Clean one-pot synthesis of 1,2,4-oxadiazoles under solvent-free conditions using microwave irradiation and potassium fluoride as catalyst and solid support

Article abstract of DOI:10.1016/j.tet.2009.11.063

Potassium fluoride was found to be an efficient catalyst and solid support for the synthesis of 3,5-disubstituted-1,2,4-oxadiazoles. In this work, a one-pot method for the synthesis of these compounds from the reaction of nitriles with hydroxylamine hydro

Full text of DOI:10.1016/j.tet.2009.11.063

Tetrahedron 66 (2010) 494–497
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Clean one-pot synthesis of 1,2,4-oxadiazoles under solvent-free conditions using
microwave irradiation and potassium fluoride as catalyst and solid support
,
a,
Shahnaz Rostamizadeh a , Hamid Reza Ghaieni b, Reza Aryan a, Ali Mohammad Amani a
*
a Department of Chemistry, K.N.Toosi University of Technology, PO Box 15875-4416, Tehran, Iran
b Faculty of Material and Chemical Engineering, Malek Ashtar University of Technology, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Potassium fluoride was found to be an efficient catalyst and solid support for the synthesis of 3,5-di-
substituted-1,2,4-oxadiazoles. In this work, a one-pot method for the synthesis of these compounds from
the reaction of nitriles with hydroxylamine hydrochloride and acyl chloride in the presence of potassium
fluoride under solvent-free conditions using microwave irradiation has been developed. The advantages
of using potassium fluoride as a solid support in comparison to conventional solid supports are simple
operation and convenient separation of the products.
Received 16 June 2009
Received in revised form
24 October 2009
Accepted 12 November 2009
Available online 17 November 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
1,2,4-Oxadiazole
Amidoxime
Nitrile
Potassium fluoride
Solvent free
1. Introduction
Recently, the use of inorganic solid supports as catalysts has been
developed for solvent free reactions, which has led to higher selec-
1,2,4-oxadiazole derivatives are important heterocyclic com-
pounds with a wide range of biological activity. This ring system has
been used as a urea bioisostere in b3 adrenergic receptor agonists.1
Furthermore, derivatives containing the 1,2,4-oxadiazole ring
systems have been employed as antitumor, anesthetic, and anti-
schistosomal agents, coronary artery dilators, muscle relaxants,
monoamine oxidase as well as aldose reductase inhibitors.2–6 In
addition, they have also been used as dipeptide mimics.7
The drive to environmentally sustainable or so-called ‘green
chemistry’ has provided one of the greatest challenges for the
chemists of today. The so-called green technologies are looking for
alternative ways to reduce drastic prerequisites for reactions. Among
the proposed solutions, solvent-free conditions hold a leading posi-
tion. So, it is now often claimed that ‘the best solvent is no solvent’.8
tivity, milder conditions, and an easier experimental procedure.
Such reagents not only simplify purification processes but also help
to prevent release of reaction residues into the environment.9
In general 1,2,4-oxadiazoles are prepared in two steps by the
O-acylation of an amidoxime with an activated carboxylic acid
derivative, typically an active acyl chloride, followed by cyclization
and cyclodehydration (Scheme 1).10–17
Recently, new methods have also been reported in the litera-
ture for the synthesis of these useful heterocycles. Among them,
reactions that require heating for long periods of time, the use of
microwave irradiation in the presence of solvent or under solvent-
free conditions have been reported as well.18–22 However, some of
these methods suffer from harsh reaction conditions which
exclude additional functionality on the oxadiazole. Many other
O
R2
OH
R2COX
N
O
N
NH2OH
N
N
O
heat
OH
R2
R1
C N
O
R1
or catalyst
reflux, EtOH
R1
NH2
R1
R2
R1
NH2
N
N
H
Scheme 1. Three-step sequence for the transformation of nitriles to 1,2,4-oxadiazoles.
methods are not one-pot procedures and require amidoximes
as starting materials.
* Corresponding author. Tel./fax: þ9821 22853650.
