Facile synthesis of 5-bromotropono[c]-fused pyrazoles and isoxazole

Article abstract of DOI:10.3906/kim-1308-27

A facile synthesis of a series of new 5-bromotropono[c ]pyrazole derivatives (3 and 10-15) as well as 5-bromotropono[c]isoxazole (17) is described, involving a condensation reaction of 3-acetyl-5-bromotropolone (1) with hydrazine monohydrate (2), arylhydrazine hydrochlorides (4-9), and hydroxylamine hydrochloride (16), respectively. All the synthesized compounds were obtained in good yields of 56%-77% and their structures were characterized by spectral data and HRMS. TUeBITAK.

Full text of DOI:10.3906/kim-1308-27

Turk J Chem
Turkish Journal of Chemistry
(2014) 38: 470 – 476
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1308-27
Research Article
Facile synthesis of 5-bromotropono[c]-fused pyrazoles and isoxazole
Yang LI, Liangyu XU, Wentao GAO∗
Institute of Superfine Chemicals, Bohai University, Jinzhou, P.R. China
Received: 09.08.2013
•
Accepted: 07.11.2013
•
Published Online: 14.04.2014
•
Printed: 12.05.2014
Abstract: A facile synthesis of a series of new 5-bromotropono[c]pyrazole derivatives (3 and 10–15) as well as 5-
bromotropono[c]isoxazole (17) is described, involving a condensation reaction of 3-acetyl-5-bromotropolone (1) with
hydrazine monohydrate (2), arylhydrazine hydrochlorides (4–9), and hydroxylamine hydrochloride (16), respectively.
All the synthesized compounds were obtained in good yields of 56%–77% and their structures were characterized by
spectral data and HRMS.
Key words: Tropono[c]pyrazole, tropono[c]isoxazole, condensation reaction, hydrazine monohydrate, arylhydrazine,
hydroxylamine
1. Introduction
The pyrazole skeleton often appears as an important structural component in both biologically active and
natural compounds exhibiting a wide range of pharmacological properties.1,2 In particular, the important role
of ring-fused pyrazoles in medicinal and pharmaceutical chemistry is indisputable and well reflected by a large
number of recent publications,3−6 including some excellent reviews.7,8 Most fused pyrazole compounds reported
in the literature comprise a common heterocyclic ring moiety, such as pyrazolopyrimidine,9 pyrazolopyridine,10
pyrazoloquinoline,11 pyrazoloindol,12 benzofuropyrazole,13 benzopyranopyrazole,14 and synthetic analogues
thereof. In addition, as is well known, the ring-fused isoxazole skeleton is also an important structural unit that
can be found in various bioactive compounds such as those with anti-HIV,15 antifungal,16 and nematicidal
properties.17 Accordingly, much work has been directed toward the design and synthesis of various ring-fused
isoxazole derivatives.18−20
It is worth mentioning that a combination of a heterocycle moiety fused with a tropone ring may increase
their biological activities or create new medicinal properties due to the different electronic distribution and the
additional basic character of the tropone ring.21−23 For example, structural variations of established drugs with
the tropone ring resulted in enhanced anticancer activity.23 In addition, some natural products and synthetic
compounds containing heterocyclic-fused tropone moiety exhibit potent biological and pharmacological activities
such as antitumor and antimalarial activities.24−26 Therefore, there is much current interest in assembling a
tropone ring by fusing with heterocyclic systems.27,28 However, there are very few examples in the literature
concerning the synthesis of tropone-fused pyrazoles or isoxazole,29 although such compounds are attractive in
the field of new drug discovery.
On the basis of these observations, and in view of structural diversity playing a prominent role in
∗Correspondence: bhuzh@163.com
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medicinal and combinatorial chemistry for a faster and more efficient route towards new drug discovery,30
the synthesis of novel tropone-fused pyrazoles as well as isoxazole is not only synthetically challenging but also
potentially biologically interesting. Therefore, in the context of our ongoing studies concerning the preparation
of potential biologically active heterocycles,31−33 and in diversifying our work on the synthesis of new tropone
compounds,34−36 we wish to report herein a simple and efficient protocol for the synthesis of structurally
novel tropone-fused pyrazoles and isoxazole, namely 1-aryl-5-bromo-3-methyltropono[c]pyrazoles and 5-bromo-
3- methyl-tropono[d]isoxazole.
