2-Aryl and 2-Heteroaryl Pyrrolo[2,3-b]quinoxalines via Copper-Catalyzed Reaction of 1-Alkynes with 2-Bromo-3-trifluoroacetamidoquinoxaline

Article abstract of DOI:10.1055/s-2003-45004

2-Aryl and 2-heteroaryl pyrrolo[2,3-b]quinoxalines have been prepared in good to high yield through the reaction of 1-alkynes with 2-bromo-3- trifluoroacetamidoquinoxaline in the presence of catalytic amounts of CuI, PPh₃, and K₂CO₃ in dioxane at 110°C. The reaction appears to tolerate a wide range of functionalized 1-alkynes, including those containing ether, alcohol, amide, aldehyde, ketone, ester, and nitro groups.

Full text of DOI:10.1055/s-2003-45004

LETTER
287
2-Aryl and 2-Heteroaryl Pyrrolo[2,3-b]quinoxalines via Copper-Catalyzed
Reaction of 1-Alkynes with 2-Bromo-3-trifluoroacetamidoquinoxaline
C
opper-Cataly
a
zedRoute
n
to 2-Aryl Py
d
rrolo[2,3-
b
]q
r
uinoxali
o
nes Cacchi,*a Giancarlo Fabrizi,a Luca M. Parisi,a Roberta Berninib
a
Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Università degli Studi ‘La Sapienza’,
P.le A. Moro 5, 00185 Roma, Italy
b
Dipartimento A.B.A.C., Università degli Studi della Tuscia, Via S. Camillo De Lellis, 01100 Viterbo, Italy
Fax +39(6)49912780; E-mail: sandro.cacchi@uniroma1.it
Received 27 October 2003
N
N
Br
Abstract: 2-Aryl and 2-heteroaryl pyrrolo[2,3-b]quinoxalines have
been prepared in good to high yield through the reaction of 1-
alkynes with 2-bromo-3-trifluoroacetamidoquinoxaline in the pres-
ence of catalytic amounts of CuI, PPh3, and K2CO3 in dioxane at
110 °C. The reaction appears to tolerate a wide range of functional-
ized 1-alkynes, including those containing ether, alcohol, amide,
aldehyde, ketone, ester, and nitro groups.
R
NHCOCF3
1
2
[Cu]
R
Key words: coupling, cyclization, copper, alkynes, heterocycles
N
N
N
R
N
N
NHCOCF3
H
The quinoxaline nucleus is present among many pharma-
ceutical agents showing antiviral,1 antiglucoma,2
anticancer3 activities. Recently, attention has been fo-
cused on pyrroloquinoxalines4 and some of them have
been proved to be potent and selective 5-HT3 receptor
ligands.4a As to pyrrolo[2,3-b]quinoxalines, to the best of
our knowledge, the construction of the pyrrole ring from
quinoxaline precursors has been achieved via palladium-
catalyzed intramolecular Heck reaction on aminoquinox-
aline scaffolds,4b cyclization of 2-alkynyl-3-chloro-
quinoxalines in the presence of primary amines,5 and
base- or acid-catalyzed cyclization of 2-ethyl-3-phenyl-
ethynylquinoxaline.5
3
4
Scheme 1
A rapid entry into the desired quinoxaline substrate is
shown in Scheme 2. Compound 5 was prepared from
commercially available phenylendiamine via reaction
with diethyloxalate,13 treatment of the resultant 1,4-dihy-
droquinoxaline-2,3-dione with PBr5 to give 2,3-
dibromoquinoxaline14 that was then converted into 515,16
(the procedure described in literature16b was slightly mod-
ified).17 Treatment of 2-amino-3-bromoquinoxaline (5)
with 2 equivalents of trifluoroacetic anhydride and 1
equivalent of triethylamine in THF at room temperature
furnished the trifluoroacetamido derivative 1 in almost
quantitative yield.18 The utilization of the trifluoroaceta-
mido group in the quinoxaline substrate was suggested by
our previous experience with palladium-19 and copper-
catalyzed12 cyclizations, that showed that this type of N-
substituent is particularly suited for favoring cyclization
processes involving h2-alkyne-organopalladium and h2-
alkyne-copper intermediates. In addition, it allows for the
formation of free N-H pyrrole nuclei (the amide bond is
broken during the reaction or/and the work-up), avoiding
troublesome and time-consuming deprotecting steps.
