One-Pot Synthesis of 4-Heteroaryl-Substituted Pyrazoles: A Gold-Catalyzed Oxidation/1,2-Heteroaryl Migration Cascade Constitutes the Key Step

Article abstract of DOI:10.1002/adsc.201600168

An efficient gold-catalyzed oxidation/1,2- heteroaryl migration cascade leading to 2-heteroaryl-substituted 1,3-diketones and its further application in the one-pot synthesis of 4-heteroaryl-substituted pyrazoles and the preparation of 4-heteroaryl-substituted isoxazoles are reported. A wide variety of 4-heteroaryl-substituted pyrazoles were obtained, indicating the broad substrate scope and functional group tolerance of this method.

Full text of DOI:10.1002/adsc.201600168

UPDATES
DOI: 10.1002/adsc.201600168
One-Pot Synthesis of 4-Heteroaryl-Substituted Pyrazoles: A
Gold-Catalyzed Oxidation/1,2-Heteroaryl Migration Cascade
Constitutes the Key Step
Xinbo Yao,a Tao Wang,a,* Xi Zhang,a Ping Wang,a Bing Zhang,a Junfa Wei,a
and Zunting Zhanga,*
a
School of Chemistry and Chemical Engineering, Shaanxi Normal University, No.620 West Chang’an Avenue, Xi’an
710119, Peopleꢀs Republic of China
E-mail: chemtao@snnu.edu.cn or zhangzunting@sina.com
Received: February 9, 2016; Revised: March 11, 2016; Published online: April 27, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600168.
Abstract: An efficient gold-catalyzed oxidation/1,2-
heteroaryl migration cascade leading to 2-heteroar-
yl-substituted 1,3-diketones and its further applica-
tion in the one-pot synthesis of 4-heteroaryl-substi-
tuted pyrazoles and the preparation of 4-heteroaryl-
substituted isoxazoles are reported. A wide variety
of 4-heteroaryl-substituted pyrazoles were obtained,
indicating the broad substrate scope and functional
group tolerance of this method.
pounds with 1,3-diketones.[3] However, multi-step syn-
thesis is required for the preparation of heteroaryl-
lead triacetate compounds. In addition, the high toxic-
ity of lead also limits the use of this method. The
other choice is the conversion of 4-heteroaryl-3,5-di-
methylisoxazoles to a-heteroaryl-substitituted 1,3-di-
ketones via hydrogenolysis followed by acid hydroly-
sis.[4] However, as the starting material, 4-heteroaryl-
3,5-dimethylisoxazoles have to be prepared in ad-
vance via multistep synthesis. The reaction of a hetero-
aryl ring with a carbene precursor is also a potent ap-
proach.[5] However, only the reaction of trisheteroar-
ylmethanes with diazo compounds was successfully
achieved. Fortunately, the impact of homogeneous
gold catalysis on organic synthesis in the last 15 years
has been significant.[6] The spectacular development
Keywords: 1,3-diketones; gold carbenes; gold catal-
ysis; heterocycles; pyrazoles
Heterocycles are widely found in bioactive natural of the a-oxo gold carbene chemistry[7,8] provides an
products and pharmaceuticals,[1] for example the opportunity for the facile synthesis of a-heteroaryl-
drugs Celebrex, Rimonabant, Sulfisoxazole and substitituted 1,3-diketones. Since its first report by
Viagra. Generally, the installation of a heteroaromatic Zhang,[9] the reaction of a pyridine N-oxide or quino-
group on a heterocyclic ring may change its biological line N-oxide with an alkyne in the presence of a gold
activity dramatically. A commonly used approach for catalyst has become a very useful strategy for the gen-
the preparation of heteroaryl-substituted heterocycles eration of an a-oxo gold carbene intermediate
such as 4-heteroaryl-substituted pyrazoles and isoxa- (Scheme 1, bottom). Based on this strategy, lots of
zoles, 5-heteroaryl-substituted pyrimidines and 3-het- useful transformations have been achieved. Herein,
eroaryl-substituted-3H-benzo[b][1,4]diazepines is the we report an efficient method for the preparation of
condensation of a-heteroaryl-substitituted 1,3-dike- a-heteroaryl-substituted 1,3-diketones via a gold-cata-
tones with additional reagents like hydrazine, hydrox- lyzed oxidation/1,2-heteroaryl migration cascade from
ylamine, guanidine and o-diaminobenzene (Scheme 1, readily accessible propargyl alcohols and its further
above). The a-furyl-substituted 1,3-diketone is rela- application in the one-pot synthesis of pyrazoles and
tively easy to obtain from a 1,3-diketone and 2,5-dihy- isoxazoles (Scheme 1, bottom). As a key step, for the
dro-2,5-dimethoxyfuran (DHDMF),[2] however no first time a highly selective 1,2-heteroaryl migration
convenient method is available for the preparation of of a gold-carbene intermediate is presented, previous
a-thienyl, a-benzofuryl and a-pyrrolyl-substituted 1,3- reports[10] only reported 1,2-migrations of aromatic
diketones (Scheme 1, middle). For example, one groups,[11] hydride,[12] alkyl,[13] alkynyl groups[10b,c,d,14]
method for the synthesis of these compounds is the and alkenyl groups.[15]
cross-coupling of heteroaryl-lead triacetate com-
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One-Pot Synthesis of 4-Heteroaryl-Substituted Pyrazoles
Scheme 1. Known routes to a-heteroaryl-substituted 1,3-diketones and the new route.
