Synthesis of Dihydrospiro Furo[2,3- c ]pyrazoles Promoted by Hypervalent Iodine in Water

Article abstract of DOI:10.1055/s-0037-1609321

A simple, green protocol has been accomplished for the synthesis of dihydrospirofuro[2,3- c ]pyrazoles in aqueous medium involving pyrazolone and aldehydes in a one-pot reaction promoted by bis(acetoxy)iodobenzene (BAIB) at ambient temperature. The protocol presented herein describes a new transformation where two molecules of pyrazolone react with an aldehyde in a Knoevenagel condensation followed by a Michael addition, and the resulting dienol was rearranged to the title compound. High compatibility, easy work-up, and excellent yields are the advantages of this protocol.

Full text of DOI:10.1055/s-0037-1609321

SYNLETT
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
©
Georg Thieme Verlag Stuttgart · New York
2018, 29, A–F
letter
A
en
Synlett
A. Kale et al.
Letter
Synthesis of Dihydrospiro Furo[2,3-c]pyrazoles Promoted by
Hypervalent Iodine in Water
Ashok Kalea,b
Nagaraju Medishettia
Chiranjeevi Bingia
O
H
O
O
I
O
O
Krishnaiah Atmakur*a,b
N
O
N
N
N
a
O
O
Division of Crop Protection Chemicals, CSIR – Indian Institute
of Chemical Technology, Tarnaka, Hyderabad 500 007, India
krishnu@iict.res.in
N
N
b
AcSIR – Indian Institute of Chemical Technology, Tarnaka, Hy-
derabad 500 007, India
Short reaction time
Yields up to 93%
Wide range of substrate scope
19 Examples
N
N
N
N
OH OH
Received: 07.12.2017
Accepted after revision: 29.01.2018
Published online: 26.02.2018
moieties used in photochromic materials owing to their
steric constraints equilibrate with the corresponding non-
spiro derivatives.9
DOI: 10.1055/s-0037-1609321; Art ID: st-2017-b0881-l
Therefore, exploration of efficient methods to prepare
the spirocyclic frame with a pyrazolone core has gained
much attention in recent years and significant effort has
been made towards the synthesis of 4-spiro-5-pyrazolones
Abstract A simple, green protocol has been accomplished for the syn-
thesis of dihydrospirofuro[2,3-c]pyrazoles in aqueous medium involv-
ing pyrazolone and aldehydes in a one-pot reaction promoted by
bis(acetoxy)iodobenzene (BAIB) at ambient temperature. The protocol
presented herein describes a new transformation where two molecules
of pyrazolone react with an aldehyde in a Knoevenagel condensation
followed by a Michael addition, and the resulting dienol was rearranged
to the title compound. High compatibility, easy work-up, and excellent
yields are the advantages of this protocol.
10
11
with a six-membered ring. For example i) Lu and
12
13
Enders and Du and co-workers developed a phosphine-
14
15
16
17
mediated [2+3] annulation, ii) Luan, You, Galias, Lam,
and Yao18 have accomplished the synthesis of spiropyra-
zolone compounds via transition-metal-catalyzed C–H
functionalization, and iii) intramolecular cyclization reac-
Key words benzaldehydes, pyrazolones, bis(acetoxy)iodobenzene
19
tions using 1,3-diones with benzaldehyde. Further, Sahu’s
Polycyclic spiro pyrazolone cores are privileged struc-
tural motifs that are found in natural products, pharmaceu-
ticals, and biologically active molecules (Figure 1), such as
group reported the synthesis of these scaffolds using iodine
in ammonium acetate. Additionally, a pseudo-multicompo-
nent electrochemical synthesis of spiro dihydrofurans was
reported by Yao18 et al. However, all these reports have used
metal catalysts, metal halides, Lewis acids, and the solvents
1
antimicrobial and antitumor, and were proven to be im-
portant synthetic targets in medicinal chemistry owing to
their activity as antibacterial agents or type-4-diesterase
which are derived from nonrenewable resources. Whereas,
2,3
20a,b
inhibitors.
