Synthesis and SAR studies of praziquantel derivatives with activity against Schistosoma japonicum

Article abstract of DOI:10.3390/molecules18089163

The synthesis and structure-activity relationship (SAR) studies of praziquantel derivatives with activity against adult Schistosoma japonicum are described. Several of them showed better worm killing activity than praziquantel and could serve as leads for further optimization.

Full text of DOI:10.3390/molecules18089163

Molecules 2013, 18, 9163-9178; doi:10.3390/molecules18089163
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis and SAR Studies of Praziquantel Derivatives with
Activity against Schistosoma japonicum
Wen-Long Wang 1,2,†, Li-Jun Song 3,†, Xia Chen 1, Xu-Ren Yin 3, Wen-Hua Fan 2, Gu-Ping Wang 1,
Chuan-Xin Yu 3,* and Bainian Feng 1,2,
*
1
School of Pharmaceutical Science, Jiangnan University, Wuxi 214122, China;
E-Mails: wenlongwang@jiangnan.edu.cn (W.-L.W.); chenxia19891219@126.com (X.C.);
p85615822@yeah.net (G.-P.W.)
2
3
Jiangshu Alpha Biopharmaceuticals, Inc., Wuxi 214122, China;
E-Mail: fanwenhua1987@163.com (W.-H.F.)
Key Laboratory on Technology for Parasitic Disease Prevention and Control, Ministry of Health,
Jiangsu Institute of Parasitic Diseases, Wuxi 214064, China; E-Mails: songlijun1025@163.com (L.-J.S.);
yinxuren@sohu.com (X.-R.Y.)
†
These authors contributed equally to this work.
* Authors to whom correspondence should be addressed; E-Mails: chxnyu@163.com (C.-X.Y.);
fengbainian@jiangnan.edu.cn (B.F.); Tel./Fax: +86-510-8519-7052 (B.F.).
Received: 25 June 2013; in revised form: 24 July 2013 / Accepted: 25 July 2013 /
Published: 31 July 2013
Abstract: The synthesis and structure-activity relationship (SAR) studies of praziquantel
derivatives with activity against adult Schistosoma japonicum are described. Several of
them showed better worm killing activity than praziquantel and could serve as leads for
further optimization.
Keywords: praziquantel derivatives; antischistosomal activity; Schistosoma japonicum;
neglected tropical disease
1. Introduction
Schistosomiasis is a neglected tropical disease (NTD) caused by blood-dwelling trematodes
belonging to the genus Schistosoma. Schistosoma haematobium, Schistosoma japonicum, and

Molecules 2013, 18
9164
Schistosoma mansoni are the main species parasitizing humans [1]. It has been estimated that some 779
million people are at risk for schistosomiasis transmission, with 207 million infected in 76 countries
and territories [1,2]. In China, the disease caused by Schistosoma japonicum remains a major public
health concern with more than 280 thousand people infected [3]. In the absence of a vaccine,
praziquantel (PZQ) has been the only drug recommended by the World Health Organization for the
treatment and control of schistosomiasis through mass drug administration (MDA) programs for
almost three decades [4]. However, PZQ does not prevent reinfection, and it is inactive against
juvenile schistosomes. Furthermore, it has only a limited effect on developed liver and spleen lesions,
as well as on the emergent schistosome phenotypes that are resistant to PZQ chemotherapy [5,6].
Therefore, it is imperative to develop new antischistosomal agents. A number of recent studies were
directed towards exploration of new chemical entities from natural products [7–10] and towards the
identification of additional drug targets for schistosomiasis [11–15].
In addition, there are two other major issues with regard to PZQ: (1) the mechanism of action of
PZQ is uncertain [16] and (2) as currently administered, PZQ is a racemic mixture [16]. It is a great
challenge to prepare (−)-PZQ inexpensively and stereoselectively [17]. The less active (+)-PZQ
enantiomer has well been documented as having a bitter and disgusting taste [18]. Recently, many
synthetic routes for PZQ and its derivatives have been developed [19,20], providing more
opportunities to make novel PZQ derivatives as potential drugs for the treatment of schistosomiasis.
Figure 1. Chemical structure of praziquanel (PZQ) and some reported derivatives.
Ever since the discovery of PZQ, there has been interest in the elucidation of the structure–activity
relationships (SAR) [21], mostly from data generated using S. mansoni, and to some extent with data
for S. japonicum. There are five positions amenable for chemical modification in the PZQ molecular
structure [16,22] (Figure 1). Among these, only position R1 has been heavily investigated [21]. Recently,
more variants at the R1 position as well as PZQ-ozonide hybrids were synthesized and tested against
juvenile and adult stages of S. mansoni in vitro and in vivo. Unfortunately, all the variants showed
decreased activity compared to the parent compound PZQ [23]. Patra and co-workers reported PZQ

Molecules 2013, 18
9165
analogues that replaced the cyclohexyl group at position 2 with different ferrocenyl moieties.
Unfortunately, the Fc-PZQ derivatives were considerably less active than PZQ when tested against
S. mansoni in vitro [22]. Recently, Gasser and coworkers reported Cr-PZQ derivatives with
comparable antischistosomal activities to PZQ when tested against adult S. mansoni [24]. In 2007, the
Todd group first reported several derivatives through nitration and amination of the C10 position of the
aromatic ring on PZQ. However, these compounds showed significantly decreased worm killing
activity against S. mansoni in vitro [25]. In 2012, the Qiao group synthesized eight PZQ/peroxide
conjugates, with some compounds showing superior worm-killing activity against S. japonicum in
in vitro assays compared to PZQ [26]. Recently, the Qiao group reported aromatic ring-modified
praziquantel derivatives with activity against both juvenile and adult S. japonicum, and a compound
with a bromide atom at C8 position showed higher potency against adult S. japonicum than PZQ
in vitro and comparable antischistosomal activity to PZQ in vivo. However, none of them showed
superior worm-killing ability to PZQ [27]. The Rao group developed an efficient synthetic route to
provide structurally diverse analogues for SAR studies. Several PZQ analogues with an indol ring were
synthesized and tested against adult S. mansoni. Unfortunately, no compounds showed better activity
than PZQ in vitro [28]. Taken together, all this data indicates that although much work has been done
on PZQ, it is still necessary to further expand the range of PZQ analogues in order to obtain chemical
probes to elucidate the real biological mechanism of PZQ, and to generate alternative new
chemotherapies. Therefore, we became interested in the synthesis of new PZQ analogues to find new
potential antischistosomal agents against S. japonicum.
2. Results and Discussion
2.1. Chemistry
Due to the lack of SAR details from S. japonicum, we initially resynthesized some PZQ analogues
with position 2 substitution and evaluated their worm-killing activities against S. japonicum (Scheme 1).
The racemic PZQ from Aldrich was hydrolyzed by 2 N HCl to yield praziquanamine (1) using the
method described by the Vennerstrom group [23], followed by coupling with acyl chlorides to yield
compounds 2–12. Reductive amination reactions of appropriate aldehydes with praziquanamine (1)
were performed with sodium triacetoxyborohydride in dichloroethane at room temperature to afford
compounds 13–15 in yields ranging from 27% to 35%. Praziquanamine (1) was reduced by lithium
aluminum hydride to yield compound 16, followed by acylation to yield compounds 17–22. To extend
the SAR studies, the aromatic moiety was replaced by a thiophene ring, and compound 23 was
synthesized using the method described by Frehel and his coworkers [29]. Compound 23 was
hydrolyzed with 2N HCl to yield amine 24, followed by acylation with appropriate acyl chlorides to
yield compounds 25–33 (Scheme 2). As in previous reports [22], we also observed the major isomer in
mixtures with its minor enantiomer with respect to the 11b position on the ring system for compounds
1–33. The results were also in agreement with the nature of the commercially available PZQ [19,30]
and its thiophene analogue 23 [20].

