Bis-1,2,4-triazolo[4,3-a:3′,4′-c]quinoxalines of pharmaceutical interest from 1,3-dipolar cycloaddition

Article abstract of DOI:10.1016/j.tetlet.2008.01.047

Various derivatives of the heterocyclic system 1,12,12a,12b-tetrahydrobis-1,2,4-triazolo[4,3-a:3′,4′-c]quin oxaline of pharmaceutical interest have been obtained by double site- and regio-selective 1,3-dipolar cycloaddition of arylnitrilimines to quinoxal

Full text of DOI:10.1016/j.tetlet.2008.01.047

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 1847–1850
Bis-1,2,4-triazolo[4,3-a:30,40-c]quinoxalines of pharmaceutical
interest from 1,3-dipolar cycloaddition
Antonino Lauria *, Annalisa Guarcello, Gaetano Dattolo, Anna Maria Almerico
`
Dipartimento Farmacochimico, Tossicologico e Biologico, Universita di Palermo, Via Archirafi 32, 90123 Palermo, Italy
Received 2 November 2007; revised 26 December 2007; accepted 9 January 2008
Available online 15 January 2008
Abstract
Various derivatives of the heterocyclic system 1,12,12a,12b-tetrahydrobis-1,2,4-triazolo[4,3-a:30,40-c]quinoxaline of pharmaceutical
interest have been obtained by double site- and regio-selective 1,3-dipolar cycloaddition of arylnitrilimines to quinoxalines. No evidence
for the formation of mono-adducts was obtained, at variance with literature reports. Specific studies to establish the exact stereo-
chemistry of the bis-cycloadducts were undertaken.
Ó 2008 Elsevier Ltd. All rights reserved.
H2N
In common with other nitrogen heterocycles, quinoxa-
N
N
R'
lines, as well as their fused-ring bioisosteric analogs, show
marked activity in many biological systems. A large num-
ber of compounds incorporating these ring systems were
found to possess antitumour and antibacterial activities.
Thus, indolo[2,3-b]quinoxalines of type 1 represent an
important series of DNA intercalating agents endowed
with antiviral and cytotoxic activities (Fig. 1). For example,
compound 2 (B-220) was found active against herpes
virus1,2 and also as chemopreventive agent in experimental
tumour models.3 The tetracyclic 1,6-diamino-bis-1,2,4-
triazolo[4,3-a:3,4-c]quinoxaline (3) and 7-chloro-2-oxo-
2H-pyrimido[20,10:5,1]-1,2,4-triazolo[4,3-a]quinoxalines of
type 4 have shown moderate and high activity, respectively,
against Gram-positive and Gram-negative micro-
organisms.4
N
N
N
N
N
N
R''
N
N
N
N
R
2
N
1
H2N
R'
3
O
N
R'
R'
N
N
N
N
R
N
N
R
R
N
N
N
N
N
R
N
N
N
N
Cl
N
R'
6
4
5
Fig. 1. Common heterocyclic quinoxalines and quinoxaline-like com-
pounds with biological activity.
Furthermore some 9H-bis-[1,2,4]triazolo[4,3-a:30,40-d]
[1,5]benzodiazepine derivatives of type 5, evaluated for
antiproliferative activity against a panel of cell lines derived
from either hematological or solid human tumours, showed
antiproliferative activity against either or both leukemia-
and lymphoma-derived cell lines in the low micromolar
range.5 On the basis of these data, we focused our studies
on bis-triazoloquinoxaline derivatives of type 6, bioisosters
of 5, to explore the effect of the ring contraction on biolog-
ical activities.
A useful synthetic strategy adopted to obtain various
polyheterocyclic systems involves the use of 1,3-dipolar
cycloaddition reactions.6 To date in the literature no
examples of this reaction applied to synthesize the bis-
triazoloquinoxaline core structure have been reported.
