P. Aroonkit et al. / European Journal of Medicinal Chemistry 94 (2015) 56e62
57
diversity in commercially available starting materials and the use of
transition metal catalysts which may contaminate the products.
Murray and co-workers [18] reported that halogen-substituted
isocryptolepine analogues, obtained via the direct halogenation of
the parent isocryptolepine, provided improved antimalarial activ-
ities and selectivities. However, the preparation of such analogues
by the direct halogenation was severely limited by low regiosele-
citivities and yields.
A number of isocryptolepine analogues have been evaluated
previously for their antiplasmodial and antiproliferative activities.
Murray and co-workers prepared and studied several halogen-
substituted analogues for their antiplasmodial activities [18]. The
study revealed that inclusion of halogen atoms in the structures
could significantly increase the activities of these compounds.
Fluorine is particularly of interest since it is well-known in the
literature that the presence of fluorine atoms or groups in medic-
inal compounds can lead to improved biological and physico-
chemical properties which may be suitable for developing into
active pharmaceutical ingredients [21]. Additionally, the studies of
the effect of other types of substituents besides halogen atoms are
rare in the literature. Based on the scarcity of available structur-
eeactivity relationship (SAR) information, we prepared new ana-
logues of isocryptolepine starting from ten arylmethyl azides
(2ae2j) and seven N-protected indoles (3ae3g) as shown in Fig. 1.
We were able to successfully prepare a library of nineteen iso-
cryptolepine analogues, including the parent compound (1a) and
the previously reported 1f, whose inclusions in this study were for
comparison purposes. Compound 1f was prepared previously by
Murray and co-workers [18] and was shown to possess potent
antiplasmodial activities against P. falciparum 3D7 and W2mef
strains with high selectivity index over the normal mouse embry-
onic fibroblasts (3T3) cell line. The yields of each step of our syn-
theses are summarized in Table 1.
Our research group demonstrated that isocryptolepine (1a)
could be efficiently synthesized in four steps from benzyl azide (2a;
1
4
2
3
R
R
¼ R ¼ R ¼ H) and N-phenylsulfonyl indole (3a; Ar ¼ Ph,
5 6
¼ R ¼ R ¼ H) via the key TfOH-promoted arylmethyl azide
rearrangement reaction [20]. The synthetic sequence provided
compound 1a in 65% overall yield without the need for metal cat-
alysts and harsh reaction conditions (Scheme 1). One could envi-
sion that this synthetic route would allow a rapid construction of a
library of diverse isocryptolepine analogues simply by starting from
both starting materials (2 and 3) with various and available struc-
tures with pre-defined substitution patterns for further investiga-
tion of their biological activities (see Scheme 2).
In this work, we report a library of new isocryptolepine ana-
logues and the investigation of their in vitro antiplasmodial activ-
ities against four Plasmodium falciparum strains, including
chloroquine-sensitive, chloroquine-resistant, and mefloquine-
resistant strains, and their in vitro antiproliferative activities
against four primary cancer cell lines as well as the cytotoxicity
against a normal human lung cell line for the determination of their
selectivity indices. In addition, isocryptolepine analogues with
various fluorine substituents were also investigated due to the
well-known properties of fluorine substituents in modifying the
biological and physicochemical properties of medicinal agents [21].
Of the nineteen isocryptolepine analogues (1ae1s), seventeen
(1ae1q) were directly prepared from arylmethyl azides 2ae2j and
N-protected indoles 3ae3g. For the remaining two analogues
(1re1s) the cis-fused N-tetracyclic compounds 4b and 4m, ob-
tained from the rearrangement/annulation reactions between
indole 3d and azides 2b and 2f, respectively, were first electro-
philically chlorinated [23] to give compounds 4r and 4s which were
subsequently subjected to the remaining reactions in the synthetic
sequence (entries 18e19, Table 1) as shown in Scheme 3.
2
. Results and discussion
2.1. Chemistry
2.2. Biological evaluation
In a recently reported preparation of isocryptolepine, our group
has utilized a TfOH-promoted rearrangement of arylmethyl azide
2) and subsequent interception and annulation of the resulting N-
aryliminium ion intermediate with a protected indole (3) as the key
step in obtaining the core cis-fused N-tetracyclic structure of the
natural product (4). DDQ oxidation of compound 4 then provided
the fully aromatic tetracycle 5. The arylsulfonyl group of compound
2.2.1. In vitro antiplasmodial assays
(
In the in vitro antiplasmodial assays, the twelve isocryptolepine
analogues, as free bases, were subjected to the two standard lab-
oratory P. falciparum isolates, chloroquine-sensitive (CQ-S) clone
3D7 and chloroquine-resistant (CQ-R) clone K1. The compounds
were also tested against the two recently adapted isolates, SKF58
and SRIV35, which were collected from patients in Srisaket and
Kanchanaburi provinces in Thailand in 2013, respectively. Both
isolates are chloroquine-resistant (CQ-R) and mefloquine-resistant
(MQ-R) phenotypes. In these studies, chloroquine, mefloquine,
quinine, and artesunate were employed as positive controls for all
strains. The compounds were also tested for their cytotoxicities
against the normal human embryonic lung cell, MRC-5, for deter-
mination of their selectivity indices.
5
was subsequently removed under the basic conditions (to 6),
followed by the N-methylation reaction [22] at the quinoline ni-
trogen to arrive at the isocryptolepine analogues (1). This synthetic
sequence has opened an opportunity for us to conveniently and
easily synthesize other analogues of isocryptolepine, starting from
various arylmethyl azides (2) and protected indoles (3), both of
which are widely available via a short synthesis and/or from com-
mercial sources.
Scheme 1. Synthesis of isocryptolepine (1a) by Tummatorn and co-workers [20].