Molecules 2013, 18
9166
Scheme 1. Synthesis of PZQ analogues 1–22.
Reagents and conditions: (a) 2 N HCl, reflux, overnight, 62%; (b) TFAA, Et3N, DCM, 62% or RCOCl, Et3N,
DCM, 29%–50%; (c) RCHO, NaBH4, HOAc, MeOH, 0 °C to 60 °C, 27%–35%; (d) LiAlH4, THF, 0 °C, 35%;
(e) RCOCl, Et3N, DCM, 32%–69%.
Scheme 2. Synthesis of PZQ analogues 24–33.
Reagents and conditions: (f) 2 N HCl, reflux, overnight, 78%; (g) RCOCl, Et3N, DCM, 40%–76%.
2.2. Biological Activities
The biological results are listed in Table 1. Among compounds with an aliphatic cyclic ring (2–4
and PZQ), the worm-killing activity was improved by increasing the ring size. Compounds with an
acetyl group (i.e., 5) and with an isobutyryl group (i.e., 6) lacked worm-killing activity. Among
compounds with a halogen atom, compound 7 with the chloroacetyl group killed 100% of the worms at
the concentration of 5 M, showing better activity than PZQ. However, compound 8 with a
2,2,2-trifluoroacetyl group lost worm killing activity. Replacement of the cyclohexyl ring with a
heterocyclic ring, (compound 11 with a thiophene-2-carbonyl group) led to a similar activity to that of
PZQ. However, compound 10 with an isonicotinoyl group and compound 12 with a furan-2-carbonyl
group did not show any worm-killing activity below the concentration of 100 M. Compound 9, with a
benzoyl group, showed similar activity as PZQ. Replacement of the carbonyl group at position 2 with
CH2 gave compounds 13–15, which showed no worm-killing activity. The results indicated that the
amide bond at position 2 is essential for activity against S. japonicum. The SAR results displayed
similarities between compounds 17–22 and compounds containing amide (3, 4, PZQ, 9, 11, and 7),
showing decreased activity. The results indicated that the amide bond at position 4 was very important

Molecules 2013, 18
9167
to maintain the activity of PZQ derivatives. The SAR results from S. japonicum adult worms were
similar with those from S. mansoni [21]. However, it still gave us valuable information to explore
novel PZQ derivatives as antischistosomal agents.
To enlarge the chemical space of potential drug candidates, replacement of the phenyl ring on PZQ
with a thiophene ring gave compounds 23–33. Among compounds with an alkyl group (i.e., 23, 25–29),
compound 23 with a cyclohexanecarbonyl group showed the best worm-killing activity, and it showed
similar activity to PZQ. However, the worm-killing activity of compounds 24–29 was dramatically
decreased. The amine-containing compound 24 had no activity. Among compounds with an aromatic
ring, compound 30 with a benzoyl group showed slight worm-killing activity, while compound 31 with
a thiophene-2-carbonyl group and compound 32 with a furan-2-carbonyl group had no activity.
Interestingly, compound 33 with a chloroacetyl group showed good worm killing activity at 10 M,
similar to PZQ derivative 7. Among the compounds reported here, compounds 7, 22 and 33 with a
chloroacetyl group showed better antischistosomal activity than PZQ. The evaluation of their
cytotoxity is in progress. There have been many compounds with a chloroacetyl group under
development [31–33]. For example, TNP470 [O-(chloroacetylcarbamoyl)fumagillol], which was
derived from the natural product fumagillin, was in clinical trials for Kaposi’s sarcoma [34], as well as
renal, brain, breast, cervical and prostate cancers [35].
Table 1. Worm-killing activity on S. japonicum adult worms in vitro by compounds 1–33.
Compound
R
X
Y
Z
Killing activity a
Conc (M) b
24 h
48 h
n.e
72 h
n.e
n.e
Vehicle
10
25
25.0%D 25.0%D 37.5%D
25.0%D 25.0%D 62.5%D
25.0%D 37.5%D 62.5%D
37.5%D 50.0%D 87.5%D
PZQ
50
100
100
1 d, e
2 d, e
3 e
None
O
O
O
CH=CH
CH=CH
CH=CH
None
O
n.e.
n.e.
n.e.
100
sluggish
25.0%
75.0%
O
10
25
n.e.
n.e.
n.e.
n.e.
n.e.
sluggish
10.0%D
50
sluggish 75.0%D
100
10
sluggish sluggish 87.5%D
sluggish sluggish 37.5%D
sluggish sluggish 50.0%D
sluggish sluggish 100%D
12.5%D 12.5%D 75.0%D
4 e
O
CH=CH
O
25
50
100

Molecules 2013, 18
9168
Table 1. Cont.
Compound
R
X
Y
Z
Killing activity a
Conc (M) b
24 h
48 h
72 h
5 d, e
6 d, e
7 c, e
CH3
O
O
O
CH=CH
CH=CH
CH=CH
O
O
O
100
n.e.
n.e.
n.e.
n.e.
n.e.
100
n.e.
20.0%D
n.e.
1
3
n.e.
n.e.
ClCH2
25.0%D 25.0%D
5
70.0%D 90.0%D 100%D
8
100%D
100%D
100%D
100%D
100%D
n.e.
100%D
100%D
100%D
100%D
100%D
n.e.
100%D
100%D
100%D
100%D
100%D
n.e.
10
25
50
100
100
8 d, e
9 e
O
O
CH=CH
CH=CH
O
O
10
25
n.e.
n.e.
55.6%D
sluggish 25.0%D 87.5%D
18.2%D 36.4%D 45.5%D
45.5%D 45.5%D 81.8%D
50
100
100
10 d, e
11 e
O
O
CH=CH
CH=CH
O
O
n.e.
n.e.
sluggish
10
25
n.e.
n.e.
n.e.
n.e.
62.5%D
62.5%D
50
25.0%D 37.5%D 87.5%D
14.3%D 28.6%D 87.5%D
100
100
12 d, e
13 d
O
O
CH=CH
CH=CH
O
n.e.
n.e.
n.e.
H2
100
n.e.
n.e.
n.e.
14 d, e
15 d
16 d, e
17 d
O
O
CH=CH
CH=CH
CH=CH
CH=CH
H2
H2
100
100
100
100
n.e.
n.e.
n.e.
n.e.
n.e.
n.e.
n.e.
n.e.
n.e.
n.e.
n.e.
n.e.
H2
H2
None
O
None
H2
H2
H2
CH=CH
CH=CH
CH=CH
O
O
O
10
25
sluggish sluggish sluggish
sluggish 18.2%D 18.2%D
10.0%D 20.0%D 50.0%D
25.0%D 25.0%D 75.0%D
18
19
50
100
10
n.e.
n.e.
n.e.
n.e.
25
14.3%D 57.1%D
50
sluggih 33.3%D 66.7%D
sluggish sluggish 85.7%D
100
100
20 d, e
n.e.
n.e.
n.e.
21 d
H2
H2
CH=CH
CH=CH
O
O
100
n.e.
n.e.
n.e.
n.e.
10
25
sluggish 50.0%D
22
ClCH2
87.5%D 100%D
100%D
100%D
100%D
50
100%D
100%D
100%D
100%D
100