Quinoxalines, as other azine systems, act as dipolarophiles
(2p component) in the dipolar 1,3-cycloadditions, thus
*
Corresponding author. Tel.: +39 091 6161606; fax: +39 091 6169999.
E-mail address: lauria@unipa.it (A. Lauria).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.01.047

1848
A. Lauria et al. / Tetrahedron Letters 49 (2008) 1847–1850
representing adequate synthons for the construction of
polycyclic systems by a simple one-pot synthesis. In partic-
ular azine or diazine systems (pyridine, quinoline, isoquino-
line, pyrazine, and benzodiazepine) were shown to undergo
1,3-dipolar cycloaddition in a highly site- and regio-selec-
tive fashion.7
In contrast with the literature data, previously reported
by Dalla Croce9 for the reaction of the quinoxaline with
chloroarylhydrazones 10d, the formation of the mono-
adduct of type 9 was never observed even though
equimolar amount of the reactants or a large excess of
dipolarophile was used. However we could verify that the
concise analytical and spectroscopic data of the compound
described by Dalla Croce9 are really in agreement with the
experimental data of the bis-1,2,4-triazolo[4,3-a:30,40-
c]quinoxaline 10d isolated by us. This should confirm that
the mono-cycloadduct 9 is probably formed as an inter-
mediate but it cannot be isolated: evidently it is much more
reactive as heterodipolarophile than the starting
quinoxaline.
In the case of the bis-triazoloquinoxalines 10a,c,d from
the reaction mixtures it was possible to isolate, in good
yields, two different solids A and B possessing analogous
properties as shown in Table 1 (solubility in common sol-
vent, similar IR spectrum) but different melting point and
Rf. In the NMR spectra of these symmetric compounds
the only considerable difference is due to the resonance of
two magnetic and chemical equivalent protons (H-12a,
H-12b).
These experimental evidences suggested that in the reac-
tion of 8a,c,d with quinoxaline 7 both possible diastereo-
isomer 2:1 cycloadducts were obtained, arising from the
double cycloaddition of the 1,3-dipole on the opposite or
on the same sides of the quinoxaline ring. These findings
should confirm furthermore the lack of diastereoselectivity
of the 1,3-dipolar cycloadditions of nitrilimine to quinoxa-
line, in contrast with the literature reports for other sym-
metrical diazine rings: pyrazine,7b benzodiazine.11 In our
hand only a moderate excess of diastereoisomer A with
respect to B was observed.
On the basis of the spectroscopic values reported for
analogous pyrazine-bisadducts,7b to confirm that each frac-
tion A consists of the racemic mixture of the diastereoiso-
mers anti, it was chosen to use the chiral lanthanide shift
reagent (CLSR) Europium tris-[3-(heptafluoropropyl-
hydroxymethylene)-(+)-camphorate] [Eu(hfc)3] in the
NMR analysis.
All the examples of 1,3-dipolar cycloaddition to quinox-
aline ring system, reported in the literature so far, demon-
strate that the dipoles react with the benzodiazine ring
exclusively at C@N bonds (site-selectivity) and always
maintain the same orientation (regio-selectivity).8,9 Like-
wise in our hands the treatment of quinoxaline 7 with
two equivalents of chloroarylhydrazones 8a–d gave rise
exclusively to the bis-cycloadduct 1,12,12a,12b-tetra-
hydro-bis-1,2,4-triazolo[4,3-a:30,40-c]quinoxalines 10a–d in
satisfactory yields (Scheme 1). No increase in yields was
registered by varying the reaction time or temperature.
These tetracyclic derivatives formally arise from a double
regio-specific 1,3-dipolar cycloaddition of the nitrilimines
onto the two dipolarophile sites C@N. The structures of
10a–d10 were assigned by NMR spectral analysis (1H and
13C). In particular, the 1H chemical shifts observed for
the protons in 12a and 12b positions were found in the
range 5.76–6.34 ppm, and those of the corresponding car-
bon atoms (C-12a, C-12b) in the range 72.3–84.5 ppm in
the 13C spectra, in agreement with the values reported for
analogous azine-adducts systems,7 unequivocally indicat-
ing double cycloaddition to the hetero-double bonds.
