A convenient and efficient H2SO4-promoted regioselective monobromination of phenol derivatives using N-bromosuccinimide

Article abstract of DOI:10.1080/00397911.2019.1711415

A convenient, rapid H₂SO₄-promoted regioselective monobromination reaction with N-bromosuccinimide was developed. The desired para-monobrominated or ortho-monobrominated products of phenol derivatives were obtained in good to excellent yields with high selectivity. Regioselective chlorination and iodination were also achieved in the presence of H₂SO₄ using N-chlorosuccinimide and N-iodosuccinimide, respectively.

Full text of DOI:10.1080/00397911.2019.1711415

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
A convenient and efficient H2SO4-promoted
regioselective monobromination of phenol
derivatives using N-bromosuccinimide
Yong-Qi Wu, Hai-Jia Lu, Wen-Ting Zhao, Hong-Yi Zhao, Zi-Yun Lin, Dong-Feng
Zhang & Hai-Hong Huang
To cite this article: Yong-Qi Wu, Hai-Jia Lu, Wen-Ting Zhao, Hong-Yi Zhao, Zi-Yun Lin, Dong-
Feng Zhang & Hai-Hong Huang (2020): A convenient and efficient H2SO4-promoted regioselective
monobromination of phenol derivatives using N-bromosuccinimide, Synthetic Communications,
DOI: 10.1080/00397911.2019.1711415
To link to this article: https://doi.org/10.1080/00397911.2019.1711415
View supplementary material
Published online: 13 Jan 2020.
Submit your article to this journal
Article views: 11
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
https://www.tandfonline.com/action/journalInformation?journalCode=lsyc20

