Praziquantel analogs with activity against juvenile Schistosoma mansoni

Article abstract of DOI:10.1016/j.bmcl.2010.03.001

Six amide and four urea derivatives of praziquantel were synthesized and tested for antischistosomal activity against juvenile and adults stages of Schistosoma mansoni in infected mice. Only one of these had significant activity against adult worms, but,

Full text of DOI:10.1016/j.bmcl.2010.03.001

Bioorganic & Medicinal Chemistry Letters 20 (2010) 2481–2484
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Praziquantel analogs with activity against juvenile Schistosoma mansoni
Yuxiang Dong a, Jacques Chollet b, Mireille Vargas b, Nuha R. Mansour c, Quentin Bickle c, Yazen Alnouti a,
Jiangeng Huang a, Jennifer Keiser b, Jonathan L. Vennerstrom a,
*
a University of Nebraska Medical Center, College of Pharmacy, 986025 Nebraska Medical Center, Omaha, NE, USA
b Department of Medical Parasitology and Infection Biology, Swiss Tropical and Public Health Institute, Socinstrasse 57, CH-4002 Basel, Switzerland
c Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, London, WC1E 7HT, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
Six amide and four urea derivatives of praziquantel were synthesized and tested for antischistosomal
activity against juvenile and adults stages of Schistosoma mansoni in infected mice. Only one of these
had significant activity against adult worms, but, unlike praziquantel, six of these had low to modest
activity against juvenile worms. A praziquantel ketone derivative had the best combination of activity
against juveniles and adults, but it had no effect on the motility of adult S. mansoni in ex vivo culture.
Cytochrome P450 metabolic stability data support the hypothesis that the major trans-cyclohexanol
metabolite of praziquantel plays an important role in the antischistosomal activity of this drug.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 26 January 2010
Revised 26 February 2010
Accepted 1 March 2010
Available online 4 March 2010
Keywords:
Praziquantel
Antischistosomal
Schistosoma mansoni
Schistosomiasis is a widespread tropical disease1 caused by
infection with parasitic trematodes (flukes). Five blood fluke spe-
cies infect humans—Schistosoma mansoni, S. haematobium, S. japon-
icum, S. intercalatum, and S. mekongi; the first three of these are the
most widely distributed and cause the highest disease burden.2
Praziquantel (PZ) is the drug of choice for schistosomiasis3,4 and
is very effective for this purpose.1,4 The mechanism of action of
PZ is uncertain, although most evidence points towards a perturba-
tion of calcium homeostasis.1,4,5 Even though no clinically signifi-
cant resistance to PZ has developed,6 schistosome isolates with
diminished sensitivity to PZ continue to be identified.7 Since the
last review of PZ SAR in 1983,8 very few new PZ analogs have been
synthesized. Nonetheless, a continued interest in PZ SAR is illus-
trated by the recent report of Ronketti et al.9 in which the authors
describe the activity of aromatic-ring substituted PZ analogs. As
one of the few liabilities of PZ is its low efficacy against juvenile
forms of schistosomes,1 we envisioned an investigation of PZ ana-
logs with potential broad-spectrum antischistosomal activity
against both juvenile and adult forms, and secondarily, with poten-
tially better metabolic stability. PZ undergoes rapid first-pass drug
metabolism3 to form a major trans-cyclohexanol metabolite5,10
(Fig. 1) that is reported to be equal to11 or 4- to 10-fold less effec-
tive3,9 than the parent drug against S. mansoni.
O
O
N
N
N
N
OH
O
O
Praziquantel (PZ)
PZ major metabolite
Figure 1. Praziquantel (PZ) and its major metabolite.
O
N
R =
R
N
X
X
N
O
5, X = CH2
6, X = O
1, X = C=O
2, X = CF2
3
4
7, X = NH
8, X = CF2
O
O
N
O
O
O
O
HN
N
O
O
N
O
O
9
10
Figure 2. Target PZ analogs 1–10.
As most data show5 that PZ metabolites, including the major
trans-cyclohexanol metabolite, are less active than the parent, part
of our design strategy (Fig. 2) was to increase metabolic stability.