N
R
N
N
N
Ar
N
N
7
i
+
R
N
N
H
Ar
Cl
9
8a-d
R
R
N
N
N
Ar
N
12b
12a
a R=COMe, Ar=Ph
b R=COPh, Ar=Ph
N
N
Ar
c R=COOEt, Ar=p-ClC6H4
d R=COOMe, Ar=Ph
Infact, in the presence of these optically active chelates,
enantiomers (that respond to lanthanide NMR shift
reagents by chelate formation between Eu3+ and probably
the lone pair of nitrogen and carbonyl oxygen) realize
10a-d
Scheme 1. Reaction conditions: (i) NEt3, THF, rt, 48 h.
Table 1
Physicochemical properties
Rf (DCM)
1H NMR 12a, 12b d
13C NMR 12a, 12b d
Mp (°C)
IR (cmꢀ1
)
Yield (%)
10aA
10aB
10cA
10cB
10dA
10dB
0.55
0.29
0.70
0.34
0.46
0.19
5.86
6.31
5.76
6.34
5.76
6.34
72.6
84.5
72.3
83.6
72.7
84.1
212–214
148–152
123–125
191–192
232–235
191–194
1685
1682
1729
1724
1735
1728
44
15
48
33
30
18

A. Lauria et al. / Tetrahedron Letters 49 (2008) 1847–1850
1849
phenylhydrazones 8a–d were prepared according to the procedure
described in the literature.13 General procedure for bis-1,2,4-triazolo
[4,3-a:30,40-c]quinoxalines 10a–d. Triethylamine (8.30 mmol) was
added to a solution of quinoxaline (3.84 mmol) and chlorophenylhyd-
razones (7.68 mmol) in anhydrous tetrahydrofuran (20 mL) and the
mixture was stirred at room temperature for 48 h. The solution was
concentrated under reduced pressure and treated/crystallized with
ethanol and collected by filtration as a colored solid. 1,12,12a,
12b-tetrahydrobis-3,10-diacetyl-1,12-diphenyl-1,2,4-triazolo[4,3-a:30,
4-c]quinoxaline (10a): The reaction mixture obtained from the
reaction of quinoxaline 7 with (1E)-2-oxo-N-phenylpropane-hydrazo-
noyl chloride (8a) was evaporated under reduced pressure and the
residue was treated with cold ethanol (5 mL). The first fraction
crystallized gave 10aA as orange solid: IR: 1685 cmꢀ1 (C@O); 1H
NMR: d 2.61 (s, 6H, 2 ꢁ CH3), 5.86 (s, 2H, H-12a, H-12b), 6.72–6.82
(m, 2H, p-NC6H5) 7.03–7.11 (m, 10H, H-6, H-7, m-NC6H5,
o-NC6H5), 7.55–7.61 (m, 2H, H-5, H-8). 13C NMR: d 28.5 (q), 72.6
(d), 113.1 (d), 120.9 (d), 122.1 (d), 122.5 (d), 124.8 (s), 128.8 (d), 142.6
(s), 146.1 (s), 188.4 (s). Anal. Calcd for C26H22N6O2: C, 69.32; H,
4.92; N, 18.65. Found: C, 69.25; H, 4.90; N, 18.39. HRMS: (M+)
calcd for C26H22N6O2 450.1804; found, 450.1819. The solution was
concentrated to afford 10aB as a dark orange solid: IR: 1682 cmꢀ1
(C@O); 1H NMR: d 2.35 (s, 6H, 2 ꢁ CH3), 6.31 (s, 2H, H-12a,
H-12b), 6.79–6.85 (m, 2H, p-C6H5) 7.00–7.13 (m, 8H, m-C6H5,
o-C6H5), 7.21–7.26 (m, 2H, H-6, H-7), 7.40–7.47 (m, 2H, H-5, H-8).