R
SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2019.1711415
A convenient and efficient H2SO4-promoted regioselective
monobromination of phenol derivatives using
N-bromosuccinimide
ꢀ
ꢀ
Yong-Qi Wu , Hai-Jia Lu , Wen-Ting Zhao, Hong-Yi Zhao, Zi-Yun Lin,
Dong-Feng Zhang , and Hai-Hong Huang
Beijing Key Laboratory of Active Substance Discovery and Druggability Evaluation, Institute of Materia
Medica, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China
ABSTRACT
ARTICLE HISTORY
Received 12 October 2019
A convenient, rapid H2SO4-promoted regioselective monobromina-
tion reaction with N-bromosuccinimide was developed. The desired
para-monobrominated or ortho-monobrominated products of phenol
derivatives were obtained in good to excellent yields with high
selectivity. Regioselective chlorination and iodination were also
achieved in the presence of H2SO4 using N-chlorosuccinimide and
N-iodosuccinimide, respectively.
KEYWORDS
N-Bromosuccinimide;
phenol analogs;
regioselective
monobromination; salicylic
acid derivatives;
sulfuric acid
GRAPHICAL ABSTRACT
Introduction
Brominated aromatic compounds are valuable precursors for synthesizing many natural
products and bioactive compounds. The bromine atom provides a critical reactive site
for the formation of carbon–carbon and carbon–heteroatom bonds in metal-catalyzed
cross-coupling reactions, such as Suzuki, Sonogashira, or Buchwald couplings. The
classic approach to obtaining bromoaromatic compounds is electrophilic aromatic
bromination with bromine or other bromonium ion sources. However, achieving
CONTACT Dong-Feng Zhang
zdf@imm.ac.cn; Hai-Hong Huang
joyce@imm.ac.cn
Beijing Key Laboratory of
Active Substance Discovery and Druggability Evaluation, Institute of Materia Medica, Chinese Academy of Medical
Sciences and Peking Union Medical College, Beijing 100050, China.
ꢀ
These authors contributed equally to this work.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
Y.-Q. WU ET AL.
monobromination and regioselectivity can be challenging, especially for highly activated
aromatic substrates, such as phenol and its derivatives.
Many selective monobromination reagents have been reported, including pyridinium
tribromide,[1–3] tetrabutylammonium tribromide,[4] tetrapropylammonium nonabro-
mide,[5] 1-butyl-3-methylimidazolium tribromide,[6] f[K.18-Crown-6]Br3gn,[7] dioxane
dibromide,[8] hexamethylenetetramine-bromine complex,[9] a DABCO-derived bromine
complex,[10] N,N,N0,N0-tetrabromobenzene-1,3-disulfonylamide,[11] ammonium bromide
and oxone,[12] and a ZrBr4/diazene mixture.[13] However, these reagents are expensive
or difficult to use.
N-Bromosuccinimide (NBS) is a readily available, cheap reagent that provides a pre-
cise stoichiometric amount of the bromonium ion and allows easy reaction handling
because the by-product, succinimide, is soluble in water. Several selective monobromi-
nation methods using NBS have been developed. Typical examples are NBS/
HBF4ꢁEt2O,[14] NBS/silica gel,[15,16] NBS/HZSM-5,[17] NBS/NaOH,[18] NBS/p-toluenesul-
fonic acid,[19] NBS/NH4OAc,[20] NBS/Amberlyst-15,[21] NBS/thioamide,[22] NBS/ionic
liquid,[23] NBS/CF3COOH/H2SO4,[24] NBS/UV irradiation,[25] and solvent-controlled
regioselective bromination using NBS.[26] However, these methods still use reagents that
are expensive, not commercially available, or corrosive acids, and the scope of substrates
in some reactions is restricted. Thus, the development of a selective monobromination
method using cheap, commercially available and environmentally friendly reagents is
still required.
5-Bromo-4-fluoro-2-hydroxybenzoic acid (1a) has become an important starting
material for us to construct a small molecule library for biological screening. However,
1a is not readily available on a large scale and is expensive as well. This prompted us to
develop a facile method to synthesize 5-bromo-4-fluoro-2-hydroxybenzoic acid.
According to a literature method,[27] we prepared 1a from 4-fluoro-2-hydroxybenzoic
acid (1) in DMF using NBS (Scheme 1A). However, in contrast to the literature, a
nearly equal mixture of 1a and dibrominated product 1b was obtained, and the mixture
was difficult to be separated. The amount of 1b could not be decreased, even at the
reaction temperatures of ꢂ40 and ꢂ70 ꢃC. The dibrominated compound 2b was the
major product when methyl 4-fluoro-2-hydroxybenzoate (2) was used as the starting
material in DMF (Scheme 1B). To decrease the amount of dibrominated product, we
increased the steric hindrance at the ortho hydroxyl group in 3, but the bromination
reaction did not occur (Scheme 1C).
The combination of NBS/H2SO4 had been used for the monobromination of highly
deactivated aromatic compounds under harsh reaction condition (conc. H2SO4 as a
solvent, 60 ꢃC).[28] Thomas Oberhauser also reported the synthesis of 3-bromo-4-
hydroxybenzonitrile using H2SO4 and NBS in CH3CN, however, the selectivity was only
71% and the reaction time was long up to 24 h.[14] Based on these results, we tried to
use NBS and H2SO4 in CH3CN to prepare 1a. To our delight, the desired monobromi-
nated product was obtained in excellent yield (Scheme 1D). In consideration of the lim-
ited research of NBS and H2SO4 as the regioselective monobromination reagents, in the
present work, we further investigate the application of the combination of NBS/H2SO4
in the regioselective monobromination of salicylic acid derivatives and other
phenol analogs.

R
SYNTHETIC COMMUNICATIONSV
3
(A)
(B)
(C)
(D)
Scheme 1. Synthesis of 5-bromo-4-fluoro-2-hydroxybenzoic acid (1a).
Results and discussion
The bromination reaction was optimized with salicylic acid (4). Bromination of salicylic
acid with NBS in CH3CN without H2SO4 gave a mixture of the starting material, the
ortho-monobrominated product (4a), the desired para-monobrominated product (4b),
and the corresponding dibromo product (4c) (Table 1, entry 1). When 1 equiv. of
H2SO4 was added to the reaction, the desired para-monobrominated product was
afforded with 92.6% selectivity, and the reaction time was only 0.5 h (Table 1, entry 2).
Lowering the number of equivalents of H2SO4 did not increase the selectivity and yield
of the desired product, and the reaction time was long (Table 1, entries 3 and 4).
Regioselective monobromination also occurred in CH2Cl2 and ethyl acetate, although
compared with the reaction in CH3CN, the selectivity and yield were decreased in
CH2Cl2 (Table 1, entry 5), and the reaction time was up to 22.5 h in ethyl acetate
(Table 1, entry 6). Consequently, 1 equiv. of H2SO4 and CH3CN was identified as the
optimal reaction conditions.
With the optimized reaction conditions in hand, we explored the scope of the mono-
bromination reaction (Table 2). The control reactions for every substrate were carried
out to verify the effectiveness of our approach. In the absence of H2SO4, the selectivity
and yield of the regioselective monobrominated products were low (Table 2, entries 2a