only two reports are available
on the synthesis of 4-
spiro-5-pyrazolones with a five-membered ring.20
N
On the other hand, the synthesis of complex biologically
active scaffolds by one-pot multicomponent reactions
(
O
OH
N
O
O
N
N
MCRs) has attracted considerable attention21 because of
N
N
N
their atom economy, convergence, multiple bond-forming
O
22
efficiency, and other suitable characteristics from the
N
O
O
N
N
N
O
S
green chemistry point of view. In fact, these features make
MCRs well suited for the easy construction of diversified
heterocyclic scaffolds.23 Additionally, the synthetic utility of
such protocols can be improved significantly by using green
solvents like water. Chemical research involving reactions
in aqueous media, including the synthesis of organic
heterocycles have been growing tremendously in the past
N
N
NH
O
Ph
O
O
Figure 1 Spiropyrazolone core containing biologically active scaffolds
Among the spiropyrazolones, dihydrospirofuro pyrazo-
lones are known to possess diverse biological activities, i.e.,
antimalarial, antimicrobial, antimicotic, potential antiox-
4
5
6
24
few decades in order to satisfy environmental concerns.
7
8
idants, and anticancer (prostate). Further, some of these
Therefore, water has great impact as it is environmentally
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–F

B
Synlett
A. Kale et al.
Letter
N
N
N
N
O O
N
H
O
O
N
I) DCE, reflux, 30 min
expected 3
II) I2, BAIB, TEA, r.t., 5 min
1
2
N
N
+
N
N
O
N
O
N
O
4, 76%
5, 21%
Scheme 1
acceptable, nonflammable, cheap, safe, natural, and eco-
nomical. Organic reactions in water have become exciting
and/or essential research endeavors. Indeed, industry pre-
fers to use water as a solvent rather than conventional toxic
thesis of 3,3′-dimethyl-1,1′,4-triphenyl-1,4-dihydrospiro-
{furo[2,3-c]pyrazole-5,4′-pyrazol}-5′(1′H)-ones 4 in a one-
pot reaction involving pyrazolone and benzladehyde fol-
lowed by sequential addition of bis(acetoxy)iodobenzene
(BAIB) in aqueous medium.
25
organic solvents.
With this background, and also as a part of our research
program on the synthesis of bioactive molecules in a one-
Initially,
3-methyl-1-phenyl-1H-pyrazol-5(4H)-one27
(1) was prepared by a reported method and 2 equiv of 1 re-
acted with benzaldehyde (2, 1 equiv) in 1,2-dichloroethane
26
pot reaction, we herein report for the first time the syn-
Table 1 Optimization of Reaction Conditions
Entry
Reagent (equiv)
Solvent
Time (min)
Yield of 4 (%)a
Yield of 5 (%)
1
2
3
4
5
6
7
8
9
I
I
I
I
2
2
2
2
+ BAIB (each 0.5 equiv) + TEA (2 equiv)
(1 equiv)
DCE
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
76
43
56
31
93
93
81
69
46
38
28
–
21
49
38
15
–
DCE
(1 equiv) + TEA (2 equiv)
(0.5 equiv) + TEA (2 equiv)
DCE
DCE
BAIB (1 equiv) + TEA (each 2 equiv)
BAIB (1 equiv)
DCE
DCE
–
BAIB (0.8 equiv)
BAIB (0.6 equiv)
BAIB 0.4 equiv
DCE
–
DCE
–
DCE
–
1
1
1
1
1
1
1
0
BAIB 0.3 equiv
DCE
–
1
BAIB 0.2 equiv
DCE
–
2
NCS/NBS/NIS (1 equiv), TEA (2 equiv)
KI/ CuI
DCE
–
3
DCE
–
–
4
BAIB (1 equiv)
toluene
methanol
ethanol
water
89
91
87
93
–
5
BAIB (1 equiv)
–
6
BAIB (1 equiv)
–
17
BAIB (1 equiv)
–
a
Isolated yields
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–F

C
Synlett
A. Kale et al.
Letter
established by H NMR and 13C NMR spectral data. Attempts
to prepare crystals for X-ray crystallographic analysis were
unsuccessful for these compounds and the derivatives.
