Design, synthesis and biological evaluation of new oxazines with potential antiparasitic activity

Article abstract of DOI:10.1016/j.tetlet.2007.02.032

A series of new oxazines with potential antiparasitic activity was prepared using Diels-Alder reactions, based on terpenes derived from eucarvone as dienes and nitrosoarenes with different electronic characteristics as dienophiles. The biological activity

Full text of DOI:10.1016/j.tetlet.2007.02.032

Tetrahedron Letters 48 (2007) 2505–2507
Design, synthesis and biological evaluation of new oxazines
with potential antiparasitic activity
Daniela Gamenara, Horacio Heinzen and Patrick Moyna*
Departamento de Qu ı´ mica Org a´ nica, Facultad de Qu ı´ mica, Avda. General Flores 2124, Montevideo 11800, Uruguay
Received 30 November 2006; revised 7 February 2007; accepted 8 February 2007
Available online 13 February 2007
Abstract—A series of new oxazines with potential antiparasitic activity was prepared using Diels–Alder reactions, based on terpenes
derived from eucarvone as dienes and nitrosoarenes with different electronic characteristics as dienophiles. The biological activity
was evaluated with in vitro assays against Plasmodium falciparum, Trypanosoma cruzi and Trypanosoma brucei rhodesiense. Some
of these oxazines have activities in the 20–50 lM range, and may be leaders for the development of novel antiparasitic drugs with
improved pharmacological properties.
Ó 2007 Elsevier Ltd. All rights reserved.
Parasitic infectious diseases due to pathogenic protozoa
H
affect more than 3 billion people in the world and occur
mainly in underdeveloped countries. The most impor-
tant protozoan infections are malaria, trypanosomiasis
O
O
H
O
1
and leishmaniasis. Malaria is principally caused by
O
Plasmodium falciparum, and is transmitted by the bite
of Anopheles mosquitoes. This infection, which affects
predominantly tropical countries, can be considered
the most important due to its high morbidity and mor-
tality rates.
O
Figure 1. Chemical structure of artemisinine.
group was responsible for this activity. The interaction
of the peroxide bond with Fe species within the proto-
Both the extensive and intensive use of drugs to treat
malaria have induced a ‘selection’ of resistant parasites.
This has frustrated attempts to eradicate the disease by
means of chemotherapy, and increased the risk of turn-
II
zoan food vacuole catalyzes the generation of cytotoxic
free radicals, which are supposed to be the active spe-
5
,6
2
cies.
While this has not been conclusively demon-
ing obsolete the drugs currently used.
strated, the hypothesis is soundly based on the results
7
–9
of biomimetic studies.
During the 1980s, the survey of plants used in Chinese
traditional medicine against ‘fevers’ was resumed. The
active metabolite artemisinine was isolated from Artemi-
sia annua L. (Asteraceae) (Qinghaosu) (Fig. 1) and
found to be one of the few natural endo-peroxides
known.
The activity derived from the peroxide bond of these
natural compounds encourages the study of the anti-
malarial properties of other structurally related
compounds.
Standard dissociation energies of peroxide bonds
This novel active product with a sesquiterpene-lactone
has a structure completely different from the quinine
derivatives that had been the main group used. Phar-
(
–O–O–) and of nitroso bonds (–N–O–) are 33.2 and
1
0
3
,4
35.8 kcal/mol, respectively. Although the bond disso-
ciation energy may vary somewhat when these bonds
constitute part of more complex moieties, they are labile
and might be prone to homolytic cleavages. Oxazines
might follow the same reaction pattern of endo-perox-
ides (isosteric –N–O– bond cleavage and free radical
macological studies established that the endo-peroxide
*
Corresponding author. Tel.: +598 2 924 7881; fax: +598 2 924 1906;
e-mail: pmoyna@fq.edu.uy
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.02.032

