aDepartment of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University, Aobayama,
Sendai 980-8578, Japan
bFaculty of Pharmaceutical Sciences, Okayama University, Tsushimanaka 1-1-1, Okayama, Japan
Received 3 December 2003; revised 20 January 2004; accepted 20 January 2004
Abstract—Several b-carboline compounds including natural products and their corresponding salts were synthesized and evaluated
for antimalarial activity and cytotoxicity levels. Quaternary carbolinium cations showed much higher potencies than neutral b-
carbolines and a good correlation was observed between p-delocalized lipophilic cationic structure and antimalarial efficacy.
# 2004 Elsevier Ltd. All rights reserved.
Malaria, which is caused by plasmodium protozoa, is
one of the most serious infectious diseases in tropical
and subtropical regions. Plasmodium parasites rapidly
develop resistance to new drugs, creating an ongoing
need for the development of new antimalarial com-
pounds.1 Recently we have reported that rhodacya-
nines, having a p-delocalized lipophilic cationic (DLC)
structure, exhibit strong antimalarial efficacies against
Plasmodium falciparum in vitro.2 The conceptual term,
DLC, was originally proposed by Chen et al. in their
anticancer research work.3 It has subsequently been
reported that several DLC compounds exhibit anti-
tumor activity by their selective accumulation in the
mitochondria of carcinoma cells.3,4
into DLC’s via quaternarization of the pyridine nitro-
gen atom and that the resulting b-carbolinium salts may
have higher antimalarial properties than neutral carbo-
lines. In this communication we report the synthesis of
b-carboline compounds and their quaternary salts, and
we evaluate their antimalarial and cytotoxic activity.
MVC (1a) was synthesized according to Cook’s proce-
dure for the synthesis of crenatine (1b) with some
modifications (Scheme 1).8 This involved a Pictet–
Spengler reaction of tryptamine hydrochloride (2) with
ethyl glyoxalate in ethanol, followed by acylation with
acetyl chloride. This furnished tetrahydro-b-carboline
(3) with a 44% yield (2 steps). Treatment of tetrahydro-
b-carboline (3) with 5 equivalents of DDQ produced 4-
oxocarboline (4), which proved difficult to isolate owing
to its instability, although the TLC profile of the reaction
b-Carboline alkaloids are widely found in a number of
plant and mammalian species. Some of these alkaloids,
such as manzamine A and akagerine (Fig. 1), exhibit a
variety of biological and pharmaceutical properties,
including antimalarial activity.5 Pavanand et al. isolated
the simple b-carboline, 4-methoxy-1-vinyl-b-carboline;
(MVC; Fig. 1, 1a)6 and related compounds as anti-
malarial components from Picrasma javanica B1, a
medicinal plant used in the treatment of malaria.7
However, their antimalarial potency is not so strong as
alternative compounds that are in current clinical use.
We envisaged that b-carbolines could be transformed
Keywords: Antimalarials; DLC’s; Total synthesis; b-Carbolines;
Selective toxicity.
* Corresponding authors. Tel.: +81-22-217-6879; fax: +81-22-217-
6877 (K.T.); tel.: +81-22-217-6887; fax: +81-22-217-6877 (M.I.);
e-mail: kay-t@mail.pharm.tohoku.ac.jp;
tohoku.ac.jp.
mihara@mail.pharm.
Figure 1. b-Carbolines having antimalarial activity.
0960-894X/$ - see front matter # 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.01.055

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K. Takasu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1689–1692
pletely consumed in the reaction of DDQ-oxidation of
3, the production of 1e was caused by the successive
dehydroxylation–aromatization of 4. Ester 1d was
reduced by DIBAL-H at ꢀ78 ꢁC to produce aldehyde 1f
(kumujancine) with a yield of 96%; Kumujancine (1f) is
a natural b-carboline isolated from the wood of
Picrasma quassioide.11 Finally, Wittig olefination of 1f
afforded 1a (51% yield). It is noteworthy that we
achieved the first total synthesis of the two natural pro-
ducts, MVC (1a) and kumujancine (1f). Crenatine (1b)
and the synthetic compound 4-methoxy-1-methyl-b-
carboline (1c) were also synthesized according to the
above procedure.8 Harmane (1g) and harmine (1i) were
obtained commercially, and compounds 1h and 1j
were synthesized according to standard procedures
(Fig. 2). b-Carbolinium cations (5), which correspond to
DLC’s, were prepared from the corresponding carbo-
lines 1 by simple quaternarization with either alkyl
tosylate or alkylhalide, resulting in good yields. Dihydro-
b-carbolinium cation 7, which lacks one conjugation in
the aromatic system (thus, non-DLC), was prepared
from 6. N-Methyl neutral analogue 8 was quantitatively
obtained by the treatment of carbolinium salt 5g with
aqueous NaOH.
