Synthesis of 2H-furo[2,3-c]pyrazole ring systems through silver(I) ion-mediated ring-closure reaction

Article abstract of DOI:10.3762/bjoc.15.62

Fused pyrazole ring systems are common structural motifs of numerous pharmaceutically important compounds. Nevertheless, access to derivatives of the aromatic 2H-furo[2,3-c]pyrazole ring system is still quite limited, and their chemistry and functional properties remain largely underexplored. The current study investigates routes to construct this system from easily accessible starting materials using metal-catalyzed reactions. A simple and efficient procedure to access the 2H-furo[2,3-c]pyrazole ring system was developed by employing the silver(I) ion-mediated ring-closure reaction of 4-alkynyl-3-hydroxy-1-phenyl-1H-pyra-zoles as a key step. The required intermediate hydroxyalkynyl substrates for this reaction were prepared by a Pd-catalyzed coupling of 4-iodo-1-phenyl-1H-pyrazol-3-ol with ethyne derivatives. The structures of the obtained target compounds were unequivocally confirmed by detailed ¹H, ¹³C and ¹⁵N NMR spectroscopic experiments, HRMS and a single-crystal X-ray diffraction analyses. This silver(I)-mediated 5-endo-dig cyclization of readily available 4-alkynyl-3-hydroxy-1H-pyrazoles can be used as an efficient method to access many novel 2,5-disubstituted 2H-furo[2,3-c]pyrazoles.

Full text of DOI:10.3762/bjoc.15.62

Synthesis of 2H-furo[2,3-c]pyrazole ring systems through
silver(I) ion-mediated ring-closure reaction
Vaida Milišiūnaitė1,2, Rūta Paulavičiūtė1, Eglė Arbačiauskienė1, Vytas Martynaitis1,
Wolfgang Holzer3 and Algirdas Šačkus*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 679–684.
1Department of Organic Chemistry, Kaunas University of Technology,
Radvilėnų pl. 19, Kaunas LT-50254, Lithuania, 2Institute of Synthetic
Chemistry, Kaunas University of Technology, K. Baršausko g. 59,
Kaunas LT-51423, Lithuania and 3Department of Pharmaceutical
Chemistry, University of Vienna, Althanstrasse 14, Vienna 1090,
Austria
doi:10.3762/bjoc.15.62
Received: 08 January 2019
Accepted: 04 March 2019
Published: 14 March 2019
Associate Editor: B. Stoltz
Email:
Algirdas Šačkus* - algirdas.sackus@ktu.lt
© 2019 Milišiūnaitė et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
5-endo-dig cyclization; 2H-furo[2,3-c]pyrazole; pyrazole; silver(I)
catalyst; Sonogashira coupling
Abstract
Fused pyrazole ring systems are common structural motifs of numerous pharmaceutically important compounds. Nevertheless,
access to derivatives of the aromatic 2H-furo[2,3-c]pyrazole ring system is still quite limited, and their chemistry and functional
properties remain largely underexplored. The current study investigates routes to construct this system from easily accessible
starting materials using metal-catalyzed reactions. A simple and efficient procedure to access the 2H-furo[2,3-c]pyrazole ring
system was developed by employing the silver(I) ion-mediated ring-closure reaction of 4-alkynyl-3-hydroxy-1-phenyl-1H-pyra-
zoles as a key step. The required intermediate hydroxyalkynyl substrates for this reaction were prepared by a Pd-catalyzed coupling
of 4-iodo-1-phenyl-1H-pyrazol-3-ol with ethyne derivatives. The structures of the obtained target compounds were unequivocally
confirmed by detailed 1H, 13C and 15N NMR spectroscopic experiments, HRMS and a single-crystal X-ray diffraction analyses.
This silver(I)-mediated 5-endo-dig cyclization of readily available 4-alkynyl-3-hydroxy-1H-pyrazoles can be used as an efficient
method to access many novel 2,5-disubstituted 2H-furo[2,3-c]pyrazoles.