4
Y.-Q. WU ET AL.
Table 1. Optimization of the reaction conditions for the bromination reaction.a
Product ratio (%)b
Entry
Acid
Solvent
CH3CN
CH3CN
CH3CN
CH3CN
CH2Cl2
EtOAc
Temp.
Time (h)
SM (4)
3 (4a)
5 (4b)
3,5 (4c)
1
2
3
4
5
6
–
rt
rt
rt
rt
rt
rt
16
0.5
1
3
0.5
22.5
15.8
0
4.2
8.0
0
4.4
3.7
4.2
4.0
5.2
3.6
63.3
92.6
84.0
80.0
87.0
90.9
16.5
3.7
7.6
8.0
7.8
5.5
H2SO4 (1.05eqv)
H2SO4 (0.5eqv)
H2SO4 (0.2eqv)
H2SO4 (1.05eqv)
H2SO4 (1.05eqv)
0
aReaction conditions: salicylic acid (4):NBS ¼ 1:1.05.
bProducts characterized by 1H NMR, SM refers to the substrate, the numbers in the product composition denote the
positions of bromination.
and 4a–6a), except for compounds 1 and 6 (Table 2, entries 1a and 3a). However, with
these substrates, the reaction times were longer than for the reaction with H2SO4. The
results demonstrated that H2SO4 was vital for promoting the bromination reaction.
With H2SO4, the bromination of compounds 1 and 5–9 afforded high para-selective
yields of brominated products (Table 2, entries 1b–6b). For compounds 5–7, the mono-
brominated products 5a, 6a, and 7a were obtained in the yields of 97%, 94%, and 98%,
respectively, and with 100% selectivity (Table 2, entries 2b–4b). H2SO4-promoted regio-
selective monobromination was suitable for 3-methoxybenzoic acid (10), providing the
para product (10a) in 91% yield and quantitative selectivity (Table 2, entry 7b). Gram-
scale synthesis of 5-bromo-4-fluoro-2-hydroxybenzoic acid (1a) was performed to
demonstrate the practical application of this method, which afforded 14.2 g 1a in an
excellent yield (95%) with straightforward manipulation (see Supporting Information).
With the bromination data in hand, we extended our work to investigate the effect of
H2SO4 on chlorination and iodination using N-chlorosuccinimide (NCS) or N-iodosuc-
cinimide (NIS), respectively, and salicylic acid (4) as a template substrate (Table 2).
Compared with the control reactions, the chlorination and iodination of compound 4
afforded regioselective monochlorination product 4d and monoiodination product 4e in
the yields of 92 and 82%, respectively, with good selectivity (Table 2, entries 8b and 9b).
The results demonstrated that H2SO4 also increases the speed and selectivity of chlorin-
ation and iodination.
The combination of H2SO4 and NBS in CH3CN was applied to the bromination of
other phenol derivatives (Table 3). Bromination of phenol gave good yield and high
regioselectivity for the para product (11a) after only 2 h of stirring (Table 3, entry 1b).
Various ortho-substituted phenol analogs containing electron-donating or -withdrawing
groups were investigated. Bromination of 13–17 afforded high para-selective yields of
brominated products, although bromination of 12 did not. Compared with the reactions
without H2SO4, the reaction times were only 2 or 3 h in the presence of H2SO4. For
para-substituted phenol analogs (18 and 19), bromination with NBS in acetonitrile with