PZ derivative 1 is the ketone oxidation product of the trans-cyclo-
hexanol metabolite. The geminal difluoro substituent in 2 would
be expected to block the formation of this or similar cyclohexanol
metabolites. Adamantyl analogs 3 and 4 are conformationally con-
strained analogs of the cyclohexane substructure of PZ. Ureas 5–8
are PZ analogs in which the cyclohexyl is replaced with a piperi-
dine (5), morpholine (6), piperazine (7), and difluoropiperidine
* Corresponding author. Tel.: +1 402 559 5362; fax: +1 402 559 9543.
E-mail address: jvenners@unmc.edu (J.L. Vennerstrom).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.03.001

2482
Y. Dong et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2481–2484
Table 1
(8). These more polar PZ ureas would be predicted to be more met-
abolically stable than PZ, and if metabolized, avoid the complicat-
ing stereochemistry that arises with formation of the cyclohexanol
metabolites of PZ. Based on the observation that antimalarial semi-
synthetic artemisinins and synthetic ozonides, unlike PZ, show
higher in vivo activity against juvenile schistosomes than adult
worms,2,12 we designed 9, a PZ-ozonide hybrid designed to create
a potential broad-spectrum molecule. Ozonide 10 is a control for
9 without the tetrahydroisoquinoline substructure of PZ. Even
though most of the antischistosomal properties of PZ are derived
from its more active R enantiomer,8,11,13 target PZ analogs 1-914
were synthesized as racemates.
Although a three-step synthesis of piperazinone 11 has been de-
scribed,15 we elected to obtain 11 directly by selective amide bond
hydrolysis of the readily available and inexpensive PZ (Scheme 1).
In this respect, the acetamide derivative of PZ was reported16 to un-
dergo hydrolysis with refluxing HCl for 3 h to give 11 in 65–80%
yields, but the concentration of HCl was not specified. Using similar,
but more precisely defined reaction conditions (refluxing 1 N HCl,
3 h), Mitsui and Arizono17 hydrolyzed PZ to afford, after neutraliza-
tion with sodium bicarbonate, 11 in 27% yield. We found that reflux-
ing PZ in 2 N HCl for 20 h afforded, after neutralization, 11 in 80%
yield. Reaction of 1115 and the corresponding carboxylicacids18 with
EDCI/HOBt/Et3N in acetonitrile afforded amides 1–4 and 9 in moder-
ate to good yields. Intermediate 12, the 4-nitrophenylcarbamate
derivative of 11, provided a convenient means to obtain ureas 5–8
in good to high yields by treatment with piperazine or the requisite
piperidine derivatives. Ozonide 10 was obtained as previously
described.19
Effects of 10 lg/mL concentrations of 1–10 on adult S. mansoni in ex vivo culture
Compd Motility
reduction (%)
Qualitative assessment
Vehicle
1
0
0
No effect, many eggs
Males and females shrunken, granular,
opaque, normal motility, no eggs
Females—slender, normal movement, males—dark,
granular, opaque, shrunken, occasional movement
of anterior ends dark, no eggs
No effect, no eggs
2
25
3
4
5
0
0
38
No effects, many eggs
All worms dark, granular, opaque, females—slender
and slow, males—shrunken with occasional
movement of anterior ends, no eggs
No effect, no eggs
No effect, many eggs
No effect, very few eggs
No effect, many eggs
No effect, no eggs
All worms dark, granular, opaque, with
tegument damage, males—curled-up, no eggs
6
7
8
9
10
PZ
0
0
0
0
0
100
Table 2
Effects of single 400 mg/kg oral doses of 1–10 administered to mice harboring
21-day-old juvenile and 49-day-old adult S. mansoni infections
Compd
Juvenile
Adult
TWRa (%)
FWRb (%)
TWR (%)
FWR (%)
Vehiclec
1
2
3
4
5
6
7
8
9
10
PZ
0
0
0
0
25
38d
9
41d
44d
9
79d
40
4
81d
39e
2
In vitro (Table 1) and in vivo (Table 2) antischistosomal activities
against S. mansoni were assessed following previously described
methods.12,20,21 Briefly, for the in vitro screen, adult S. mansoni
worms are obtained from infected mice by portal perfusion using
warm perfusion medium (Dulbecco’s Modified Eagle’s Medium
[DMEM], 2 mM L-glutamine, 100 U/mL penicillin, 100 lg/mL strep-
tomycin, 20 mM Hepes, 10 U/mL heparin [Sigma, UK]). Host red
blood cells were removed, and the medium was replaced with com-
42d
27d
14
25
38d
0
56d
34d
22
23
38d
0
0
1
0
0
12
0
0
9
0
0
0
0
85d,f
7
85d,f
18
1
13
96d
98d
plete medium (DMEM plus 10% fetal calf serum, 2 mM
and 100 g/mL each of penicillin and streptomycin). Test com-
pounds were then added from 10 mg/mL DMSO stock solutions to
achieve concentrations of 10 g/mL. At 120 h, compound activity
L-glutamine,
a
b
c
TWR = total worm reduction rate.