Intrigued by the formation of compound 4 and 5, we fo-
cused our attention to establish the optimum parameters
for the exclusive formation of 4. Accordingly, we conducted
1
at reflux temperature followed by addition of I (0.5 equiv),
2
bis(acetoxy)iodobenzene (0.5 equiv) and triethylamine (2
equiv) at room temperature presuming the formation of
28
product 3. However, formation of the two products 4 in
6% and 5 in 21% yield was observed instead of the expect-
7
ed 3 (Scheme 1). The structures of compounds 4 and 5 were
R3
R1
R1
R1
N
H
O
O
N
N
N
I) water, reflux, 30 min
II) BAIB, r.t., 5 min
N
O
O
+
N
R3
R2
R2
R2
1
2
4–4o
O
O
N
N
N
N
N
N
N
N
N
O
O
N
O
O
N
O
O
N
N
N
O
O
N
N
4, 93%
4a, 89%
4b, 87%
4c, 83%
O
F
Cl
Br
O
N
N
N
N
N
N
N
N
N
O
O
N
O
O
N
O
N
N
N
N
O
O
N
O
4g, 84%
4d, 85%
4e, 87%
4f, 86%
CN
CF3
O
N
N
N
N
N
N
N
N
N
N
O
O
N
O
N
O
N
N
N
N
N
O
O
O
O
4h, 85%
4i, 87%
4j, 91%
4k, 85%
F3C
N
CF3
N
N
N
N
N
N
N
N
O
O
N
O
O
N
O
N
N
N
N
O
O
N
O
4l, 88%
4m, 89%
4n, 78%
Cl
4o, 86%
Cl
Scheme 2 Synthesis of 3,3′-dimethyl-1,1′,4-triphenyl-1,4-dihydrospiro{furo[2,3-c]pyrazole-5,4′-pyrazol}-5′(1′H)-one (4). Reagents and conditions: pyr-
azolone (1, 3.6 mmol), benzaldehyde (2, 1.8 mmol) in water (10 mL) at reflux temperature followed by addition of bis(acetoxy)iodobenzene (1.8 mmol)
at room temperature.
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–F

D
Synlett
A. Kale et al.
Letter
studies by employing I , BAIB, and triethylamine in varied
combinations. First, pyrazolone 1 was reacted with benzal-
dehyde (2) in 1,2-dichloroethane at reflux temperature fol-
ne under similar reaction conditions as in Scheme 2, giving
the corresponding spiro products 4aa–ac (Scheme 3). Fur-
thermore, a reaction was also conducted at a 10 g scale and
similar results were obtained, confirming the feasibility of
this protocol even at large scale.
2
lowed by addition of only I (1 equiv) at room temperature
2
resulted in the formation of 4 and 5 in 43% and 49% yield
(Table 1, entry 2). A similar experiment was carried out by
employing I (1 equiv) and triethylamine (2 equiv) and the
yield of 4 increased to 56% and the yield of 5 decreased to
2
R
O
O
H
O
38% (Table 1, entry 3). Then, the next reaction was conduct-
I) water, reflux, 30 min
II) BAIB, r.t., 5 min
O
+
ed with 0.5 equiv of I and triethylamine (2 equiv), which
2
O
O
O
R
resulted in a reduction of the yields of 4 and 5 to 31% and
1a
2a–c
4aa–ac
1
5% along with the recovery of a dimer (A) in 23% yield (Ta-
ble 1, entry 4 and Scheme 4) which was characterized by
29a
the spectral data. Based on the outcome of these studies,
the next reaction was conducted by employing bis(ace-
toxy)iodobenzene (1 equiv) in 1,2-dichloroethane and tri-
ethylamine (2 equiv). Interestingly, this reaction indicated
the formation of 4 in 93% yield (Table 1, entry 5). Likewise,
another experiment was conducted by employing only BAIB
O
O
O
O
O
O
O
O
O
O
O
O
4aa, 91%
4ac, 83%
4
ab, 89%
(
1 equiv). Amazingly, this time also exclusive formation of
Scheme 3 Synthesis of 4′,4′,6,6-tetramethyl-3-phenyl-6,7-dihydro-3H-
spiro[benzofuran-2,1′-cyclohexane]-2′,4,6′(5H)-trione (4aa). Reagents
and conditions: dimedone (1, 3.6 mmol), benzaldehyde (2, 1.8 mmol) in
water (10 mL) at reflux temperature followed by addition of bis(ace-
toxy)iodobenzene (1.8 mmol) at room temperature. Yields refer to
isolated pure products.
product 4 was observed with the same yield of 93% (Table 1,
entry 6). Formation of 4 as a sole product may be attribut-
able to the strong electrophilic nature of hypervalent iodine
making it susceptible to nucleophilic attack by pyrazolone
along with the leaving group ability of the phenyliodinio
group.