2. Experimental
2.1. Materials and reagents
The chemicals used in this work were obtained from Fluka and were used without purification. The melting
points were determined by using a WRS-1B melting point apparatus and were uncorrected. The IR spectra
of the compounds in KBr pellets were obtained in the range of 400–4000 cm−1 on a Shimadzu FTIR-8400S
spectrophotometer. 1 H NMR was measured with a BRUKER BRX 400 at 400 MHz using CDCl3 or DMSO-
d6 as the solvent. The reported chemical shifts (δ values) are given in parts per million downfield from
tetramethylsilane (TMS) as the internal standard. HRMS (ESI) data were acquired on a Bruker Customer
micrOTOF-Q 125 high-resolution mass spectrometer with ESI. The progress of reactions was monitored by
thin-layer chromatography (TLC) on silica gel GF254 using ethyl acetate/petroleum ether (1:3) as eluent.
2.2. General procedure for the synthesis of 5-bromo-3-methyltropono[c]pyrazole derivatives (3
and 10–15)
To a stirred solution of 3-acetyl-5-bromotropolone 1 (0.24 g, 1 mmol) in 10 mL of MeOH was added the
respective hydrazine monohydrate (80%) (2) and arylhydrazine hydrochlorides (2 mmol) (4–9). The resulting
mixture was heated at reflux temperature for 12 h. After the reaction was complete (TLC), the mixture was
cooled to room temperature followed by addition of 10 mL of water to it. The resulting precipitate was collected
by filtration and purified by recrystallization from ethanol to give products 3 and 10–15 in 56%–77% yields.
5-Bromo-3-methyltropono[c]pyrazole (3)
This compound was obtained as a white solid, yield 72%, mp 142–144 ◦ C. IR (KBr): 3139 (NH), 1620
(C=O), 1584 (C=N), 1533, 1510, 1416, 1400, 1331, 1228, 1214, 1125, 1013 cm−1 1 H NMR (CDCl3 , 400 MHz):
;
δ 2.56 (s, 3H, Me), 6.89 (d, 1H, J = 10.5 Hz, Tropone-H), 7.41 (dd, 1H, J = 10.5, 1.5 Hz, Tropone-H), 7.84 (d,
1H, J = 1.5 Hz, Tropone-H), 13.67 (br s, 1H, NH). HRMS (ESI): m/z [M+Na]+ Calcd for C9 H779 BrN2 NaO+ :
260.9632. Found: 260.9636.
5-Bromo-3-methyl-1-phenyltropono[c]pyrazole (10)
This compound was obtained as a yellow solid, yield 77%, mp 183–184 ◦ C. IR (KBr): 1626 (C=O), 1591
(C=N), 1548, 1497, 1460, 1389, 1337, 1205, 1146, 1075, 1008 cm−1 1 H NMR (CDCl3 , 400 MHz): δ 2.59
;
(s, 3H, Me), 6.77 (d, 1H, J = 10.6 Hz, Tropone-H), 7.34–7.36 (m, 2H, ArH), 7.43–7.47 (m, 4H, ArH), 7.81 (d,
1H, J = 1.5 Hz, Tropone-H). HRMS (ESI): m/z [M+Na]+ Calcd for C15 H1719 BrN2 NaO+ : 336.9945. Found:
336.9936.
5-Bromo-1-(4-methoxyphenyl)-3-methyltropono[c]pyrazole (11)
This compound was obtained as colorless crystals, yield 75%, mp 194–195 ◦ C. IR (KBr): 1628 (C=O),
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1593 (C=N), 1546, 1511, 1448, 1403, 1328, 1304, 1266, 1177, 1069, 1029 cm−1 1 H NMR (CDCl3 , 400 MHz): δ
;
2.58 (s, 3H, Me), 3.86 (s, 3H, OMe), 6.76 (d, 1H, J = 10.5 Hz, Tropone-H), 6.96 (d, J = 8.4 Hz, 2H, Ph-H), 7.28
(d, J = 8.4 Hz, 2H, Ph-H), 7.42 (dd, 1H, J = 10.5, 1.6 Hz, Tropone-H), 7.80 (d, 1H, J = 1.5 Hz, Tropone-H).
HRMS (ESI): m/z [M+Na]+ Calcd for C16 H1739 BrN2 NaO2+ : 367.0051. Found: 367.0043.
5-Bromo-1-(4-chlorophenyl)-3-methyltropono[c]pyrazole (12)
This compound was obtained as yellow crystals, yield 74%, mp 251–252 ◦ C. IR (KBr): 1621 (C=O), 1589
(C=N), 1561, 1497, 1461, 1400, 1331, 1252, 1204, 1137, 1088, 1004 cm−1 1 H NMR (DMSO-d6 , 400 MHz): δ
;
2.58 (s, 3H, Me), 6.77 (d, 1H, J = 10.5 Hz, Tropone-H), 7.29 (d, J = 8.4 Hz, 2H, Ph-H), 7.42 (d, J = 8.4 Hz,
2H, Ph-H), 7.45 (dd, 1H, J = 10.5, 1.5 Hz, Tropone-H), 7.81 (d, 1H, J = 1.5 Hz, Tropone-H). HRMS (ESI):
m/z [M+Na]+ Calcd for C15 H7109 Br35 ClN2 NaO+ : 370.9555. Found: 370.9548.