R
SYNTHETIC COMMUNICATIONSV
5
Table 2. Substrate scope for phenol derivatives (salicylic acid analogs).a
Product ratio (%)c
Time
(h)
Yield
(%)d
Entryb
1a
Compds
Product
SM
9.9
3
-
5
3,5
-
24
90.1
1b
2a
1
7.4
-
92.6
42.7
-
85
30
20.1
3.9
33.3
2b
3a
3b
2
24
3
-
0.3
-
-
-
-
100
99.4
100
-
0.3
-
97
94
4a
48
15
12.5
-
72.5
4b
5a
5b
6a
4
-
100
4.1
-
-
-
-
-
-
98
76
48
48
48
95.9
14.5
35.9
77.5
44.0
8.0
20.1
6b
7a
48
48
8.3
91.7
4.8
-
-
-
82
91
15.2
80.0
7b
8a
0.5
48
-
-
100
-
-
23.5
23.0
53.5
8b
9a
9b
2.5
48
-
-
-
5.3
10.6
11.0
94.7
59.2
84.8
-
92
82
30.2
4.2
1.5
aReaction conditions: H2SO4 (285 lL, 1.05 equiv.) was added to CH3CN (10 mL) containing the
substrate (5 mmol) at room temperature, and after 5 min, NBS, NCS, or NIS (1.05 equiv.) was
added to the mixture. The reaction was monitored using TLC until completion or extended
to 48 h. The product was isolated by chromatography.
bEntry a: without H2SO4, entry b: with H2SO4 (1.05 equiv.).
cProducts characterized by 1H NMR, SM refers to the substrate, the numbers in the product
composition denote the positions of bromination.
dIsolated yield.

6
Y.-Q. WU ET AL.
Table 3. Scope of the bromination reaction.a
Time
(h)
Product ratio (%)c
Yield
(%)d
Entryb
1a
Compds
Product
SM
2
4
6
-
2,4
2,6
1.4
4,6
-
2,4,6
24.1
56
10.4
4.4
41.3
18.4
-
1b
2a
2
-
2.8
-
94.1
38.9
-
3.1
-
-
-
-
-
-
79
-
24
36.2
1.9
23.0
2b
3a
3b
4a
24
24
2
26.5
15.1
-
-
-
-
-
55.2
37.7
97.1
23.1
3.3
9.4
2.9
6.9
-
-
-
-
-
-
-
-
15.0
37.8
-
-
-
-
-
-
-
80
-
24
31.5
38.5
4b
5a
3
6
-
-
-
91.7
21.9
5.5
5.5
-
-
2.8
-
-
80
-
31.6
-
41.0
5b
6a
6b
7a
7b
8a
8b
9a
9b
2
7
5.3
26.8
-
-
-
-
-
-
88.5
46.9
88.5
80.6
93.5
-
6.2
8.0
6.2
8.1
3.7
-
-
-
-
-
-
-
-
-
-
-
-
18.3
5.3
6.5
2.8
-
-
-
-
-
-
-
-
-
-
73
-
-
2
-
80
-
2
4.8
-
-
2
-
46.1
-
77
-
9
34.1 19.8
2
-
100
87.0
92.6
-
-
-
92
-
24
6
7.0
7.4
-
-
6.0
-
-
-
-
-
78
10a
48
34.7 61.5
-
3.8
-
-
-
-
-
10b
11a
2
-
98.0
-
-
2.0
9.5
-
-
-
-
-
-
-
-
95
-
48
11.1
79.4
11b
3
-
-
87.1
8.6
-
-
4.3
-
86
12a
12b
48
1
21.4
1.0
-
-
46.0
97.1
0.9
1.0
-
-
-
-
31.7
0.9
-
-
-
89
aReaction conditions: H2SO4 (285 lL, 1.05 equiv.) was added to CH3CN (10 mL) con-
taining the substrate (5 mmol) at room temperature, and after 5 min, NBS, NCS, or
NIS (1.05 equiv.) was added to the mixture. The reaction was monitored using TLC
until completion. The product was isolated by chromatography.
bEntry a: without H2SO4, entry b: with H2SO4 (1.05 equiv.).
cProducts characterized by 1H NMR or GC-MS (11a), SM refers to the substrate, the
numbers in the product composition denote the positions of bromination.
dIsolated yield.

