Br?nsted acidic ionic liquid-catalyzed tandem reaction: An efficient approach towards regioselective synthesis of pyrano[3,2-: C] coumarins under solvent-free conditions bearing lower E-factors

Article abstract of DOI:10.1039/c7gc01158j

1-Butane sulfonic acid-3-methylimidazolium tosylate, [BSMIM]OTs, is found to be a remarkable catalyst for the tandem cyclization of 4-hydroxycoumarin with chalcones for the syntheses of pyrano[3,2-c]coumarins under solvent-free conditions. The developed p

Full text of DOI:10.1039/c7gc01158j

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Brønsted acidic ionic liquid-catalyzed tandem reaction: an
efficient approach towards regioselective synthesis of pyrano[3,2-
c]coumarins under solvent-free conditions bearing lower E-
factors†
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Sachinta Mahato,a Sougata Santra,b Rana Chatterjee,a Grigory V. Zyryanov,b,c Alakananda Hajraa
and Adinath Majeea*
www.rsc.org/
1-Butane sulfonic acid-3-methylimidazolium tosylate, [BSMIM]OTs is found to be a remarkable catalyst for the tandem
cyclization of 4-hydroxycoumarin with chalcones for the syntheses of pyrano[3,2-c]coumarins under solvent-free
conditions. The developed protocol is applicable for the construction of biologically important pyranocoumarins from
easily accessible chalcones having various substituents. This reaction possibly proceeds through Michael addition followed
by cyclization. The feasibility of the catalyst recycling is also demonstrated. This method produced only water as the
byproduct and represents a green synthetic protocol. The catalytic reaction proceeded very smoothly under solvent-free
conditions and showed high regioselectivity. Clean reaction, non-chromatographic purification technique, easily accessible
reactants, metal and solvent-free and environmentally friendly reaction conditions are the notable advantages of this
procedure. In addition, this method possesses lower E-Factors.
Room-temperature ionic liquids (RTILs) have been explored as
greener alternatives to the volatile organic solvents due to
their unique properties, such as negligible vapor pressure, high
Introduction
Tandem reactions are one of the most powerful and atom
economical methodologies in modern organic synthesis.1 The
conjugate addition of nucleophiles to α,β-unsaturated
compounds (such as α,β-unsaturated carbonyl compounds,
esters, nitriles, phosphates, sulfones, and nitroalkenes) is
known as the Michael addition.2 These reactions usually
provide processes in a more efficient and environmentally
benign manner than conventional procedures by omitting the
steps of separation and purification of the reaction
intermediates. It is a versatile method for carbon-carbon and
carbon-heteroatom bond formations. Both Michael and
hetero-Michael additions are widely used in inter- and
intramolecular reactions to provide biologically important
scaffolds. α,β-Unsaturated ketones are easily accessible from
readily available aldehydes and ketones by the condensation
reaction. These are good Michael acceptors and have been
used for the construction of biologically important scaffolds by
exploring their bielectrophilic properties in tandem fashion.3
thermal stability, large liquid temperature range, easy
recyclability, excellent chemical stability, and strong dissolving
power for a wide range of organic and inorganic molecules.4
However, their uses in large quantities as solvents have
limitations such as combustibility, biodegradability, toxicity,
and high cost.5 As a consequence, now a days ionic liquids are
generally prepared in such a way that they can act as a proper
working system rather than only as media for organic
transformations.6 By changing the cations and/or anions, the
properties of ILs can be turned into many different ways. So
far, a large number of functionalized ILs has been prepared for
different purposes by choosing the appropriate cations and
anions.7
After the report by Forbes and Davis for the synthesis of
the new class of phosphonium- and imidazolium ion-based
ionic liquids equipped with a pendant acidic sulfonic acid
moiety, Brønsted acidic ionic liquids (BAILs) have drawn a
considerable attention in the field of catalysis.8 BAILs are more
preferable over the mineral acids as BAILs show strong acidity
with usual properties of ionic liquids. As a result many
transformations have been carried out involving acidic proton
of these ILs. Majority of these reported methodologies rely on
this on the acidic proton. In this context, it is noteworthy to