Scheme 1. Conditions: (a) EtO2CCHO, EtOH; (b) AcCl, cat. DMAP,
Et3N, CH2Cl2 (44% for 2 steps); (c) DDQ, THF–H2O, ꢀ78 ꢁC to rt;
(d) Me2C(OMe)2, p-TsOH, benzene; then p-chloranil, rt (1d: 30%, 1e:
7% for 2 steps); (e) DIBALH, CH2Cl2 (96%); (f) Ph3P=CH2, THF
(51%).
mixture indicated an almost pure single spot with no
remaining starting material. It is known that treatment
of tetrahydro-b-carbolines with DDQ promotes C1- and
C4-oxidation in a competitive reaction.8 In this case,
the introduction of an electron-withdrawing group as a
C-1 substituent may electrochemically inhibit C-1 oxi-
dation. Crude product 4 was used in the subsequent
reaction. The reaction of product 4 with dimethoxy-
propane in the presence of PTSA under azeotropic
conditions, followed by oxidative aromatization with p-
chloranil,9 produced 1-ethoxycarbonyl-4-methoxy-b-
carboline (1d) along with demethoxy compound (1e;
kumujian A)10 with yields of 30% and 7% (2 steps),
respectively. Because the starting material was com-
The antimalarial potencies of the b-carbolines, their
corresponding salts and related compounds were eval-
uated in vitro against P. falciparum (the antimalarial
drug sensitive FCR-3 strain) and their cytotoxicities
were determined using mouse mammary tumor FM3A
cells. Selective toxicities, defined by the ratio EC50
(FM3A)/EC50 (P. falciparum), were determined.12 The
biological results are summarized in Table 1. Neutral
b-carbolines 1a, c, i and j display weak to medium inhi-
bitory effects against P. falciparum (EC50=0.5 to
3.1ꢂ10ꢀ5 M), and their cytotoxicities are comparable to
their antimalarial activity levels (runs 1–4). In contrast,
N-alkyl carbolinium salts 5 display considerably
enhanced antimalarial activities (runs 5–14) except for
Figure 2. b-Carbolines and b-carbolinium salts.

K. Takasu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1689–1692
Table 1. Antimalarial activity and cytotoxicity
1691
Run
Compd
EC50
Selective
toxicityc
P. falciparuma
FM3Ab
1
2
3
4
5
6
7
8
1a
1c
1i
5.0ꢂ10ꢀ6
2.2ꢂ10ꢀ5
2.2ꢂ10ꢀ5
3.1ꢂ10ꢀ5
1.1ꢂ10ꢀ6
1.3ꢂ10ꢀ7
9.5ꢂ10ꢀ7
3.7ꢂ10ꢀ7
1.9ꢂ10ꢀ5
2.0ꢂ10ꢀ6
1.3ꢂ10ꢀ5
1.1ꢂ10ꢀ6
4.6ꢂ10ꢀ7
1.1ꢂ10ꢀ6
1.8ꢂ10ꢀ5
1.6ꢂ10ꢀ5
4.8ꢂ10ꢀ6
1.1ꢂ10ꢀ7
3.8ꢂ10ꢀ6
1.8ꢂ10ꢀ5
1.8ꢂ10ꢀ5
3.2ꢂ10ꢀ5
8.4ꢂ10ꢀ6
1.0ꢂ10ꢀ5
>3.0ꢂ10ꢀ5 d
3.0ꢂ10ꢀ5
2.2ꢂ10ꢀ5
3.5ꢂ10ꢀ5
>2.3ꢂ10ꢀ5 e
1.9ꢂ10ꢀ5
2.2ꢂ10ꢀ5
>7.1ꢂ10ꢀ5 f
4.5ꢂ10ꢀ5
>3.8ꢂ10ꢀ5 g
2.9ꢂ10ꢀ5
1.0ꢂ10ꢀ4
0.76
0.82
0.82
1.0
7.6
77
>32
81
1.2
18
>1.8
17
48
>65
2.1
>2.4
6.0
910
Scheme 2.
