Unexpected regioselectivity in cycloisomerization of 2-alkynyl-3-nitro- thiophenes

Article abstract of DOI:10.1055/s-0030-1259053

The intramolecular cycloizomerization of 2-alkynyl-3-nitrothiophenes was catalyzed by AuCl₃ or CF₃CO₂Ag to produce the corresponding thieno[3,2-c]isoxazoles bearing carbonyl functionality in position 3 instead of expected

Full text of DOI:10.1055/s-0030-1259053

LETTER
3027
Unexpected Regioselectivity in Cycloisomerization of 2-Alkynyl-3-nitro-
thiophenes
C
ycloiso
n
merization o
g
f
2-Alkynyl-
a
3-nitrothiophene
s
Cikotiene,*a Rokas Sazinas,a Rita Mazeikaite,b Linas Labanauskasb
a
Department of Organic Chemistry, Faculty of Chemistry, Vilnius University, Naugarduko 24, 03225 Vilnius, Lithuania
Fax +370(5)2330987; E-mail: inga.cikotiene@chf.vu.lt
b
Center for Physical Sciences and Technology, Institute of Chemistry, Akademijos 7, 08412 Vilnius, Lithuania
Received 7 October 2010
bond of the starting compounds is electron poor, so the cyclo-
isomerization reaction takes only a few minutes and does
not require transition-metal catalysts.5
Abstract: The intramolecular cycloizomerization of 2-alkynyl-3-
nitrothiophenes was catalyzed by AuCl3 or CF3CO2Ag to produce
the corresponding thieno[3,2-c]isoxazoles bearing carbonyl func-
tionality in position 3 instead of expected 5-substituted 6H-
thieno[3,2-b]pyrrol-6-one 4-oxides.
Encouraged by knowledge from literature and our previ-
ous results, we envisioned that cycloizomerization of all
aromatic and heteroaromatic compounds, bearing triple
bond and nitro group in close proximity to each other,
would always lead to the formation of pyrolone N-oxide
ring containing compounds. However, to our great sur-
prise, we observed different regioselectivity in cyclo-
isomerization of 2-alkynyl-3-nitrothiophenes, so herein
we would like to report on these unexpected results.
Key words: 2-alkynyl-3-nitrothiophenes, thieno[3,2-c]isoxazoles,
cycloizomerization, catalysis
The cyclization of alkynes containing proximate nucleo-
philic centers is currently of great interest and developing
into a most effective strategy for heterocyclic ring con-
struction. This chemistry provides a straightforward ap-
proach to the synthesis of functionalized carbo- and
heterocycles through the regio- and stereoselective addi-
tion of a nucleophile and an unsaturated carbon unit
across the carbon–carbon triple bond.1 Thus a variety of
transition-metal or electrophile-induced 5-exo-dig or 6-
endo-dig cyclization reactions have been reported.2 The
ortho-alkynyl nitrobenzenes are well known as precursors
for the preparation of isatogens (Scheme 1). The classical
cycloisomerizations of starting compounds can be initiat-
ed by pyridine,3a–e nitrosobenzene,3e,f concentrated sulfu-
ric acid,3g,h tetrabutylammonium fluoride (TBAF),3c and
UV light.3i–k Usually these reactions take long times and
require high temperatures. However, few years ago, it was
shown that the presence of transition-metal salts such as
gold(III) bromide4a or silver (I) nitrate4b initiates smooth
cyclization reaction.
The starting compounds 1 were synthesized by well-
known Sonagashira reaction6 from 2-bromo-3-
nitrothiophene (Scheme 2).7 First of all, it should be noted
that classical pyridine initiated cycloisomerization of the
title compounds was not successful. During the long heat-
ing of 3-nitro-2-phenylethynylthiophene 1a in dry pyri-
dine, no changes of the starting material were observed by
TLC. This result can be easily explained by the fact that
triple bonds in starting compounds are electron-rich due to
neighboring electron-donating thiophene ring, so nucleo-
philic activation becomes impossible. So we decided to
take advantages of transition-metal salts catalytic poten-
cy. When we added 5 mol% of gold(III) chloride to the
solution of 3-nitro-2-phenylethynylthiophene (1a) in dry
dichloromethane, the quick and selective conversion of
the starting material was observed by TLC. The NMR, IR,
and microanalysis data of isolated product were not at
variance with the expected structure – 5-phenyl-6H-
thieno[3,2-b]pyrrol-6-one 4-oxide (3a), but we had doubt
because of the substance’s slightly yellowish color, which
is not common for structural analogues of isatogens (the
latter compounds are usually from deep red to deep vio-
let). And we were really intrigued to see the results from
X-ray analysis, which enabled the outcome of the reaction
to be elucidated unambiguously (Figure 1).8 We were sur-
prised when crystallographic data showed that during
cycloisomerization of the starting compound phenyl-
thieno[3,2-c]isoxazol-3-yl methanone (2a) was formed.