mention that during last few years we are actively engaged in
the field of catalysis using BAILs as well as imidazole based
zwitterions.9
a.Department of Chemistry, Visva-Bharati (A Central University), Santiniketan-
731235, India; E-mail: adinath.majee@visva-bharati.ac.in
b.Department of Organic & Biomolecular Chemistry, Chemical Engineering Institute,
Ural Federal University, 19 Mira Str., Yekaterinburg, 620002, Russian Federation
c. I. Ya. Postovskiy Institute of Organic Synthesis, Ural Division of the Russian
Academy of Sciences, 22 S. Kovalevskoy Str., Yekaterinburg, 620219, Russian
Federation
†Electronic Supplementary Information (ESI) available: [X-ray crystallographic data
,
E-factor calculations, copies of 1H and 13C NMR spectra]. See
for 3n
DOI: 10.1039/x0xx00000x
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The concept of sustainable development now plays an americanum), khellactone (isolated from Ligusticum elatum),
important role in deciding strategies for chemical synthesis arisugacins, and pyripyropenes.16
and, consequently, the search for efficient, economic and eco-
Considering these important uses, their syntheses have
friendly synthetic methods has become a major concern. In been received much attention in the field of medicinal and
1992, the E-Factor, or Environmental Impact Factor, was pharmaceutical chemistry. A number of methodologies for the
introduced by Roger Sheldon.10 This metric helps to quantify synthesis of this important framework has been developed by
the amount of waste generated per kilogram of product. It is a various groups.17 Most of the methodologies were developed
means to assess the “environmental acceptability” of a via the reaction of 4-hydroxycoumarin with various types of
manufacturing process. It has been well accepted that solvents electrophiles such as 1,3-diarylallylic compounds,17a α,β-
are the main reason for an insufficient E-factor, especially in unsaturated aldehydes17b–d or ketones17e,f and propargylic
the synthesis of fine chemicals and pharmaceutical alcohols.17g,h Various catalysts as well as reagents have been
industries.11 Based on the current working practice with used for tandem reaction between 4-hydroxycoumarin and
special emphasize on Green Chemistry, it is now often claimed α,β-unsaturated carbonyl compounds such as Brønsted acid,17c
that “the best solvent is no solvent”. The crucial advantages of POCl3,17e AuCl3/3AgOTf,17f etc. Multicomponent reaction (MCR)
solvent-free reactions are cost saving, by easy work-up, strategies have also been employed to synthesize
purification, and remarkable rate acceleration with less energy pyranocoumarins.18 However, most of these methodologies
consumption. Owing these importances, a library of organic have been developed using expensive reagents (for example
reactions has been carried out under solvent and catalyst-free AuCl3/3AgOTf is highly expensive), Brønsted acid which is not
conditions in the last decade for the synthesis of various easily available and POCl3 which is not environmentally
biologically active compounds.12
friendly. In addition, regio-selectivity is also a major issue for
Pyranocoumarin derivatives are a class of fused oxygen these cyclizations. Therefore, finding a new methodology for
containing heterocycles that have drawn much attention due the synthesis of pyrano[3,2-c]coumarins in terms of efficiency,
to their potential biological and pharmaceutical activities operational simplicity, availability of reagents, and economic
including antifungal, insecticidal, anti-cancer, anti-HIV, anti- practicability is highly desirable. As a part of our ongoing
inflammatory, antioxidants, and antibacterial activities.13 program on catalysis by ionic liquid,9 herein we report the
Pyranocoumarin scaffolds are also important in medicinal catalytic effect of Brønsted acidic task specific ionic liquid, 1-
chemistry for the treatment of skin disorder.14 A large number butane
sulfonic
acid-3-methylimidazolium
tosylate,
of pyranocoumarin derivatives have been isolated from nature [BSMIM]OTs (BAIL-1) on the tandem reaction between 4-
having several structural arrays between the coumarin and the hydroxycoumarin with α,β-unsaturated carbonyl compounds
pyran rings.15 Particularly a few important pyranocoumarins for the formation of pyrano[3,2-c]coumarins (Scheme 1). The
are xanthyletin (predominantly isolated from Zanthoxylum reactions proceeded under neat conditions and no need to
perform column chromatography for purification.
