Geraniol-derived 1,2,4-trioxanes with potent in-vivo antimalarial activity

Article abstract of DOI:10.1016/S0960-894X(03)00782-0

Geraniol, an abundantly available naturally occurring allylic alcohol, has been used as a starting material to prepare a series of 6-[α-(3′ -aryl-3′-hydroxypropyl)vinyl]-1,2,4-trioxanes. Some of these novel trioxanes have shown very promising antimalarial

Full text of DOI:10.1016/S0960-894X(03)00782-0

Bioorganic & Medicinal Chemistry Letters 13 (2003) 3447–3450
Geraniol-Derived 1,2,4-Trioxanes with Potent In-Vivo
Antimalarial Activity§
Chandan Singh,a,* Nitin Guptaa and Sunil K. Purib
aDivision of Medicinal Chemistry, Central Drug Research Institute, Lucknow 226001, India
bDivision of Parasitology, Central Drug Research Institute, Lucknow 226001, India
Received 25 April 2003; accepted 15 July 2003
Abstract—Geraniol, an abundantly available naturally occurring allylic alcohol, has been used as a starting material to prepare a
series of 6-[a-(30-aryl-30-hydroxypropyl)vinyl]-1,2,4-trioxanes. Some of these novel trioxanes have shown very promising anti-
malarial activity against multi-drug resistant Plasmodium yoelii in mice by both intramuscular (im) and oral routes.
# 2003 Elsevier Ltd. All rights reserved.
Artemisinin 1, the antimalarial principle of Chinese tra-
ditional drug Artemisia annua and its derivatives are
highly potent antimalarials, active against multi-drug
resistant malaria.1 The fact that these compounds owe
their antimalarial activity to peroxide group, present in
the form of a 1,2,4-trioxane in their molecular struc-
tures, has led to the current interest in synthetic 1,2,4-
trioxanes.2
In continuation with our studies in this area,2g,3 we have
recently reported the synthesis and antimalarial activity
of a series of geraniol-based 1,2,4-trioxanes.4 Trioxane
3, the best compound in this series showed only moder-
ate activity against multi-drug resistant Plasmodium
yoelii in mice. The methodology for preparation of these
trioxanes also suffered from two serious limitations; (i)
photooxygenation of geraniol 2, the key step in the
synthesis, gave a complex mixture of hydroperoxides
resulting in very poor yield of the desired compounds,
(ii) only limited structural variations were possible. In
the present study, we have used aldehyde acetate 4,
easily accessible from geranyl acetate in two steps,5 to
prepare a series of new hydroxy-functionalized 1,2,4-
trioxanes. Several of these trioxanes have shown prom-
ising antimalarial activity against multi-drug resistant P.
yoelii in mice by both oral and intramuscular (im)
routes.
Scheme 1. Reaction conditions: (a) (i) ArMgBr, dry Et2O, 0 ꢀC to rt,
3 h; (ii) H2O, 0 ꢀC; (b) hn, O2, methylene blue, MeCN, 0 ꢀC, 4–6 h;
(c) ketone, TsOH, CH2Cl2, rt, 1h.
§CDRI Communication no.: 6401.