O–
N+
NO2
R2
R1
R2
R1
O
Scheme 1 Literature results
On the other hand, recently, we have developed a novel,
concise, and high-yielding formation of pyrrolo[3,2-d]-
pyrimidin-7-one 5-oxides via smooth cycloisomerization
of 2,4-disubstituted 6-arylethynyl-5-nitropyrimidines in
the presence of pyridine. We have shown that the triple
Moreover, it was interesting to note that optically inactive
phenylthieno[3,2-c]isoxazol-3-yl methanone (2a) in its
solid state forms chiral crystals with symmetry space
group P212121.8
SYNLETT 2010, No. 20, pp 3027–3030
x
x
.x
x
.2
0
1
0
Advanced online publication: 17.11.2010
DOI: 10.1055/s-0030-1259053; Art ID: G30510ST
© Georg Thieme Verlag Stuttgart · New York
Thus, we decided to look for the best conditions to trigger
the cycloisomerization of the title compounds and to study

3028
I. Cikotiene et al.
LETTER
the influence of the catalyst for reaction rate and regio- less effective (entries 4, 6–8). Cycloisomerization of 1a
selectivity. It should be noted that in all successful cases on silica gel in microwave oven was unsuccessful (entry
phenylthieno[3,2-c]isoxazol-3-yl methanone (2a) was 9) and the use of pyridine did not initiate the reaction at all
formed and no formation of 5-phenyl-6H-thieno[3,2- (entries 1, 2). So, we have found that the preliminarily op-
b]pyrrol-6-one 4-oxide (3a) was observed.
timal reaction conditions are: 5 mol% of gold(III) chloride
in dichloromethane at room temperature, or 10 mol% of
silver trifluoroacetate in dichloroethane at reflux.
NO2
N
O
i
Encouraged by these results we decided to perform the
cycloisomerization reactions of the other 2-alkynyl-3-
nitrothiophenes 1. The results are summarized in Table 2.
It is noteworthy that the nature of the substituent on alky-
nyl group does not have influence on the cycloisomeriza-
tion regioselectivity – in all cases the corresponding
substituted thieno[3,2-c]isoxazol-3-yl methanones have
been formed. In the case of 3-nitro-2-trimethylsilylethy-
nylthiophene (1e), slow decomposition of the starting ma-
terial was observed (Table 2, entry 5). We solved this
problem by performing the deprotection of alkynyl moi-
ety, followed by cycloisomerization of obtained 2-ethy-
nyl-3-nitrothiophene (1f). The latter reaction was smooth
enough and the thieno[3,2-c]isoxazol-3-ylcarbaldehyde
(2f) was isolated in high yield (Table 2, entry 6).
R
S
S
R
S
O
1
2
O–
N+
R
O
3
not observed
Scheme 2 Cycloisomerization reaction of 2-alkynyl-3-nitrothio-
phenes. Reagents and conditions: i) AuCl3 (5 mol%), CH2Cl2, r.t., 15–
45 min or CF3CO2Ag (10 mol%), DCE, reflux, 2 h.
Moreover, we decided to study the cycloisomerization of
3-alkynyl-2-nitrothiophenes (4a,b). The latter two com-
pounds were prepared from the corresponding 3-bromo-
2-nitrothiophene,9 using Sonogashira coupling reaction. It
should be noted, that compounds 4a,b underwent the
same regioselective cycloisomerization reaction to form
arylthieno[2,3-c]isoxazol-3-yl methanones 5a,b. But on
the other hand, the conversions of starting materials were
incomplete after long reactions times, and the yields of the
final products were not high (Table 2, entries 9 and 10).
