Synthesis of new isoxazolylnaphthoquinones as potential trypanocidal and antibacterial agents

Article abstract of DOI:10.1039/a806065g

The synthesis of three new isoxazolytnaphthoquinones with an ethyl group on the isoxazole ring is reported.

Full text of DOI:10.1039/a806065g

View Article Online / Journal Homepage / Table of Contents for this issue
110 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
1999, 110±111$
Synthesis of New Isoxazolylnaphthoquinones as
Potential Trypanocidal and Antibacterial Agents$
Gladys E. Granero, Marõa M. de Bertorello* and
Margarita C. Brinon
Departamento de Farmacia, Facultad de Ciencias Quõmicas, Universidad Nacional de Cordoba,
5000 Cordoba, Argentina
The synthesis of three new isoxazolylnaphthoquinones with an ethyl group on the isoxazole ring is reported.
Previous studies on the biological behavior of isoxazolyl-
naphthoquinones1 have demonstrated that some members
of the series exhibit important biological activity against
the causative agent of Chagas' disease1 and Staphylococcus
aureus.3 These compounds have poor solubility in organic
solvents so we focused our synthetic e€orts towards more
lipophilic compounds. Here we describe the preparation of
three new isoxazolylnaphthoquinones with an ethyl group
on the isoxazole ring by condensing sodium 1,2-naphtho-
7.25 (s, OH), 7.66±7.82 (m, H5,6,7K, H6,7E, 7.94 (br, NH),
8.16±8.22 (m, H8K, H5E), 8.53±8.56 (d, H8E); E ˆ enol,
K ˆ keto] indicated the presence of two forms (d 5.58 for
keto and d 6.49 for enol), 40:60. When CF3COOD was
added to the CDCl3 solution, the tautomeric equilibrium
was shifted towards the keto structure: dH (CDCl3±
CF3COOD) 1.34 (t, 3H, CH3), 2.36 (s, 3H, CH3), 2.85
(q, 2H, CH2), 4.04 (s, 1H, NH), 6.45 (s, 1H, H3), 8.00±8.06
(m, 2H, H6,7), 8.36±8.45 (m, 2H, H5,8). When NaOD was
added, the equilibrium was displaced to the tautomeric form
3 to yield its sodium salt 7: dH (CDCl3±NaOD) 0.73 (t, 3H,
CH3), 2.07 (s, 3H, CH3), 2.18 (q, 2H, CH2), 5.20 (s, 1H,
H3), 6.99±7.14 (m, 2H, H6,7), 7.46 (m, 1H, H5), 7.93 (m, 1H,
H8). No evidence of a tautomeric equilibrium was found in
the 1H NMR spectrum of 3 in (CD3)2SO, presumably due
to H-bonding of (CD3)2SO to OH.
quinone-4-sulfonate 1 with 3-methyl-5-ethyl-4-aminoisox-
azole 2 in aqueous solution (Scheme 1).
3-Methyl-5-ethyl-4-aminoisoxazole
2 was obtained by
treatment of 3,5-dimethylisoxazole with n-butyllithium3 in
THF at 78 8C and quenching the reaction mixture with
MeI followed by nitration4 and reduction.5
Compounds 3, 4 and 5 were characterized by IR, MS and
1
NMR. The H NMR data of 3±7 are shown in Table 1.
We investigated the e€ects of 7 on the motility and
vitality of trypomastigote forms of Trypanosoma cruzi on
blood samples obtained by bleeding of infected Albino
Swiss mice. These bioassays showed that 7 inhibits the
motility of ¯agellated forms (30 h) in a dose-related manner,
the more e€ective blood concentrations being 150, 300 and
400 mg/ml. Furthermore, parasites in blood samples from
animals injected with trypomastigote forms immobilized
in vitro with 7 were not detected.7
Scheme 1
The method proposed is relatively simple and versatile
allowing, through variation of the pH of the medium, the
reaction to be directed towards the formation of products
with a variable degree of lipophilicity. In neutral and alka-
line aqueous solutions, the reaction between 1 and 2 yielded
48 and 67% of 2-hydroxy-N-(3-methyl-5-ethylisoxazol-4-yl)-
The evaluation of the antimicrobial properties of 3 and 7
revealed that both compounds show in vitro antibacterial
1 8
.