*Corresponding author. Tel.: +91-522-222-4273; fax: +91-522-222-
3405; e-mail: chandancdri@yahoo.com
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00782-0

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C. Singh et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3447–3450
Table 1. In vivo antimalarial activity of trioxanes against P. yoelii in Swiss mice
Compd
Dose
(mg/kg/day)
Route
% Suppression
on day 4a
Mice alive
on day 28
Mean survival timeb
(MST)ꢁSE
3
96
48
24
100.0
100.0
67.7
2/5
0/5
0/5
19.57ꢁ2.14
14.80ꢁ2.23
12.20ꢁ2.24
im
7a
7b
7c
7d
96
48
96
100.0
97.0
30.0
0/5
0/5
0/5
14.80ꢁ1.06
12.60ꢁ1.12
08.20ꢁ0.53
im
oral
96
48
96
100.0
98.4
29.6
2/5
0/5
0/5
14.57ꢁ0.83
14.75ꢁ0.75
07.40ꢁ0.51
im
oral
96
48
96
100.0
93.4
81.3
1/5
0/5
0/5
14.75ꢁ1.48
15.75ꢁ2.37
10.40ꢁ0.93
im
oral
96
48
24
96
48
100.0
100.0
85.2
100.0
94.3
2/5
0/5
0/5
0/5
0/5
17.33ꢁ1.75
16.75ꢁ1.23
11.80ꢁ1.53
16.00ꢁ1.22
12.00ꢁ1.14
im
oral
8a
8b
8c
8d
96
48
96
94.5
90.5
65.0
0/5
0/5
0/5
14.40ꢁ1.21
12.60ꢁ1.21
11.60ꢁ1.12
im
oral
96
48
96
100.0
97.10/5 6.00
88.8
5/5
1
0/5
>28
ꢁ1.93
im
oral
10.20ꢁ0.53
96
48
96
100.0
93.4
68.4
4/5
0/5
0/5
16.00ꢁ0.00
14.20ꢁ1.33
13.00ꢁ1.67
im
oral
96
48
96
48
96.4
81.0
100.0
77.5
4/5
0/5
0/5
0/5
14.00ꢁ0.00
10.60ꢁ0.68
13.80ꢁ0.53
11.20ꢁ1.36
im
oral
9a
9b
9c
9d
96
48
96
48
100.0
5/5
1
2/5
0/5
>28
im
98.12/5 5.57
100.0
62.4
ꢁ0.33
16.57ꢁ1.21
09.00ꢁ0.71
oral
96
48
96
48
100.0
78.9
100.0
90.8
5/5
0/5
5/5
0/5
>28
15.00ꢁ0.95
im
>28
10.20ꢁ0.85
oral
96
96
48
24
im
72.13
100.0
100.0
87.10/5 0.40
2/5
3/5
2/5
1
15.33ꢁ2.31
17.00ꢁ3.00
15.57ꢁ1.75
ꢁ1.24
oral
96
96
48
24
48
24
im
79.0
100.0
100.0
90.4
100.0
100.0
3/5
2/5
2/5
0/5
5/5
4/5
14.50ꢁ0.50
17.33ꢁ1.45
17.57ꢁ2.02
08.80ꢁ0.53
>28
oral
Artemisinin
Chloroquine
im
16.00ꢁ0.00
96
48
—
100.0
100.0
—
4/5
2/5
20.00ꢁ0.00
17.60ꢁ1.33
07.00ꢁ0.14
oral
Vehicle control
—
0/15
aPercent suppression=[(CꢂT)/C]ꢃ100; where C=parasitaemia in control group, and T=parasitaemia in treated group.
bMST calculated for the mice which died during 28-day observation period.

C. Singh et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3447–3450
3449
Chemistry
Both these trioxanes are derived from 2-adamantanone.
The other two trioxanes derived from 2-adamantanone
(9c and 9d) also show significant activities. 9c provides
60% protection at 96 mg/kg and 40% protection at 48
mg/kg when given orally. Similarly trioxane 9d provides
40% protection when given im at 96 and 48 mg/kg.
Both these trioxanes show complete clearance of para-
sitaemia on day 4 when given orally at 96 and 48 mg/kg.
Trioxanes 8b and 8c derived from cyclohexanone also
show complete clearance of parasitaemia on day 4 at 96
mg/kg given im and provide 100 and 80% protection,
respectively. Trioxane 8d though shows only 96%
clearance of parasitaemia on day 4 but provides 80%
protection when given im. Trioxanes 7b, 7c, and 7d
derived from cyclopentanone also show complete clear-
ance of parasitaemia on day 4 at 96 mg/kg given by im
route and provide 40, 20 and 40% protection, respec-
tively. Trioxanes 7c and 7d also show significant sup-
pression of parasitaemia on day 4 when given orally but
none of the treated tice survives beyond day 28. Simi-
larly, trioxanes 7a and 8a show significant suppression
of parasitaemia on day 4 at 96 and 48 mg/kg given im,
but none of the treated animals survive, beyond day 28.
Trioxane 3, the best compound of the earlier series,
shows 100% clearance of parasitaemia at 96 and 48 mg/
kg on day 4 when given im but provides only 40% pro-
tection at 96 mg/kg (Table 1).