Molecules 2013, 18
9169
Table 1. Cont.
Compound
23 e
R
X
Y
Z
Killing activity a
24 h 48 h
Conc (M) b
72 h
O
S
O
10
25
sluggish 75.0%D 75.0%D
sluggish 87.5%D 87.5%D
50
25.0%
75.0%D 87.5%D
100
100
28.6%D 71.4%D 71.4%D
24 d
25 d
None
O
O
S
S
None
O
n.e.
n.e.
n.e.
n.e.
100
100
12.5%D 37.5%D
sluggish sluggish
26 d
O
S
O
n.e.
27 d
28 d
29 d
30 e
O
O
O
O
S
S
S
S
O
O
O
O
100
100
100
n.e.
n.e.
n.e.
n.e.
sluggish
n.e.
sluggish sluggish sluggish
10
25
n.e.
n.e.
n.e.
n.e.
n.e.
sluggish sluggish
14.3%D 14.3%D
50
100
100
sluggish 37.5%D 75.0%D
31 d, e
O
S
O
n.e.
n.e.
n.e.
32 d, e
33 c
O
O
S
S
O
O
100
n.e.
n.e.
n.e.
1
3
n.e.
n.e.
n.e.
n.e.
n.e.
ClCH2
n.e.
5
sluggish
75%D
100%D
100%D
100%D
100%D
25%D
37.5%D
8
87.5%D 87.5%D
10
25
50
100
100%D
100%D
100%D
100%D
100%D
100%D
100%D
100%D
a
Data collected by visual examination of worm movement and shape (the values were the averages of two
tests); n.e. (no effect): all worms are scored as active in culture with typical appearance; sluggish: worm
movement is significantly reduced; % D = number of dead worms/total number of worms observed, and dead
worms judged by lack of movement within 2 minutes in addition to morphological and tegumental alterations;
b The concentration of the chemicals on S. japonicum adult worms in vitro; c Starting concentration of 1 M
d
when the chemicals killed 100% of the worms at the concentration of 10 M; Data only showed the
concentration of 100 M when the chemicals have no effect on the worms at 10, 25 and 50 M; e Reported in
References [20,21,29] and references therein.
3. Experimental
3.1. Chemistry
All chemicals were reagent grade and used as purchased. All reactions were performed under an
inert atmosphere of dry argon or nitrogen using distilled dry solvent. 1H-NMR spectra (400 MHz) were
recorded on a Bruker AVⅢ 400 MHz spectrometer. The chemical shifts were reported in (ppm) using

Molecules 2013, 18
9170
the 7.26 signal of CDCl3 (1H-NMR) as internal standards. Mass spectra (MS) were obtained on a
Waters Micromass Platform LCZ Mass Spectrometer.
3.1.1. Procedure for the Preparation of Compound 1
A mixture of PZQ (10 g, 32 mmol) and 2 N HCl (50 mL) was refluxed overnight. After PZQ was
consumed, the reaction solution was cooled to room temperature, neutralized with NaHCO3 (aq.) and
then extracted with DCM/MeOH (V/V 10/1, 100 mL × 3). The organic layer was washed with water
and brine, dried with anhydrous Na2SO4 and concentrated to get the crude product. The crude product
was washed with petroleum ether/EtOAc (V/V 10/1) to yield compound 1 (4 g, 62%) as a yellow solid.
1H-NMR (CDCl3) δ: 2.73–3.03 (m, 4H), 3.48–3.77 (m, 3H), 4.79–4.89 (m, 2H), 7.13–7.27 (m, 4H);
MS (ESI): m/z calcd for C12H15N2O [M+H]+: 203.1, found: 203.4.
3.1.2. Procedure for the Preparation of Compound 8
A stirred solution of compound 1 (500 mg, 2.5 mmol) in DCM (20 mL) was added to trifluoroacetic
anhydride (275 L, 2 mmol) and triethylamine (275 L, 2 mmol). After the addition, the mixture was
stirred at room temperature overnight. The reaction residue was poured into water, and extracted with
EtOAc (50 mL × 3). The organic phases were then processed in the usual way and chromatographed
1
(1:1 petroleum ether/EtOAc) to afford compound 8 (450 mg, 62%) as a white solid. H-NMR
(CDCl3) δ:2.96 (m), 3.38 (dd, J = 10.8 Hz, 14.0 Hz) (total 4H), 4.00 (d, J = 18.4 Hz), 4.26 (d, J = 17.6 Hz)
(total 1H), 4.61 (dd, J = 14.0Hz, 18.0 Hz, 1H), 4.84-5.10 (m, 3H), 7.16–7.34 (m, 4H); MS (ESI): m/z
calcd. for [M+H]+: 299.1, found: 299.2.
3.1.3. General Procedure for the Preparation of Derivatives 2–7 and 9–12
A stirred solution of compound 1 (500 mg, 2.5 mmol) in DCM (50 mL) was added to
cyclopropanecarbonyl chloride (420 L, 3.7 mmol) at 0 °C slowly. After the addition, the mixture was
stirred at room temperature overnight. The reaction residue was poured into NaHCO3 (aq.), extracted
with DCM (250 mL), washed with water and brine/dried over Na2SO4. The organic phases were then
processed in the usual way and chromatographed (1:1 petroleum ether/EtOAc) to afforded compound 2
1
(350 mg, 49%). H-NMR (CDCl3) δ: 0.89 (m, 2H), 1.04-1.15 (m, 2H), 1.73 (m, 1H), 2.79–3.02
(m, 4H), 4.23 (d, J = 17.2 Hz, 1H), 4.71 (d, J = 17.6 Hz, 1H), 4.84-4.87 (m, 2H), 5.15 (d, J = 12.8 Hz,
1H), 7.20–7.28 (m, 4H); MS (ESI): m/z calcd. for [M+H]+: 271.1, found: 271.2. The following
compounds were similarly prepared.
2-(Cyclobutanecarbonyl)-2,3,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-4(11bH)-one (3). Yield = 40%;
1H-NMR (CDCl3) δ: 1.88-2.11 (m, 2H), 2.19–2.27 (m, 2H), 2.32–2.50 (m, 2H), 2.78–3.03 (m, 4H),
3.15–3.42 (m, 1H), 3.85 (d, J = 18.8 Hz), 3.99 (d, J = 17.2 Hz) (total 1H), 4.27 (d, J = 17.6 Hz, 1H),
4.74–4.91 (m, 2H), 5.14 (m, 1H), 7.17–7.33 (m, 4H); MS (ESI): m/z calcd. for [M+H]+: 285.2,
found: 285.3.
2-(Cyclopentanecarbonyl)-2,3,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-4(11bH)-one (4). Yield = 43%;
1H-NMR (CDCl3) δ: 1.62–1.96 (m, 8H), 2.78–3.02 (m, 5H), 3.54–3.90 (m, 1H), 4.10 (d, J = 17.2 Hz),