As part of our ongoing studies on the utilization of transi-
tion metals in the synthesis of heterocycles, we decided to
explore the development of a copper-catalyzed approach
to this class of compounds. The low cost of copper-based
methods (and hence their potential in large scale prepara-
tions) have stimulated a growing interest in copper-cata-
lyzed procedures in recent years.6–11 In addition, our
recent success in the construction of the substituted pyr-
role ring incorporated into the indole system via a copper-
catalyzed reaction12 prompted us to investigate the feasi-
bility of a pyrrole cyclization process onto a 1,4-diazine
moiety by using similar chemistry. Therefore, we investi-
gated the development of a coupling-cyclization domino
reaction as outlined in Scheme 1.
Compound 1 was treated without further purification with
a variety of 1-alkynes in the presence of catalytic amounts
of air stable CuI, PPh3, and K2CO3 in dioxane at 110 °C.20
Under these conditions (no anhydrous or oxygen-free
conditions are required) the reaction affords 2-aryl and 2-
heteroaryl pyrrolo[2,3-b]quinoxalines in good to high
yield and tolerates a wide range of functionalized 1-
alkynes, including those containing ether, alcohol, amide,
aldehyde, ketone, ester, and nitro groups. No evidence
of formation of 1,3-diyne byproducts derived from the
Here we report that 1-alkynes 2 and 2-bromo-3-trifluoro-
acetamidoquinoxaline (1) form 2-aryl and 2-heteroaryl
pyrrolo[2,3-b]quinoxalines (4) through a copper-cata-
lyzed reaction employing a straightforward procedure.
SYNLETT 2004, No. 2, pp 0287–0290
0
2.
0
2.
2
0
0
4
Advanced online publication: 18.12.2003
DOI: 10.1055/s-2003-45004; Art ID: G29003ST
© Georg Thieme Verlag Stuttgart · New York

288
S. Cacchi et al.
LETTER
[PdCl2(PPh3)2, CuI, Et2NH, DMF, 40 °C, 1 h: the cou-
pling product was isolated in 85% yield], trifluoroacetyl-
ation [(CF3CO)2O, Et3N, THF, room temperature, 1.5 h:
3f was obtained in quantitative yield and used without
further purification], and copper-catalyzed cyclization
[CuI, PPh3, K2CO3, dioxane, 110 °C, 2 h: 4f was isolated
in 70% yield].
N
N
Br
NH2
5
(CF3CO)2O
Et3N
THF
r.t.
Triphenylphosphine plays an important role in favoring
the reaction, most probably generating a soluble catalyst
system. The pyrroloquinoxaline product was isolated in
only 17% yield after 8 hours when 1 was treated with 2f
under our standard conditions omitting the phosphine
ligand (the starting material was recovered in 4% yield
and 2-amino-3-bromoquinaxoline (5), derived from the
hydrolysis of 1, was isolated in 32% yield).
R
N
Br
N
N
CuI, PPh3
R
dioxane
110 °C
N
N
NHCOCF3
H
1
4
Scheme 2
coupling of terminal alkynes (quite common in palladi-
um-catalyzed cross-coupling reactions) was observed, so
that the process can be performed by using of a slight
excess of terminal alkynes. Our preparative results are
shown in Table 1.
The remarkable role of copper in the cyclization step is
emphasized by the following experiment. Compound 3f
(R = Ph) was treated with K2CO3 in dioxane at 110 °C for
24 hours. The indole product was isolated in only 22%
yield after 24 hours along with 40% of 2-amino-3-phenyl-
Remarkably, the domino process was found to give higher ethynylquinoxaline, derived from the hydrolysis of the
yields than the stepwise protocol based on the Sonoga- starting material.