R2
OH
O
O
N
R
N
O
BAIL-1 (5 mol%)
100 oC, 3 h
R
SO3H
R1
+
R2
R1
OTs
O
O
O
2
1
BAIL-1:
[BSMIM]OTs
3
26 examples
up to 90% yields
Scheme 1 BAIL-catalyzed synthesis of pyrano[3,2-c]coumarins
methanol, toluene, PEG and dioxane the desired product was
obtained in 18-44% yields (Table 1, entries 4–8). The targeted
product was obtained with maximum yield (84%) under
solvent-free conditions at 100 °C for 3 h (Table 1, entry 10).
These results indicate the detrimental effect of the solvents for
this transformation. Next the effects of reaction time and
temperature on the reaction were also investigated. No
significant amount of the desired product was formed when
the reaction was carried out at room temperature (Table 2,
entry 1). The best effective reaction temperature was found to
be 100 °C (Table 2, entry 4) and the yield of the reaction did
not improve by increasing the reaction time from 3 h to 6 h
(Table 2, entry 5). Increasing the temperature was not
beneficial (Table
Results and Discussion
For the initial study, 4-hydroxycoumarin 1a and 3-phenyl-1-(p-
tolyl)prop-2-en-1-one 2a were taken as the model substrates
to optimize the reaction conditions employing 5 mol% 1-
butane
sulfonic
acid-3-methylimidazolium
tosylate,
[BSMIM]OTs (BAIL-1) as the catalyst. Initially the reaction was
carried out in various organic solvents such as DMF, DMSO,
methanol, ethanol, toluene, polyethylene glycol (PEG), 1,4-
dioxane, DCE, etc. as well as in aqueous medium. The results
are summarized in Table 1. Either no formation or trace
amount of the desired product was observed in DMF, DMSO,
water and DCE (Table 1, entries 1–3, 9). In case of ethanol,
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Table 2 Temperature effect on the tandem cyclization reactiona
2, entry 6) while decreasing the reaction temperature
decreased the yield of the reaction (Table 2, entries 2 & 3).
Entry
Temp. (°C)
RT
Time (h)
Yields (%)b
Table 1 Screening of the solvent effectsa
1
2
3
4
5
6
24
3
<10
48
67
84
84
81
60
Me
80
3
100
3
OH
O
O
100
6
BAIL-1 (5 mol%)
100 oC, 3 h
+
120
3
O
O
O
Me
a Reaction conditions: A mixture of 1 (1 mmol), 2a (1 mmol) and BAIL-1 (5 mol%).
b Isolated yields.
2a
1a
O
3a
Table 3 Effect of various ionic liquids and catalyst loadinga
Entry
1
Solvents (2 mL)
DMF
Yieldsb (%)
<5
<5
<5
18
23
44
24
30
<5
84
2
DMSO
Water
EtOH
3c
4c
5c
6
MeOH
Toluene
PEG
7
8c
9c
10
Dioxane
DCE
Neat
a
Reaction conditions: A mixture of 1a (1 mmol) and 2a (1 mmol) was heated at
100 °C for 3 h. b Isolated yield. c Under reflux.
We have examined few other ionic liquids (ILs) synthesized
in our laboratory as shown in Table 3. It has been observed
that other ILs (BAIL-2, BAIL-3, BAIL-4 and IL-2) are not effective
like BAIL-1 for this tandem cyclization process. When 5 mol%
of BAIL-2 is used (Table 3, entry 2) the yield is considerably
lower (72%) than BAIL-1 (84%). Similarly BAIL-3 and BAIL-4
were also less effective and afforded the desired product with
76% and 65% yields respectively (Table 3, entries 3 & 4). The
yields of the reaction decreased when the chain length of the
ionic liquid (BAIL-3) was reduced (Table 3, entry 3).19 No
significant amount of product was obtained using IL-2 (Table 3,
entry 5). Accordingly, we chose BAIL-1 as catalyst for this
tandem cyclization. Finally, the catalyst loading was checked
under these reaction conditions using BAIL-1 as the catalyst for
the same reaction. We have observed that 5 mol% of BAIL-1
afforded better yield (84%) compared to that of 2 mol% (72%
yield) (Table 3, entry 6). Similarly on using 10 mol% of the
catalyst no improvement of the yield was observed (Table 3,
entry 7). Other acid catalysts such as Brønsted once like p-TSA
and H2SO4 were not effective for this reaction and gave 52%
and 43% yields respectively (Table 3, entries 8 & 9). In the
absence of any catalyst no conversion has been detected
(Table 3, entry 10). Thus the optimal yield (84%) was obtained
when the reaction was carried out employing 5 mol% BAIL-1 at
100 °C under solvent-free conditions for 3 h.