1j
5a
5a0
5b
5c
5d
5g
5h
5i
5j
5j0
6
7
conditions (Scheme 2). Replacement of methyl group as
an R5 substituent by an ethyl or hydroxyethyl group
results in both an increase in antimalarial potency (5a
versus 5a0) and a decrease in cytotoxicity (5j versus 5j0),
respectively. Among our tested compounds, 2-ethyl-4-
methoxy-1-vinyl-b-carbolinium tosylate (5a0) was found
to display good antimalarial efficacy. Its selective toxi-
city is less than that of quinine, which is an antimalarial
medicine in current clinical use, but its antimalarial
activity against P. falciparum is comparable. It is addi-
tionally noteworthy that 5c is effective even against
multidrug resistant parasites (P. falciparum K1 strain)
with an EC50 value of 3.6ꢂ10ꢀ7 M.
9
10
11
12
13
14
15
16
17
18
8
Quinine
a Chloroquine sensitive strain (FCR-3).
b Mouse mammary tumor FM3A cells representing a model of host.
c Selective toxicity=EC50 value for FM3A/EC50 for P. falciparum.
d EC37 value (63% growth of FM3A was observed).
e EC10 value (90% growth).
In summary, we have synthesized b-carbolines and their
corresponding salts, including several naturally occur-
ring compounds, and evaluated them for their anti-
malarial potency in vitro. The b-carbolinium salts,
which have a DLC (p-delocalized lipophilic cationic)
structure, show a higher antimalarial potency and better
selective toxicity than non-DLC compounds. Thus, this
study indicates that transformation of specific com-
pounds into a DLC structure may prove to be a highly
effective modification methodology in antimalarial
medicinal chemistry. Currently, we are carrying out
further structural optimization of b-carbolinium salts
based on structure–activity relationships uncovered thus
far.
f EC14 value (86% growth).
g EC16 value (84% growth).
5d and h. Introduction of a methyl or ethyl group on the
pyridine nitrogen atom of 1a results in a 5-fold and a
39-fold increase in antimalarial activity, respectively (1a
versus 5a or 5a0). In addition, the cytotoxicity levels of
these substances are decreased 2–3-fold by quarter-
narization. A similar enhancement of antimalarial
effectiveness was observed by the transformation of
carboline 1 into carbolinium salts 5 (1c versus 5c, 1i
versus 5i, and 1j versus 5j or 5j0). In particular, 5j shows
a 67-fold increase in antimalarial activity compared to
the corresponding 1j. These results indicate that DLC
compounds have increased antimalarial potency within
the class of b-carbolines. In contrast, quarternarization
of dihydro-b-carboline has no effect on its biological
activity; both the neutral molecule 6 and cationic salt 7
display similarly low antimalarial activities (runs 15 and
16). The neutral N-methylharmane analogue 8 is less
active than the corresponding cation 5g (run 17).
Accordingly, this further indicates that a broad deloca-
lized system and the cationic charge of the compound,
that is a DLC structure, are critical factors in the bio-
logical properties of this family of compounds.13
Acknowledgements
We thank to Prof. Y. Ohsima and Prof. S. Kurata for
their help in using microscpic instruments. This work
was financially supported by a Grant-in-Aid from the
Tokyo Biochemical Research Foundation and a Grant-
in-Aid for Exploratory Research (No. 15659024) from
the Ministry of Education, Culture, Science, and Tech-
nology, Japan.
A preliminary structure–activity relationship analysis
concerning R1–R5 substituents was also undertaken.14
The introduction of a methoxyl group as an R1 or R2
substituent causes a 2-fold (5g versus 5i) or 5-fold (5g
versus 5c) enhancement in activity against P. falci-
parum, whereas the cytotoxicities of these substances are
decreased by these substitutions. In contrast, substitu-
tion of R3 with phenyl or ethoxycarbonyl groups results
in a remarkable decrease in antimalarial activity (runs 9
and 11). The introduction of a substituent at the indole-
NH by a methyl group causes an increase in activity (5i
versus 5j), and this protection might inhibit conversion
into an electronically neutral molecule under biological
References and notes
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K. Takasu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1689–1692
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was observed despite of cationic salts having different
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