Figure 1 ORTEP view of phenylthieno[3,2-c]isoxazol-3-yl meth-
anone (2a)
Firstly, we noted, that the use of gold(III) chloride (5
mol%) in dichloromethane at room temperature gave the
best result (Table 1, entry 3). While CF3CO2Ag in boiling
dichloroethane provided a slightly lower yield of the de-
sired product 2a (Table 1, entry 5), CF3CO2Ag, CuI, and
PdCl2(PPh3)2 in dichloromethane at room temperature as
well as AgNO3 in boiling dichloroethane proved to be far
After a thorough literature search, we had found find only
one literature source where the formation of isoxazole
ring from the neighboring nitro and ethynyl groups was
described.4a In 2003, Asao and co-workers, during inves-
tigations of gold-catalyzed intramolecular cyclizations of
o-(alkynyl)nitrobenzenes, found that, besides the produc-
tion of the desired isatogens, anthranyls were also formed
Table 1 Reaction Conditions for the Cycloisomerization of 3-Nitro-2-phenylethynylthiophene (1a)
Entry
Reaction conditions
Time
10 h
12 h
20 min
50 h
2 h
Conversion
Yield of 2a (%)
1
2
3
4
5
6
7
8
9
pyridine, reflux
0a
0a
0
0
pyridine, DMAP, reflux
AuCl3 (5 mol%), CH2Cl2, r.t.
CF3CO2Ag (5 mol%), CH2Cl2, r.t.
CF3CO2Ag (10 mol%), DCE, reflux
CuI (5 mol%), CH2Cl2, r.t.
PdCl2(PPh3)2 (5 mol%), CH2Cl2, r.t.
AgNO3 (10 mol%), DCE, reflux
SiO2, MW
100
43
97
33
93
11
40
77
0
100
30
50 h
50 h
8 h
44
80
20 min
0a
a Starting material 1a was isolated.
Synlett 2010, No. 20, 3027–3030 © Thieme Stuttgart · New York

LETTER
Cycloisomerization of 2-Alkynyl-3-nitrothiophenes
3029
as side products in low yields.4a The authors proposed a
mechanism for these reactions and postulated therein that
water present in the solvents played a key role in the
cycloisomerization step.
Acknowledgment
We thank the Lithuanian Research Council for the financial support
from the Global Grant Programme (Grant No. VP1-3.1-ŠMM-07-
K-01-002). Rokas Sazinas acknowledges Student Research Fel-
lowship Award from the Lithuanian Research Council.
However, we speculate that the cycloisomerization of ar-
omatic or heteroaromatic compounds bearing nitro group
and triple bond moiety in close proximity to each other
takes place via intramolecular oxidation–reduction pro-
cesses via subsequent ring cleavage and recyclization re-
actions. Moreover, we performed the cycloisomerization
reactions in absolute solvent and precisely avoided the
presence of water; so therefore, we think that oxygen in
carbonyl function came from the nitro group, not from the
water.
References and Notes
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(g) Cacchi, S.; Fabrizi, G.; Marinelli, F.; Moro, L.; Pace, P.
Synlett 1997, 1363. (h) Arcadi, A.; Cacchi, S.; Del Rosario,
M.; Fabrizi, G.; Marinelli, F. J. Org. Chem. 1996, 61, 9280.
(i) Chowdhury, C.; Kundu, N. G. Chem. Commun. 1996,
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Table 2 Cycloisomerization of 2-Alkynyl-3-nitrothiophenes 1 and
3-Alkynyl-2-nitrothiophenes 4 by the Presented Method
Entry Starting material
Product
Yield (%)
N
NO2
O
R
R
S
S
O
1
2
3
4
5
1a, R = Ph
2a, R = Ph
97
92
98
89
-
1b, R = 4-MeC6H4 2b, R = 4-MeC6H4
1c, R = 4-EtC6H4
1d, R = 2-pyridyl
1e, R = SiMe3
2c, R = 4-EtC6H4
21d, R = 2-pyridyl
slow decomposition of
starting material
6
7
8
1f, R = H
2f, R = H
87
90
88
(2) (a) Takeda, A.; Kamijo, S.; Yamamoto, Y. J. Am. Chem.
Soc. 2000, 122, 5662. (b) Gabriele, B.; Salerno, G.; Fazio,
A. Org. Lett. 2000, 2, 351. (c) Arcadi, A.; Cacchi, S.;
Del Rosario, M.; Fabrizi, G.; Marinelli, F. J. Org. Chem.
1996, 61, 9280. (d) Cacchi, S.; Fabrizi, G.; Moro, L.
Tetrahedron Lett. 1998, 39, 5101. (e) Roesh, K. R.; Larock,
R. C. Org. Lett. 1999, 1, 553. (f) Roesh, K. R.; Larock, R. C.
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Lett. 2004, 1581. (k) Yue, D.; Della Ca, N.; Larock, R. C.
J. Org. Chem. 2005, 3381.
1g, R = Bu
1h, R = t-Bu
2g, R = Bu
2h, R = t-Bu
N
O
NO2
S
S
R
R
O
9
4a, R = Ph
5a, R = Ph
45a
41
10
4b, R = 4-FC6H4
5b, R = 4-FC6H4
a incomplete conversion of starting material.