Molecules 2013, 18
9171
4.51 (d, J = 17.2 Hz) (total 1H), 4.82–4.4.91 (m), 5.18 (dd, J = 2.8 Hz, 13.2 Hz)(total 3H), 7.16–7.30
(m, 4H); MS (ESI): m/z calcd. for [M+H]+: 299.2, found: 299.4 [M+H]+.
1
2-Acetyl-2,3,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-4(11bH)-one (5). Yield = 43%; H-NMR
(CDCl3) δ: 2.19 (s), 2.25 (s) (total 3H), 2.79–3.01 (m), 3.29–3.35 (m) (total 4H), 3.92 (d, J = 18.4 Hz),
4.11 (d, J = 17.6 Hz) (total 1H), 4.31–4.47 (m), 4.38 (d, J = 17.6 Hz) (total 1H), 4.77–4.89 (m, 2H),
5.13–5.18 (m, 1H), 7.20–7.33 (m, 4H); MS (ESI): m/z calcd. for [M+H]+: 245.1, found: 245.1.
1
2-Isobutyryl-2,3,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-4(11bH)-one (6). Yield = 43%; H-
NMR (CDCl3) δ: 1.10 (d, J = 6.8 Hz), 1.13 (d, J = 6.8Hz) (total 6H), 2.71–2.97 (m), 3.21 (m) (total
5H), 3.81 (d, J = 18.4 Hz), 4.01–4.09 (m) (total 1H), 4.41(d, J = 17.2 Hz, 1H), 4.76 (m, 2H), 5.11 (dd,
J = 2.4 Hz, 12.8 Hz, 1H), 7.12–7.23 (m, 4H); MS (ESI): m/z calcd. for [M+H]+: 273.2, found: 273.0.
2-(2-Chloroacetyl)-2,3,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-4(11bH)-one (7). Yield = 44%;
1H-NMR (CDCl3) δ: 2.78–2.96 (m), 3.28–3.34 (m) (total 4H), 3.89 (d, J = 18.4 Hz), 4.19–4.26 (m) (total
1H), 4.19 (s), 4.15 (s) (total 2H), 4.42–4.46 (m, 1H), 4.77–4.88 (m, 2H), 5.02–5.08 (m, 1H), 7.19–7.28
(m, 4H); MS (ESI): m/z calcd. for [M+H]+: 279.1/281.1, found: 279.2/281.3.
1
2-Benzoyl-2,3,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-4(11bH)-one (9). Yield = 34%; H-NMR
(CDCl3) δ: 2.77–3.10 (m, 4H), 4.06–4.11 (m, 1H), 4.25–4.39 (m, 1H), 4.82–4.99 (m, 2H), 5.25
(m, 1H), 7.19–7.49 (m, 9H); MS (ESI): m/z calcd for [M+H]+: 307.1, found: 307.2.
2-Isonicotinoyl-2,3,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-4(11bH)-one (10). Yield = 29%;
1H-NMR (CDCl3) δ: 2.80–3.30 (m, 4H), 4.10–4.27 (m, 2H), 4.81–5.24 (m, 3H), 6.75 (m), 7.23–7.39
(m) (total 6H), 8.79 (2H); MS (ESI): m/z calcd. for [M+H]+: 308.1, found: 308.4.
2-(Thiophene-2-carbonyl)-2,3,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-4(11bH)-one (11). Yield = 29%;
1H-NMR (CDCl3) δ: 2.80–2.85 (m, 1H), 2.90–3.04 (m, 2H), 3.12–3.18 (m, 1H), 4.27 (d, J = 17.6 Hz,
1H), 4.86 (m, 2H), 5.03 (dd, J = 4.0 Hz, 10.8 Hz, 1H), 5.11 (d, J = 11.2 Hz, 1H), 7.14 (dd, J = 3.6 Hz,
4.8 Hz, 1H), 7.22-7.30 (m, 4H), 7.46 (d, J = 3.6 Hz, 1H), 7.57 (d, J = 4.4 Hz, 1H); MS (ESI): m/z calcd.
for [M+H]+: 313.1, found: 313.2.
2-(Furan-2-carbonyl)-2,3,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-4(11bH)-one (12). Yield = 50%;
1H-NMR (CDCl3) δ: 2.80–3.19 (m, 4H), 4.29(m, 1H), 4.87(d, J = 11.2 Hz, 1H), 5.01(d, J = 15.2 Hz,
2H), 5.19 (d, J = 11.2 Hz, 1H), 6.56 (d, J = 3.2 Hz, 1H), 7.19 (d, J = 3.6 Hz, 1H), 7.22–7.31 (m, 4H),
7.58 (s, 1H); MS (ESI): m/z calcd. for [M+H]+: 297.1, found: 297.2.
3.1.4. General Procedure for the Preparation of Derivatives 13–15
To a solution of compound 1 (305 mg, 1.5 mmol) in methanol (10 mL), cyclohexanecarbaldehyde
(186 μL, 1.5 mmol) was added at 0 °C, followed by acetic acid (170 L, 3.0 mmol) addition. The
mixture was thus maintained at 0 °C for 1 h. Then it was heated at 60 °C for 2 h. After cooled to 0 °C,
NaBH4 (0.45 g, 12.0 mmol) was added by portions. The reaction mixture was stirred at 60–70 °C for
12 h, followed by evaporation to remove methanol. The residue was diluted with water (30 mL) and