FWR = female worm reduction rate.
3% ethanol and 7% Tween 80 solubilizing vehicle.
p <0.05 from the Kruskal–Wallis test comparing the medians of the responses
l
d
l
was assessed by motility disturbances and morphological changes.
For the in vivo screen, mice were infected with ca. 80 S. mansoni
cercariae on day 0 followed by administration of single 400 mg/kg
oral doses of test compounds dissolved or suspended in a solubiliz-
ing 3% ethanol and 7% Tween 80 vehicle to groups of three mice on
day 21 post-infection (juvenile stage) and groups of five mice on
day 49 post-infection (adult stage). At 28 days post-treatment, ani-
mals were sacrificed by the CO2 method and dissected to assess total
and female worm reduction as described in detail by Xiao et al.12
between the treatment and control groups.
e
p = 0.053 from the Kruskal–Wallis test comparing the medians of the responses
between the treatment and control groups.
f
Data from a single 200 mg/kg oral dose.
For the target compounds tested against adult S. mansoni
in vitro (Table 1), only 2 and 5 reduced worm motility (by 25
and 38%, respectively). For comparison, PZ killed 100% of adults
and had an IC50 of 0.37 lg/mL in this experiment. Similarly, a qual-
itative assessment showed that 2 and 5 had the most significant ef-
fect on worm function, although 1 also produced significant
morphological abnormalities. From the in vivo adult worm reduc-
tion data (Table 2), a somewhat different activity profile for these
three compounds was evident. These data revealed that 1 was
the only compound with significant activity, whereas 2 had mar-
ginal activity and 5 was inactive. With the exception of 1, the much
lower to no activity of PZ analogs 2–10 against adult worms paral-
lels earlier SAR trends where most alterations22 of the cyclohexane
substructure (e.g., cyclopentane, cycloheptane, tetrahydropyran) of
PZ8,22 lowered in vivo activity by an order of magnitude or greater.
Like PZ, 3, 6, and 7 had no significant activity against juvenile
worms (Table 2). However, PZ derivatives 1, 2, 4, 5, and 8 had mod-
est activity against juvenile worms (female worm burden reduc-
tions of 34–56%). Like other structurally related ozonides,12
ozonide 10 had high activity against the juvenile worms, whereas
O
NO2
O
N
N
b
a
PZ
N
NH
O
11, 80%
12, 68%
O
c
d
1, 50%
2, 38%
3, 74%
4, 27%
9, 82%
5, 73%
6, 85%
7, 57%
8, 55%
Scheme 1. Reagents and conditions: (a) 2 N HCl, reflux, 24 h, then 20% NaOH; (b) 4-
nitrophenyl chloroformate, pyridine/CH2Cl2, 4 h; (c) carboxylic acid derivative,
EDCl, HOBt, Et3N, CH3CN, rt, 24 h; (d) piperazine/piperidine derivative, CH3CN,
80 °C, 24 h.

Y. Dong et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2481–2484
2483
2.23 (m, 4H), 2.29–2.68 (m, 4H), 2.72–3.12 (m, 5H), 4.17 (d, J = 17.0 Hz, 1H),
4.52 (d, J = 17.1 Hz, 1H), 4.69–4.89 (m, 2H), 5.14 (d, J = 13.2 Hz, 1H), 7.09–7.39
(m, 4H); 13C NMR d 28.44, 28.52, 28.64, 38.09, 39.02, 39.69, 45.18, 48.97, 54.68,
54.75, 125.27, 126.90, 127.44, 129.23, 132.34, 134.56, 163.79, 172.65, 209.43.