Propargyl alcohol 1a was prepared facilely by treat- methyl migration product 3a’. Encouraged by this
ing phenylacetylene with n-butyllithium and the fol- result, we started to optimize the reaction conditions.
lowing one-pot addition of 1-(thiophen-2-yl)ethanone. First the N-oxides were varied (Table 1, entries 1–5).
In our initial study, 1a was treated with 4-methylpyri- The electron-deficient pyridine N-oxide (2b; entry 2)
dine N-oxide (2a) and methanesulfonic acid in the as well as a pyridine N-oxide with large steric hin-
presence of IPrAuCl/AgNTf2 [IPr=1,3-bis(2,6-diiso- drance (2c; entry 3) reacted with 1a just gave trace
propylphenyl)imidazol-2-ylidene, Tf=trifluoromethyl- amount of 1,3-diketone 3a. Fortunately, 8-methylqui-
sulfonyl] at room temperature (Table 1, entry 1). Ac- noline 1-oxide (2e) afforded 3a in high yield (87%)
cording to literature,[7c] acid was added to remove the with high chemoselectivity (>20:1). The control ex-
pyridine by-product so that it would not deactivate periment revealed that methanesulfonic acid was es-
the gold catalyst. The pyridine N-oxide played not sential for this reaction (entry 6). In the next step, var-
only the role of an oxidant, but also buffers the acidi- ious catalysts were examined (Table 1, entries 7–11)
ty of the reaction considering the instability of the ter- by the use of 8-methylquinoline 1-oxide (2e) as
tiary alcohol under the acidic conditions. To our de- oxygen-transfer reagent. Among the tested catalysts,
light, a 1,2-thienyl migration product 3a was obtained (2,4-t-Bu-C6H3O)3PAuCl together with AgNTf2 gave
with high selectivity over the also conceivable 1,2- the best result (entry 8). Besides dichloromethane, the
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Xinbo Yao et al.
Table 1. Optimization of the reaction conditions for the synthesis of a-thienyl 1,3-diketone.[a]
Entry
N-Oxide
Catalyst
Solvent/Time
Yield of 3a [%][b]
3a:3a’
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2e
2e
2e
2e
2e
2e
2e
2e
2e
2e
2e
2e
2e
2e
IPrAuCl/AgNTf2
IPrAuCl/AgNTf2
IPrAuCl/AgNTf2
IPrAuCl/AgNTf2
IPrAuCl/AgNTf2
IPrAuCl/AgNTf2
Ph3PAuCl/AgNTf2
(2,4-t-Bu-C6H3O)3PAuCl/AgNTf2
JohnPhosAuCl/AgNTf2
(2,4-t-Bu-C6H3O)3PAuCl/AgOTf
(2,4-t-Bu-C6H3O)3PAuCl/AgSbF6
(2,4-t-Bu-C6H3O)3PAuCl/AgNTf2
(2,4-t-Bu-C6H3O)3PAuCl/AgNTf2
(2,4-t-Bu-C6H3O)3PAuCl/AgNTf2
(2,4-t-Bu-C6H3O)3PAuCl/AgNTf2
(2,4-t-Bu-C6H3O)3PAuCl/AgNTf2 (3 mol%)
(2,4-t-Bu-C6H3O)3PAuCl/AgNTf2 (1 mol%)
Zn(OTf)2
DCM/15 min
DCM/15 min
DCM/15 min
DCM/2 h
DCM/15 min
DCM/12 h
DCM/8 h
DCM/2 h
DCM/5 h
DCM/8 h
DCM/8 h
THF/24 h
DCE/8 h
CH3CN/24 h
toluene/24 h
DCM/4 h
DCM/4 h
DCM/12 h
DCM/12 h
70
13:1
–
–
12:1
15:1
–
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
–
>20:1
>20:1
>20:1
–
trace
trace
52
87
<10[c]
32
88
50
80
84
77
57
trace
89
90
9
10
11
12
13
14
15
16
17
18
19
73
ND[d]
ND[d]
Cu(OTf)2
–
[a]
All reactions were carried out on a 0.2 mmol scale in 2 mL of solvent at room temperature.