Entry
Catalyst
BAIL-1
BAIL-2
BAIL-3
BAIL-4
IL-2
Catalyst loading (mol%)
Yields (%)b
1
2
5
5
84
72
3
5
76
4
5
65
5
5
<10
72
6
BAIL-1
BAIL-1
p-TSAc
H2SO4
2
7
10
10
10
--
84
8
52
9
43
NDd
10
No Catalyst
a Reaction conditions: Carried out with 0.5 mmol of 1a and 0.5 mmol of 2a in the
b
c
presence of various catalysts under neat conditions. Isolated yields. p-TSA =
para-toluenesulfonic acid. dND = not detected in TLC.
After getting the optimized reaction conditions in hand,
various chalcones (
hydroxycoumarin
2
) were introduced to react with 4-
(
1a) as well as substituted 4-
hydroxycoumarin to prove the general applicability of this
methodology. First, the effect of substituents on the both
phenyl moieties of chalcone was tested. The results are
summarized in Table 4. In the most cases the desired products
were obtained with good yields (3a
electron donating substituents like –Me, –OMe (3a
3p 3q 3r) and electron withdrawing substituents such as –I,
–Cl, –F, –Br, –NO2 reacted well to afford the corresponding
pyrano[3,2-c]coumarin derivatives (3d 3e 3f 3g 3l 3m 3q
–
3r). The chalcones bearing
,
3c, 3k 3o
,
,
,
&
,
,
,
,
,
,
&
3r). The acid sensitive group containing chalcone was
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unaffected under the present reaction conditions which signify and hydroxyl groups afforded the corresponding
the mildness of the reaction conditions (3h). In addition, pyranocoumarins with high to excellent yields. Chalcones
heteroaryl chalcones reacted well without accompanying self- bearing functional groups like Me (3t
, 3y), OMe (3u) as well as
condensation or ring cleavage (3i, 3j, 3n). Then our attention acid sensitive functionalities were unaffected under the
was turned to the use of substituted 4-hydroxycoumarins to reaction conditions and the desired products were obtained in
prove the general applicability of the reaction conditions as 79–85% yields.
summarized in Table 5. 4-Hydroxycoumarin bearing methyl
Table 4 Synthesis of various pyrano[3,2-c]coumarin derivatives in presence of BAIL-1a,b
a Reaction conditions: 1 mmol of 1a and 1 mmol of 2 in presence of BAIL-1 (5 mol%) at 100 °C for 3 h. b All are isolated yields.
4 | J. Name., 2012, 00, 1-3
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Table 5 Substrate scopes of 4-hydroxycoumarinsa,b
a Reaction conditions: 1 mmol of 1 and 1 mmol of 2 in presence of BAIL-1 (5 mol%) at 100 °C for 3 h. b All are isolated yields.
Me
OH
O
O
BAIL 1 (5 mol%)
100 oC, 3 h
+
O
O
O
Me
2a
1a
(10 mmol)
(10 mmol)
O
3a
, 2.93 g, 80%
Scheme 2 Gram-scale reaction.
All these reactions were carried out in open atmosphere
and are not sensitive to air and moisture. In addition, the
reaction is highly regio-selective. No other regio-isomer was
isolated under the present reaction conditions. For purification
no need to perform column chromatography. After completion
of the reaction, water was added to the reaction mixture and
filtered it off. The pure product was obtained by recrystallizing
the residue from hot ethanol. The reaction conditions are mild
and give no decomposition of the products or polymerization
of the starting materials. We have not observed any by-
products for all reaction combinations which are supported
with high yields and regio-selectivity of the protocol. All of the
known synthesized compounds have been characterized by
Figure
1 X-ray crystal structure of 4-phenyl-2-(thiophen-2-yl)-4H,5H-pyrano[3,2-
c]chromen-5-one (3n).