activity against S. aureus with a MIC of 32 mg ml
1,4-naphthoquinone-4-imine
3 respectively. The reaction
Table 1 Proton chemical shifts (dH) and coupling constants
(J/H2) calculated by LAOCOON III program
between 1 and 2 in acidic aqueous solution gave 90% of the
bisisoxazolylnaphthoquinone imine 4. By hydrolysis with
acidic aqueous solutions at 80 8C in EtOH, compound 4 was
converted to the diketone derivative 5, a tautomer of 3 in
99% yield. Reaction of 3 with ethereal diazomethane led to
the O-methyl derivative 6, whereas treatment of 5 with dia-
zomethane led to no reaction. The IR spectrum of 3 (KBr)
showed one absorption band at 3309 cm (O±H) and one
1
band at 1703 cm (carbonyl of the quinone ring). The IR
spectrum of 5 showed one absorption band at 3283 cm
1
(N±H) and two bands at 1689 and 1623 cm (two carbonyl
Compd.
H-3c
H-5
3
4
5
6
7
5.23a
±
5.32a
5.62b
7.94a
8.09b
7.78a
7.70b
7.85a
7.76b
8.06a
8.12b
0.034a
0.167b
6.26a
6.21b
8.08a
8.22b
7.76a
7.66b
7.84a
7.74b
8.44a
8.49b
0.077a
0.118b
5.29a
±
5.64b
±
±
8.04a
7.88a
±
8.23b
±
±
H-6
7.75a
7.52a
±
7.67b
±
±
H-7
7.89a
7.61a
1
±
7.76b
±
±
H-8
8.22a
8.36a
±
8.53b
±
±
rmsd
0.092a
±
0.001a
±
1
0.218b
J56
J57
J58
J67
J68
J78
7.43a
±
7.94a
8.60b
2.51a
1.64b
1.88a
1.35b
7.24a
8.65b
1.87a
1.74b
7.46a
8.75b
8.08a
6.98b
1.46a
1.18b
1.63a
1.11b
7.25a
7.39b
1.03a
1.22b
8.21a
7.70b
7.86a
stretching vibrations of the quinone ring). The 1H NMR
spectrum of 5 in CDCl3 gave no indication of a second
tautomer (see Experimental). For enol 3 a complex 1H
NMR spectrum [dH (CDCl3) 1.22±1.29 (t, CH3), 2.19
(s, CH3), 2.59±2.77 (m, CH2), 5.58 (s, H3K), 6.49 (s, H3E),
±
7.92b
±
±
1.83a
1.05a
±
1.39b
±
±
0.75a
1.81a
±
0.61b
±
±
7.54a
8.63a
±
7.38b
±
±
0.81a
0.99a
±
1.11b
±
±
8.01a
±
8.20a
±
*To receive any correspondence (e-mail: marcor@dqo.fcq.unc.edu.ar).
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
8.23b
aRecorded in (CD3)2SO. bRecorded in CDCl3. cExperimental data
20.01 ppm. dErrors in Hz.

View Article Online
J. CHEM. RESEARCH (S), 1999 111
(CDCl3) 1.12 (t, 3H, CH3), 1.18 (t, 3H, CH3), 2.10 (s, 3H, CH3),
2.11 (s, 3H, CH3), 2.46 (q, 2H, CH2), 2.61 (q, 2H, CH2), 5.64
(s, 1H, 3-H), 6.47 (br, s, 1H, NH), 7.68 (m, 1H, 6-H), 7.76 (m, 1H,
7-H), 8.23 (m, 1H, 5-H); 8.53 (m, 1H, 8-H); ꢀC [(CD3)2SO] 8.7
(CH3), 9.1 (CH3), 10.3 (CH3), 10.5 (CH3), 18.0 (CH2), 18.3 (CH2),
95.0 (C-3), 113.3 (C), 124.7 (C), 125.2 (C), 125.6 (C), 129.6 (C), 130.8
(C), 133.5 (C), 134.5 (C), 144.0 (C), 154.6 (C), 157.0 (C), 157.7 (C),
Experimental
Melting and boiling points were determined by the capillary
method on a Buchi 510 melting point apparatus and are un-
corrected. The IR spectra were recorded for potassium bromide
discs on a Nicolet 5 SXC FT IR spectrophotometer and only
selected absorptions are reported. 1H and 13C NMR spectra were
recorded at 200.1 and 50.3 MHz in CDCl3 and (CD3)2SO on a
Bruker AC 200E spectrometer; chemical shifts are given in ppm (d)
with Me4Si as internal standard. The proton spectra were simulated
using the LAOCOON III program. The mass spectra at 70 eV of
1
157.8 (C), 168.2 (CNH), 180.3 (CO); ꢁmax/cm (KBr) 3335 (NH),
.