Introduction
Heterocyclic ring systems possessing a pyrazole ring fused to often represent the main structural motifs of anticancer [2-4],
an oxygen-containing six-membered heterocycle are present in anti-inflammatory [5], and antidiabetic agents [6]. The numer-
a wide variety of biologically active compounds. For example, ous known methods for the preparation of these compounds are
the 1,4- and 2,4-dihydropyrano[2,3-c]pyrazole ring systems [1] generally based on multicomponent reactions of an aromatic
679

Beilstein J. Org. Chem. 2019, 15, 679–684.
aldehyde, a β-keto ester, a hydrazine and malononitrile [7]. In a scaffold to obtain the 2H-pyrazolo[4,3-c]pyridine [27,28], pyra-
similar reaction using a pyridinium ylide instead of malononi- zolo[4,3-f][1,2,3]triazolo[5,1-c][1,4]oxazepine [29] and pyra-
trile, the dihydro-1H-furo[2,3-c]pyrazole system, consisting of zolo[4’,3’:3,4]pyrido[1,2-a]benzimidazole [30] ring systems as
an aromatic pyrazole ring fused with a five-membered 2,3- well as to prepare building blocks for the construction of opto-
dihydrofuran ring, was formed [8]. Derivatives of the latter electronic materials and fluorescent organic nanoparticles [31-
system are known for their antimicrobial [9] and antiprolifera- 34].
tive activities [10]. Nevertheless, methods to access
2H-furo[2,3-c]pyrazoles, where both the fused pyrazole and
furan moieties are aromatic, are still quite limited, and their
chemistry and functional properties remain largely underex-
plored. Huang et al. have reported the synthesis and the evalua-
tion of antiplatelet and anti-allergic activities of 2,3,4-trisubsti-
tuted 2H-furo[2,3-c]pyrazole-5-carboxylic acids and related
carboxamides [11,12]. The aforementioned carboxylic acids
have been prepared by bromination of the corresponding
pyrano[2,3-c]pyrazol-6(1H)-one derivatives followed by
heating of the obtained 5-bromo derivatives in the presence of
sodium alkoxide [11]. Notably, the 2H-furo[2,3-c]pyrazole ring
system is structurally similar to the benzo[b]furan system,
which is a privileged motif in natural products and biologically
active compounds [13,14]. As a result, numerous strategies
have been developed for the construction of benzo[b]furan and
Scheme 1: Preparation of hydroxyalkynyl substrates from 1-phenyl-
1H-pyrazol-3-ol (1).
its derivatives [15-18]. For example, Damera et al. reported an
efficient synthesis of 2-substituted benzo[b]furans with good
yields by the base-promoted cyclization of easily accessible
2-alkynylphenols [19]. Recently, 2-arylbenzo[b]furans were The iodination of 1 with iodine in the presence of KOH in DMF
conveniently synthesized by the one-pot tandem Hiyama afforded 4-iodo-1H-pyrazol-3-ol 2 [26]. We have previously
alkynylation/cyclization reaction between 2-iodophenol and shown that the Sonogashira-type coupling of the aforemen-
(triethoxysilyl)alkynes [20]. In recent years silver and gold salts tioned iodinated compound with phenylacetylene under stan-
have found application as versatile and mild catalysts to access dard reaction conditions (Pd(PPh3)2Cl2, CuI, and TEA) gives
the benzo[b]furan ring system through intramolecular cycliza- 4-(phenylethynyl)-substituted pyrazolol 3a [26]. When various
tion of 2-alkynylphenol substrates [21], including preparation of het(aryl)- and alkylacetylenes were used in the coupling with 2
2-benzofuranmethanamines [22] and 4-indolylbenzo[b]furans under the same reaction conditions, compounds 3b–j were ob-
[23] in the presence of AgNO3 and AgOTf, respectively, and tained in 64–85% yield (Scheme 1).