13C NMR: d 26.7 (q), 84.5 (d), 115.4 (d), 121.8 (d), 125.8 (d), 126.0 (d),
128.4 (d), 135.0 (s), 142.7 (s), 146.4 (s), 187.6 (s). Anal. Calcd for
C26H22N6O2: C, 69.32; H, 4.92; N, 18.65. Found: C, 69.25; H, 4.95; N,
18.39. HRMS: (M+) calcd for C26H22N6O2 450.1804; found, 450.1815.
1,12,12a,12b-tetrahydrobis-3,10-dibenzoyl-1,12-diphenyl-1,2,4-triazol-
o[4,3-a:30,4-c]quinoxaline (10b): The reaction mixture obtained from
the reaction of quinoxaline 7 with 1(E)-2-oxo-N, 2-diphenylethane-
hydrazonoyl chloride (8b) crystallized from ethanol giving 10b as an
orange solid: yield 23%, mp 200–201 °C; IR 1657 cmꢀ1 (CO); 1H
NMR: d 6.01 (s, 2H, H-12a, H-12b), 6.81–7.28 (m, 14H, H-6, H-7,
2 ꢁ NC6H5, p-COC6H5) 7.67, (d, 4H, m-COC6H5, J = 7.0 Hz), 7.78
(d, 2H, H-5, H-8) 8.20 (d, 4H, o-COC6H5, J = 7.0 Hz). 13C NMR: d
73.7 (d), 113.5 (d), 120.5 (d), 121.1 (d), 122.7 (d), 124.8 (s), 128.7 (d),
128.9 (d), 130.4 (d), 134.5 (d), 135.8 (s), 143.6 (s), 145.6 (s), 183.5 (s).
Anal. Calcd for C36H26N6O2: C, 75.25; H, 4.56; N, 14.62. Found: C,
75.31; H, 4.45; N, 14.51. HRMS: (M+) calcd for C36H26N6O2
574.2117; found, 574.2112. 1,12,12a,12b-tetrahydrobis-3,10-diethoxy-
carbonyl-1,12-4-chlorophenyl-1,2,4-triazolo[4,3-a:30,4-c]quinoxaline (10c):
The reaction mixture obtained from the reaction of quinoxaline 7 with
ethyl (2Z)-chloro[(4-chlorophenyl)hydrazono]acetate (8c) was evapo-
rated under reduced pressure, the residue was washed with ethanol
(5 mL) and chromatographed on a silica gel column using DCM as
eluent. The first fraction eluted gave 10cA as a clear yellow solid: IR:
1729 cmꢀ1 (C@O); 1H NMR: d 1.32 (t, 6H, J = 6.6 Hz, 2 ꢁ CH3),
4.39 (q, 4H, J = 6.6 Hz, 2 ꢁ CH2), 5.76 (s, 2H, H-12a, H-12b), 7.00–
7.20 (m, 10H, m-C6H5, o-C6H5, H-6, H-7), 7.45–7.49 (m, 2H, H-5,
H-8); 13C NMR: d 13.8 (q), 62.6 (t), 72.3 (d), 114.7 (d), 121.1 (d),
123.2 (d), 124.4 (s), 124.5 (s), 128.7 (d), 141.7 (s), 142.4 (s), 158.1 (s).