Geranyl acetate was converted to aldehyde acetate 4
using a known procedure.5 Reaction of 4 with excess of
Grignard reagents prepared from bromobenzene, 4-
bromochlorobenzene, 1-bromonaphthalene and 4-bro-
mobiphenyl furnished allylic alcohols 5a–d in 60-75%
yields. Methylene blue sensitized photooxygenation of
allylic alcohols 5a–d in MeCN furnished b-hydroxy-
hydroperoxides 6a–d in 30–45% yield, as inseparable
mixture of diastereomers. Acid catalyzed condensation
of b-hydroxyhydroperoxides 6a–d with cyclopentanone,
cyclohexanone,
and
2-adamantanone
furnished
hydroxy-functionalized 1,2,4-trioxanes 7a–d, 8a–d, 9a–
d in 50–74% yields (Scheme-1), again as inseparable
mixture of diastereomers. In most of the cases these
diastereomers were indistinguishable even by 1H
NMR, and only 13C NMR could differentiate them
(Scheme 1).6
A high order of activity shown by trioxanes having
adamantane moiety (9a–d) is in agreement with our
earlier observations.3 In this series, the introduction of
very hydrophobic aryl groups such as naphthyl and
biphenyl (9c and 9d) leads to decrease in the activity.
For trioxanes with adamantane moiety a phenyl/
chlorophenyl in the side chain provides the desired level
of hydrophobicity to these molecules. Introduction of
naphthyl and biphenyl groups in the side chain of the
trioxanes derived from cyclohexanone (8c and 8d) has
favorable effect on the activity.
Antimalarial Activity
Trioxanes 7a–d, 8a–d, and 9a–d were initially screened
for their antimalarial activity against multi-drug resis-
tant P. yoelii in Swiss mice7 at a highest dose of 96 mg/
kg8 by both intramuscular (im) and oral routes. The
trioxanes showing activity at 96 mg/kg by either routes
were further evaluated at 48 and 24 mg/kg. The results
are shown in Table 1.
Conclusion
Using geraniol derived trioxane 3 as lead, we have pre-
pared a new series of hydroxy-functionalized 1,2,4-
trioxanes, several of which have shown very promising
activity against multi-drug resistant P. yoelii in mice
both by oral and im routes.
Results and discussion
Acknowledgements
Trioxane 9b is the best compound in the series. It shows
100% clearance of parasitaemia on day 4 at 96 mg/kg
by both im and oral routes and all the animals survive
beyond day 28. Trioxane 9a is the next best compound
in the series. At 96 mg/kg given im, this compound
shows complete clearance of parasitaemia on day 4 and
all the animals survive beyond day 28. Even at 48 mg/
kg, this compound shows almost complete clearance of
parasitaemia on day 4 and 40% of the animals survive
beyond day 28. This trioxane also shows complete
clearance of parasitaemia on day 4 at 96 mg/kg by oral
route and 40% of the animals survive beyond day 28.
Nitin Gupta is grateful to the Council of Scientific and
Industrial Research (CSIR), New Delhi for award of
Senior Research Fellowship.
References and Notes
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37, 253. (e) Zhou, W. S.; Xu, X. X. Acc. Chem. Res. 1994, 27,
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eomers): FT-IR (neat, cmꢂ1) 1597.5, 1645.4, 3443.8; 1H NMR
(200 MHz, CDCl3) d 1.42–1.57 (m, 8H), 1.84–2.31 (m, 6H),
3.68 (dd, 1H, J=11.8, 3.0), 3.86–4.01 (m, 1H), 4.69 (dd, 1H,
J=10.2, 3.0), 5.04 and 5.06 (2ꢃs, 2H), 5.45 (m, 1H), 7.45–8.08
(m, 7H); 13C NMR (50 MHz, CDCl3) d 22.67 (t), 22.71(t),
25.94 (t), 29.41(t), 30.67 (t), 30.72 (t), 35.00 (t), 36.77 (t), 36.82
(t), 62.53 (t), 62.57 (t) 70.80 (d), 70.95 (d), 81.68 (d), 81.75 (d),
102.84 (s), 114.64 (t) 114.87 (t), 123.20 (d), 123.31 (d), 123.49
(d), 125.81 (d), 125.98 (d), 126.48 (d), 126.52 (d), 128.44 (d),
128.48 (d), 129.31 (d), 130.77 (s), 134.28 (s), 140.49 (s)140.58
(s), 143.94 (s), 144.01 (s); FABMS (m/z) 369 (M++1). Anal.
calcd C 74.97%, H 7.66%; found C 75.20%, H 7.49%.