Molecules 2013, 18
9172
extracted with ethyl acetate (30 mL × 3). The organic phases were then processed in the usual way and
chromatographed (2:1 petroleum ether/EtOAc) to afforded compound 13 (140 mg, 32%) as a white
1
solid. H-NMR (CDCl3) δ: 0.89–0.95 (m, 4H), 1.59–1.83 (m, 6H), 2.23–2.35 (m, 3H), 2.75–2.99
(m, 4H), 3.46–3.60 (m, 3H), 4.80–4.90 (m, 2H), 7.16–7.28 (m, 4H); MS (ESI): m/z calcd. for [M+H]+:
299.2, found: 299.3. The following compounds were similarly prepared.
1
2-Benzyl-2,3,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-4(11bH)-one (14). Yield = 35%; H-NMR
(CDCl3) δ: 2.37 (t, J = 10.8 Hz, 1H), 2.75 (d, J = 14.8 Hz, 1H), 2.86–2.97 (m, 3H), 3.49–3.59 (m, 2H),
3.64 (AB, J = 13.2 Hz, 2H), 4.78 (m, 1H), 4.87 (m, 1H), 6.97-7.36 (m, 9H); MS (ESI): m/z calcd. for
[M+H]+: 293.2, found: 293.4.
2-(Thiophen-2-ylmethyl)-2,3,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-4(11bH)-one (15). Yield =
1
27%; H-NMR (CDCl3) δ: 2.37 (t, J = 10.4 Hz, 1H), 2.75 (d, J = 14.8 Hz, 1H), 2.89–3.02 (m, 3H),
3.49–3.63 (m, 2H), 3.86 (s, 2H), 4.79 (m, 1H), 4.89(m, 1H), 6.97–7.29 (m, 7H). MS (ESI): m/z calcd.
for [M+H]+: 299.1, found: 299.0.
3.1.5. Procedure for the Preparation of Compound 16
Compound 1 (3.03 g, 15 mmol) was added to a solution of lithium aluminum hydride (1.14 g,
30 mmol) in tetrahydrofuran (63 mL) and the mixture was refluxed for 10 h After cooled to 0 °C, the
reaction was quenched with aqueous sodium hydroxide (15%, 1.2 mL), and the precipitate was
filtrated. The organic phases were then processed in the usual way and chromatographed
1
(10:1 CH2Cl2/MeOH) to afforded compound 16 (1 g, 35%) as red oil. H-NMR (CDCl3) δ: 2.39–2.56
(m, 2H), 2.62–2.68 (m, 2H), 2.83–2.91 (m, 2H), 2.97 (dd, J = 2.8 Hz, 8.4 Hz, 2H), 3.07–3.15 (m, 1H),
3.25 (d, J = 10.4 Hz, 1H), 3.52 (dd, J = 2.8 Hz, 12.0 Hz, 1H), 4.58 (brs, 1H), 7.04–7.13 (m, 4H);
MS (ESI): m/z calcd. for [M+H]+: 189.1, found:189.3.
3.1.6. General Procedure for the Preparation of Derivatives 17–22
To a stirred solution of compound 16 (500 mg, 2.7 mmol) in DCM (50 mL), cyclohexanecarbonyl
chloride (532 L, 4.0 mmol) was added at 0 °C. The mixture was stirred at room temperature
overnight. The reaction was quenched with NaHCO3 (aq.), extracted with DCM (50 mL × 3). The
organic phases were then processed in the usual way and chromatographed (1:1 petroleum ether/
1
EtOAc) to afforded compound 19 (350 mg, 43%) as white solid. H-NMR (CDCl3) δ:1.24–1.87 (10H),
2.50 (m, 1H), 2.65 (m, 2H), 2.78 (m, 1H), 2.90–3.13 (m, 3H), 3.28 (m, 2H), 3.44 (m, 1H), 3.92 (d,
J = 12.4 Hz), 4.37 (d, J =12.8 Hz) (total 1H), 4.65 (d, J = 13.2 Hz), 5.19 (d, J = 12.4 Hz) (total 1H),
7.13–7.33 (4H); MS (ESI): m/z calcd. for [M+H]+: 299.2, found: 299.1. The following compounds
were similarly prepared.
Cyclobutyl(3,4,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-2(11bH)-yl)methanone (17). Yield = 35%;
1H-NMR (CDCl3) δ: 1.92–2.01 (m, 2H), 2.15–2.25 (m, 2H), 2.33–2.50 (m, 3H), 2.54–2.64 (m, 2H), 2.75
(m, 1H), 2.87–3.04 (m, 3H), 3.15–3.46 (m, 3H), 3.68 (d, J = 12.8 Hz), 4.11-4.18 (m) (total 1H), 4.60
(d, J = 12.0 Hz), 5.16 (d, J = 12.4 Hz) (total 1H), 7.14–7.33 (m, 4H); MS (ESI): m/z calcd. for [M+H]+:
271.2, found: 271.3.

Molecules 2013, 18
9173
Cyclopentyl(3,4,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-2(11bH)-yl)methanone (18). Yield = 32%;
1H-NMR (CDCl3) δ: 1.58–1.98 (8H), 2.46 (m, 1H), 2.61 (m, 2H), 2.76 (d, J = 17.2 Hz, 1H)，2.91–3.10
(m, 3H), 3.23 (m, 2H), 3.40 (m, 1H), 3.97 (d, J = 12.4 Hz), 4.45 (d, J = 12.8 Hz) (total 1H), 4.66 (d,
J = 12.8 Hz), 5.21 (d, J = 12.8 Hz) (total 1H), 7.13–7.33 (m, 4H); MS (ESI): m/z calcd. for [M+H]+:
285.2, found: 285.1.
Phenyl(3,4,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-2(11bH)-yl)methanone (20). Yield = 69%;
1H-NMR (CDCl3) δ: 2.40–2.60 (m, 2H), 2.66–2.71 (m, 1H), 2.79 (m, 1H), 2.97 (m, 2H), 3.13–3.33 (m,
3H), 3.67 (m), 4.18 (m) (total 1H), 4.64 (m), 5.21 (m) (total 1H), 6.58–7.38 (9H); MS (ESI): m/z calcd.
for [M+H]+: 293.2, found: 293.2.
(3,4,6,7-Tetrahydro-1H-pyrazino[2,1-a]isoquinolin-2(11bH)-yl)(thiophen-2-yl)methanone (21). Yield = 44%;
1H-NMR (CDCl3) δ: 2.61 (m, 2H), 2.77 (m, 1H), 3.00–3.08 (m, 3H), 3.18–3.43 (m, 3H), 4.44 (brs, 1H),
5.04 (brs, 1H), 7.12 (t, J = 4.4 Hz, 1H), 7.15–7.21 (m, 4H), 7.40 (d, J = 2.8 Hz, 1H), 7.51 (d, J = 5.2
Hz, 1H); MS (ESI): m/z calcd. for [M+H]+: 299.1, found: 299.3.
2-Chloro-1-(3,4,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-2(11bH)-yl)ethanone (22). Yield = 36%;
1H-NMR (CDCl3) δ: 2.52–2.78 (m, 3H), 2.95–3.51 (m, 6H), 3.86 (m), 4.34 (m) (total 1H), 4.15 (AB,
J = 12.0 Hz), 4.22 (AB, J = 12.0 Hz) (total 2H), 4.57 (m, ), 5.10 (m) (total 1H), 7.15–7.31 (m, 4H);
MS (ESI): m/z calcd. for [M+H]+: 265.1/267.1, found: 265.2/267.1.
3.1.7. The Procedure for the Preparation of Compound 24
A stirred solution of compound 23 (1.0 g, 3.1 mmol) in HCl (2 N, 10 mL) was heated to 110 °C
overnight. The reaction mixture was neutralized with solid NaHCO3, and extracted with DCM/MeOH
(V/V 5/1, 30 mL × 3). The organic phases were then processed in the usual way and chromatographed
(10:1 CH2Cl2/MeOH) to afforded compound 24 (500 mg, 78%). 1H-NMR (CDCl3) δ: 2.80–2.88 (m, 3H),
2.95–3.01 (m, 1H), 3.54 (d, J = 17.6 Hz, 1H), 3.66 (m, 1H), 3.67 (d, J = 17.2 Hz, 1H), 4.71 (m, 1H),
5.12 (m, 1H), 6.80 (d, J = 5.2 Hz, 1H), 7.19 (d, J = 5.2 Hz, 1H); MS (ESI): m/z calcd. for [M+H]+:
209.1, found: 209.2.
3.1.8. General Procedure for the Preparation of Compound 25–33
To a stirred solution of compound 24 (208 mg, 1 mmol) in DCM (10 mL), cyclobutanecarbonyl
chloride (136 L, 1.2 mmol) was added and stirred at room temperature overnight. The reaction was
quenched with NaHCO3 (aq.), extracted with DCM. The organic phases were then processed in the
usual way and chromatographed (1:1 petroleum ether/EtOAc) to afforded compound 26 (150 mg, 52%).
1H-NMR (CDCl3) δ: 1.92–2.37 (m, 6H), 2.75 (t, J = 11.2 Hz, 1H), 2.84–2.96 (m, 3H), 3.27 (m, 1H),
3.75 (d, J = 18.8 Hz) 3.94 (d, J = 17.6 Hz) (total 1H), 4.18 (d, J = 12.8 Hz), 4.25 (d,
J = 17.6 Hz) (total 1H), 4.71 (d, J = 10.4 Hz, 1H), 4.91 (d, J = 19.2 Hz), 5.04 (dd, J = 3.2 Hz, 12.8 Hz)
(total 1H), 5.10 (dd, J = 2.8 Hz, 13.2 Hz, 1H), 6.81 (d, J = 5.2 Hz), 6.91 (d, J = 5.2 Hz) (total 1H), 7.19
(d, J = 4.8 Hz), 7.23 (d, J = 4.8 Hz) (total 1H); MS (ESI): m/z calcd. for [M+H]+: 291.1, found: 291.2.