2-phenylbenzo[b]furans, where AuCl3 or a mixture thereof with
AgOTf has been used to promote the appropriate cyclization Having prepared the series of hydroxyethynyl substrates 3a–j,
[24].
the optimal conditions for the subsequent cyclization reaction
were next investigated using 3a as the model compound
In the present work, we describe a method for the construction (Table 1). First, we attempted the desired cyclization of 3a to
of the 2H-furo[2,3-c]pyrazole ring system by a Sonogashira- 2H-furo[2,3-c]pyrazole 4a using the synthetic protocol involv-
type alkynylation of 4-iodopyrazol-3-ol and subsequent intra- ing Cs2CO3 in dry DMF at 60 °C, which has been previously
molecular 5-endo-dig cyclization of the obtained hydroxy- successfully employed for the preparation of benzo[b]furans by
alkynyl substrate mediated by a Ag(I) catalyst.
the cyclization of o-alkynylphenols [19]. Unfortunately, no ad-
dition of the hydroxy group across the carbon–carbon triple
bond was observed even after heating the reaction mixture
Results and Discussion
The synthetic strategy designed to construct the 2H-furo[2,3- under analogous reaction conditions for 24 hours (Table 1, entry
c]pyrazole ring system employs a hydroxyethynyl substrate that 1).
contains adjacent ethyne and hydroxy groups on the pyrazole
core (Scheme 1). As a starting material, we used 1-phenyl-1H- The formation through a 5-endo-dig cyclization of some amount
pyrazol-3-ol (1), which is readily accessible from the oxidation of compound 4a was detected by LC–MS measurements only
of 1-phenyl-3-pyrazolidinone [25,26]. Recently, we used this when the reaction mixture contained Cs2CO3 as a base and was
680

Beilstein J. Org. Chem. 2019, 15, 679–684.
Table 1: Cyclization of compound 3a.
Entry
Base
Catalyst
Temp (°C)
t (h)
Yield 4a
1
2
3
4
5
6
7
8
Cs2CO3
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
–
–
60
24
96
96
14
14
96
96
96
no product
traces
47%
–
120
120
120
120
80
–
[(Ph3P)Au]Cl (10 mol %)
AgOTf (10 mol %)
AgOTf (10 mol %)
AgOTf (1 equiv)
AgOTf (10 mol %)
69%
92%
15%
120
120
traces
69%
Cs2CO3
heated at 120 °C for 4 days (Table 1, entry 2). Surprisingly, With the optimized conditions for the 5-endo-dig cyclization
when K2CO3 was used instead of Cs2CO3, the desired reaction identified, the scope of this transformation for the prep-
2-phenyl-2H-furo[2,3-c]pyrazole was obtained in a significant- aration of several 2,5-disubstituted 2H-furo[2,3-c]pyrazoles was
ly higher yield (47%, Table 1, entry 3).
explored (Scheme 2). For substrates 3b–j, the reactions were
complete after 14 hours at 120 °C, and products 4b–j were
Some of the most effective catalysts for the electrophilic activa- generated in fair to excellent yields.
tion of alkynes under homogeneous conditions are gold(I) [35]
and silver(I) [36] salts or complexes, and a broad range of
versatile synthetic methods has been developed for the con-
struction of carbon–heteroatom bonds using these types of cata-
lysts. For example, the gold(I) catalyst [(Ph3PAu)3O]BF4 was
applied in the regioselective intramolecular cyclization of
alkynols to construct bicyclic ethers [37], while the silver(I)
catalyst AgOTf efficiently catalyzed the intramolecular cycliza-
tion of phenoxyethynyl diols into 2,3-unsaturated lactones [38].
Scheme 2: Cyclization of hydroxyalkynyl substrates to 2,5-disubsti-
tuted 2H-furo[2,3-c]pyrazoles.