5-Bromo-1-(4-bromophenyl)-3-methyltropono[c]pyrazole (13)
This compound was obtained as a yellow solid, yield 72%, mp 259–260 ◦ C. IR (KBr): 1628 (C=O), 1590
(C=N), 1498, 1452, 1400, 1332, 1256, 1207, 1064, 1005 cm−1 1 H NMR (CDCl3 , 400 MHz): δ 2.58 (s, 3H,
;
Me), 6.78 (d, 1H, J = 10.5 Hz, Tropone-H), 7.25 (d, J = 7.8 Hz, 2H, Ph-H), 7.45 (dd, 1H, J = 10.5, 1.5 Hz,
Tropone-H), 7.58 (d, J = 7.8 Hz, 2H, Ph-H), 7.80 (d, 1H, J = 1.5 Hz, Tropone-H). HRMS (ESI): m/z[M+H]+
HRMS (ESI): m/z [M+Na]+ Calcd for C15 H1709 Br2 N2 NaO+ : 414.9050. Found: 414.9057.
5-Bromo-1-(4-cyanotrophenyl)-3-methyltropono[c]pyrazole (14)
This compound was obtained as an orange solid, yield 60%, mp 188–190 ◦ C. IR (KBr): 2221 (C≡N),
1651 (C=O), 1598 (C=N), 1574, 1561, 1518, 1492, 1397, 1325, 1257, 1220, 1139, 1089, 1042 cm−1 1 H NMR
;
(CDCl3 , 400 MHz): δ 2.66 (s, 3H, Me), 6.81 (d, 1H, J = 10.5 Hz, Tropone-H), 7.46 (d, 2H, J = 7.8 Hz, Ph-H),
7.69 (dd, 1H, J = 10.5, 1.6 Hz, Tropone-H), 7.62 (d, J = 7.8 Hz, 2H, Ph-H), 7.84 (d, 1H, J = 1.5 Hz, Tropone-
H), 8.38 (d, 1H, J = 2.0 Hz, Ph-H). HRMS (ESI): m/z [M+Na]+ Calcd for C16 H1790 BrN3 NaO+ : 361.9897.
Found 361.9889.
5-Bromo-1-(2,4-dinitrophenyl)-3-methyltropono[c]pyrazole (15)
This compound was obtained as a yellow solid, yield 56%, mp 194–196 ◦ C. IR (KBr): 1647 (C=O), 1609,
1585 (C=N), 1554, 1513, 1484, 1402, 1338, 1260, 1214, 1147, 1097, 1038 cm−1 1 H NMR (CDCl3 , 400 MHz):
;
δ 2.62 (s, 3H, Me), 6.77 (d, 1H, J = 10.5 Hz, Tropone-H), 7.55 (dd, 1H, J = 10.5, 1.6 Hz, Tropone-H), 7.76 (d,
1H, J = 7.8 Hz, Ph-H), 7.87 (d, 1H, J = 1.5 Hz, Tropone-H), 8.56 (dd, 1H, J = 7.8, 2.0 Hz, Ph-H), 9.02 (d, 1H,
J = 2.0 Hz, Ph-H). HRMS (ESI): m/z [M+Na]+ Calcd for C15 H799 BrN4 NaO+5 : 426.9646. Found 426.9652.
2.3. Procedure for the synthesis of 5-bromo-3-methyl-8H -tropono[d]isoxazole (17)
To a stirred solution of 3-acetyl-5-bromotropolone 1 (0.24 g, 1 mmol) in 10 mL of MeOH was added hydrox-
ylamine hydrochloride (16) (0.14 g, 2 mmol). The resulting mixture was heated at reflux for 10 h. After the
reaction was complete (TLC), the mixture was cooled to room temperature and 10 mL of water was added to
it. The resulting precipitate was collected by filtration and purified by recrystallization from ethanol to give
product 17. This compound was obtained as a brown solid, yield 70%, mp 164–166 ◦ C. IR (KBr): 1624 (C=O),
1578, 1519, 1403, 1325, 1252, 1219, 1132, 1023 cm−1 1 H NMR (DMSO-d6 , 400 MHz): δ 2.60 (s, 3H, Me),
;
7.08 (d, 1H, J = 10.5 Hz, Tropone-H), 7.63 (dd, 1H, J = 10.5, 1.5 Hz, Tropone-H), 7.65 (d, 1H, J = 1.4 Hz,
Tropone-H). HRMS (ESI): m/z [M+Na]+ Calcd for C9 H679 BrNNaO+2 : 261.9472. Found 261.9467.