Excited over the exclusive formation 4, studies were
conducted to scrutinize the quantity of bis(acetoxy)iodo-
benzene with 0.8 and 0.6 equiv. The outcome of these stud-
ies showed that, with 0.8 equiv, the yields of the product 4
were 81% and of dimer A 12%. With 0.6 equiv, formation of
compound 4 was observed in 69% yield and with 0.4, 0.3,
and 0.2 equiv of bis(acetoxy)iodobenzene the yields of 4 de-
creased even more (Table 1, entries 7–11). Furthermore, ex-
periments were also carried out by replacing bis(ace-
toxy)iodobenzene with NCS, NBS and NIS and we observed
no formation of any product. Since dimer A was recovered
from the reaction with 0.8 equiv of BAIB, we also verified
the exclusive formation of product 4 in 93% yield from di-
mer A which was prepared by reacting pyrazolone 1 and
benzaldehyde (2) in DCE at reflux temperature. Having es-
tablished the amount of BAIB (1 equiv) for exclusive forma-
tion of 4 in excellent yields, solvent effect was also verified
by conducting the reactions in ethanol, methanol, and wa-
ter. Interestingly, the reaction in water as solvent also gave
the product 4 in excellent yields (Scheme 2).
Having prepared the desired product 4 by reacting pyra-
zolone (1) and benzaldehyde (2) and by sequential addition
of bis(acetoxy)iodobenzene in water medium. Scope and
compatibility of this protocol was various electron-rich,
neutral, electron-deficient aromatic, aliphatic, and het-
eroaromatic aldehydes (Scheme 2). Gratifyingly, all the al-
dehydes produced the similar results. Further, this protocol
was also carried out by varying substitution on the pyra-
zolone as well as by replacing the pyrazolone with dimedo-
Based on the previous reports a tentative mechanism
for the formation of compound 4 is proposed in Scheme 4
where two molecules of pyrazolone condense with benzal-
dehyde to form the intermediate dienol A. Next, nucleophil-
ic addition of hypervalent iodine onto C-3 carbon of pyra-
zolone lead to B. Then, attack of the lone-pair electrons of
the hydroxy functionality from the other pyrazolone ring
impelled by the strong leaving-group ability of the
phenyliodinio group facilitates the concurrent elimination
of iodobenzene and acetic acid leading to 4 (Scheme 4).
In conclusion, we have presented a new protocol to syn-
thesize 3,3′-dimethyl-1,1′,4-triphenyl-1,4-dihydrospiro{fu-
ro[2,3-c]pyrazole}-5,4′-pyrazol]-5′(1′H)-ones 4 from pyra-
zolones
with
various
aldehydes
promoted
by
bis(acetoxy)iodobenzene in aqueous medium. Simple reac-
tion conditions, no tedious workup and only a flash chro-
matography purification to obtain pure products are the
advantages of this protocol which can be exploited further.
Funding Information
We are grateful to SERB-DST, New Delhi for financial support (grant
no. EEQ/20l6/000066). Ashok is thankful to UGC, New Delhi for SRF.
)(
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1609321.
S
u
p
p
ortioIgnfmr oaitn
S
u
p
p
o
nrtogI
i
f
rm oaitn
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–F

E
Synlett
A. Kale et al.
Letter
N
• •
O
O
N
N
N
N
N
O
HO
N
N
N
N
+
+
OH
OH
–
H2O
1
2
5
A
AcO
OAc
I
OAc
Ph
I
N
N
N
N
N
N
N
O
N
– OAc
N
N
N
N
OH
O
OH
O
OH
4
B
A
Scheme 4 Reaction mechanism
References
(9) (a) Fisher, E.; Hirshberg, Y. J. Chem. Soc. 1952, 11, 4522.
b) Hirshberg, Y. J. Am. Chem. Soc. 1956, 78, 2304. (c) Berkovic,
G.; Krongauz, V.; Weiss, V. Chem. Rev. 2000, 100, 1741.
d) Witiak, D. T.; Cavestri, R. C.; Newman, H. A.; Baldwin, J. R.;
(
(1) (a) Horton, D. A.; Bourne, G. T.; Smythe, M. L. Chem. Rev. 2003,
03, 893. (b) Chauhan, P.; Mahajan, S.; Enders, D. Chem.
1
(
Commun. 2015, 51, 12890. (c) Kimata, A.; Nakagawa, H.;
Ohyama, R.; Fukuuchi, T.; Ohta, S.; Suzuki, T.; Miyata, N. J. Med.
Chem. 2007, 50, 5053. (d) Kojima, M.; Hosoda, H.; Date, Y.;
Nakazato, M.; Matsuo, H.; Kangawa, K. Nature 1999, 402, 656.
2) (a) Che, M. S.; Barve, P. A.; Suryanarayan, V. J. Heterocycl. Chem.