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spectral data and the new compounds by spectral and product was obtained by recrystallizing the residue from hot
analytical data and the X-ray crystallographic analysis of 4- ethanol. The catalyst maintained its high level of activity even
phenyl-2-(thiophen-2-yl)-4H,5H-pyrano[3,2-c]chromen-5-one
after being recycled five times for synthesizing 3a as shown in
(
3n) was performed to confirm the structure of the product as Table 6.
shown in Figure 1.20
Table 6 Recycling of BAIL for synthesizing 3a.a
Furthermore, the potential synthetic applicability of this
method was investigated on a gram scale using the model
reaction. As shown in Scheme 2, the reaction could afford 2.93
g of 3a in 80% yield without any significant loss of its
efficiency, demonstrating the potential applications of the
present method for a large scale synthesis of pyrano[3,2-
c]coumarin derivatives.
No. of cycle
Yields (%)b
Catalyst recovery (%)
1
2
3
4
5
84
82
80
80
78
96
92
89
85
82
Next, we turn our attention towards the recovery and
reusability of the catalyst. For this purpose, we have chosen
the reaction of 4-hydroxycoumarin (1a) with 3-phenyl-1-(p-
tolyl)prop-2-en-1-one (2a) in presence of 5 mol% acidic ionic
liquid (BAIL-1) at 100 °C as the model reaction. After
completion of the reaction water was added to the reaction
mixture. The reaction mixture was then filtered off and the
ionic liquid was recovered by evaporating the water. Pure
a Carried out with 1 mmol of 1a and 1 mmol of 2a in presence of catalyst (BAIL-1)
at 100 °C for 3 h. b Isolated yields.
In addition, we have developed a greener reaction
condition bearing lower E-factors10,11 in cases of synthesizing
the desired products
3 (Table 7, see also ESI) which is
consistent with the principles of the atom economy.
Table 7 E-Factors in the synthesis of pyrano[3,2-c]coumarins (3)a
#
1
4-Hydroxycoumarin (1)
Chalcones (2)
Products (3)
Yieldsb
(%)
E-Factor (kg waste
per kg product)c
84
0.25
2
85
0.24
3
80
0.31
4
80
0.31
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5
82
0.28
6
83
0.26
7
82
0.28
8
83
0.26
O
Ph
O
2h
O
9
78
0.35
10
75
0.41
Ph
O
O
O
O
3j
11
82
0.28
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12
80
0.31
13
79
0.31
14
80
0.31
15
O
78
0.34
Me
Me
2o
16
83
0.26
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17
82
0.27
18
O
78
0.34
OMe
F
OMe
2r
O
O
O
F
3r
19
79
0.33
20
82
0.27
Me
O
Me
Ph
O
O
3t
21
81
0.29
22
83
0.26
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23
85
0.23
O
Ph
O
2h
O
24
78
0.34
25
80
0.31
26
82
0.27
O
Ph
O
2h
O
a All reactions were performed on 1 mmol scale in presence of BAIL-1 (5 mol%) at 100 °C for 3 h. b Isolated yields. c No solvent was used for further purification and the
catalyst was recovered and recycled.
A plausible mechanistic pathway for this tandem reaction is
O
O
outlined in Scheme 3. Probably the first step is the Michael
addition of 4-hydroxycoumarin ( ) to the α,β-unsaturated
ketone ( The intermediate
) which gave the intermediate A 17f
on intramolecular cyclization afforded the intermediate
which finally afforded the pyranocoumarin ( ) on subsequent
OH
O
R2
R1
OH
R2
1
R
2
R
2
.
R1
A
B
O
H+
O
O
1
3
A
removal of water. The acidic ionic liquid activates the
unsaturated ketone through protonation of the carbonyl group
and increased the electrophilic character at the β-carbon
which promote the Michael adduct formation. In addition the
acidic ionic liquid also helps to facilitate the intramolecular
cyclization step by the protonation of carbonyl group of
H+
R2 OH
R2
O
O
H+
R
R
R1
R1
intermediate A.
O
O
H2O
O
O
3
B
Scheme 3 Plausible reaction pathway.
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To a mixture of 4-hydroxycoumarin (1a, 1.62 g, 10 mmol) and
3-phenyl-1-(p-tolyl)prop-2-en-1-one (2a, 2.22 g, 10 mmol), the
5 mol% BAIL-1 was added and the mixture was stirred at 100
ºC for 3 h (TLC). After completion of the reaction, water was
added to the reaction mixture. Then the product was filtered
off and the crude product was recrystallized from hot ethanol
to afford the pure product as white solid in 80% yield (2.93 g).