E-mail address: shrostamizadeh@yahoo.com (S. Rostamizadeh).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.11.063

S. Rostamizadeh et al. / Tetrahedron 66 (2010) 494–497
495
Adib and co-workers have reported a one-pot, three-component
reaction between nitriles, hydroxylamine, and aldehydes under
microwave irradiation and solvent-free conditions, which shows
a huge improvement over previous methods, but aerial oxidation
and higher temperature (160 ꢀC) are required for the completion of
the reaction.23 Also, we knew that for the conversion of nitriles to
amidoximes with hydroxylamine hydrochloride, basic media is
required because in acidic media the nitrile would undergo
hydrolysis to the corresponding amide (Scheme 2).24 So, for a one-
pot synthesis of 1,2,4-oxadiazoles using nitrile as starting material
we need basic media.
under solvent-free conditions is a cleaner methodology. Thus
by combining these two facts, the synthesis of 3,5-disubstituted
1,2,4-oxadiazoles was performed under optimized condition
(Scheme 3).
R2COCl
OH
N
O
KF, NH2OH.HCl
N
R1
C
N
R1
MW, 100 °C, 5 min
R2
450W, 130 °C, 10 min
N
R1
NH2
Scheme 3. Optimized condition for the synthesis of 3,5-disubstituted 1,2,4-
oxadiazoles.
The reactions were carried out by first mixing the nitrile and
hydroxylamine hydrochloride in the presence of potassium fluoride
under microwave irradiation. After a few minutes and nearly
complete conversion as indicated by TLC monitoring, to the cor-
responding amidoxime intermediate, acyl halide was added and
the mixture again irradiated for the appropriate time. After com-
pletion of the reaction, water was added which dissolved the by-
products and the desired 1,2,4-oxadiazole was easily separated by
simple filtration.
Inordertofind outwhatwouldbe obtainedusingother bases, the
reaction of benzonitrile with p-chlorobenzoyl chloride and hy-
droxylamine hydrochloride in the presence of sodium carbonate or
sodium bicarbonate was performed under similar conditions, which
gave only the corresponding O-acylamidoxime as the major product
and yield for the required product was very low (Scheme 4). The
same reactionwas also performed at higher temperature and power,
which again gave the O-acylamidoxime.
O
OH
N
NH2OH.HCl
basic media
NH2OH.HCl
N
acidic media
R
R
C
R
NH2
NH2
Scheme 2. Conversion of nitriles to amide and amidoxime in acidic and basic media.
Bases such as NaH or NaOEt at room temperature or pyridine
with heating25–29 and tetrabutylammonium fluoride30,31 in
dichloromethane have been reported for the cyclization and
cyclodehydration of formed intermediates. Therefore, it seems that
strong bases can promote this reaction. Consequently, we chose
potassium fluoride as a strong base as well as a reagent for the
sequesterisation of by-products. An aqueous potassium fluoride
solution can dissolve by-products such as amidoxime in-
termediates at the end of the reaction and 1,2,4-oxadiazole can be
separated by simple filtration.
Potassium fluoride has proven to be a versatile reagent in
organic synthesis. Examples relevant to the discussion presented
here include decarboxylation,32 Michael addition,33 and various
Knoevenagel reactions.34–37 As part of our ongoing interest on
developing efficient methods for the synthesis of useful hetero-
cycles,38–44 we wish to report a general and convenient method for
the synthesis of 1,2,4-oxadiazoles from readily available starting
materials.
To understand the ability of this new methodology in the syn-
thesis of 3,5- disubsituted 1,2,4-oxadiazoles, we chose a variety of
structurally divergent nitriles and acyl chlorides possessing a wide
range of functional groups. The results of which are summarized in
Table 1. The electronic effects and the nature of the substituents on
the nitrile and acyl chloride did not show strongly obvious effects in
terms of yields.
Finally, we compared this method with methods that have been
presented in the literature (Table 2). These results prove superior
catalytic activity of KF as solid support and strong base in this
transformation.