R
SYNTHETIC COMMUNICATIONSV
7
Scheme 2. The proposed pathway of sulfuric acid-promoted regioselective monobromination.
H2SO4 gave excellent yields and selectivity of the desired products (Table 3, entries 8b
and 9b). The effectiveness of the monobromination reaction was demonstrated further
in the reaction of chromen (20), which furnished the ortho product (20a) in 95% yield
and 98% selectivity (Table 3, entry 10b).
The effect of H2SO4 on chlorination and iodination of phenol derivatives was exam-
ined using 2-bromo-phenol (15) as a template substrate (Table 3). Compared with the
control reactions, the chlorination and iodination of compound 15 afforded regioselec-
tive monochlorination product 18a and monoiodination product 15b in yields of 86%
and 89% with 87.1% and 97.1% selectivity, respectively (Table 3, entries 11b and 12b).
The selectivity and yield were better than the results in the literatures,[19,29] and we
found that the chlorination could be achieved without H2SO4 after sufficient reaction
time. The results showed that H2SO4 increases the speed and selectivity of chlorination
and iodination of other phenol derivatives.
To gain further insight into the NBS-H2SO4 method, 13C NMR measurements were
performed. Immediately after the addition of equal equivalent of concentrated H2SO4 to
NBS in CD3CN, the carbonyl and methylene signals showed changes (see Supporting
Information). The carbonyl signal (175.36 ppm) was split into two signals (178.42 and
181.64 ppm), and the methylene signal (29.21 ppm) was split into two signals (29.40 and
29.24 ppm). The results showed that NBS was protonated by H2SO4 to form an
NBS–H2SO4 complex via hydrogen bonding, and the N–Br bond was polarized. The
polarized Br attacked the position activated by the OH group to form the desired prod-
uct (Scheme 2). The bromination was accelerated by the formation of the NBS–H2SO4
complex and polarized N–Br bond.
Conclusion
An efficient, convenient aromatic monobromination method was developed for bromi-
nating phenol derivatives, especially salicylic acid analogs. Rapid, regioselective mono-
bromination was accomplished using NBS as a safe bromine source in the presence of
H2SO4, and good to excellent selectivity and yield were obtained. Furthermore, the reac-
tion is also applicable to chlorination and iodination using NCS and NIS, respectively.

8
Y.-Q. WU ET AL.
The mild reaction conditions, rapid conversion, straightforward manipulation, and
excellent selectivity are notable advantages of the method and make it potentially suit-
able for large-scale aromatic halogenation.
Experimental
General information
All the substrates and reagents were purchased from commercial suppliers and were
1
used without further purification. H NMR or 13C NMR spectra were recorded on
Varian 400 MHz or 500 MHz spectrometer using CDCl3, DMSO-d6 or acetone-d6 as sol-
vents and tetramethylsilane (TMS) as an internal standard. High-resolution mass spectra
were recorded using a Thermo Exactive Orbitrap plus mass spectrometer (ESI).
Representative procedure for the preparation of compound 1a
To a solution of 4-fluoro-2-hydroxybenzoic acid (780.6 mg, 5 mmol) in MeCN (10 mL)
was added con. H2SO4 (285 lL, 1.05 equiv.) at room temperature, the mixture was
stirred for 5 min. Then, NBS (934.4 mg, 1.05 equiv.) was added to the mixture. The
reaction was monitored using TLC analysis. The mixture was evaporated to dryness and
MeCN (1 mL) was added to the flask. The mixture was stirred for 10 min and filtered.
The filter cake was washed by water and dried to obtain the product as a white solid;
1
yield: 85% (0.99 g). H NMR (400 MHz, DMSO-d6) d: 8.01 (d, J ¼ 8.0 Hz, 1 H), 7.05 (d,
J ¼ 10.0 Hz, 1 H).[27]
13C NMR (100 MHz, DMSO-d6) d: 169.9, 162.1 (d, JC-F ¼ 13.1 Hz), 161.9 (d, JC-F
¼
249.8 Hz), 134.5 (d, JC-F ¼ 3.2 Hz), 112.0, 105.5 (d, JC-F ¼ 24.9 Hz), 97.1 (d, JC-F
¼ 22.0 Hz).
Representative procedure for the preparation of compound 11a
To a solution of phenol (470.6 mg, 5 mmol) in MeCN (10 mL) was added con. H2SO4
(285 lL, 1.05 equiv.) at room temperature, the mixture was stirred for 5 min. Then,
NBS (934.4 mg, 1.05 equiv.) was added to the mixture. The reaction was monitored
using TLC analysis. The mixture was evaporated to dryness. H2O (15 mL) was added to
the residue and extracted with CH2Cl2 (3 ꢄ 15 mL). The combined organic layers were
washed with water and brine, dried over Na2SO4, and concentrated under reduced pres-
sure to afford the crude product. The major product was isolated using silica gel chro-
1
matography to obtain the product as light yellow liquid; yield: 79% (0.69 g). H NMR
(400 MHz, CDCl3) d: 7.31 (dd, J ¼ 8.8 Hz, 2 H), 6.71 (dd, J ¼ 8.8 Hz, 2 H), 5.67 (brs,
1 H). 13C NMR (100 MHz, CDCl3) d: 154.7, 132.5, 117.2, 112.9.[22]
1
All synthesized compounds were characterized using H NMR and 13C NMR spectra
which can be found in Supporting Information. The products reported herein are
known compounds (except 20a) which spectroscopic data are in agreement with the
public literatures.