Anal. Calcd for C28H24Cl2N6O4: C, 58.04; H, 4.17; N, 14.50. Found:
C, 58.09; H, 4.13; N, 14.41. HRMS: (M+) calcd for C28H24Cl2N6O4
578.1236; found, 578.1244. The second fraction eluted afforded 10cB
as a clear yellow solid: IR: 1724 cmꢀ1 (C@O); 1H NMR: d 1.15 (t, 6H,
J = 7.0 Hz, 2 ꢁ CH3), 4.16 (q, 4H, J = 7.0 Hz, 2 ꢁ CH2), 6.34 (s, 2H,
H-12a, H-12b), 6.93 (d, 4H, 4o-C6H5, J = 8.4 Hz), 7.10 (d, 4H,
4m-C6H5, J = 8.4 Hz), 7.32–7.39 (m, 2H, H-6, H-7), 7.45–7.51 (m,
2H, H-5, H-8); 13C NMR: d 13.7 (q), 61.7 (t), 83.6 (d), 116.2 (d), 124.7
(s), 126.7 (d), 126.9 (d), 128.3 (d), 134.9 (s), 141.81 (s), 141.83 (s),
156.9 (s). Anal. Calcd for C28H24Cl2N6O4: C, 58.04; H, 4.17; N, 14.50.
Found: C, 57.99; H, 4.19; N, 14.35. HRMS: (M+) calcd for
C28H24Cl2N6O4 578.1236; found, 578.1229. 1,12,12a,12b-tetrahydr-
obis-3,10-dimethoxycarbonyl-1,12-diphenyl-1,2,4-triazolo[4,3-a:30,4-
c]quinoxaline (10d): The reaction mixture obtained from the reaction
of quinoxaline 7 with methyl (2E)-chloro (phenylhydrazono)-acetate
diastereoisomeric complexes spectroscopically separated
even if in a racemic mixture.
For both fractions A and B of derivative 10d, 1H NMR
spectra were registered in CDCl3, adding increasing
amounts (10% p/p of Eu(hfc)3 with respect to the amount
of analysed sample) before each measurements.12 Hence
it was possible to observe the splitting of the singlet due
to 12a, 12b protons, only in the spectrum of 10dA. These
results confirm that fraction A of derivative 10d, and of
the other strictly related derivatives 10aA and 10cA, con-
sists of the racemic mixture (RR and SS) of anti isomers.
In conclusion, 1,3-dipolar cycloadditions constitute a
versatile and useful synthetic strategy to obtain polycon-
densed nitrogen heterocycles in one-pot reaction. The 1,3-
dipolar cycloadditions of nitrilimines to quinoxalines are
highly site- and regio-selective but not diastereoselective.
Specific studies to establish the exact stereochemical config-
uration of the derivatives synthesized were carried out, by
using CLSRs in NMR analysis.
Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.tetlet.
2008.01.047.
References and notes
1. Harmenberg, J.; Aakesson-Johansson, A.; Graeslund, A.; Malmfors,
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2. Harmenberg, J.; Wahren, B.; Bergman, J.; Akerfeldt, S.; Lundblad, L.
Antimicrob. Agent Chemother. 1988, 32, 1720.
3. Skarin, T.; Rozell, B. L.; Bergman, J.; Toftagard, R.; Moller, L.
Chem. Biol. Interact. 1999, 122, 89.
4. Nasr, M.; Nasr, A. Arch. Pharm. Med. Chem. 2002, 8, 389.
5. Di Braccio, M.; Grossi, G.; Ceruti, M.; Rocco, F.; Loddo, R.; Sanna,
G.; Busonera, B.; Murreddu, M.; Marongiu, M. E. Il Farmaco. 2005,
60, 113.
6. (a) Lauria, A.; Patella, C.; Diana, P.; Barraja, P.; Montalbano, A.;
Cirrincione, G.; Dattolo, G.; Almerico, A. M. Tetrahedron Lett. 2006,
47, 2187; (b) Lauria, A.; Patella, C.; Diana, P.; Barraja, P.;
Montalbano, A.; Cirrincione, G.; Dattolo, G.; Almerico, A. M.
Heterocycles 2004, 60, 2669; (c) Lauria, A.; Diana, P.; Barraja, P.;
Montalbano, A.; Cirrincione, G.; Dattolo, G.; Almerico, A. M.
Tetrahedron 2002, 58, 9723; (d) Lauria, A.; Diana, P.; Barraja, P.;
Almerico, A. M.; Cirrincione, G.; Dattolo, G. J. Heterocycl. Chem.