Molecules 2013, 18
9174
2-(Cyclopentanecarbonyl)-2,3,6,7-tetrahydro-1H-thieno[3',2':3,4]pyrido[1,2-a]pyrazin-4(10bH)-one (25).
1
Yield = 52%; H-NMR (CDCl3) δ: 1.61–1.91 (m, 8H), 2.74 (t, J = 11.2 Hz, 1H), 2.86–2.97 (m, 4H),
3.76 (d, J = 18.4 Hz), 4.04 (d, J = 17.6 Hz) (total 1H), 4.50 (d, J = 17.6 Hz, 1H), 4.71 (d, J = 9.2 Hz,
1H), 4.93 (d, J = 19.2 Hz), 5.04 (dd, J = 5.2 Hz, 12.8 Hz) (total 1H), 5.15 (dd, J = 2.4 Hz, 13.2 Hz, 1H),
6.90 (d, J = 4.8 Hz, 1H), 7.19 (d, J = 5.2 Hz, 1H); MS (ESI): m/z calcd. for [M+H]+: 305.1, found: 305.1.
2-(Cyclopropanecarbonyl)-2,3,6,7-tetrahydro-1H-thieno[3',2':3,4]pyrido[1,2-a]pyrazin-4(10bH)-one (27).
Yield = 57%; 1H-NMR (CDCl3) δ: 1.01 (m, 2H), 1.12 (m, 2H), 1.70 (m, 1H), 2.72–2.98 (m, 4H), 4.18 (d,
J = 17.6 Hz, 1H), 4.72 (m, 2H), 5.04–5.14 (m, 2H), 6.90 (d, J = 4.8 Hz, 1H), 7.19 (d, J = 4.8 Hz, 1H);
MS (ESI): m/z calcd. for [M+H]+: 277.1, found: 276.9.
2-Isobutyryl-2,3,6,7-tetrahydro-1H-thieno[3',2':3,4]pyrido[1,2-a]pyrazin-4(10bH)-one (28). Yield = 46%;
1H-NMR (CDCl3) δ: 1.15 (d, J = 6.8 Hz), 1.17 (d, J = 6.8 Hz), (total 6H), 2.70–2.80 (m, 1H), 2.86–2.98
(m), 3.16–3.25 (m) (4H), 3.77 (d, J = 18.4 Hz), 4.06 (d, J = 17.6 Hz) (total 1H), 4.34–4.40 (m), 4.48 (d,
J = 17.6 Hz) (total 1H), 4.71–4.78 (m, 1H), 4.89–5.06 (m, 1H), 5.15 (d, J = 13.2 Hz, 1H), 6.90 (d,
J = 5.2 Hz, 1H), 7.20 (d, J = 5.2 Hz, 1H); MS (ESI): m/z calcd. for [M+H]+: 279.1, found: 279.2.
2-Pivaloyl-2,3,6,7-tetrahydro-1H-thieno[3',2':3,4]pyrido[1,2-a]pyrazin-4(10bH)-one (29). Yield = 76%;
1H-NMR (CDCl3) δ: 1.34 (s, 9H), 2.81–2.97 (m, 4H), 3.96 (d, J = 18.0 Hz, 1H), 4.76 (m, 1H), 4.82 (d,
J = 17.6 Hz, 1H), 5.04(dd, J = 4.8 Hz, 12.4Hz, 2H), 6.87 (d, J = 5.2 Hz, 1H), 7.21 (d, J = 5.2 Hz, 1H);
MS (ESI): m/z calcd. for [M+H]+: 293.1, found: 293.1[M+H]+.
2-Benzoyl-2,3,6,7-tetrahydro-1H-thieno[3',2':3,4]pyrido[1,2-a]pyrazin-4(10bH)-one (30). Yield = 51%;
1H-NMR (CDCl3) δ: 2.86–3.02 (m, 4H), 4.08 (m, 1H), 4.36–4.38 (m, 1H), 4.88 (m, 1H), 5.01 (m, 1H),
5.19–5.25 (m, 1H), 6.98–7.62 (m, 6H), 8.10 (d, J = 7.6 Hz, 1H); MS (ESI): m/z calcd. for [M+H]+:
313.1, found: 313.2.
2-(Thiophene-2-carbonyl)-2,3,6,7-tetrahydro-1H-thieno[3',2':3,4]pyrido[1,2-a]pyrazin-4(10bH)-one (31).
Yield = 40%; 1H-NMR (CDCl3) δ: 2.87–2.97 (m, 4H), 4.20–4.24 (m, 1H), 4.85–5.10 (m, 4H), 6.90–7.88
(m, 5H); MS (ESI): m/z calcd. for [M+H]+: 319.1, found: 319.0.
2-(Furan-2-carbonyl)-2,3,6,7-tetrahydro-1H-thieno[3',2':3,4]pyrido[1,2-a]pyrazin-4(10bH)-one (32).
Yield = 46%; 1H-NMR (CDCl3) δ: 2.85–3.03 (m, 4H), 4.21–4.25 (m, 1H), 4.88 (m, 1H), 5.01–5.07 (m,
2H), 5.15 (m, 1H), 6.55 (d, J = 2.8 Hz, 1H), 6.94 (brs, 1H)，7.17 (d, J = 3.2 Hz, 1H), 7.22 (d, J = 5.2 Hz,
1H), 7.56 (s, 1H); MS (ESI): m/z calcd. for [M+H]+: 303.1, found: 303.2 [M+H]+.
2-(2-Chloroacetyl)-2,3,6,7-tetrahydro-1H-thieno[3',2':3,4]pyrido[1,2-a]pyrazin-4(10bH)-one
(33).
Yield = 76%; 1H-NMR (CDCl3) δ: 2.82–2.98 (m), 3.24–3.29 (m) (total 4H), 3.81 (d, J = 18.8 Hz), 4.23
(d, J = 12.4 Hz) (total 1H), 4.13 (s), 4.17 (s) (total 2H), 4.38–4.46 (m, 1H), 4.75–5.06 (m, 3H), 6.87 (d,
J = 5.2 Hz), 6.90 (d, J = 5.2 Hz) (total 1H), 7.22 (d, J = 5.2 Hz), 7.24 (d, J = 5.2 Hz) (total 1H); MS
(ESI): m/z calcd. for [M+H]+: 285.0/287.0, found: 285.1/287.1 [M+H]+.