2. Results and discussions
Due to the beneficial pharmacological properties of certain
molecules containing the 1,2,4-oxadiazole moiety, a new synthetic
route, which could provide a greener procedure in which some
aspects of green chemistry could be met is desirable. Our main
strategy in this work was to develop a microwave-induced organic
reaction enhancement methodology, which is extremely fast,
cleaner than conventional reactions and lead to higher atom
economy (less chemical waste). We have also found that the use of
KF as a strong base and solid support for the synthesis of these
compounds, starting from nitrile and hydroxylamine hydrochloride
3. Conclusion
In summary, we have developed a microwave-enhanced, rapid,
and efficient method for the synthesis of 3,5-disubstituted 1,2,4-
oxadiazoles under solvent-free condition. The main advantages of
this method are: (i) a one pot reaction with use of nitrile as starting
material, (ii) solvent-free conditions, (iii) microwave irradiation,
(iv) elimination of insoluble solid support such as MgO, Al2O3,
which is an complicating factor in the work-up step, (v) avoiding
N
OH
450W, 130 °C, 10 min
N
O
KF
MW, 100 °C, 5 min
p-ClC6H4COCl
NH2
N
Cl
Cl
+ NH2OH.HCl
CN
O
C
N
OH
N
O
Na2CO3 or NaHCO3
MW, 100 °C, 5 min
450W, 130 °C, 10 min
p-ClC6H4COCl
NH2
NH2
Scheme 4. Effect of condition on the formation of products.

496
S. Rostamizadeh et al. / Tetrahedron 66 (2010) 494–497
Table 1
4.2. General experimental procedure for the preparation
of 1,2,4-oxadizaole derivatives
One-pot synthesis of 1,2,4-oxadiazoles under solvent-free condition using micro-
wave irradiation
1- KF, NH2OH.HCl
A mixture of nitrile (2 mmol), hydroxylamine hydrochloride
(2.2 mmol, finely ground), potassium fluoride (1 g) were mixed and
ground in a mortar and pestle until a fine homogeneous powder
was obtained (5 min). The mixture was then irradiated under
microwave irradiation for 5 min at 100 ꢀC. In the next step, the
required acyl chloride (3 mmol) was added to the mixture, shaken
for 5 min and irradiated again for 10 min at 450 W at 130 ꢀC. Water
(10 mL) was then added to the reaction mixture and the
precipitated solids were filtered after vigorous stirring. The crude
1,2,4-oxadiazole was further recrystallized from 95% ethanol.
Spectral data for selected known compounds are:
N
O
MW, 100 °C, 5 min
R1
C
N
R1
R2
2-R2COCl
450W, 130 °C, 10 min
N
Entry
R1
R2
Yielda (%)
1
2
3
4
5
6
7
8
C6H5
C6H5
C6H5
C6H5
C6H5
94
92
92
91
95
95
93
93
92
97
95
89
90
89
93
95
92
p-CH3OC6H4
p-NO2C6H4
m-NO2C6H4
p-Cl C6H4
p-Cl C6H4
p-Br C6H4
m-NO2C6H4
p-ClC6H4
m-NO2C6H4
p-ClC6H4
m-NO2C6H4
p-CH3C6H4
C6H4
C6H5
p-ClC6H4
p-BrC6H4
p-ClC6H4
p-CH3C6H4
p-BrC6H4
o-ClC6H4
o-ClC6H4
2,4-Cl2C6H4
2,4-Cl2C6H4
p-BrC6H4
C6H5CH2
C6H5CH2
3-Phenyl-5-(p-methoxyphenyl)-1,2,4-oxadiazole10(Table 1, En-
try 2, 0.496 g, 92%) as a white solid, mp 107–108 ꢀC; nmax (KBr)
9
3028, 2919, 1608, 1558 cmꢁ1
; dH (300 MHz, DMSO-d6) 3.87 (3H, s,
OMe), 7.17–7.20 (2H, d, J 9 Hz, CH), 7.55–7.62 (3H, m, CH), 8.06–8.09
(2H, m, CH), 8.12–8.15 (2H, d, J 9 Hz, CH); dC (75 MHz, DMSO-d6)
10
11
12
13
14
15
16
17
d
175.2, 168.1, 163.1, 131.5, 129.9, 129.2, 127.0, 126.2, 115.6, 55.6.
3-Phenyl-5-(m-nitrophenyl)-1,2,4-oxadiazole10(Table 1, Entry
p-FC6H4
p-NO2C6H4
p-FC6H4
4, 0.491 g, 91%) as a white solid, mp 145–147 ꢀC; nmax (KBr) 2916,
1618, 1529, 1450 cmꢁ1
CH), 7.93 (1H, t, J 8.01 Hz, CH), 8.06–8.08 (2H, m, CH), 8.50–8.56
; dH (300 MHz, DMSO-d6) 7.54–7.64 (3H, m,
a
Isolated yield (over two steps, based on nitrile).