In our case, the addition of 10 mol % chloro(triphenylphos-
phine)gold(I) improved the yield of product 4a to 69% (Table 1,
entry 4). However the best results were obtained when AgOTf
was used as a catalyst. In this case, the target product 4a was The structural assignments of 4a–j were based on multinuclear
obtained in an excellent 92% yield (Table 1, entry 5).
NMR and IR spectroscopy as well as high-resolution mass
spectrometry (HRMS) data and a single-crystal X-ray diffrac-
It is important to note that the reaction temperature had a signif- tion analysis. The 1H NMR spectra of compounds 4a–j revealed
icant effect on the yield of the product. When the temperature the characteristic H-3 proton singlet in the δ 7.60–7.77 ppm
was lowered to 80 °C, the yield of the product did not exceeded region as well as a H-4 proton singlet in the δ 6.06–6.74 ppm
15% (Table 1, entry 6). The presence of a base also plays a region. The 13C NMR spectra of 4a–j exhibited signals for
crucial role in the cyclization described herein, and the transfor- the five carbon atoms of the 2H-furo[2,3-c]pyrazole ring system
mation of 3a to 4a did not occur in the presence of only the in the regions of δ 115.0–116.3 for C-3, δ 113.1–113.9 for
catalyst and without base (Table 1, entry 7). Finally, the reac- C-3a, δ 94.5–96.8 for C-4, δ 156.1–163.8 for C-5 and
tion optimization experiments showed that the Cs2CO3/AgOTf δ 168.4–169.0 ppm for C-6a. The 15N spectra of 4b–e exhib-
system does not offer any advantages for this cyclization com- ited signals for two nitrogen atoms in the regions of δ −169.4 to
pared to the K2CO3/AgOTf system (Table 1, entry 8).
−172.6 for N-2 and δ −127.5 to −128.0 ppm for N-1.
681

Beilstein J. Org. Chem. 2019, 15, 679–684.
The structure of 4d was investigated additionally by single identical, they differ in bond lengths and dihedral angles Φ
crystal X-ray analysis (Figure 1) [39].
from each other. The corresponding bond lengths of
2H-furo[2,3-c]pyrazole systems in 4d(A) and 4d(B) are given
The asymmetric unit of the crystal consists of two molecules in Table 2.
4d(A) and 4d(B) held together by the weak CH···N type hydro-
gen bonds C19–H···N51 (C···N = 3.470(11) Å, H···N = 2.60 Å, The lengths of the C–F bonds for molecules 4d(A) and 4d(B)
C–H···N = 151°) and C69–H···N1 (C···N = 3.355(11) Å, H···N = are 1.356(10) Å and 1.392(11) Å, respectively. All ring atoms
2.51 Å, C–H···N = 144°), respectively, whose length and angles of the 2H-furo[2,3-c]pyrazole moiety lie in almost one plane,
are typical for crystals of aza-heterocycles [40]. Although at but the phenyl substituents are slightly turned in relation to this
first glance the molecules 4d(A) and 4d(B) appear to be plane (Table 3).
Figure 1: a) ORTEP diagram of the asymmetric unit consisting of two independent molecules 4d(A) and 4d(B); b) view (normal to (100)) of molecu-
lar packing in the crystal.
Table 2: Selected bond lengths of the 2H-furo[2,3-c]pyrazole core.
Molecule 4d(A)
d, Å
Molecule 4d(B)
d, Å
N1–N2
N2–C3
C3–C8
C8–C4
C4–C5
C5–O6
O6–C7
C7–N1
C7–C8
1.391(7)
1.378(8)
1.378(10)
1.457(11)
1.362(8)
1.401(8)
1.354(9)
1.317(11)
1.396(8)
N51–N52
N52–C53
C53–C58
C58–C54
C54–C55
C55–O56
O56–C57
C57–N51
C57–C58
1.407(8)
1.358(9)
1.371(11)
1.450(11)
1.337(9)
1.424(8)
1.365(10)
1.298(12)
1.405(9)
Table 3: Dihedral angles Φ formed by the phenyls and the 2H-furo[2,3-c]pyrazole core in 4d(A) and 4d(B).