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3. Results and discussion
As far as we know, 3-acetyl-5-bromotropolone (1) has been synthesized conveniently for many years, but the
further modification of it was very limited.37 In this regard, we have reported the synthesis of a series of
flavonoid-like troponoids employing 3-acetyl-5-bromotropolone (1) as the starting compound.38 In the context
of our ongoing studies on troponoid chemistry and further extending the diversity of our previous work, we have
become interested in the synthesis of new 5-bromo-tropono[c]pyrazole derivatives by employing 1 for current
medicinal chemistry needs.
Initially, we investigated the condensation reaction of 1 with hydrazine monohydrate (2) by refluxing 1
and 2.0 equiv. of hydrazine monohydrate (98%) in methanol (10 mL) as shown in Scheme 1. After the reaction
was completed as monitored by TLC, the desired product 3 was obtained in a good yield of 74%. Further, when
conducting the reaction using relatively cheap hydrazine monohydrate (80%) (Scheme 1), we were delighted to
find that the reaction also proceeded well and the product was obtained in a comparable yield of 72%.
Scheme 1. Synthesis of 5-bromo-3-methyltropono[c]pyrazole (3).
Subsequently, we examined the reaction of 1 with some free arylhydrazines (4–9) (2.0 equiv.) under the
same reaction conditions. However, we found that the condensation reaction proceeded poorly, and the desired
products were obtained in low yields of 21%–48% even after 48 h. It is worth mentioning that the observation in
our case was very similar to that reported by Lee et al., where the use of free arylhydrazines also gave products
in poor yields.39 Considering these results, we attempted to use the corresponding arylhydrazine hydrochloride
as the reaction partner as shown in Scheme 2.
Scheme 2. Synthesis of 1-aryl-5-bromo-3-methyltropono[c]pyrazoles (10–15).
To our delight, an improvement in terms of yields and reaction time was achieved, and the desired 1-
aryl-5-bromo-3-methyltropono[c]pyrazoles (10–15) were obtained in satisfactory yields of 56%–77% after 12 h.
The reaction results are summarized in the Table.
As shown in the Table, all the desired products were obtained in satisfactory yields except for compounds
14 and 15 (entries 6 and 7). The products 14 and 15 were obtained in moderate yields of 60% and 52%,
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respectively, which we attributed to the strong electron-withdrawing effects of the CN and NO2 groups, which
rendered the condensation reaction unfavorable.
Table. Yields of the newly synthesized 5-bromo-tropono[c]pyrazoles (3, 10–15).
Entry
1
Product
3, 10 -15
Yielda /%
3
72
2
3
10
11
77
75
4
12
74
5
6
72
60
13
14
7
15
52
a Isolated yield.
Additionally, in diversifying our work on new tropone-fused heterocyles, the same reaction conditions
were further applied to the reaction with hydroxylamine hydrochloride (16) with the aim of constructing a
novel tropone-fused isoxazole system as shown in Scheme 3. Interestingly, hydroxylamine hydrochloride was
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equally amenable to the conditions, only varying the reaction time according to TLC monitoring, and the
corresponding 5-bromo-3-methyl-tropono[d]isoxazole (17) was obtained in a comparable yield of 70%.
Scheme 3. Synthesis of 5-bromo-3-methyltropono[d]isoxazole (17).
To the best of our knowledge, none of the newly synthesized compounds 3, 10–15, and 17 have yet been
reported and their structures were easily established based on spectral data and HRMS. As an example, the
main features of the 1 H NMR data of compound 10 showed the absence of the signal belonging to OH moiety
of the precursor, along with the signals for 8 aromatic protons exactly matching its structure in the range of
the aromatic region of 6.77–7.81 ppm. Further, its molecular formula was established to be C15 H11 BrN2 O in
accordance with the suggested molecular structure by its HRMS spectrum, which showed a pseudo-molecular-
ion peak at m/z 336.9936 ([M+Na]+ ; calc. for C15 H1719 BrN2 NaO+ : 336.9945), indicating the presence of 11◦
of unsaturation.
4. Conclusions
We have achieved a facile synthesis of previously unattainable 5-bromo-tropono[c]pyrazole derivatives. These
compounds could be potentially applied for the development of biologically and pharmaceutically important
drugs. In addition, it is important to mention that these newly synthesized compounds contain a derivatizable
bromo group on the tropone ring, which makes them particularly appealing, since the functional group provides
ample opportunity for further synthetic manipulation, for example, by cross coupling reactions to obtain more
complex compounds. Further elaboration of these compounds to a variety of other functional groups is ongoing
and will be covered in further publications.
Acknowledgments
The authors thank the Foundation of Liaoning Province Key Laboratory of Applied Chemistry (Grant No.
2008s001) for its financial support.
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