Sober, C. L.; Feller, D. R. J. Med. Chem. 1978, 21, 1198.
(
10) (a) Companyó, X.; Zea, A.; Alba, A.-N.; Mazzanti, R.; Moyano, A.;
Rios, A. R. Chem. Commun. 2010, 46, 6953. (b) Zea, A.; Alba, A.-
N.; Mazzanti, R.; Moyano, A.; Rios, A. R. Org. Biomol. Chem. 2011,
(
9
, 6519. (c) Liang, J.; Chen, Q.; Liu, L.; Jiang, X.; Wang, R. Org.
2007, 44, 49. (b) Schlemminger, I.; Schmidt, B.; Flockerzi, D.;
Biomol. Chem. 2013, 11, 1441. (d) Zhang, J.-X.; Li, N.-K.; Liu, Z.-
M.; Huang, X.-F.; Geng, Z.-C.; Wang, X.-W. Adv. Synth. Catal.
Tenor, H.; Zitt, C.; Hatzelmann, A.; Marx, D.; Braun, C.; Kuelzer,
R.; Heuser, A.; Kley, H.-P.; Sterk, G. J. WO 2010055083, 2008.
2013, 355, 797. (e) Chauhan, P.; Mahajan, S.; Loh, C. C. J.; Raabe,
(c) Schmidt, B.; Scheufler, C.; Volz, J.; Feth, M. P.; Hummel, R.-P.;
G.; Enders, D. Org. Lett. 2014, 16, 2954.
Hatzelmann, A.; Zitt, C.; Wohlsen, A.; Marx, D.; Kley, H.-P.;
Ockert, D.; Heuser, A.; Christiaans, J. A. M.; Sterk, G. J.; Menge,
W. M. P. B. WO 2008138939, 2010.
(
11) Han, X.; Yao, W.; Wang, T.; Tan, Y. R.; Yan, Z.; Kwiatkowski, J.; Lu,
Y. Angew. Chem. Int. Ed. 2014, 53, 5643.
(
12) Hack, D.; Dürr, A. B.; Deckers, K.; Chauhan, P.; Seling, N.;
Rübenach, L.; Mertens, L.; Raabe, G.; Schoenebeck, F.; Enders, D.
Angew. Chem. Int. Ed. 2016, 55, 1797.
(
3) (a) Kumar, V.; Kaur, K.; Gupta, G. K.; Sharma, A. K. Eur. J. Med.
Chem. 2013, 69, 735. (b) Fustero, S.; Sánchez-Roselló, M.; Barrio,
P.; Simón-Fuentes, A. Chem. Rev. 2011, 111, 6984. (c) Narender
Reddy, E.; Chiranjeevi, B.; Sudha Sravanti, K.; Ramesh, U.;
Jagadeesh Babu, N.; Krishnaiah, A. Bioorg. Med. Chem. Lett. 2015,
(
(
13) Li, J.-H.; Du, D.-M. Adv. Synth. Catal. 2015, 357, 3986.
14) Nan, J.; Zuo, Z.; Luo, L.; Bai, L.; Zheng, H.; Yuan, Y.; Liu, J.; Luan,
X.; Wang, Y. J. Am. Chem. Soc. 2013, 135, 17306.
25, 2918.
(15) (a) Zheng, J.; Wang, S.-B.; Zheng, C.; You, S.-L. J. Am. Chem., Soc.
(
(
(
4) Zou, B.; Yap, P.; Sonntag, L.-S.; Leong, S. Y.; Yeung, B. K. S.; Keller,
2
2
015, 137, 4880. (b) Zheng, C.; Zheng, J.; You, S.-L. ACS Catal.
016, 6, 262.
T. H. Molecules 2012, 17, 10131.
5) (a) Okita, T.; Isobe, M. Tetrahedron 1994, 50, 11143. (b) Kornet,
M. J.; Tnio, A. P. J. Med. Chem. 1976, 19, 892.
(16) Seoane, A.; Casanova, N.; Quiñones, N.; Mascareñas, J. L.; Gulías,
M. J. Am. Chem. Soc. 2014, 136, 7607.
6) (a) The Merck Index; Budavari, S., Ed.; Merck: Rahway, NJ, 1989,
(17) (a) Chidipudi, S. R.; Khan, I.; Lam, H. W. Angew. Chem. Int. Ed.
11th ed. 715. (b) The Merck Index; Merck: Whitehouse Station,
2012, 51, 12115. (b) Dooley, J. D.; Reddy Chidipudi, S.; Lam, H.