Substitutent
phenyl
Molecule 4d(A)
Φ, deg
Molecule 4d(B)
Φ, deg
C10–C9–N2–N1
C14–C9–N2–C3
C16–C15–C5–C4
C20–C15–C5–O6
7.64
10.09
9.71
6.51
C60–C59–N52–N51
C64–C59–N52–C53
C66–C65–C55–C54
C70–C65–C55–O56
6.97
10.68
5.06
8.89
4-fluorophenyl
682

Beilstein J. Org. Chem. 2019, 15, 679–684.
8. Tangeti, V. S.; Siva Prasad, G. V.; Panda, J.; Varma, K. R.
The molecules in the crystal are located in columns made up of
asymmetric units held by hydrogen bonds (Figure 1b).
Synth. Commun. 2016, 46, 878–884.
doi:10.1080/00397911.2016.1174781
9. Bondock, S.; Khalifa, W.; Fadda, A. A. Eur. J. Med. Chem. 2011, 46,
2555–2561. doi:10.1016/j.ejmech.2011.03.045
Conclusion
In conclusion, we have demonstrated a new, three-step synthe-
tic route to 2H-furo[2,3-c]pyrazoles starting from commercially
available 1-phenylpyrazol-3-ol. Iodination of the latter com-
pound with iodine in DMF smoothly afforded 1-phenyl-4-
iodopyrazol-3-ol, which can undergo a Pd-catalyzed coupling
with terminal alkynes to give the corresponding 4-alkynyl-3-
hydroxy-1-phenyl-1H-pyrazoles. The desired 5-endo-dig cycli-
zation leading to the formation of the 2H-furo[2,3-c]pyrazole
ring system is easily achieved by heating the aforementioned
hydroxyalkynyl substrates with a base in DMF in the presence
of a silver(I) catalyst.
10.Tangeti, V. S.; Vasundhara, D.; Satyanarayana, K. V. V. V.;
Pavan Kumar, K. S. Asian J. Chem. 2017, 29, 1525–1532.
doi:10.14233/ajchem.2017.20550
11.Kuo, S.-C.; Huang, L.-J.; Teng, C.-M. 2,3,4,5-Substituted
furo[2,3-c]pyrazole derivatives. U.S. Pat. Appl. US5240947A, Aug 31,
1993.
12.Huang, L.-J.; Kuo, S.-C.; Wang, J.-P.; Ishii, K.; Nakamura, H.
Chem. Pharm. Bull. 1994, 42, 2036–2041. doi:10.1248/cpb.42.2036
13.Lütjens, H.; Scammells, P. J. Tetrahedron Lett. 1998, 39, 6581–6584.
doi:10.1016/s0040-4039(98)01371-9
14.Heravi, M. M.; Zadsirjan, V.; Hamidi, H.; Tabar Amiri, P. H. RSC Adv.
2017, 7, 24470–24521. doi:10.1039/c7ra03551a
15.Yudin, A. K., Ed. Catalyzed Carbon-Heteroatom Bond Formation;
Wiley-VCH Verlag GmbH: Weinheim, Germany, 2011.
doi:10.1002/9783527633388
Supporting Information
16.Heravi, M. M.; Zadsirjan, V. Curr. Org. Synth. 2016, 13, 780–833.
doi:10.2174/1570179413666151218202156
Supporting Information File 1
17.Godoi, B.; Schumacher, R. F.; Zeni, G. Chem. Rev. 2011, 111,
2937–2980. doi:10.1021/cr100214d
Experimental details and characterization data.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-62-S1.pdf]
18.Wu, X.-F.; Neumann, H.; Beller, M. Chem. Rev. 2013, 113, 1–35.
doi:10.1021/cr300100s
19.Damera, K.; Ke, B.; Wang, K.; Dai, C.; Wang, L.; Wang, B. RSC Adv.