.
1662 (C O), 1616 (C N); ꢂmax/nm 239, 300, 350, 442; m/z (%) 390
ꢀ
‡
(3, M ), 333 (28), 307 (17), 293 (20), 236 (70), 225 (10), 209 (16),
195 (60), 169 (6), 139 (15), 57 (100) (Found: C, 67.99; H, 5.78; N,
14.59. C22H22N4O3 requires C, 67.69; H, 5.64; N, 14.36%).
3
and 4 were recorded and the analyses were performed by
UMYMFOR Laboratories, Buenos Aires, Argentina. The mass
spectra of 2 and 5 were recorded on a Finnigan Model 3300 F-100
Quadrupole Mass Spectrometer. UV spectra were recorded for
solutions in ethanol (95%) on a Shimadzu UV-160 A UV-VIS
spectrometer. A Chromatotron model 7924 T was used for preparative
radial chromatography (PRC). Column chromatography was per-
formed on silica gel 60 (Macherey Nagel 0.05±0.2 mm). Precoated
silica gel (Merck) was used for thin layer chromatography (TLC).
All chemicals and reagents were of analytical grade. THF was freshly
distilled over Na wire and LiAlH4. Organic extracts were dried over
anhydrous Na2SO4. n-Butyllithium in diethyl ether solution was
prepared and the concentration was determined by Gilman's pro-
cedure.6 3,5-Dimethylisoxazole was purchased from Aldrich.
3-Methyl-5-ethylisoxazole.ÐBp 159±160 8C, prepared by treat-
ment of 3,5-dimethylisoxazole (47.9 mmol) with n-butyllithium
(1.33 M; 36 ml) followed by addition of CH3I (47.9 mmol) (70%).3
ꢀH (CDCl3) 1.27 (t, 3H, CH3), 2.25 (s, 3H, CH3), 2.72 (q, 2H, CH2),
5.80 (s, 1H, 4-H).
4-(3-Methyl-5-ethylisoxazol-4-ylamino)-1,2-naphthoquinone 5.Ð
After re¯uxing 4 (0.098 g, 0.25 mmol) in EtOH (10.05 ml) for 5 min,
0.5 M HCl (5 ml) was added. The mixture was heated at 80 8C
for 40 min. After cooling to room temp. the organic solvent was
evaporated under reduced pressure and the aqueous phase was
extracted with CH2Cl2 (5Â 10 ml). The organic extract was dried
(Na2SO4) and the solvent removed to give 0.064 g (90%) of 5 as an
orange solid. The crude product was puri®ed by PRC (benzene
100%, benzene±CH2Cl2 90:10, 80:20, 60:40), mp 130±131 8C; ꢀH
(CDCl3) 1.29 (t, 3H, CH3), 2.24 (s, 3H, CH3), 2.70 (q, 2H, CH2),
5.62 (s, 1H, 3-H), 6.63 (br s, 1H, NH), 7.68±7.81 (m, 2H, 6-, 7-H),
8.08±8.14 (m, 2H, 5-, 8-H); ꢀH [(CD3)2SO], 1.18 (t, 3H, CH3), 2.10
(s, 3H, CH3), 2.66 (q, 2H, CH2), 5.32 (s, 1H, 3-H), 7.73±7.95 (m,
3H, 5-, 6-, 7-H), 8.03±8.06 (d, 1H, 8-H), 8.70 (br s, 1H, NH); ꢀc
[(CD3)2SO] 8.9 (CH3), 10.6 (CH3), 18.2 (CH2), 99.6 (C), 102.3 (C-3),
113.01 (C), 125.1 (C), 125.8 (C), 130.3 (C), 132.4 (C), 134.6 (C),
1
142.4 (C), 148.4 (C), 157.7 (C), 168.2 (CO), 182.0 (CO); ꢁmax/cm
.
.
.
(KBr) 3283 (N±H), 1689 (C O), 1623 (C O), 1597 (C O), 1597
.
3-Methyl-5-ethyl-4-nitroisoxazole.ÐBp 121 8C (30 mmHg) (lit.