Conclusions
To conclude, we have successfully developed an efficient and
regioselective methodology for the synthesis of pyrano[3,2-
c]coumarin derivatives by the coupling of 4-hydroxycoumarin
with chalcones catalyzed by Brønsted acidic ionic liquid (BAIL-
1). This tandem reaction proceeds through the Michael
addition followed by annulation reaction and only water is
produced as the byproduct. A library of pyranocoumarin
derivatives having variety of substituents on the pyran moiety
have been synthesized employing this atom efficient
methodology. During optimization it was estimated that no
external solvent was needed to carry out the reaction. The
catalyst BAIL-1 is easily recyclable without the significant loss
of catalytic activities. The notable advantages of the present
methodology are clean reaction, ease of product
4-Phenyl-2-(p-tolyl)-4H,5H-pyrano[3,2-c]chromen-5-one
(3a).17f White solid (307 mg, 84%), mp: 157-158 °C (lit.17f mp:
1
156-158 °C); H NMR (CDCl3, 400 MHz): δ 8.03-8.01 (m, 1H),
7.63 (d, J = 8.0 Hz, 2H), 7.58-7.54 (m, 1H), 7.45-7.43 (m, 2H),
7.39-7.31 (m, 4H), 7.27-7.22 (m, 3H), 5.79 (d, J = 5.2 Hz, 1H),
4.70 (d, J = 4.8 Hz, 1H), 2.42 (s, 3H); 13C NMR (CDCl3, 100 MHz):
δ 161.6, 155.9, 152.9, 147.1, 143.8, 139.5, 132.1, 129.9, 129.5,
128.7, 128.6, 127.3, 124.7, 124.3, 122.8, 116.9, 114.7, 103.8,
103.0, 36.7, 21.5.
isolation/purification
(no
need
to
use
column
chromatography), easily accessible reactants, metal and
solvent-free and environmentally friendly reaction conditions
and water is the only byproduct. These features make this
procedure to be a green synthetic protocol. The proposed
approach is consistent with the principles of the atom
economy, and also allows varying the nature of the
substituents in the final compounds in a fairly wide range. We
believe that our present methodology will open up a new
route for the synthesis of bioactive pyrano[3,2-c]coumarin
derivatives.
2,4-Diphenyl-4H,5H-pyrano[3,2-c]chromen-5-one
(3b).17f
White solid (299 mg, 85%), mp: 171-172 °C (lit.17f mp: 170-171
1
°C ); H NMR (CDCl3, 400 MHz): δ 8.06-8.04 (m, 1H), 7.78-7.76
(m, 2H), 7.62-7.57 (m, 1H), 7.50-7.34 (m, 9H), 7.28-7.25 (m,
1H), 5.88 (d, J = 5.2 Hz, 1H), 4.74 (d, J = 4.8 Hz, 1H); 13C NMR
(CDCl3, 100 MHz): δ 161.5, 155.8, 152.8, 147.0, 143.6, 132.7,
132.1, 129.4, 128.8, 128.7, 128.6, 127.3, 124.8, 124.3, 122.8,
116.9, 114.6, 103.8(2C), 36.7.
2-Phenyl-4-(p-tolyl)-4H,5H-pyrano[3,2-c]chromen-5-one
(3c).17f White solid (293 mg, 80%), mp: 186-188 °C (lit.17f mp:
1
188-189 °C); H NMR (CDCl3, 400 MHz): δ 8.03-8.01 (m, 1H),
Experimental Section
General. Melting points were determined on a glass disk with
1
an electric hot plate and are uncorrected. H NMR (400 MHz)
7.64-7.62 (m, 2H), 7.59-7.54 (m, 1H), 7.44-7.30 (m, 6H), 7.27-
7.23 (m, 3H), 5.79 (d, J = 4.8 Hz, 1H), 4.70 (d, J = 5.2 Hz, 1H),
2.41 (s, 3H); 13C NMR (CDCl3, 100 MHz): δ 161.6, 155.9, 152.8,
147.0, 143.8, 139.4, 132.0, 129.9, 129.4, 128.7, 128.6, 127.3,
124.7, 124.2, 122.8, 116.9, 114.7, 103.8, 103.0, 36.7, 21.4.