2506
D. Gamenara et al. / Tetrahedron Letters 48 (2007) 2505–2507
generation), resulting in a similar biological activity.
Our group has already reported the design, synthesis
and biological evaluation of 12 oxazines obtained by
Diels–Alder cycloaddition using dienes with the benzo-
tropolone structure (tri-O-methylpurpurogallin) and
rule predicts that the R isomer will be esterified prefer-
ably. This way, we have (S)-eucarvol and (R)-acetyl-
eucarvol as dienes for the Diels–Alder cycloaddition.
1
3
The oxazines were synthesized through Diels–Alder
reaction using these dienes and nitrosobenzene, 4-nitroso-
benzaldehyde, 4-chloronitrosobenzene and 4-nitroso-
toluene as dienophiles (Fig. 2). Yields observed are
reported in Figure 2. Compound 6 was previously syn-
1
1
nitrosobenzene as dienophile.
In this Letter, the design, synthesis and biological eval-
uation of another 12 oxazines using the same hetero
Diels–Alder reaction is reported. As dienes, eucarvone
and its reduced and acetylated derivatives were used.
As dienophiles, nitrosobenzene and p-substituted deriv-
atives (aldehyde, chloro and methyl groups) were used
1
4
thesized and characterized by Hart et al. Compounds
1
13
7–17 were characterized by IR, MS, H and C NMR.
The dienes were treated with a slight excess of the substi-
tuted nitrosoarenes at room temperature, using tetra-
hydrofuran as solvent. The Diels–Alder reaction was
clean and resulted in good yields (70–98%). In most
cases the reactions were completed in less than 24 h. In
the cases of adducts 10–17, 15 and 17, only the anti
stereoisomer was obtained. Oxazines 14 and 16 were
obtained as mixtures of the anti and syn stereoisomers
in a anti/syn adduct ratio = 9/1. The regiochemistry of
these hetero Diels–Alder reactions is the one shown in
(
Fig. 2).
Eucarvone was synthesized from carvone using hydro-
gen bromide in glacial acetic acid according to Corey
1
2
and Burke’s technique. The product obtained was
reduced with NaBH in methanol, resulting in a racemic
4
mixture of the corresponding alcohols. The stereoselec-
tive acetylation of eucarvol was catalyzed by Candida
antarctica lipase (Chirazyme L5 Lyo) and carried out
using vinyl acetate as acyl donor agent. Kazlauskas’s
1
13
Figure 2, which was established from the H and
C
OH
O
OH
OAc
O
6
7
8
9
1
: R = H; 12 hs, 98%
: R = CHO; 10 hs, 95%
: R = Cl; 15 hs, 87%
a,b)
c)
d)
+
: R = CH ; 24 hs, 90%
5
3
2
3
4
0: R = H; 24 hs, 92%
e)
e)
1
e)
11: R = CHO; 24 hs, 95%
2: R = Cl; 24 hs, 95%
13: R = CH ; 72 hs, 70%
1
H
HO
O
OAc
H
3
1
4: R = H; 24 hs, 95%
O
O
N
O
N
15: R = CHO; 24 hs, 95%
16: R = Cl; 72 hs, 80%
N
1
7: R = CH ; 72 hs, 70%
3
R
R
10 - 13
14 - 17
R
6
- 9
Figure 2. Route to oxazines 6–17. Reagents and conditions: (a) HBr, HAc, À30 °C; (b) KOH, MeOH, reflux; (c) NaBH
CH COOCHCH , Chirazime L5 Lyo, 30 °C, 48 h; (e) NOPhR, THF, rt yields.
4
, MeOH, rt; (d)
3
2
Table 1. In vitro antiprotozoal activity of oxazines 6–17 against P. falciparum, T. cruzi and T. brucei rhodesiense
Compound
P. falciparum (IC50, lM)
T. cruzi (IC50, lM)
T. brucei rhodesiense (IC50, lM)
Cytotoxicity (lM)
6
7
8
9
n.a.
n.a.
n.a.
43.9
n.a.
n.a.
99.5
n.a.
n.a.
n.a.
n.a.
72.9
112.7
n.a.
n.a.
31.7
n.a.
62.3
28.3
39.5
77.3
n.a.
13.9
103
73
110
310
120
60
740
250
180
450
450
220
110
42.2
53.8
n.a.
n.a.
59.2
94.7
23.9
52.2
19.7
32
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
37.8
42.5
Chloroquine (IC50 3.0 nM), benznidazol, (IC50 4.7 lM), pentamidine (IC50 1.7 lM) were used as positives for P. falciparum, T. cruzi and T. brucei
rhodesiense respectively. Cytotoxicity was assayed against KB cells, using Podophyllotoxin (LD50 0.2 nM) as a standard. All determinations were
performed in triplicate.

D. Gamenara et al. / Tetrahedron Letters 48 (2007) 2505–2507
2507
spectra of the products and later confirmed by X-ray
diffraction studies performed on oxazine 6.
References and notes
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