shira coupling,21 trifluoroacetylation, and copper-cata-
lyzed cyclization. For example, 4f was isolated in 59%
Though no attempts were made to get experimental
evidences of coupling intermediates under our standard
overall yield (74% overall yield in the domino process)
conditions, we believe that the present synthesis of pyr-
through the reaction of 5 with phenyl acetylene
roloquinaxolines does proceed according to our working
hypothesis and involves a copper-catalyzed coupling-cy-
clization process. The coupling step should proceed
according to the mechanism proposed for the copper-cat-
Table 1 Copper-Catalyzed Synthesis of 2-Aryl Pyrrolo[2,3-b]qui-
noxalinesa
alyzed reaction of 1-alkynes with aryl and vinyl iodides.6a
The cyclization step (Scheme 3; phosphine ligands have
been omitted for clarity) most probably involves the in-
tramolecular nucleophilic attack of the nitrogen nucleo-
phile across the carbon-carbon triple bond, which is
activated by the formation of the h2-alkyne-copper com-
plex 6.22 Subsequent substitution of the carbon-copper
Entry 1-Alkyne 2
Time (h) Yield of 4b (%)
R
1
2
p-MeO-C6H4-
m-MeO-C6H4-
o-MeO-C6H4-
m-HOCH2-C6H4-
3,5-Me2-C6H3-
Ph
2a
2b
2c
2d
2e
2f
24
15
7
80
63
91
52
80
74
75
70
53
75
65
76
4a
4b
4c
4d
4e
4f
3
4
24
27
8
Cu(I)
5
R
R
N
N
N
N
6
Cu(I)
7
p-MeCONH-C6H4- 2g
30
24
5
4g
4h
4i
NHCOCF3
Cu(I)
NHCOCF3
3
8
6
8
p-MeCO-C6H4-
p-OHC-C6H4-
p-MeOOC-C6H4-
p-O2N-C6H4-
2h
2i
BH
B
9
10
11
12
2j
2k
2l
24
24
6
4j
N
N
N
N
BH
R
R
4k
4l
Cu(I)
B
N
N
COCF3
COCF3
[H2O]
7
CF3COOH
N
N
a Reactions were conducted on a 0.31 mmol scale in starting 2-bromo-
3-trifluoroacetamidoquinoxaline (1) in dioxane (2 mL) at 110 °C us-
ing 1 equiv of 1, 1.1 equiv of 2, 0.15 equiv of CuI, 0.3 equiv of PPh3,
and 2 equiv of K2CO3.
R
N
H
N
4
b Yields are given for isolated products.
Scheme 3
Synlett 2004, No. 2, 287–290 © Thieme Stuttgart · New York

LETTER
Copper-Catalyzed Route to 2-Aryl Pyrrolo[2,3-b]quinoxalines
289
(6) For recent references on the Cu-catalyzed formation of C-C
bonds, see: (a) Okuro, K.; Furuune, M.; Enna, M.; Miura,
M.; Nomura, M. J. Org. Chem. 1993, 58, 4716.
bond of the resultant intermediate 7 with a carbon-hydro-
gen bond (the hydrogen for such a substitution can for-
mally arise from the original trifluoroacetamido group)
affords the trifluoroacetylpyrroloquinoxaline 8. The de-
sired product 4 is obtained via hydrolysis of the amide
bond during the reaction and/or work-up.
(b) Gujadhur, R. K.; Bates, C. G.; Venkataraman, D. Org.
Lett. 2001, 3, 4315. (c) Bates, C. G.; Saejueng, P.; Murphy,
J. M.; Venkataraman, D. Org. Lett. 2002, 4, 4727.
(d) Hennessy, E.; Buchwald, S. L. Org. Lett. 2002, 4, 269.
(e) Zanon, J.; Klapars, A.; Buchwald, S. L. J. Am. Chem.
Soc. 2003, 125, 2890. (f) Knöpfel, T. F.; Carreira, E. M. J.
Am. Chem. Soc. 2003, 125, 6054. (g) Krauss, I. J.; Leighton,
J. L. Org. Lett. 2003, 5, 3201.
In conclusion, we have shown that CuI in the presence of
PPh3 serves as an efficient catalyst for the reaction of 1-
alkynes with 2-bromo-3-trifluoroacetamidoquinoxaline
under relatively mild conditions. This method provides a
straightforward access to many novel 2-aryl and 2-het-
eroaryl pyrrolo[2,3-b]quinaxolines and is of particular
value because of the low cost of the catalyst system, func-
tional group compatibility, and experimental simplicity.
(7) For recent references on the Cu-catalyzed formation of C-N
bonds, see: (a) Zhang, H.; Larock, R. C. Tetrahedron Lett.
2002, 43, 1359. (b) Roesch, K. R.; Larock, R. C. J. Org.
Chem. 2002, 67, 86. (c) Job, G. E.; Buchwald, S. L. Org.
Lett. 2002, 4, 3703. (d) Kwong, F. Y.; Klapars, A.;
Buchwald, S. L. Org. Lett. 2002, 4, 581. (e) Antilla, J. C.;
Klapars, A.; Buchwald, S. L. J. Am. Chem. Soc. 2002, 124,
11684. (f) Hiroya, K.; Itoh, S.; Ozawa, M.; Kanamori, Y.;
Sakamoto, T. Tetrahedron Lett. 2002, 43, 1277.