Table 2
Comparison of different methods for the synthesis of 1,2,4-oxadiazole derivatives
Entry
Condition
Time
Temperature ꢀC
Power level W
Yield %
Ref.
1
2
Solvent (DMF)a
Solid supporta,b
6 h
12 h
115
25–35
d
d
45–69
21–52
16
13
/
solvent (DMF, pyridine)
Solventa (THF)
3
4
5
6
7
1–24 h
6–12 h
4 min
15 min
3 min
25–35
120
60
150
160
d
d
d
d
d
20–98
62–91
37–58
90–98
92–97
30
22
19
20
23
Solventa (Toluene)
Solid supporta,b
PASPa,b,c/solvent (THF)
Solvent freed/
air oxidation
8
9
Solid supportd/
solvent free
2–4 min
10 min
d
600
450
41–70
89–97
24
Solid supportd,e
solvent free
/
130
This method
a
These methods used amidoxime as starting material.
These solid supports can act as an complicating in the aqueous work-up.
Polymer-assisted solution-phase.
These methods used nitrile as starting material.
This solid support is dissolved in water at the end of reaction.
b
c
d
e
the use of silica gel for purification of the products, (vi) performing
the work-up in aqueous media and (vii) higher yields and milder
reaction conditions.
(2H, m, CH), 8.78–8.79 (1H, m, CH); dC (75 MHz, DMSO-d6) 173.6,
168.4, 148.1, 133.8, 131.8, 131.4, 129.3, 127.5, 127.1, 125.7, 124.7,
122.4.
3,5-Bis(p-chlorophenyl)-1,2,4-oxadiazole17(Table 1, Entry 6,
0.553 g, 95%) as a white solid, mp 183 ꢀC; nmax (KBr) 2921, 1606,
4. Experimental
4.1. General
1555, 1478 cmꢁ1
; dH (300 MHz, DMSO-d6) 7.67 (2H, d, J 8.4 Hz, CH),
7.74 (2H, d, J 8.4 Hz, CH), 8.09 (2H, d, J 9.0 Hz, CH), 8.19 (2H, d, J
9.0 Hz, CH); dC (75 MHz, DMSO-d6) 174.8, 167.5, 138.3, 136.5, 129.8,
129.5, 128.9, 125.2, 124.8, 122.1.
Melting points were recorded on a Buchi B-540 apparatus and
are uncorrected. IR spectra were recorded on an ABB Bomem Model
FTLA 200–100 instrument. 1H and 13C NMR spectra were measured
with a Bruker DRX-300 Avance spectrometer at 300 and 75 MHz
3-(p-Chlorophenyl)-5-(m-nitrophenyl)-1,2,4-oxadiazole26(Table 1,
Entry8,0.560 g, 93%)asamilkysolid, mp167–168 ꢀC; nmax (KBr)2925,
1621, 1597, 1540 cmꢁ1
; dH (300 MHz, DMSO-d6) 7.63–7.67 (2H, d, J
using TMS as an internal standard. Chemical shifts are reported (
d
)
9 Hz, CH), 7.94 (1H, t, J 8.1 Hz, CH), 8.09 (2H, d, J 9 Hz, CH), 8.51–8.57
(2H, m, CH), 8.79–8.80 (1H, m, CH); dC (75 MHz, DMSO-d6) 173.8,167.6,
148.1, 136.6, 133.9, 131.5, 129.5, 128.9, 127.6, 124.6, 122.2.
relative to TMS, and coupling constants (J) are reported in Hertz
(Hz). Mass spectra were recorded on Shimadzu QP 1100 EX mass
spectrometer with 70 eV ionization potential. Elemental analysis of
new compounds was done with a Vario EL III 0 Serial No. 11024054
instrument and their results favorably agreed with calculated
values. The reactions were performed in microwave oven (Delongi,
model number CE290DN). Chemicals were obtained from Merck
and Sigma–Aldrich and used without further purification.