(3) (a) Pfeiffer, P. Justus Liebigs Ann.Chem. 1916, 411, 72.
(b) Bond, C. C.; Hooper, M. J. Chem. Soc. 1969, 2453.
(c) Price, D. W.; Dirk, S. M.; Maya, F.; Tour, J. M.
Tetrahedron 2003, 59, 2497. (d) Ruggli, C. Helv. Chim.
Acta 1944, 27, 649. (e) Nepveu, F.; Kim, S.; Boyer, J.;
Ibrahim, H.; Reybier, K.; Monje, M.-C.; Chevalley, S.;
Perio, P.; Lajoie, B. H.; Bouajila, J.; Deharo, E.; Sauvain,
M.; Valentin, A.; Chatriant, O.; Petit, S.; Nallet, J.-P.; Tahar,
R.; Basco, L.; Pantaleo, A.; Turini, F.; Arese, P.; Thompson,
E.; Vivas, L. J. Med. Chem. 2010, 53, 699. (f) Adams, D.
B.; Hooper, M.; Morpeth, A. G.; Raper, E. S.; Clegg, W.;
Stoddart, B. J. Chem. Soc., Perkin Trans. 2 1990, 1269.
(g) Baeyer, A. Ber. Dtsch. Chem. Ges. 1881, 14, 1741.
(h) Baeyer, A. Ber. Dtsch. Chem. Ges. 1882, 15, 775.
(i) Pfeiffer, P. Ber. Dtsch. Chem. Ges. 1912, 45, 1819.
(j) Pfeiffer, P.; Kramer, E. Ber. Dtsch. Chem. Ges. 1913, 46,
In conclusion, we have presented unexpected regioselec-
tive cycloisomerization reactions of 2-alkynyl-3-
nitrothiophenes. The novel, simple, and high-yielding
synthetic method of thieno[3,2-c]isoxazole framework
was proposed. Extension of regioselective cycloizomer-
izations of aromatic and heteroaromatic compounds, bear-
ing triple bond and nitro group in close proximity to each
other, is currently under way in our laboratory. More de-
tailed study of the mechanism of the cycloisomerizations
together with their scope and limitations are in progress,
and the results will be published in due course.
Synlett 2010, No. 20, 3027–3030 © Thieme Stuttgart · New York

3030
I. Cikotiene et al.
LETTER
3655. (k) Abromovich, R. A.; Cue, B. W. J. Org. Chem.
1980, 45, 5316.
(4) (a) Asao, N.; Sato, K.; Yamamoto, Y. Tetrahedron Lett.
2003, 44, 5675. (b) Soderberg, B. C. G.; Gorugantula, S. P.;
Howerton, C. R.; Petersen, J. L.; Dantale, S. W. Tetrahedron
2009, 65, 7357.
nitrothiophene 1 (0.3 mmol) in dry CH2Cl2 (5 mL) AuCl3 (5
mol%) was added. The resulting reaction mixture was stirred
for 15–60 min at r.t. After the evaporation of solvent, the
crude was purified by column chromatography, eluting with
benzene and hexane mixtures.
Method B
(5) (a) Susvilo, I.; Brukstus, A.; Tumkevicius, S. Synlett 2003,
1151. (b) Cikotiene, I.; Pudziuvelyte, E.; Brukstus, A.;
Tumkevicius, S. Tetrahedron 2007, 63, 8145. (c)Cikotiene,
I.; Pudziuvelyte, E.; Brukstus, A. J. Heterocycl. Chem. 2008,
45, 1615.
(6) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 16, 4467.
To a solution of the corresponding 2-arylethynyl-3-
nitrothiophene 1 (0.3 mmol) in dry DCE (5 mL) silver
trifluoroacetate (10 mol.%) was added. The resulting
reaction mixture was refluxed for 1.5–2 h. After the
evaporation of solvent, the crude was purified by column
chromatography, eluting with benzene and hexane mixtures.
Phenylthieno[3,2-c]isoxazol-3-yl Methanone (2a)
Yield 97%; mp 114–115 °C. IR (KBr): nmax = 1635 (C=O)
cm–1. 1H NMR (300 MHz, CDCl3): d = 7.15 [1 H, d, J = 5.4
Hz, C(6)H], 7.57–7.63 (2 H, m, ArH), 7.68–7.73 (1 H, m,
ArH), 7.70 [1 H, d, J = 5.4 Hz, C(5)H], 8.35–8.38 (2 H, m,
ArH) ppm. 13C NMR (75 Hz, CDCl3): d = 112.0 [C(6)],
127.0 [C(3a)], 128.8 (ArC), 130.1 (ArC), 134.0 (ArC), 135.0
(ArC), 143.1 [C(5)], 158.2 [C(6a)], 170.5 [C(3)], 179.3
(C=O) ppm. Anal. Calcd for C12H7NO2S: C, 62.87; H, 3.08;
N, 6.11. Found: C, 62.90; H, 3.10; N, 6.08.