Molecules 2013, 18
9175
3.2. Killing Activity of Compounds 1–33 on S. Japonicum Adult Worms in Vitro [36]
Stock solutions of compounds 1–33 and praziquantel were prepared by dissolving 1 mg of the drugs
in 0.4 ml dimethyl sulfoxide (DMSO) and adding 0.6 mL RPMI 1640 medium. S. japonicum worms
obtained from mice (C57BL/6, female, 22–24 g, each infected with 50 cercariae) were washed in
RPMI 1640 medium, kept at pH 7.5 with HEPES 20 mM and supplemented with penicillin
(100 UI/mL), streptomycin (100 mg/mL) and 10% fetal bovine serum (FBS, Gibco) . After washing,
8–15 adult worms were transferred to each well of a 24-well culture plate containing 2 mL of the same
medium. The worms were cultured for 30 to 60 min at 37 °C in a humid atmosphere containing 5%
CO2, and then different concentrations of compounds 1–33 (10, 25, 50, 100 µM) diluted with RPMI
1640 medium were added. Control worms were treated with equal volumes of RPMI 1640 or DMSO,
and worms treated with 10, 25, 50, 100 µM praziquantel were also observed. The worm mobility,
tegumental alterations and parasite survival were monitored under an inverted microscope (Leica,
Wetzlar, Germany) at 24, 48 and 72 h. Parasite death was defined as having no motor activity during 2
min of continuous observation as well as morphological and tegumental alterations. The tests were
repeated two times when compouds showed worm killing activity below the concentration of 100 M.
4. Conclusions
Given the widespread morbidity and mortality derived from schistosomiasis and the possible
emergence of PZQ-resistant parasites in the near future, it is undoubtedly urgent to develop new
chemotherapy for the disease. In this work, we first reinvestigated the SAR of PZQ analogues at
positions 2 and 4, and we identified that the introduction of a chloroacetyl group at position 2 led to
compound 7 with higher worm-killing activity against adult S. japonicum than PZQ in vitro. Second,
we extended the SAR studies by replacing the aromatic moiety with a thiophene ring, which displayed
similar activity as PZQ derivatives, further confirming the respective SAR. Notably, compound 33
with the chloroacetyl group killed 100% of the worms at concentrations as low as 10 M, which
represents a more potent compound than PZQ. The study of these structurally diverse compounds will
likely provide meaningful information for the design of new PZQ analogues. After the initial in vitro
screening, in vivo activity and toxicity will have to be tested. Design and synthesis of more potent
antischistosomal agents are currently undergoing in our laboratory.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 30972581), the
Natural Science Foundation of Jiangsu Province (No. BK2008110), the Health Promotion Project
Foundation of Jiangsu Province (No. ZX201108), the National S & T Major Program (No. 2012ZX
10004-220), and the Fundamental Research Funds for the Central Universities (JUSRP1040). We also
thank Sergio C. Chai at St. Jude Children’s Research Hospital for his suggestions.
Conflict of Interest
The authors declare no conflict of interest.

Molecules 2013, 18
9176
References
1. Danso-Appiah, A.; Olliaro, P.L.; Donegan, S.; Sinclair, D.; Utzinger, J. Drugs for treating
Schistosoma mansoni infection. Cochrane. Database. Syst. Rev. 2013, 2, CD000528.
2. Gryseels, B.; Polman, K.; Clerinx, J.; Kestens, L. Human schistosomiasis. Lancet 2006, 368,
1106–1118.
3. Zheng, H.; Zhang, L.J.; Zhu, R.; Xu, J.; Li, S.Z.; Guo, J.G.; Xiao, N.; Zhou, X.N. Schistosomiasis
situation in People's Republic of China in 2011. Chin. J. Schisto. Control. 2012, 24, 621–626.
4. Caffrey, C.R.; Secor, W.E. Schistosomiasis: From drug deployment to drug development.
Curr. Opin. Infect. Dis. 2011, 24, 410–417.
5. Stelma, F.F.; Talla, I.; Sow, S.; Kongs, A.; Niang, M.; Polman, K.; Deelder, A.M.; Gryseels, B.
Efficacy and side effects of praziquantel in an epidemic focus of Schistosoma mansoni. Am. J.
Trop. Med. Hyg. 1995, 53, 167–170.
6. Wang, W.; Wang, L.; Liang, Y.S. Susceptibility or resistance of praziquantel in human
schistosomiasis: a Review. Parasitol. Res. 2012, 111, 1871–1877.
7. Caixeta, S.C.; Magalhaes, L.G.; de Melo, N.I.; Wakabayashi, K.A.; Aguiar Gde, P.; Aguiar Dde, P.;
Mantovani, A.L.; Alves, J.M.; Oliveira, P.F.; Tavares, D.C.; et al. Chemical composition and in
vitro schistosomicidal activity of the essential oil of Plectranthus neochilus grown in Southeast
Brazil. Chem. Biodivers. 2011, 8, 2149–2157.
8. Moraes, J.; Almeida, A.A.; Brito, M.R.; Marques, T.H.; Lima, T.C.; Sousa, D.P.; Nakano, E.;
Mendonca, R.Z.; Freitas, R.M. Anthelmintic activity of the natural compound (+)-limonene
epoxide against Schistosoma mansoni. Planta Med. 2013, 79, 253–258.
9. Porto, T.S.; da Silva Filho, A.A.; Magalhaes, L.G.; dos Santos, R.A.; Furtado, N.A.;
Arakawa, N.S.; Said, S.; de Oliveira, D.C.; Gregorio, L.E.; Rodrigues, V.; et al. Fungal
transformation and schistosomicidal effects of pimaradienoic acid. Chem. Biodivers. 2012, 9,
1465–1474.
10. Ndjonka, D.; Rapado, L.N.; Silber, A.M.; Liebau, E.; Wrenger, C. Natural Products as a Source
for Treating Neglected Parasitic Diseases. Int. J. Mol. Sci. 2013, 14, 3395–3439.
11. Jao, S.C.; Chen, J.; Yang, K.; Li, W.S. Design of potent inhibitors for Schistosoma japonica
glutathione S-transferase. Bioorg. Med. Chem. 2006, 14, 304–318.
12. Rai, G.; Sayed, A.A.; Lea, W.A.; Luecke, H.F.; Chakrapani, H.; Prast-Nielsen, S.; Jadhav, A.;
Leister, W.; Shen, M.; Inglese, J.; et al. Structure Mechanism Insights and the Role of Nitric
Oxide Donation Guide the Development of Oxadiazole-2-Oxides as Therapeutic Agents against
Schistosomiasis. J. Med. Chem. 2009, 52, 6474–6483.
13. Rai, G.; Thomas, C.J.; Leister, W.; Maloney, D.J. Synthesis of oxadiazole-2-oxide analogues as
potential antischistosomal agents. Tetrahedron Lett. 2009, 50, 1710–1713.
14. Sayed, A.A.; Simeonov, A.; Thomas, C.J.; Inglese, J.; Austin, C.P.; Williams, D.L. Identification
of oxadiazoles as new drug leads for the control of schistosomiasis. J. Nat. Med. 2008, 14, 407–412.
15. Liu, J.; Dyer, D.; Wang, J.; Wang, S.; Du, X.; Xu, B.; Zhang, H.; Wang, X.; Hu, W.
3-Oxoacyl-ACP Reductase from Schistosoma japonicum: Integrated In Silico-In Vitro Strategy
for Discovering Antischistosomal Lead Compounds. PLoS One 2013, 8, e64984.
16. Domling, A.; Khoury, K. Praziquantel and schistosomiasis. Chemmedchem 2010, 5, 1420–1434.