Anal. Calcd for C19H22N2O3: C, 69.92; H, 6.79; N, 8.58. Found: C, 70.10; H, 6.94;
PZ-ozonide hybrid 9 was completely inactive, similar to what was
recently reported23 for six 1,2,4-trioxane-PZ hybrids. In sum,
although we identified new PZ derivatives with activity against
juvenile S. mansoni, none of these had activity against adult S. man-
soni equal to that of PZ. Of these PZ derivatives, ketone 2 had the
best combination of activity against juveniles and adults.
To assess whether the two geminal difluoro-substituted 2 and 8
were more metabolically stable than their methylene counterparts
PZ and 5, in vitro metabolic stability of these compounds were
determined using human liver microsomes. Briefly, compounds
N,
8.80.
2-[(4,4-Difluorocyclohexyl)carbonyl]-1,2,3,6,7,11b-hexahydro-4H-
pyrazino[2,1-a]isoquinolin-4-one (2) (4:1-mixture of two rotamers): mp 183–
185 °C; For the major rotamer: 1H NMR d 1.65–2.09 (m, 6H), 2.13–2.34 (m, 2H),
2.51–2.63 (m, 1H), 2.75–3.03 (m, 4H), 4.11 (d, J = 17.6 Hz, 1H), 4.43 (d,
J = 17.1 Hz, 1H), 4.69–4.88 (m, 2H), 5.14 (dd, J = 13.6, 2.9 Hz, 1H), 7.09–7.38 (m,
4H); 13C NMR d 25.32 (t, J = 9.2 Hz), 25.43 (t, J = 9.2 Hz), 28.61, 32.74 (t,
J = 25 Hz), 32.75 (t, J = 25 Hz), 38.23, 39.09, 45.22, 49.02, 54.81, 122.34 (t,
J = 241 Hz), 125.37, 126.98, 127.51, 129.30, 132.45, 134.64, 163.91, 172.79.
Anal. Calcd for C19H22F2N2O2: C, 65.50; H, 6.36; N, 8.04. Found: C, 65.75; H,
(1 lM) were incubated with 0.5 mg/mL microsomes at 37 °C and
6.24;
N,
8.26.
2-[(1-Adamantyl)carbonyl]-1,2,3,6,7,11b-hexahydro-4H-
the reactions were initiated by the addition of a NADPH regenerat-
ing system and were terminated by the addition of ice-cold MeOH.
Loss of parent compound over a 120 min incubation period was
monitored using LC–MS/MS. Similar to a previous CYP450 study
of PZ,24 dehydrogenation and monohydroxylation were the major
metabolic pathways for PZ and its analogs. Predicted human hepa-
tic extraction ratios (ER) were calculated using appropriate scaling
factors as previously described by Obach.25 ER values of 0.44 and
0.59 for 2 and PZ and 0.36 and 0.43 for 8 and 5 revealed that gem-
inal difluoro substitution produced only small increases in meta-
bolic stability. Similarly, these data also reveal that metabolic
stability increased only marginally when the exocyclic amide in
PZ was replaced with a urea in 5. In sum, these data suggest that
other metabolic pathways besides CYP450-mediated oxidation of
the cyclohexyl distal methylene carbon are predominant in 2 and
8, and support the hypothesis3 that the major trans-cyclohexanol
metabolite26 of PZ plays an important role in the antischistosomal
activity of this drug. Indeed, it is possible that the relatively high
activity of ketone 2 against adult S. mansoni may derive from its
in vivo reduction to this trans-cyclohexanol metabolite. Experi-
ments to test this hypothesis and probe other metabolic ambigui-
ties of PZ are in progress.