(7) Carpanelli, L. Ann. Chim. (Rome) 1961, 51, 181.
(8) Crystal Structure Analysis for 2a
C12H7NO2S, Mr = 229.25 g mol–1, orthorombic, space group
P212121, a = 3.86590 (10), b = 9.5511 (4), c = 26.7855 (12)
Å, a = b = g = 90.00, V = 989.02 (7) Å3, r = 1.540 g/cm3,
F(000) = 472. X-ray diffraction data were collected on a
Nonius Kappa CCD diffractometer at the temperature 293 K
using graphite-monochromated MoKa radiation (l =
0.71073 Å). Structure 2a was solved by direct methods with
SIR97 program10 and refined by full-matrix least squares
techniques with anisotropic nonhydrogen atoms. Hydrogen
atoms were refined in the riding model. The refinement
calculations were carried out with the help of SHELX97
program.11 ORTEP12 view of the molecule is shown in
Figure 1. Crystallographic data for structure 2a have been
deposited at the Cambridge Crystallographic Data Centre
(CCDC number 795865. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif).
Thieno[3,2-c]isoxazole-3-carbaldehyde (2f)
Yield 87%; mp 66–67 °C. IR (KBr): nmax = 1683 (C=O) cm–1.
1H NMR (300 MHz, CDCl3): d = 7.15 [1 H, d, J = 5.4 Hz,
C(6)H], 7.68 [1 H, d, J = 5.4 Hz, C(5)H], 10.14 (1 H, s,
CHO) ppm. 13C NMR (75 Hz, CDCl3): d = 111.7 [C(6)],
123.4 [C(3a)], 142.4 [C(5)], 156.8 [C(6a)], 171.0 [C(3)],
177.3 (C=O) ppm. Anal. Calcd for C6H3NO2S: C, 47.05; H,
1.97; N, 9.15. Found: C, 47.00; H, 2.01; N, 9.18.
1-Thieno[3,2-c]isoxazol-3-yl-1-pentanone (2g)
Yield 90%; mp 62–63 °C. IR (KBr): nmax = 1681 (C=O) cm–1.
1H NMR (300 MHz, CDCl3): d = 0.95 (3 H, t, J = 7.5 Hz,
CH2CH2CH2CH3), 1.44 (2 H, sext, J = 7.5 Hz,
(9) Steinkopf, W.; Jacob, H.; Penz, H. Justus Liebigs Ann.
Chem. 1934, 512, 136.
(10) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Spagna,
R. J. Appl. Crystallogr. 1999, 115.
(11) Sheldrick, G. M. SHELXL97, Program for the Refinement of
Crystal Structures; University of Göttingen: Germany,
1997, .
(12) Johnson, C. K. ORTEP-II, Report ORNL-5138; Oak Ridge
National Laboratory: Oak Ridge TN, 1976, .
(13) Typical Procedures for the Cycloisomerization of
2-Alkynyl-3-nitrothiophenes 1
CH2CH2CH2CH3), 1.76 (2 H, sext, J = 7.5 Hz,
CH2CH2CH2CH3), 3.04 (2 H, t, J = 7.5 Hz,
CH2CH2CH2CH3), 7.09 [1 H, d, J = 5.4 Hz, C(6)H], 7.63
[1 H, d, J = 5.4 Hz, C(5)H] ppm. 13C NMR (75 Hz, CDCl3):
d = 13.8 (CH2CH2CH2CH3), 22.3 (CH2CH2CH2CH3), 25.5
(CH2CH2CH2CH3), 39.3 (CH2CH2CH2CH3), 111.9 [C(6)],
123.4 [C(3a)], 142.7 [C(5)], 157.4 [C(6a)], 170.9 [C(3)],
188.6 (C=O) ppm. Anal. Calcd for C10H11NO2S: C, 57.39;
H, 5.30; N, 6.69. Found: C, 57.45; H, 5.31; N, 6.88.
(14) Compounds 1a–h, 2b–d,h, 4a,b, and 5a,b were also fully
characterized by IR, 1H NMR, 13C NMR spectroscopic and
microanalytical data.
Method A
To a solution of the corresponding 2-arylethynyl-3-
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