50 8C at 0.7 mmHg), obtained in 76% yield by treatment of 3-
methyl-5-ethylisoxazole with a 1:3 mixture of HNO3 (65%) and
H2SO4 (96%).4 ꢀH (CDCl3) 1.38 (t, 3H, CH3), 2.56 (s, 3H, CH3),
3.22 (q, 2H, CH2).
ꢀ
‡
(C N); ꢂmax/nm 218, 264, 325, 435; m/z (%) 283 (27, M +1), 253
(29), 241 (31), 227 (9), 199 (6), 185 (100), 157 (16), 129 (21), 101
(23), 89 (1), 75 (9), 57 (19) (Found: C, 68.48; H, 5.18; N, 10.19.
C
16H14N2O3 requires C, 68.09; H, 4.96; N, 9.93%).
2-Methoxy-N-(3-methyl-5-ethylisoxazol-4-yl)-1,4-naphthoquinone
3-Methyl-5-ethyl-4-aminoisoxazole 2.ÐTo 3-methyl-5-ethyl-4-
nitroisoxazole (0.493 g, 3.16 mmol) magnetically stirred at 0±5 8C
(ice bath), 16 ml of distilled water and 4.01 g of NH4Cl were added.
The reaction mixture was stirred for an additional 5 min and sub-
sequently zinc dust (1.76 g) was added in portions with stirring.
After the mixture had been stirred at 0 8C for 10 min, EtOAc
(18 ml) was added and the mixture was ®ltered through Celite.
The layers were separated and the aqueous layer extracted
with EtOAc (3Â 20 ml). Removal of solvent under reduced pressure
provided an oil (0.277 g, 70% yield) of suitable purity, bp 184±
185 8C, for the next reaction: ꢀH (CDCl3) 125 (t, 3H, CH3), 2.19
4-Imine 6.ÐObtained in 85% yield by reaction of 3 with an ethereal
solution of diazomethane.9 Mp 148±149 8C; ꢀH (CDCl3) 1.26 (t, 3H,
CH3), 2.21 (s, 3H, CH3), 2.65 (q, 2H, CH2), 3.79 (s, 3H, OCH3),
6.21 (s, 1H, 3-H), 7.62±7.78 (m, 2H, 6-, 7-H), 8.22 (m, 1H, 5-H),
8.49 (m, 1H, 8-H); ꢀH [(CD3)2SO] 1.16 (t, 3H, CH3), 2.16 (s, 3H,
CH3), 2.65 (q, 2H, CH2), 3.78 (s, 3H, OCH3), 6.26 (s, 1H, 3-H),
7.71±7.88 (m, 2H, 6-, 7-H), 8.07 (m, 1H, 5-H), 8.44 (m, 1H, 8-H);
.
1
ꢁ
max/cm (KBr) 2977 (CH3), 1667 (C O); ꢂmax/nm 236, 292, 340,
ꢀ
‡
438; m/z (%) 296 (2.88, M ), 239 (5.82), 213 (14), 199 (100), 184
(34), 172 (11), 156 (17), 127 (19), 101 (17), 57 (44).
Sodium 4-(3-Methyl-5-ethylisoxazol-4-ylimino)-1-oxo-1,4-dihydro-
naphthalene-2-olate 7.ÐObtained in 82% yield by reaction of 3 with
a methanolic solution of NaMeO (5 M).10 ꢀH [(CD3)2SO] 1.12 (t, 3H,
CH3), 1.99 (s, 3H, CH3), 2.56 (q, 2H, CH2), 5.29 (s, 1H, 3-H), 7.46±
(s, 3H, CH3), 2.51±2.58 (br s, 2H, NH2), 2.65 (q, 2H, CH2); ꢁmax
/
1
cm (KBr) 3406±3238, 2981, 2944, ꢀ1668, 1485, 1434, 1389, 1339,
1273, 1053, 1023; m/z (%) 126 (3, M ), 111 (100), 85 (23), 70 (21),
‡
69 (16), 57 (94), 56 (31), 42 (34), 29 (36), 28 (35).
2-Hydroxy-N-(3-methyl-5-ethylisoxazol-4-yl)-1,4-naphthoquinone
4-Imine 3.ÐTo 1 (1.037 g, 3.98 mmol) in 38 mL of bu€er consisting
7.66 (m, 2H, 6-, 7-H), 7.68 (m, 1H, 5-H), 8.36 (m, 1H, 8-H); ꢁmax
/
1
cm (KBr) 1636 (C N), 1648 (C O).