R
SYNTHETIC COMMUNICATIONSV
9
Funding
This work was supported by the National Natural Science Foundation of China under Grant
[No. 81502917].
ORCID
Dong-Feng Zhang
Hai-Hong Huang
http://orcid.org/0000-0003-0870-3782
http://orcid.org/0000-0003-1641-1309
References
[1] Bliman, D.; Pettersson, M.; Bood, M.; Grøtli, M. Tetrahedron Lett. 2014, 55, 2929–2931.
DOI: 10.1016/j.tetlet.2014.03.084.
[2] Borikar, S. P.; Daniel, T.; Paul, V. Tetrahedron Lett. 2009, 50, 1007–1009. DOI: 10.1016/j.
tetlet.2008.12.053.
[3] Kavala, V.; Naik, S.; Patel, B. K. J. Org. Chem. 2005, 70, 4267–4271. DOI: 10.1021/
jo050059u.
[4] Gao, S.; Bethel, T. K.; Kakeshpour, T.; Hubbell, G. E.; Jackson, J. E.; Tepe, J. J. J. Org.
Chem. 2018, 83, 9250–9255. DOI: 10.1021/acs.joc.8b01251.
[5] Beck, T. M.; Haller, H.; Streuff, J.; Riedel, S. Synthesis 2014, 46, 740–747. DOI: 10.1055/s-
0033-1340705.
[6] Le, Z.-G.; Chen, Z.-C.; Hu, Y.; Zheng, Q.-G. Synthesis 2004, 17, 2809–2812. DOI: 10.1055/
s-2004-831239.
[7] Zolfigol, M. A.; Chehardoli, G.; Salehzadeh, S.; Adams, H.; Ward, M. D. Tetrahedron Lett.
2007, 48, 7969–7973. DOI: 10.1016/j.tetlet.2007.09.050.
[8] Chaudhuri, S. K.; Roy, S.; Saha, M.; Bhar, S. Synth. Commun. 2007, 37, 579–583. DOI: 10.
1080/00397910601055081.
[9] Heravi, M. M.; Abdolhosseini, N.; Oskooie, H. A. Tetrahedron Lett. 2005, 46, 8959–8963.
DOI: 10.1016/j.tetlet.2005.10.041.
[10] Liu, H.; Jiang, G.; Pan, X.; Wan, X.; Lai, Y.; Ma, D.; Xie, W. Org. Lett. 2014, 16,
1908–1911. DOI: 10.1021/ol5004109.
[11] Ghorbani-Vaghei, R.; Jalili, H. Synthesis 2005, 7, 1099–1102. DOI: 10.1055/s-2005-861851.
[12] Naresh, M.; Kumar, M. A.; Reddy, M. M.; Swamy, P.; Nanubolu, J. B.; Narender, N.
Synthesis 2013, 45, 1497–1504. DOI: 10.1055/s-0033-1338431.
ꢀ
[13] Stropnik, T.; Bombek, S.; Kocevar, M.; Polanc, S. Tetrahedron Lett. 2008, 49, 1729–1733.
DOI: 10.1016/j.tetlet.2008.01.072.
[14] Oberhauser, T. J. Org. Chem. 1997, 62, 4504–4506. DOI: 10.1021/jo9622993.
[15] Konishi, H.; Aritomi, K.; Okano, T.; Kiji, J. Bull. Chem. Soc. Jpn. 1989, 62, 591–593. DOI:
10.1246/bcsj.62.591.
[16] Das, B.; Venkateswarlu, K.; Krishnaiah, M.; Holla, H. Tetrahedron Lett. 2006, 47,
8693–8697. DOI: 10.1016/j.tetlet.2006.10.029.
[17] Paul, V.; Sudalai, A.; Daniel, T.; Srinivasan, K. V. Tetrahedron Lett. 1994, 35, 7055–7056.
DOI: 10.1016/0040-4039(94)88224-X.
[18] Auerbach, J.; Weissman, S. A.; Blacklock, T. J.; Angeles, M. R.; Hoogsteen, K. Tetrahedron
Lett. 1993, 34, 931–934. DOI: 10.1016/S0040-4039(00)77457-0.
[19] Bovonsombat, P.; Ali, R.; Khan, C.; Leykajarakul, J.; Pla-On, K.; Aphimanchindakul, S.;
Pungcharoenpong, N.; Timsuea, N.; Arunrat, A.; Punpongjareorn, N. Tetrahedron 2010,
66, 6928–6935. DOI: 10.1016/j.tet.2010.06.041.
[20] Das, B.; Venkateswarlu, K.; Majhi, A.; Siddaiah, V.; Reddy, K. R. J. Mol. Catal. A: Chem.
2007, 267, 30–33. DOI: 10.1016/j.molcata.2006.11.002.
[21] Baharfar, R.; Alinezhad, H.; Azimi, S.; Salehian, F. J. Chil. Chem. Soc. 2011, 56, 863–865.
DOI: 10.4067/S0717-97072011000400008.