and 13C NMR (100 MHz) spectra were run in DMSO-
͘ and
6
CDCl3 solutions. Commercially available substrates were
freshly distilled before the reaction. Solvents, reagents, and
chemicals were purchased from Aldrich, Fluka, Merck, SRL,
Spectrochem and Process Chemicals. All reactions involving
moisture-sensitive reactants were executed using oven-dried
glassware. Brønsted acidic ionic liquids were prepared
according to the previously reported method.21
4-(4-Fluorophenyl)-2-phenyl-4H,5H-pyrano[3,2-c]chromen-5-
one (3d).17j White solid (296 mg, 80%), mp: 143-145 °C (lit.17j
1
mp: 142-143 °C); H NMR (CDCl3, 400 MHz): δ 8.03-8.01 (m,
1H), 7.75-7.73 (m, 2H), 7.61-7.56 (m, 1H), 7.49-7.44 (m, 3H),
7.41-7.34 (m, 4H), 7.02-6.98 (m, 2H), 5.82 (d, J = 4.8 Hz, 1H),
4.71 (d, J = 5.2 Hz, 1H); 13C NMR (CDCl3, 100 MHz): δ 162.1 (d,
General procedure for the synthesis of pyrano[3,2-
3
1JC-F = 244 Hz), 161.6, 155.9, 147.2, 139.4 (d, JC-F = 2 Hz),
c]coumarins
132.6, 132.3, 130.2 (d, 4JC-F = 8 Hz), 129.5, 128.9, 128.6, 128.2,
To a mixture of 4-hydroxycoumarin (1 mmol) and α,β-
unsaturated ketone (1 mmol), the 5 mol% BAIL-1 was added
and the mixture was stirred at 100 °C for 6 h (TLC). After
completion of the reaction, water was added to the reaction
mixture. Then the product was filtered off and the ionic liquid
was recovered by evaporating the water. The recovered ionic
liquid was reused for a subsequent fresh batch of the reaction
after reactivation. The crude product was recrystallized from
hot ethanol to afford the pure product.
2
124.8, 124.4, 122.8, 117.0, 115.5 (d, JC-F = 21 Hz), 114.6,
103.6, 36.0.
4-(4-Chlorophenyl)-2-phenyl-4H,5H-pyrano[3,2-c]chromen-5-
one (3e).17g White solid (296 mg, 80%), mp: 194-196 °C
1
(lit.17g mp: 196-197 °C); H NMR (CDCl3, 400 MHz): δ 8.03-8.01
(m, 1H), 7.75-7.72 (m, 2H), 7.60-7.56 (m, 1H), 7.49-7.43 (m,
3H), 7.40-7.33 (m, 4H), 7.29-7.27 (m, 2H), 5.80 (d, J = 4.8 Hz,
1H), 4.68 (d, J = 4.8 Hz, 1H); 13C NMR (CDCl3, 100 MHz): δ
161.5, 155.9, 152.8, 147.2, 142.1, 133.1, 132.5, 132.3, 130.0,
129.5, 129.3, 128.8, 124.8, 124.4, 122.8, 116.9, 114.5, 103.3,
103.2, 36.2.
Typical procedure for the synthesis of 4-Phenyl-2-(p-tolyl)-
4H,5H-pyrano[3,2-c]chromen-5-one (3a)
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J. Name., 2013, 00, 1-3 | 11
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ARTICLE
Journal Name
4-(4-Bromophenyl)-2-phenyl-4H,5H-pyrano[3,2-c]chromen-5- 131.9, 128.6, 128.5, 127.1, 126.1, 125.1, 124.1, 122.6, 116.7,
one (3f).17j White solid (357 mg, 83%), mp: 170-171 °C (lit.17j 114.6, 114.0, 103.7, 101.9, 55.4, 36.6.