Acknowledgment
Work carried out in the framework of the National Project ‘Stereo-
selezione in Sintesi Organica. Metodologie ed Applicazioni’
supported by MURST, Rome. Financial support of this research by
the University ‘La Sapienza’ is also gratefully acknowledged. We
also thank Mrs. Jeanne Alladoum for her contribution within the
Erasmus program.
(g) Evindadr, G.; Batey, R. A. Org. Lett. 2003, 5, 133.
(h) Mallesham, B.; Rajesh, B. M.; Reddy, R.; Srinivas, D.;
Trhan, S. Org. Lett. 2003, 5, 963. (i) Frederick, M. O.;
Mulder, J. A.; Tracey, M. R.; Hsung, R. P.; Huang, J.; Kurtz,
K. C. M.; Shen, L.; Douglas, C. J. J. Am. Chem. Soc. 2003,
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2003, 5, 793. (k) Jiang, L.; Job, G. E.; Klapars, A.;
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Y.-J. Tetrahedron Lett. 2003, 44, 3385. (m) He, H.; Wu,
Y.-J. Tetrahedron Lett. 2003, 44, 3445. (n) Wu, Y.-J.;
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Chem. 2003, 68, 7041. (p) Lu, Z.; Twieg, R. J.; Huang, S. D.
Tetrahedron Lett. 2003, 44, 6289.
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(9) For recent references on the Cu-catalyzed formation of C-P
bonds, see: (a) Gelman, D.; Jiang, L.; Buchwald, S. L. Org.
Lett. 2003, 5, 2315. (b) Van Allen, D.; Venkataraman, D. J.
Org. Chem. 2003, 68, 4590.
(10) For recent references on the Cu-catalyzed formation of C-S
bonds, see: (a) Baskin, J. M.; Wang, Z. Org. Lett. 2002, 4,
4423. (b) Bates, C. G.; Gujadhur, R. K.; Venkataraman, D.
Org. Lett. 2002, 4, 2803.
(11) For the Cu-catalyzed conjugate reduction, see: (a)Moritani,
Y.; Appella, D. H.; Jurkauskas, V.; Buchwald, S. L. J. Am.
Chem. Soc. 2000, 122, 6797. (b) Jurkauskas, V.; Sadighi, J.
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S.; Hamon, M.; Novellino, E.; Greco, G.; Cagnotto, A.;
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1069.
(5) Ames, D. E.; Brohi, M. I. J. Chem. Soc., Perkin Trans. 1
1980, 1384.
(15) Synthesis of 2-amino-3-bromoquinoxaline (5): To a
solution of 2,3-dibromo-quinoxaline (2.00 g, 6.92 mmol) in
DMSO/MeCN (8 mL/12 mL), NH4OH (1.8 mL, 13.84
mmol, solution 30%) and K2CO3 (0.96 g, 6.92 mmol) were
added. The solution was stirred at 50 °C for 120 h. After
cooling, the reaction mixture was diluted with EtOAc,
washed with H2O, dried over Na2SO4 and concentrated
under reduced pressure. The residue was purified by
Synlett 2004, No. 2, 287–290 © Thieme Stuttgart · New York

290
S. Cacchi et al.
LETTER
13C NMR (DMSO-d6): d = 157.8, 157.6 (q, J = 31.7 Hz),
141.8, 141.4, 138.6, 130.0, 127.8, 127.3, 126.7, 119.1 (q,
J = 290.1 Hz). 19F NMR {H} (DMSO-d6): d = –73.2.
(19) Battistuzzi, G.; Cacchi, S.; Fabrizi, G. Eur. J. Org. Chem.
2002, 2671.
chromatography (silica gel, 130 g; n-hexane/EtOAc 75:25)
to give 1.29 g of 5 (83% yield). Mp 163–164 °C. IR (KBr):
3290, 3137 cm–1. 1H NMR (CDCl3): d = 7.86 (dd, J1 = 8.0
Hz, J2 = 1.0 Hz, 1 H), 7.72–7.55 (m, 2 H), 7.49–7.40 (m, 1
H), 5.55 (bs, 2 H). 13C NMR (CDCl3): d = 149.8, 141.0,
138.2, 130.6, 130.3, 128.5, 125.9, 125.7. Anal. Calcd for
C8H7BrN3: C, 42.88; H, 2.70; N, 18.75. Found: C, 42.80; H,
2.68; N, 18.73.