3-(p-Methylphenyl)-5-(p-chlorophenyl)-1,2,4-oxadiazole27(Table
1, Entry 9, 0.479 g, 92%) as a white solid, mp 138–140 ꢀC; nmax (KBr)
3087, 2953, 1608, 1493 cmꢁ1
; dH (300 MHz, DMSO-d6) 7.40 (2H, d, J
8.1 Hz, CH), 7.73 (2H, d, J 8.5 Hz, CH), 7.97 (2H, d, J 8.1 Hz, CH), 8.18 (2H,
d, J 8.5 Hz, CH); dC (75 MHz, DMSO-d6) 174.3,168.2,141.6,138.0,129.7,
129.6, 126.9, 123.1, 122.2.

S. Rostamizadeh et al. / Tetrahedron 66 (2010) 494–497
497
3-(p-Bromophenyl)-5-(m-nitrophenyl)-1,2,4-oxadiazole21(Table
1, Entry 10, 0.669 g, 97%) as a milky solid, mp 168–169 ꢀC; nmax (KBr)
120.0, 116.9, 116.6, 31.3; MS(70 eV) m/z 254 (59Mþ), 123(100),
103(58), 95(67%).
2918, 1595, 1524, 1397 cmꢁ1
; dH (300 MHz, DMSO-d6) 7.79–7.83
(2H, m, CH), 7.96 (1H, t, J 8.1 Hz, CH), 8.02–8.07 (2H, m, CH), 8.53–
8.60 (2H, m, CH), 8.29 (1H, t, J 1.8 Hz, CH); dC (75 MHz, DMSO-d6)
173.9,167.7,148.2,133.9,132.4,131.5,129.1,127.6,125.5,124.9,124.6,
122.5.
Acknowledgements
The Authors gratefully acknowledge Research Council of K.N.
Toosi University of Technology for partial financial support of this
work.
3-(o-Chlorophenyl)-5-(p-chlorophenyl)-1,2,4-oxadiazole12(Table
1, Entry 11, 0.553 g, 95%) as a white solid, mp 154–156 ꢀC; nmax (KBr)
2950, 1612, 1486, 1467 cmꢁ1
; dH (300 MHz, DMSO-d6) 7.57 (1H, dt, J
References and notes
7.0, 1.0 Hz, CH), 7.63 (1H, dt, J 7.5, 1.7 Hz, CH), 7.70–7.71 (1H, m, CH),
7.75 (2H, d, J 8.5 Hz, CH), 7.99(1H, dd, J 7.6, 0.9 Hz, CH), 8.20 (2H, d, J
8.5 Hz, CH); dC (75 MHz, DMSO-d6) 174.0, 167.1, 138.2, 132.6, 132.1,
131.7, 130.7, 129.7, 127.6, 125.2, 121.9.
1. Mathvink, R. J.; Barritta, A. M.; Candelore, M. R.; Cascieri, M. A.; Deng, L.; Tota,
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3-(o-Chlorophenyl)-5-(m-nitrophenyl)-1,2,4-oxadiazole26(Table
1, Entry 12, 0.537, 89%) as a pale yellow solid, mp 154–156 ꢀC; nmax
(KBr) 3082, 2873, 1616, 1594 cmꢁ1
; dH (300 MHz, DMSO-d6) 7.59
(1H, dt, J 4.4, 8 Hz, CH), 7.64 (1H, dt, J 1.4, 7.4 Hz CH), 7.71 (1H, dd, J
1.1, 7.2 Hz, CH), 7.96 (1H, t, J 7.3 Hz, CH), 8.03 (1H, dd, J 1.5, 6.4 Hz,
CH), 8.54–8.60 (2H, m, CH), 8.83 (1H, s, CH); dC (75 MHz, DMSO-d6)
173.2,167.3, 148.2, 133.9, 132.8, 132.2, 131.8, 131.5, 130.8, 127.7, 127.6,
125.0, 124.5, 122.5.
3-(2,4-Dichlorophenyl)-5-(p-methylphenyl)-1,2,4-oxadiazole18
(Table 1, Entry 13, 0.542 g, 90%) as a yellow solid, mp 145–146 ꢀC;
7. Borg, S.; Vollinga, R. C.; Labarre, M.; Payza, K.; Terenius, L.; Luthman, K. J. Med.
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8. Tanaka, K. Solvent-free Organic Synthesis; Wiley VCH: Weinheim, 2003.