pyrazino[2,1-a]isoquinolin-4-one (3) (4:1 mixture of two rotamers): mp 119–
121 °C; For the major rotamer: 1H NMR d 1.57–2.43 (m, 14H), 2.72–3.07 (m,
5H), 4.06 (d, J = 17.1 Hz, 1H), 4.37 (d, J = 17.6 Hz, 1H), 4.69–4.93 (m, 2H), 5.19
(d, J = 11.7 Hz, 1H), 7.05–7.39 (m, 4H); 13C NMR d 27.14, 27.82, 28.72, 30.11,
30.18, 32.66, 32.68, 37.47, 38.79, 38.93, 39.05, 45.24, 46.52, 49.78, 54.90,
125.40, 126.93, 127.37, 129.29, 132.87, 134.69, 164.48, 174.10. Anal. Calcd for
C23H28N2O2: C, 75.79; H, 7.74; N, 7.69. Found: C, 75.63; H, 7.58; N, 7.90. 2-[(2-
Adamantyl)carbonyl]-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a]isoquinolin-4-
one (4): mp 165–167 °C; 1H NMR d 1.69–1.85 (m, 6H), 1.98–2.17 (m, 9H), 2.72–
3.07 (m, 4H), 4.00 (d, J = 18.0 Hz, 1H), 4.79–4.89 (m, 2H), 4.92 (dd, J = 17.8,
1.2 Hz, 1H), 5.14 (ddd, J = 13.7, 4.0, 2.0 Hz, 1H), 7.15–7.35 (m, 4H); 13C NMR d
28.10, 28.20, 28.28, 28.67, 36.40, 38.78, 41.73, 48.44, 50.13, 54.88, 54.92,
125.11, 126.85, 127.31, 129.35, 132.71, 134.85, 164.63, 175.79. Anal. Calcd for
C23H28N2O2: C, 75.79; H, 7.74; N, 7.69. Found: C, 76.00; H, 7.54; N, 8.00. 2-(1-
Piperidinylcarbonyl)-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a]isoquinolin-4-
one (5): mp 137–139 °C; 1H NMR d 1.49–1.71 (m, 6H), 2.73–3.05 (m, 4H), 3.19–
3.35 (m, 4H), 3.95 (d, J = 17.0 Hz, 1H), 4.12 (dd, J = 17.0, 1.2 Hz, 1H), 4.41 (ddd,
J = 13.7, 3.9, 2.0 Hz, 1H), 4.84 (ddd, J = 12.7, 4.9, 2.4 Hz, 1H), 5.07 (dd, J = 10.2,
3.4 Hz, 1H), 7.17–7.33 (m, 4H); 13C NMR d 24.54, 25.65, 28.82, 38.80, 47.60,
49.29, 51.47, 54.40, 125.37, 126.79, 127.14, 129.29, 133.29, 134.84, 163.03,
165.17. Anal. Calcd for C18H23N3O2: C, 68.98; H, 7.40; N, 13.41. Found: C, 69.34;
H, 7.34; N, 13.94. 2-(4-Morpholinylcarbonyl)-1,2,3,6,7,11b-hexahydro-4H-
pyrazino[2,1-a]isoquinolin-4-one (6): mp 138–140 °C; 1H NMR d 2.73–3.05 (m,
4H), 3.19–3.44 (m, 4H), 3.65–3.82 (m, 4H), 3.97 (d, J = 17.1 Hz, 1H), 4.16 (d,
J = 17.5 Hz, 1H), 4.41–4.47 (m, 1H), 4.81–4.87 (m, 1H), 5.06 (dd, J = 10.2, 3.0 Hz,
1H), 7.17–7.33 (m, 4H); 13C NMR d 28.75, 38.83, 46.99, 49.02, 51.23, 54.25,
66.48, 125.32, 126.83, 127.24, 129.32, 132.98, 134.78, 162.65, 164.68. Anal.
Calcd for C17H21N3O3: C, 64.74; H, 6.71; N, 13.32. Found: C, 64.80; H, 6.54; N,
13.50.
2-(1-Piperazinylcarbonyl)-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-
a]isoquinolin-4-one (7): mp 116–118 °C; 1H NMR d 2.73–3.05 (m, 8H), 3.21–
3.43 (m, 4H), 3.96 (d, J = 17.1 Hz, 1H), 4.15 (dd, J = 17.5, 1.4 Hz, 1H), 4.43 (ddd,
J = 13.2, 3.9, 2.0 Hz, 1H), 4.83 (ddd, J = 12.2, 4.4, 2.4 Hz, 1H), 5.06 (dd, J = 10.2,
3.9 Hz, 1H), 7.17–7.33 (m, 4H); 13C NMR d 28.76, 38.83, 45.73, 47.66, 49.14,
51.33, 54.31, 125.34, 126.81, 127.19, 129.30, 133.10, 134.79, 162.82, 164.93.