2012, 2, 9403–9405. doi:10.1039/c2ra21302h
20.Ao, J.; Liu, Y.; Jia, S.; Xue, L.; Li, D.; Tan, Y.; Qin, W.; Yan, H.
Tetrahedron 2018, 74, 433–440. doi:10.1016/j.tet.2017.11.049
21.Blanc, A.; Bénéteau, V.; Weibel, J.-M.; Pale, P. Org. Biomol. Chem.
2016, 14, 9184–9205. doi:10.1039/c6ob01468b
ORCID® iDs
Vaida Milišiūnaitė - https://orcid.org/0000-0002-5338-4621
Eglė Arbačiauskienė - https://orcid.org/0000-0003-1365-9990
Vytas Martynaitis - https://orcid.org/0000-0003-1147-6584
Algirdas Šačkus - https://orcid.org/0000-0002-1514-4934
22.Wongsa, N.; Sommart, U.; Ritthiwigrom, T.; Yazici, A.;
Kanokmedhakul, S.; Kanokmedhakul, K.; Willis, A. C.; Pyne, S. G.
J. Org. Chem. 2013, 78, 1138–1148. doi:10.1021/jo302554v
23.Ye, Y.; Fan, R. Chem. Commun. 2011, 47, 5626–5628.
doi:10.1039/c1cc10137d
References
1. Das, D.; Banerjee, R.; Mitra, A. J. Chem. Pharm. Res. 2014, 6,
24.Zhang, Y.; Xin, Z.-J.; Xue, J.-J.; Li, Y. Chin. J. Chem. 2008, 26,
1461–1464. doi:10.1002/cjoc.200890265
108–116.
2. Salama, S. K.; Mohamed, M. F.; Darweesh, A. F.; Elwahy, A. H. M.;
Abdelhamid, I. A. Bioorg. Chem. 2017, 71, 19–29.
doi:10.1016/j.bioorg.2017.01.009
25.König, H.; Götz, N.; Klein, U.; Eller, K. Process for producing
N-substituted 3-hydroxypyrazoles. WO Patent WO1997003969, Feb 6,
1997.
3. Yan, C.; Theodorescu, D.; Miller, B.; Kumar, A.; Kumar, V.; Ross, D.;
Wempe, M. F. Bioorg. Med. Chem. Lett. 2016, 26, 5815–5818.
doi:10.1016/j.bmcl.2016.10.021
Chem. Abstr. 1997, 126, 199566.
26.Arbačiauskienė, E.; Vilkauskaitė, G.; Eller, G. A.; Holzer, W.;
Šačkus, A. Tetrahedron 2009, 65, 7817–7824.
4. Kashtoh, H.; Muhammad, M. T.; Khan, J. J. A.; Rasheed, S.; Khan, A.;
Perveen, S.; Javaid, K.; Atia-tul-Wahab; Khan, K. M.; Choudhary, M. I.
Bioorg. Chem. 2016, 65, 61–72. doi:10.1016/j.bioorg.2016.01.008
5. Zaki, M. E. A.; Soliman, H. A.; Hiekal, O. A.; Rashad, A. E.
Z. Naturforsch., C: J. Biosci. 2006, 61, 1–5.
doi:10.1016/j.tet.2009.07.017
27.Milišiūnaitė, V.; Arbačiauskienė, E.; Řezníčková, E.; Jorda, R.;
Malínková, V.; Žukauskaitė, A.; Holzer, W.; Šačkus, A.; Kryštof, V.
Eur. J. Med. Chem. 2018, 150, 908–919.
doi:10.1016/j.ejmech.2018.03.037
doi:10.1515/znc-2006-1-201
28.Arbačiauskienė, E.; Martynaitis, V.; Krikštolaitytė, S.; Holzer, W.;
Šačkus, A. ARKIVOC 2011, No. xi, 1–21.