2000, 37, 747.
7. (a) Aversa, M. C.; Bonaccorsi, P.; Giannetto, P. J. Heterocycl. Chem.
1989, 26, 1619; (b) Grubert, L.; Patzel, M.; Jugelt, W.; Riemer, B.;
Liebscher, J. Liebigs Ann. Chem. 1994, 1005; (c) Grubert, L.; Jugelt,
W.; Breb, J.; Koppel, H.; Strietzel, U.; Dombrowski, A. Liebigs Ann.
Chem. 1992, 885.
8. Grassi, G.; Risitano, F.; Foti, F. Tetrahedron 1995, 51, 11855.
9. Dalla Croce, P. J. Heterocycl. Chem. 1975, 12, 1133.
10. Experimental: Melting points (uncorrected) were taken on a Buchi-
Tottoli capillary apparatus; IR spectra were determined in bromo-
form with a Jasco FT/IR 5300 spectrophotometer; 1H and 13C NMR
spectra were measured at 200 and 50.3 MHz, respectively, in
(CD3)2SO solution, using a Bruker AC-E series 200 MHz spectro-
meter (TMS as internal reference). Column chromatography was
performed with Merck silica gel 230–400 mesh ASTM. Chloro-

1850
A. Lauria et al. / Tetrahedron Letters 49 (2008) 1847–1850
(8d) was evaporated under reduced pressure. The residue was washed
2 ꢁ CH3), 6.34 (s, 2H, H-12a, H12b), 6.76–6.83 (m, 2H, p-C6H5) 6.95–
7.10 (m, 8H, m-C6H5, o-C6H5), 7.27–7.32 (m, 2H, H-6, H-7), 7.41–
7.51 (m, 2H, H-5, H-8); 13C NMR: d 52.6 (q), 84.1 (d), 114.9 (d), 121.3
(d), 125.77 (d), 126.4 (d), 128.4 (d), 134.7 (s) 141.0 (s), 143.3 (s), 157,7
(s). Anal. Calcd for C26H22N6O4: C, 64.72; H, 4.60; N, 17.42. Found:
C, 64.80; H, 4.67; N, 17.34. HRMS: (M+) calcd for C26H22N6O4
482.1702; found, 482.1714.
with ethanol (5 mL) and chromatographed on a silica gel column
using DCM as eluent. The first fraction eluted gave 10dA as a clear
yellow solid: IR: 1735 cmꢀ1 (C@O); 1H NMR: d 3.91 (s, 6H,
2 ꢁ CH3), 5.76 (s, 2H, H-12a, H-12b), 6.74–6.81 (m, 2H, p-C6H5)
7.03–7.15 (m, 10H, m-C6H5, o-C6H5, H-6, H-7), 7.46–7.51 (m, 2H,
H-5, H-8); 13C NMR: d 53.2 (q), 72.7 (d), 113.2 (d), 120.9 (d), 121.1
(d), 123.1 (d), 124.5 (s), 128.9 (d), 141.3 (s), 143.8 (s), 158.6 (s). Anal.
Calcd for C26H22N6O4: C, 64.72; H, 4.60; N, 17.42. Found: C, 64.66;
H, 4.63; N, 17.32. HRMS: (M+) calcd for C26H22N6O4 482.1702;
found, 482.1695. The second fraction eluted afforded 10dB as a clear
yellow solid: IR: 1728 cmꢀ1 (C@O); 1H NMR: d 3.74 (s, 6H,
11. Nabih, K.; Baouid, A.; Hasnaoui, A.; Selkti, M.; Compain, P. New J.
Chem. 2003, 27, 1644.
12. Supplementary data: 1H NMR spectra of 10dA and 10dB, registered
in CDCl3, by adding portionwise amounts of Eu(hfc)3.
13. Dieckmann, W.; Platz, L. Chem. Ber. 1905, 38, 2989.