10
Y.-Q. WU ET AL.
[22] Bovonsombat, P.; Teecomegaet, P.; Kulvaranon, P.; Pandey, A.; Chobtumskul, K.;
Tungsirisurp, S.; Sophanpanichkul, P.; Losuwanakul, S.; Soimaneewan, D.;
Kanjanwongpaisan, P.; et al. Tetrahedron 2017, 73, 6564–6572. DOI: 10.1016/j.tet.2017.10.
005.
[23] Pingali, S. R. K.; Madhav, M.; Jursic, B. S. Tetrahedron Lett. 2010, 51, 1383–1385. DOI:
10.1016/j.tetlet.2010.01.002.
[24] Duan, J.; Zhang, L. H.; Dolbier, W. R. Jr. Synlett 1999, 8, 1245–1246. DOI: 10.1055/s-
1999-2818.
[25] Chhattise, P. K.; Ramaswamy, A. V.; Waghmode, S. B. Tetrahedron Lett. 2008, 49,
189–194. DOI: 10.1016/j.tetlet.2007.10.126.
[26] Zou, L.; Li, Y.; Li, P.; Zhou, J.; Wu, Z. Eur. J. Org. Chem. 2018, 2018, 5639–5643. DOI:
10.1002/ejoc.201800994.
[27] Van Zandt, M. C.; Sibley, E. O.; McCann, E. E.; Combs, K. J.; Flam, B.; Sawicki, D. R.;
Sabetta, A.; Carrington, A.; Sredy, J.; Howard, E.; et al. Bioorg. Med. Chem. 2004, 12,
5661–5675. DOI: 10.1016/j.bmc.2004.07.062.
[28] Rajesh, K.; Somasundaram, M.; Saiganesh, R.; Balasubramanian, K. K. J. Org. Chem. 2007,
72, 5867–5869. DOI: 10.1021/jo070477u.
[29] Bovonsombat, P.; Leykajarakul, J.; Khan, C.; Pla-On, K.; Krause, M. M.; Khanthapura,
P.; Ali, R.; Doowa, N. Tetrahedron Lett. 2009, 50, 2664–2667. DOI: 10.1016/j.tetlet.2009.
03.128.