(20) Typical Procedure for the Preparation of 2-Aryl and 2-
Heteroaryl Pyrrolo[2,3-b]quinoxalines (4, Table 1, entry
12): To a solution of 1 (0.100 g, 0.31 mmol) in 1,4-dioxane
(2 mL), 3-ethynylquinoline (0.530 g, 0.344 mmol), CuI
(0.009 g, 0.047 mmol), PPh3 (0.025 g, 0.090 mmol), and
K2CO3 (0.087 g, 0.630 mmol) were added. The solution was
stirred at 110 °C for 6 h. After cooling, the reaction mixture
was suspended on silica gel and the solvent evaporated. The
residue was purified by chromatography (silica gel, 40 g;
CHCl3/CH2Cl2/MeOH 1:1:0.1) to give 0.070 g of 4l (76%
yield): mp 365–367 °C. IR (KBr): 3422, 3090 cm–1. 1H
NMR (DMSO-d6): d = 9.32 (s, 1 H), 9.03 (s, 1 H), 7.76 (d,
J = 8.5 Hz, 2 H), 7.65–7.56 (m, 3 H), 7.49–7.28 (m, 4 H),
7.10 (s, 1 H). 13C NMR (DMSO-d6): d = 149.2, 148.2, 147.0,
144.6, 143.9, 141.1, 139.6, 133.5, 131.3, 129.5, 129.3 (2
Caryl-H isochrone resonances, as established by inverse gated
1H-decoupling experiments), 128.5, 128.2, 128.1, 127.7,
127.1, 124.3, 98.8. Anal Calcd for C19H12N4: C, 77.01; H,
4.08; N, 18.91. Found: C, 76.95; H, 4.07; N, 18.89.
(21) (a) Sonogashira, K. In Metal-Catalyzed Cross-Coupling
Reactions; Diederich, F.; Stang, P. J., Eds.; Wiley-WCH:
Weinheim, 1998, 203. (b) Sonogashira, K. In Handbook of
Organopalladium Chemistry for Organic Synthesis, Vol. 1;
Negishi, E., Ed.; John Wiley and Sons: New York, 2002,
493.
(16) (a) Tanaka, K.; Takahashi, H.; Takimoto, K.; Sugita, M.;
Mitsuhashi, K. J. Heterocycl. Chem. 1992, 29, 771.
(b) Iwata, S.; Sakajyo, M.; Tanaka, K. J. Heterocycl. Chem.
1994, 31, 1433.
(17) When we attempted the conversion of 2,3-
dibromoquinoxaline into 5 under the conditions described in
ref.16b (NH4CO3, DMF) at 60 °C – instead of r.t. – for 20 h,
compound 5 was isolated in 30% yield and the starting 2,3-
dibromoquinoxaline was recovered in 50% yield. Switching
to DMSO as the solvent led to a higher conversion.
However, compound 5 was isolated in only 25% yield, the
main by-product being 2,3-diaminoquinoxaline. After some
experimentation we arrived at the conditions described in
ref.15 that afforded the desired product in 83% yield.
(18) Synthesis of 2-bromo-3-trifluoroacetamidoquinoxaline
(1): To a solution of 5 (0.500 g, 2.23 mmol) and Et3N (310
mL, 2.23 mmol) in THF (10 mL) trifluoroacetic anhydride
(630 mL, 4.46 mmol) was added at 0 °C dropwise. The
solution was stirred at r.t. for 1 h. Then, the reaction mixture
was diluted with EtOAc, washed with a sat. NaHCO3
solution, dried over Na2SO4 and concentrated under reduced
pressure. Compound 1 was obtained in quantitative yield:
mp 215–217 °C. IR (KBr): 3423, 1632, 1603 cm–1. 1H NMR
(DMSO-d6): d = 7.83 (d, J = 7.9 Hz, 1 H), 7.75 (d, J = 8.0
Hz, 1 H), 7.67 (t, J = 7.2 Hz, 1 H), 7.53 (t, J = 7.1 Hz, 1 H).
(22) The formation of an h2-alkyne-Cu(I) complex has been
reported: Macomber, D. W.; Rausch, M. D. J. Am. Chem.
Soc. 1983, 105, 5325.
Synlett 2004, No. 2, 287–290 © Thieme Stuttgart · New York