.
.
3
2
of 7.4Â 10 M Na3PO4 and 8.0Â 10 M Na2HPO4, pH 11, cooled
with an ice±NaCl bath was added a suspension of 2 (0.227 g,
1.81 mol) in 6 ml of distilled water. The reaction mixture was stirred
(0 8C, 30 min) and then acidi®ed with HCl until precipitation of
product was complete. The resulting orange precipitate was
collected, washed with water until neutral and dried in vacuum
to give 0.343 g (67%) of 3 as an orange solid. The product was
chromatographed on silica gel. Elution with benzene±ethyl acetate
(40:60) gave 0.251 g (49%) of 3, mp (decomp.) 225±226 8C, ꢀH
[(CD3)2SO] 1.18 (t, 3H, CH3), 2.14 (s, 3H, CH3), 2.73 (q, 2H, CH2),
5.23 (s, 1H, 3H), 7.74 (m, 1H, 6-H), 7.90 (m, 1H, 7-H), 8.04 (m,
1H, 5-H), 8.22 (m, 1H, 8-H), 9.27 (s, 1H, OH); ꢀC [(CD3)2SO] 8.8
(CH3), 10.6 (CH3), 18.2 (CH2), 100.1 (C-3), 112.9 (C), 123.5 (C),
128.0 (C), 129.1 (C), 130.0 (C), 131.0 (C), 131.3 (C), 134.2 (C),
We thank CONICET, CONICOR and SECYT-UNC
of Argentina for ®nancial support.
Received, 3rd August 1998; Accepted, 23rd October 1998
Paper E/8/06065G
References
1 M. N. Schwarcz de Tarlovsky, S. G. Goijman, M. P. Molina
Portella and A. O. M. Stoppani, Experientia, 1990, 46, 502;
P. Amuchastegui, E. Moretti, B. Basso, N. Sperandeo and M. M.
de Bertorello, Rev. Argent. Microbiol., 1990, 22, 199.
2 I. Albesa, P. Bogdanov, A. Eraso, N. R. Sperandeo and M. M.
de Bertorello, J. Appl. Bacteriol., 1995, 78, 373.
1
155.2 (C), 157.6 (C), 175.8 (COH), 180.6 (CO); ꢁmax/cm (KBr)
.
.
3309 (OH), 1703 (C O, 1598 (C N); ꢂmax/nm 239, 269, 446; m/z
(%) 282 (4, Mꢀ ), 241 (5), 199 (5), 186 (10), 185 (92), 156 (7), 102
‡
(24), 101 (18), 77 (20), 57 (100) (Found: C, 68.31; H, 5.19; N, 9.64.
C16H14N2O3 requires C, 68.09; H, 4.96; N, 9.93%).
2-(3-Methyl-5-ethylisoxazol-4-ylamino)-N-(3-methyl-5-ethylisoxazol-
4-yl)-1,4-naphthoquinone 4-Imine 4.ÐTo 2 (0.301 g, 2.39 mmol) sus-
pended in 0.5 M HCl (20 ml) and cooled with an ice±NaCl bath was
3 D. J. Brunelle, Tetrahedron Lett., 1981, 22, 3699.
4 J. D. Perez and D. Wunderlin, J. Org. Chem., 1987, 53, 3637.
5 A. Pascual, Helv. Chim. Acta, 1989, 72, 556; 1991, 74, 531.
6 H. Gilman, Org. React., 1975, 6, 339.
7 G. E. Granero, P. Paglini, A. R. Fernandez and M. M. de
Bertorello, Medicina, 1997, 57, 44.
added
1 (0.654 g, 2.51 mmol) in 15 ml of distilled water. The
reaction mixture was stirred (0 8C, 30 min) and then extracted with
CH2Cl2 (5Â35 ml). The organic extract was washed with 5 M
NaOH (3Â35 ml), dried (Na2SO4) and the solvent removed to give
0.420 g (90%) of 4 as a red solid. Crystallization from H2O±EtOH
(1:1) gave 0.339 g (72.9%) of red crystals, mp 115±116 8C; ꢀH
8 P. Bogdanov, G. Granero, I. Albesa and M. M. de Bertorello,
to be published.
9 Vogel's Textbook of Practical Organic Chemistry, 4th edn.,
Longman, London, 1978, p. 291.
10 H. Gilman and A. H. Blatt, Org. Synth., 1941, vol. I, p. 206.