2. (a) O’Neill, P. M.; Pugh, M.; Davies, J.; Ward, S. A.; Park,
B. K. Tetrahedron Lett. 2001, 42, 4569. (b) Bloodworth, A. J.;
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Tetrahedron 1993, 49, 2729. (d) Posner, G. H.; Oh, C. H.; Til-
hous, W. K. Tetrahedron Lett. 1991, 34, 4235. (e) Bunelle,
W. H.; Isbell, T. A.; Bames, C. L.; Qualls, S. J. Am. Chem.
Soc. 1991, 113, 8168. (f) Avery, M. A.; Jennings-White, C.;
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Tetrahedron Lett. 1990, 33, 6901. (h) Kepler, J. A.; Philip, A.;
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Med. Chem. Lett. 1992, 2, 497. (b) Singh, C.; Misra, D.;
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Trioxane 9d (mixture of diastereomers): FT-IR (neat, cmꢂ1
)
1
1600.8, 1646.5, 3407.3; H NMR (200 MHz, CDCl3) d 1.50–
2.18 (m, 17H), 2.88 (bs, 1H), 3.72 (dd, 1H, J=11.8, 3.0 Hz),
3.94 (dd, 1H, J=11.8, 10.5 Hz), 4.70–4.80 (m, 2H), 5.07 (s,
2H), 7.33–7.60 (m, 9H); FABMS (m/z) 447 (M++1). Anal.
calcd: C 78.00%, H 7.67%; found C 77.69%, H 8.01%.
7. The in vivo efficacy of compounds was evaluated against P.
yoelii (MDR) in Swiss mice model. The colony bred Swiss
mice (25ꢁ1g) were inoculated with 1 ꢃ106 parasitised RBC on
day zero and treatment was administered to a group of five
mice at each dose, from day 0 to 3, in two divided doses daily.
The drug dilutions were prepared so as to contain the required
amount of the drug (1.2 mg for a dose of 96 mg/kg and 0.6 mg
for a dose of 48 mg/kg) in 0.1mL and administered either
intramuscularly or orally for each dose. Parasitaemia level
were recorded from thin blood smears between days 4 and 28.9
Mice treated with artemisinin and chloroquine served as posi-
tive controls.
5. Dodd, D. S.; Oehlschlager, A. C.; Georgopapadakou, N. H.;
Polok, A.-M.; Hartman, P. G. J. Org. Chem. 1992, 57, 7226.
6. Selected data: Trioxane 7b (mixture of diastereomers): FT-
1
IR (neat, cmꢂ1) 1597.4, 1646.8, 3435.1; H NMR (200 MHz,
CDCl3) d 1.70–1.92 (m, 9H), 2.06–2.16 (m, 2H), 2.38–2.44 (m,
1H), 3.78–3.82 (m, 2H), 4.64–4.76 (m, 2H), 5.05 (s, 2H), 7.20–
7.40 (m, 4H); 13C NMR (50 MHz, CDCl3) d 23.33 (t), 24.70
(t), 29.84 (t), 32.73 (t), 36.97 (t), 37.27 (t), 64.46 (t), 73.04 (d),
73.07 (d), 81.14 (d), 81.22 (d), 114.47 (s), 114.50 (t), 127.23
(2ꢃd), 128.67 (2ꢃd), 133.31 (s), 142.97 (s), 143.10 (s); FABMS
(m/z) 339 and 341(M ++1). Anal. calcd C 63.81%, H 6.84%;
found C 63.58%, H 7.02%. Trioxane 8c (mixture of diaster-
8. Since artemisinin is active at a dose of 48 mg/kg, we have
chosen 96 mg/kg as the highest dose in the primary screening
of trioxanes.
9. Puri, S. K.; Singh, N. Expl. Parasit. 2000, 94, 8.