NJ, 2006, 14th ed..
W. J. Am. Chem. Soc. 2013, 135, 10829.
(
7) (a) Cooper, D. H.; Wright, D. L. Molecules 2013, 18, 2438.
(
(
(
18) Yao, C.; Wang, Y.; Li, T.; Yu, C.; Li, L.; Wang, C. Tetrahedron 2013,
(b) Jiangsu New Medical College: Dictionary of Chinese Tradi-
6
9, 10593.
19) Sahu, D. P.; Giri, S. K.; Varshney, V.; Kumar, S. Synth. Commun.
009, 39, 3406.
20) (a) Zheng, J.; Li, P.; Gu, M.; Lin, A.; Yao, H. Org. Lett. 2017, 19,
829. (b) Zheng, J.; Wang, S.-B.; Zheng, C.; You, S.-L. Angew.
tional Medicines; Shanghai Science and Technology Publishing
House: Shanghai, 1977, 1967. (c) Wang, Y.; Mo, S.-Y.; Wang, S.-J.;
Li, S.; Yang, Y.-C.; Shi, J.-G. Org. Lett. 2005, 7, 1675.
2
(
8) (a) Huggins, C.; Hodges, C. V. Cancer Res. 1941, 1, 293. (b) Gittes,
R. F. N. Engl. J. Med. 1991, 24, 236. (c) Albert, B. J.;
Sivaramakrishnan, A.; Naka, T.; Czaicki, N. L.; Koide, K. J. Am.
Chem. Soc. 2007, 129, 2648. (d) Balachran, R.; Hopkins, T. D.;
Thomas, C. A.; Wipf, P.; Day, B. W. Chem. Biol. Drug. Des. 2008,
2
Chem. Int. Ed. 2017, 56, 4540.
(21) Samai, S.; Nandi, G. C.; Kumar, R.; Singh, M. S. Tetrahedron Lett.
2009, 50, 7096.
(22) Madhu, C.; Narender Reddy, E.; Chiranjeevi, B.; Jagadeesh Babu,
71, 117.
N.; Krishnaiah, A. Green Chem. 2014, 16, 3237.
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–F

F
Synlett
A. Kale et al.
Letter
(
23) Savaiko, B. F.O. Licht's World Ethanol Biofuels Rep. 2004, 2, 20.
(26) (a) Ashok, K.; Madhu, C.; Chiranjeevi, B.; Jagadeesh Babu, N.;
Krishnaiah, A. Org. Biomol. Chem. 2016, 14, 582. (b) Ashok, K.;
Chiranjeevi, B.; Sarada, S.; Ganesh Kumar, C.; Krishnaiah, A.
Bioorg. Med. Chem. Lett. 2016, 26, 4899.
(27) Sun, B.; Liu, K.; Han, J.; Zhao, L.-Y.; Su, X.; Lin, B.; Zhao, D.-M.;
Cheng, M.-S. Bioorg. Med. Chem. 2015, 23, 6763.
(
24) (a) Polshettiwar, V.; Varma, R. S. Aqueous Microwave Assisted
Chemistry; RSC Green Chemistry 2010, Chap. 1. (b) Organic Syn-
thesis in Water; Grieco, P. A., Ed.; Blackie Academic and Profes-
sional: London, 1998. (c) Butler, R. N.; Coyne, A. G. Chem. Rev.
2010, 110, 6302. (d) Organic Reactions in Water; Lindstrom, U.
M., Ed.; Blackwell Publishing: Oxford, 2007. (e) McGonagle, F. I.;
Sneddon, H. F.; Jamieson, C.; Watson, A. J. B. ACS Sustainable
Chem. Eng. 2014, 2, 523.
(28) Prasun, M.; Das, A. R. J. Org. Chem. 2017, 82, 2794.
(29) (a) Monireh, Z.; Batool, A. RSC Adv. 2015, 5, 87769. (b) Li, X.-L.;
Wang, Y.-M.; Tian, B.; Matsuura, T.; Meng, J.-B. J. Heterocycl.
Chem. 1998, 35, 129.
(
25) (a) Blackmond, D. G.; Armstrong, A.; Coombe, V.; Wells, A.
Angew. Chem. Int. Ed. 2007, 46, 3798. (b) Li, C. J. Chem. Rev. 2005,
105, 3095. (c) Lindstrom, U. M. Chem. Rev. 2002, 102, 2751.
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–F