Molecules 2013, 18
9177
17. Woelfle, M.; Seerden, J.P.; de Gooijer, J.; Pouwer, K.; Olliaro, P.; Todd, M.H. Resolution of
praziquantel. PLoS. Negl. Trop. Dis. 2011, 5, e1260.
18. Meyer, T.; Sekljic, H.; Fuchs, S.; Bothe, H.; Schollmeyer, D.; Miculka, C. Taste, a new incentive
to switch to (R)-praziquantel in schistosomiasis treatment. PLoS Negl. Trop. Dis. 2009, 3, e357.
19. Cao, H.; Liu, H.; Domling, A. Efficient multicomponent reaction synthesis of the schistosomiasis
drug praziquantel. Chemistry 2010, 16, 12296–12298.
20. Liu, H.; William, S.; Herdtweck, E.; Botros, S.; Domling, A. MCR synthesis of praziquantel
derivatives. Chem. Biol. Drug. Des. 2012, 79, 470–477.
21. Andrews, P.; Thomas, H.; Pohlke, R.; Seubert, J. Praziquantel. Med. Res. Rev. 1983, 3, 147–200.
22. Patra, M.; Ingram, K.; Pierroz, V.; Ferrari, S.; Spingler, B.; Keiser, J.; Gasser, G. Ferrocenyl
derivatives of the anthelmintic praziquantel: Design, synthesis, and biological evaluation. J. Med.
Chem. 2012, 55, 8790–8798.
23. Dong, Y.X.; Chollet, J.; Vargas, M.; Mansour, N.R.; Bickle, Q.; Alnouti, Y.; Huang, J.G.; Keiser, J.;
Vennerstrom, J.L. Praziquantel analogs with activity against juvenile Schistosoma mansoni.
Bioorg. Med. Chem. Lett. 2010, 20, 2481–2484.
24. Patra, M.; Ingram, K.; Pierroz, V.; Ferrari, S.; Spingler, B.; Gasser, R.B.; Keiser, J.; Gasser, G.
[(eta(6)-Praziquantel)Cr(CO)3] derivatives with remarkable in vitro anti-schistosomal activity.
Chemistry 2013, 19, 2232–2235.
25. Ronketti, F.; Ramana, A.V.; Chao-Ming, X.; Pica-Mattoccia, L.; Cioli, D.; Todd, M.H.
Praziquantel derivatives I: Modification of the aromatic ring. Bioorg. Med. Chem. Lett. 2007, 17,
4154–4157.
26. Duan, W.W.; Qiu, S.J.; Zhao, Y.; Sun, H.; Qiao, C.; Xia, C.M. Praziquantel derivatives exhibit
activity against both juvenile and adult Schistosoma japonicum. Bioorg. Med. Chem. Lett. 2012,
22, 1587–1590.
27. Wang, Z.X.; Chen, J.L.; Qiao, C. Praziquantel Derivatives with Anti-Schistosomal Activity:
Aromatic ring Modification. Chem. Biol. Drug. Des. 2013, 82, 216–225.
28. Sadhu, P.S.; Kumar, S.N.; Chandrasekharam, M.; Pica-Mattoccia, L.; Cioli, D.; Rao, V.J.
Synthesis of new praziquantel analogues: Potential candidates for the treatment of
schistosomiasis. Bioorg. Med. Chem. Lett. 2012, 22, 1103–1106.
29. Frehel, D.; Maffrand, J.P. Synthesis of the new tricyclic system thieno[3',2':3,4]pyrido[1,2-
a]pyrazin-4-one. Heterocycles 1984, 22, 143–149.
30. Todd, M.H.; Ndubaku, C.; Bartlett, P.A. Amino acid derived heterocycles: Lewis acid catalyzed
and radical cyclizations from peptide acetals. J. Org. Chem. 2002, 67, 3985–3988.
31. Bissinger, E.M.; Heinke, R.; Spannhoff, A.; Eberlin, A.; Metzger, E.; Cura, V.; Hassenboehler, P.;
Cavarelli, J.; Schule, R.; Bedford, M.T.; et al. Acyl derivatives of p-aminosulfonamides and
dapsone as new inhibitors of the arginine methyltransferase hPRMT1. Bioorg. Med. Chem. 2011,
19, 3717–3731.
32. Kim, D.J.; Reddy, K.; Kim, M.O.; Li, Y.; Nadas, J.; Cho, Y.Y.; Kim, J.E.; Shim, J.H.; Song, N.R.;
Carper, A.; et al. (3-Chloroacetyl)-indole, a Novel allosteric AKT inhibitor, Suppresses colon
cancer growth in vitro and in vivo. Cancer Prev. Res. 2011, 4, 1842–1851.
33. Nam, K.N.; Koketsu, M.; Lee, E.H. 5-Chloroacetyl-2-amino-1,3-selenazoles attenuate microglial
inflammatory responses through NF-kappa B inhibition. Eur. J. Pharmacol. 2008, 589, 53–57.

Molecules 2013, 18
9178
34. Dezube, B.J.; Von Roenn, J.H.; Holden-Wiltse, J.; Cheung, T.W.; Remick, S.C.; Cooley, T.P.;
Moore, J.; Sommadossi, J.P.; Shriver, S.L.; Suckow, C.W.; et al. Fumagillin analog in the
treatment of Kaposi’s sarcoma: A phase I AIDS Clinical Trial Group Study. J. Clin. Oncol. 1998,
16, 1444–1449.
35. Kruger, E.A.; Figg, W.D. TNP-470: An Angiogenesis inhibitor in clinical development for
cancer. Expert Opin. Invest. Drug 2000, 9, 1383–1396.
36. Song, L.J.; Li, J.H.; Xie, S.Y.; Qian, C.Y.; Wang, J.; Zhang, W.; Yin, X.R.; Hua, Z.C.; Yu, C.X.
Thioredoxin Glutathione Reductase as a Novel Drug Target: Evidence from Schistosoma
japonicum. PLoS One 2012, 7, e31456.
Sample Availability: Samples of the compounds 1–33 are available from the authors.
© 2013 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/3.0/).