6. Qvortrup, K.; Jensen, J. F.; Sørensen, M. S.; Kouskoumvekaki, I.;
Petersen, R. K.; Taboureau, O.; Kristiansen, K.; Nielsen, T. E.
PLoS One 2017, 12, e0162642. doi:10.1371/journal.pone.0162642
7. Myrboh, B.; Mecadon, H.; Rohman, M. R.; Rajbangshi, M.;
Kharkongor, I.; Laloo, B. M.; Kharbangar, I.; Kshiar, B.
Org. Prep. Proced. Int. 2013, 45, 253–303.
doi:10.3998/ark.5550190.0012.b01
29.Arbačiauskienė, E.; Laukaitytė, V.; Holzer, W.; Šačkus, A.
Eur. J. Org. Chem. 2015, 5663–5670. doi:10.1002/ejoc.201500541
30.Milišiūnaitė, V.; Arbačiauskienė, E.; Bieliauskas, A.; Vilkauskaitė, G.;
Šačkus, A.; Holzer, W. Tetrahedron 2015, 71, 3385–3395.
doi:10.1016/j.tet.2015.03.092
doi:10.1080/00304948.2013.798566
683

Beilstein J. Org. Chem. 2019, 15, 679–684.
31.Arbačiauskienė, E.; Kazlauskas, K.; Miasojedovas, A.; Juršėnas, S.;
Jankauskas, V.; Holzer, W.; Getautis, V.; Šačkus, A. Synth. Met. 2010,
160, 490–498. doi:10.1016/j.synthmet.2009.11.038
32.Arbačiauskienė, E.; Kazlauskas, K.; Miasojedovas, A.; Juršėnas, S.;
Jankauskas, V.; Holzer, W.; Getautis, V.; Šačkus, A. Dyes Pigm. 2010,
85, 79–85. doi:10.1016/j.dyepig.2009.10.007
33.Kazlauskas, K.; Kreiza, G.; Arbačiauskienė, E.; Bieliauskas, A.;
Getautis, V.; Šačkus, A.; Juršėnas, S. J. Phys. Chem. C 2014, 118,
25261–25271. doi:10.1021/jp507707f
34.Kazlauskas, K.; Miasojedovas, A.; Dobrovolskas, D.;
Arbačiauskienė, E.; Getautis, V.; Šačkus, A.; Juršėnas, S.
J. Nanopart. Res. 2012, 14, No. 877. doi:10.1007/s11051-012-0877-6
35.Dorel, R.; Echavarren, A. M. Chem. Rev. 2015, 115, 9028–9072.
doi:10.1021/cr500691k
36.Fang, G.; Bi, X. Chem. Soc. Rev. 2015, 44, 8124–8173.
doi:10.1039/c5cs00027k
37.Kubota, M.; Saito, T.; Miyamoto, K.; Hirano, K.; Wang, C.;
Uchiyama, M. Chem. Pharm. Bull. 2016, 64, 845–855.
doi:10.1248/cpb.c16-00204
38.Egi, M.; Ota, Y.; Nishimura, Y.; Shimizu, K.; Azechi, K.; Akai, S.
Org. Lett. 2013, 15, 4150–4153. doi:10.1021/ol401824v
39.The CCDC deposition number of
5-(4-fluorophenyl)-2-phenyl-2H-furo[2,3-c]pyrazole (4d) is 1862867;
formula C17H11FN2O; unit cell parameters: a, 5.4969(3) b, 16.2128(7)
c, 28.8194(16), space group Pbn21.
40.Shivakumar, K.; Vidyasagar, A.; Naidu, A.; Gonnade, R. G.;
Sureshan, K. M. CrystEngComm 2012, 14, 519–524.
doi:10.1039/c1ce05997a
License and Terms
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/4.0). Please note
that the reuse, redistribution and reproduction in particular
requires that the authors and source are credited.
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(https://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.15.62
684