Yields of isolated products.
Without the addition of MeSO3H, the conversion of 1a was less than 80%.
No product was detected.
[b]
[c]
[d]
reaction also worked well in tetrahydrofuran (THF) 4-heteroaryl-substituted pyrazoles. As summarized in
and 1,2-dichlorethane (DCE). However, the use of Scheme 2, thiophene-derived propargyl alcohols were
the coordinative solvent acetonitrile resulted in deac- examined first. Propargyl alcohols with a substituted
tivation of the catalyst (entry 14). On lowering of the phenyl group on the alkyne terminal afforded the cor-
amount of the catalyst from 5 to 3 mol%, no drop in responding pyrazoles in good yield, no matter if the
yield was observed (entry 16). However, reducing cat- substituents were electron-donating groups or elec-
alyst loading to 1 mol% led to a sharp decrease of the tron-withdrawing groups (Scheme 2, 4a–4f). A 1-
yield (entry 17). Literature reports revealed that naphthyl group (4g) or a cyclohexenyl group (4h) was
zinc[16] and copper[17] salts could also promote the readily tolerated in the one-pot reaction. Aliphatic
oxygen transfer reaction from pyridine N-oxides to al- propargyl alcohols also worked well leading to the
kynes. However, neither Zn(OTf)2 nor Cu(OTf)2 corresponding pyrazoles in good yields. For example,
could catalyze this reaction (entries 18 and 19).
the cyclopropyl-substituted pyrazole 4i and the n-
With the optimized conditions for the preparation butyl-substituted pyrazole 4j were isolated in 72%
of 1,3-diketones in hand, we hypothesized that this re- and 46% yields, respectively. However, 4k and its
action could be used in a one-pot synthesis of pyra- isomer 4k’ were obtained as a mixture in 41% total
zoles because the acid which was added in the first yield. This might be explained by the decreased steric
step could also promote the following condensation bulk of the PhOCH2- group, which resulted in the
of hydrazine and 1,3-diketone.[18] To prove this, phe- poor regioselectivity in the condensation step. Inter-
nylhydrazine was added after the propargyl alcohol estingly, in analogy to the above-mentioned results,
1 was fully consumed. Gratifyingly, a pyrazole product when R2 was an H atom instead of a methyl group,
was obtained in good yield as we expected. Scheme 2 only a 1,2-thienyl migration product 4l was isolated.
shows the reaction scope of the one-pot synthesis of A methyl group in the thiophene ring did not affect
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One-Pot Synthesis of 4-Heteroaryl-Substituted Pyrazoles
Scheme 2. One-pot synthesis of 4-heteroaryl-substituted pyrazoles. Yields given in this Scheme refer to isolated products.
this reaction, delivering the desired product 4m in benzofuran derived (5d) propargyl alcohols worked
good yield. 3-Methyl-1,5-diphenyl-4-(thiophen-3-yl)- smoothly, delivering 4-heteroaryl-substituted isoxa-
1H-pyrazole (4n) was also obtained in good yield. Be- zoles in good yields.
sides thiophene, furan (4o, 4p and 4q), benzofuran
(4r) and N-methylpyrrole (4s) derived propargyl alco-
hols also worked smoothly leading to 4-heteroaryl-
substituted pyrazoles in good yields, respectively. The
structure of 4b was confirmed by an X-ray diffraction
single crystal structure analysis (Figure 1).[19]
As a key step, the gold-catalyzed oxidation/1,2-het-
eroaryl group migration approach was also used for
the synthesis of 4-heteroaryl-substituted isoxazoles.
Scheme 3 shows some examples. Isoxazoles were ob-
tained via the condensation of isolated 1,3-diketones
and hydroxylamine hydrochloride. The yield given is
the total yield of the two-step reaction. As depicted,
thiophene (5a), furan (5b), N-methylpyrrole (5c) and Figure 1. Solid-state molecular structure of 4b.
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UPDATES
Xinbo Yao et al.
(ESI): m/z=267.0447, calcd. for C14H12O2SNa [M]+Na:
267.0456.
General Procedure for the One-Pot synthesis of
Pyrazoles
According to Procedure A, after the propargyl alcohol was
fully consumed (monitored by TLC), 3 equiv. of hydrazine
(0.6 mmol) were added directly to the solution. The result-
ing mixture was allowed to stir at room temperature until
the diketone was fully consumed (monitored by TLC).
After that the solvent was removed and the residue was pu-
rified by column chromatography on silica gel affording the
desired product.