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nmax (KBr) 3025, 1616, 1591, 1555 cmꢁ1
; dH (300 MHz, DMSO-d6)
10. Eloy, F. Fortschr. Chem. Forsch. 1965, 4, 807.
3.22 (3H, s, CH3), 7.44 (2H, d, J 8.03 Hz, CH), 7.62 (1H, dd, J 8.44,
1.83 Hz, CH), 7.57 (1H, d, J 1.92, CH), 8.01 (1H, d, J 9.00 Hz, CH), 8.03
(2H, d, J 8.4 Hz, CH); dC (75 MHz, DMSO-d6) 174.9, 166.1, 143.7,
136.3, 133.1, 132.7, 130.2, 129.9, 127.7, 124.3, 120.2, 21.0.
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3-(2,4-Dichlorophenyl)-5-(phenyl)-1,2,4-oxadiazole24(Table 1,
Entry 14, 0.519 g, 89%) as a yellow solid, mp 141–143 ꢀC; nmax (KBr)
3070, 1609, 1592, 1568 cmꢁ1
; dH (300 MHz, DMSO-d6) 7.64–7.69
(3H, m, CH), 7.72–7.77 (1H, m, CH), 7.90 (1H, d, J 2.0 Hz, CH), 8.03
(1H, d, J 8.4 Hz, CH), 8.16–8.21 (2H, m, CH); dC (75 MHz, DMSO-d6)
175.0, 166.4, 136.6, 133.5, 133.2, 133.0, 130.4, 129.6, 128.1, 128.0,
124.4, 123.0.
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Spectral data for new compounds are: 3-(p-bromophenyl)-5-(p-
flourophenyl)-1,2,4-oxadiazole (Table 1, Entry 15, 0.634 g, 93%) as
a white solid, mp 177–179 ꢀC; Found: C, 52.47; H, 2.49; N, 8.75.
C14H8 BrFN2O requires C, 52.68; H, 2.51; N, 8.78; nmax (KBr) 2924,
22. Amarasinghe, K. K. D.; Maier, M. B.; Srivastava, A.; Gray, J. L. Tetrahedron Lett.
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1612, 1500, 1407 cmꢁ1
; dH (300 MHz, DMSO-d6) 7.01 (1H, s, CH),
7.34 (1H, s, CH), 7.49–8.00 (4H, m, CH), 8.24 (2H, s, CH); dC (75 MHz,
DMSO-d6) 174.7, 167.5, 165.9, 132.3, 132.3, 128.9, 125.2, 119.9, 116.8,
115.4; MS(70 eV) m/z 320 (77 Mþþ2), 318 (75 Mþ), 199(85),
197(85), 123(74), 90 (76%).
3-Benzyl-5-(p-nitrophenyl)-1,2,4-oxadiazole (Table 1, Entry 16,
0.530 g, 95%) as a white solid, mp 139–141 ꢀC; Found: C, 63.75; H,
4.21; N, 14.41. C15H11N3O3 requires C, 64.06; H, 3.91; N, 14.94; nmax
(KBr) 3020, 2922, 1607, 1518 cmꢁ1
; dH (300 MHz, DMSO-d6) 4.20
(2H, s, CH2), 7.23–7.37 (5H, m, CH), 8.31 (2H, d, J 9 Hz, CH), 8.38 (2H,
d, J 9 Hz, CH); dC (75 MHz, DMSO-d6) 173.4, 170.3, 149.8, 135.4,
129.3, 128.6, 128.5, 126.9, 124.5, 31.33; MS(70 eV) m/z 281 (63Mþ),
150(100), 131(91), 91(50%).
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3-Benzyl-5-(p-flourophenyl)-1,2,4-oxadiazole (Table 1, Entry
17, 0.463 g, 92%) as a white solid, mp 107–108 ꢀC; Found: C,
70.21; H, 4.20; N, 10.58. C15H11FN2O require C, 70.86; H, 4.33; N,
11.02; nmax (KBr) 3031, 2926, 1612, 1567 cmꢁ1
; dH (300 MHz,
DMSO-d6) 4.15 (2H, s, CH2), 7.24–7.32 (5H, m, CH), 7.38–7.46 (2H,
m, CH), 8.08–8.15 (2H, m, CH); dC (75 MHz, DMSO-d6) 174.2,
169.9, 166.4, 163.1, 135.6, 130.7, 130.5, 128.9, 128.5, 126.8, 120.0,