Anal. Calcd for C17H22N4O2: C, 64.95; H, 7.05; N, 17.82. Found: C, 64.86; H, 7.02;
Acknowledgments
We thank Josefina Santo Tomas for assistance in performing the
antischistosomal assays. This investigation received financial sup-
port from the UNDP/WORLD BANK/WHO Special Programme for
Research and Training in Tropical Diseases (TDR ID No. A30135
and A70463), and the Swiss National Science Foundation
(PPOOA-114941).
N,
17.62.
2-[(4,4-Difluoropiperidinyl)carbonyl]-1,2,3,6,7,11b-hexahydro-4H-
pyrazino[2,1-a]isoquinolin-4-one (8). mp 121–123 °C; 1H NMR d 1.93–2.17 (m,
4H), 2.73–3.06 (m, 4H), 3.28–3.53 (m, 4H), 3.99 (d, J = 17.1 Hz, 1H), 4.14 (dd,
J = 17.1, 1.3 Hz, 1H), 4.43 (ddd, J = 13.7, 3.9, 1.9 Hz, 1H), 4.83 (ddd, J = 12.7, 4.8,
2.4 Hz, 1H), 5.06 (dd, J = 10.7, 3.9 Hz, 1H), 7.16–7.33 (m, 4H); 13C NMR d 28.73,
33.77 (t, J = 23 Hz), 38.83, 43.52 (t, J = 5.3 Hz), 49.10, 51.33, 54.19, 121.53 (t,
J = 242 Hz), 125.29, 126.83, 127.27, 129.33, 132.90, 134.78, 162.29, 164.56.
Anal. Calcd for C18H21F2N3O2: C, 61.88; H, 6.06; N, 12.03. Found: C, 61.61; H,
6.06; N, 12.16. 2-[(Adamantane-2-spiro-30-10,20,40-trioxaspiro[4.5]decane-80-
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5.64.
2-[(4-Nitrophenoxy)carbonyl]-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-
a]isoquinolin-4-one (12) (7:3-mixture of two rotamers): mp 133–135 °C; 1H
NMR d 2.74–3.41 (m, 4H), 4.07 (d, J = 18.0 Hz, 0.3H), 4.18 (d, J = 17.5 Hz, 0.7H),
4.53–5.09 (m, 4H), 7.17–7.34 (m, 4H), 7.35 (d, J = 8.8 Hz, 2H), 8.29 (d, J = 8.8 Hz,
2H); 13C NMR d 28.67, 38.93, 47.35, 47.61, 47.89, 48.33, 54.90, 55.18, 122.15,
122.33, 125.19, 125.23, 125.31, 125.35, 126.93, 127.03, 127.67, 127.76, 129.44,
129.60, 131.83, 131.97, 134.88, 135.22, 145.10, 151.36, 151.65, 155.52, 163.93,
164.37. Anal. Calcd for C19H17N3O5: C, 62.12; H, 4.66; N, 11.44. Found: C, 61.76;
H, 4.47; N, 11.75.
15. Kim, J. H.; Lee, Y. S.; Park, H.; Kim, C. S. Tetrahedron 1998, 54, 7395.
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carboxyadamantane (Stetter, H.; Tillmanns, V. Chem. Ber. 1972, 105, 735) and
the ozonide acid cis-adamantane-2-spiro-30-80-carboxy-10,20,40-trioxaspiro-
[4.5]decane (Tang, Y.; Dong, Y.; Karle, J. M.; DiTusa, C. A.; Vennerstrom, J. L. J.
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14. 1H and 13C NMR spectra were recorded on a 500 MHz spectrometer in CDCl3
solvent. All chemical shifts are reported in parts per million (ppm) and are
relative to internal (CH3)4Si (0 ppm) for 1H, and CDCl3 (77.0 ppm) for 13C NMR.
Combustion analysis confirmed that all target compounds possessed purities
P95%. Melting points are uncorrected. 2-[(4-Oxocyclohexyl)carbonyl]-
1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a]isoquinolin-4-one (1) (3:1-mixture
of two rotamers): mp 130–132 °C; For the major rotamer: 1H NMR d 1.97–
19. Dong, Y.; Wittlin, S.; Sriraghavan, K.; Chollet, J.; Charman, S. A.; Charman, W.
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