3-Methyl-1,5-diphenyl-4-(thiophen-2-yl)-1H-pyrazole (4a):
1
yield: 85%; yellow solid; mp 152–1538C; H NMR (CDCl3,
400 MHz): d=7.33–7.20 (m, 8H), 7.20–7.15 (m, 3H), 6.99–
6.93 (m, 1H), 6.82 (d, 1H, J=3.0 Hz), 2.47 (s, 3H);
13C NMR (CDCl3, 100 MHz): d=148.2, 140.9, 139.9, 134.6,
130.6, 130.2, 128.8, 128.7, 128.5, 127.1, 127.0, 126.3, 125.0,
124.8, 115.0, 13.2; IR (KBr): n=3061, 2922, 2856, 2733,
1963, 1892, 1798, 1728, 1665, 1591, 1499, 1431, 1369, 1225,
1161, 1026, 966, 835, 760, 689, 455; HRMS (ESI): m/z=
339.0926, calcd. for C20H16N2SNa [M]+Na: 339.0930.
General Procedure for the Synthesis of Isoxazoles
Scheme 3. Synthesis of 4-heteroaryl-substituted isoxazoles.
1,3-Diketone (0.20 mmol), hydroxylamine hydrochloride
(0.22 mmol) and K2CO3 (0.22 mmol) were dissolved in etha-
nol (1.0 mL). The reaction mixture was allowed to reflux for
4–8 h. After that the solvent was removed and the residue
was purified by column chromatography on silica gel afford-
ing the desired product.
In conclusion, we have described an efficient gold-
catalyzed oxidation/1,2-heteroaryl group migration
cascade leading to a-heteroaryl-substituted 1,3-dike-
tones and its further application in the one-pot syn-
thesis of 4-heteroaryl-substituted pyrazoles and the
preparation of isoxazoles. The approach showed
a broad substrate scope and functional group toler-
ance. Research on the biological activity of the prod-
ucts obtained is currently underway in our laboratory.
3-Methyl-5-phenyl-4-(thiophen-2-yl)isoxazole (5a): yield:
1
70%; yellow oil; H NMR (CDCl3, 400 MHz): d=7.62–7.59
(m, 2H), 7.43 (d, 1H, J=6.8 Hz), 7.39–7.33 (m, 3H), 7.12–
7.11 (m, 1H), 7.02 (d, 1H, J=4.4 Hz), 2.29 (s, 3H);
13C NMR (CDCl3, 100 MHz): d=165.6, 160.6, 130.9, 130.2,
128.8, 128.6, 128.4, 127.9, 127.7, 127.2, 109.6, 10.8; IR (KBr):
n=3520, 3443, 3323, 3069, 2959, 2864, 1962, 1894, 1807,
1728, 1628, 1441, 1286, 1227, 1078, 1032, 926, 843, 771, 700,
592, 546, 494 cmÀ1; HR-MS (ESI): m/z=264.0449, calcd. for
C14H11NOSNa [M]+Na: 264.0459.
Experimental Section
General Procedure for the Preparation of 1,3-
Diketones (Procedure A)
8-Methylquinoline 1-oxide (0.4 mmol) and MeSO3H
(0.24 mmol) were added successively to a stirring suspension
of (2,4-t-Bu-C6H3O)3PAuCl (6.0 mmol) and AgNTf2
(6.0 mmol) in dichloromethane (2.0 mL) at room tempera-
ture. After that the propargyl alcohol (0.2 mmol) was added
and the combined solution was then stirred at room temper-
ature until the reaction was completed (monitored by TLC).
After evaporation, the residue was purified by column chro-
matography on silica gel affording the desired product.
1-Phenyl-2-(thiophen-2-yl)butane-1,3-dione (3a): yield:
Acknowledgements
Financial support by the National Natural Science Founda-
tion of China (21502110 and 21542002) and the Fundamental
Research Funds for the Central Universities (GK201503034
and GK261001095) are greatly appreciated. We thank Prof.
Dr. A. Stephen K. Hashmi (Heidelberg University) for many
helpful discussions.
1
90%; H NMR (CDCl3, 400 MHz): d=17.50 (s, 1H), 7.30 (d,
2H, J=7.6 Hz), 7.20–7.15 (m, 2H), 7.12–7.06 (m, 2H), 6.86– References
6.83 (m, 1H), 6.70 (d, 1H, J=3.2 Hz), 2.05 (s, 3H);
13C NMR (CDCl3, 100 MHz): d=197.9, 184.3, 138.3, 136.0,
130.7, 129.9, 128.7, 127.8, 127.3, 127.2, 106.2, 25.6; IR (KBr):
n=3553, 3476, 3414, 3238, 3098, 2963, 2027, 1616, 1387,
1271, 1078, 1022, 878, 835, 789, 714, 619, 482 cmÀ1; HR-MS
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tallographic data for this paper. These data can be ob-
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Adv. Synth. Catal. 2016, 358, 1534 – 1539
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