1
mp: 168-169 °C); H NMR (CDCl3, 400 MHz): δ 8.03-8.01 (m,
1H), 7.74-7.72 (m, 2H), 7.61-7.56 (m, 1H), 7.49-7.43 (m, 5H), 2-(4-Chlorophenyl)-4-phenyl-4H,5H-pyrano[3,2-c]chromen-5-
7.41-7.29 (m, 4H), 5.80 (d, J = 4.8 Hz, 1H), 4.68 (d, J = 4.8 Hz, one (3l).17i White solid (309 mg, 80%), mp: 171-173 °C; 1H NMR
1H); 13C NMR (CDCl3, 100 MHz): δ 161.5, 156.0, 152.9, 147.3, (CDCl3, 400 MHz): δ 7.95-7.93 (m, 1H), 7.63-7.60 (m, 2H), 7.55-
142.7, 132.5, 132.3, 131.8, 130.4, 129.6, 128.9, 124.8, 124.4, 7.51 (m, 1H), 7.38-7.34 (m, 5H), 7.30-7.17 (m, 4H), 5.78 (d, J =
122.8, 121.3, 117.0, 114.5, 103.3, 103.2, 36.3.
4.8 Hz, 1H), 4.65 (d, J = 5.2 Hz, 1H); 13C NMR (CDCl3, 100 MHz):
δ 161.4, 155.7, 152.8, 146.1, 143.4, 135.2, 132.2, 131.2, 129.7,
129.0, 128.8, 128.5, 127.4, 126.0, 124.3, 122.7, 117.0, 114.5,
4-(3-Nitrophenyl)-2-phenyl-4H,5H-pyrano[3,2-c]chromen-5-
one (3g).17j White solid (325 mg, 82%), mp: 185-186 °C; H 104.3, 36.7.
1
NMR (CDCl3, 400 MHz): δ 8.25-8.24 (m, 1H), 8.12-8.04 (m, 2H),
7.82-7.73 (m, 3H), 7.64-7.59 (m, 1H), 7.52-7.35 (m, 6H), 5.80 2-(4-Iodophenyl)-4-phenyl-4H,5H-pyrano[3,2-c]chromen-5-
(d, J = 5.2 Hz, 1H), 4.86 (d, J = 4.8 Hz, 1H); 13C NMR (CDCl3, 100 one (3m): White solid (378 mg, 79%), mp: 214-215 °C; 1H NMR
MHz): δ 161.5, 156.5, 153.0, 148.7, 147.9, 145.8, 134.9, 132.7, (CDCl3, 400 MHz): δ 7.91-7.88 (m, 1H), 7.71-7.68 (m, 2H), 7.52-
132.3, 129.8, 129.6, 128.9, 128.7, 124.9, 124.6, 123.5, 123.0, 7.47 (m, 1H), 7.39-7.36 (m, 2H), 7.33-7.30 (m, 3H), 7.28-7.22
122.5, 117.1, 114.3, 102.4, 36.8.
(m, 3H), 7.18-7.16 (m, 1H), 5.77 (d, J = 4.8 Hz, 1H), 4.61 (d, J =
5.2 Hz, 1H); 13C NMR (CDCl3, 100 MHz): δ 161.4, 155.7, 152.8,
146.2, 143.3, 138.0, 137.9, 132.2, 128.8, 128.5, 127.4, 126.4,
4-(Benzo[d][1,3]dioxol-5-yl)-2-phenyl-4H,5H-pyrano[3,2-
c]chromen-5-one (3h).17f White solid (329 mg, 83%), mp: 176- 124.3, 122.7, 117.0, 114.5, 104.5, 103.7, 95.2, 36.7. Anal. Calcd
177 °C (lit.17f mp: 177-178 °C); H NMR (CDCl3, 400 MHz): δ For C24H15IO3: C, 60.27; H, 3.16%; Found: C, 60.21; H, 3.24%.
1
8.03-8.00 (m, 1H), 7.74-7.72 (m, 2H), 7.60-7.55 (m, 1H), 7.48-
7.34 (m, 5H), 6.91-6.89 (m, 2H), 6.76-6.74 (m, 1H), 5.91 (s, 2H), 4-Phenyl-2-(thiophen-2-yl)-4H,5H-pyrano[3,2-c]chromen-5-
1
5.82 (d, J = 5.2 Hz, 1H), 4.63 (d, J = 4.8 Hz, 1H); 13C NMR (CDCl3, one (3n): White solid (286 mg, 80%), mp: 201-202 °C; H NMR
100 MHz): δ 161.6, 155.8, 152.9, 148.0, 146.9, 146.8, 137.8, (CDCl3, 400 MHz): δ 8.00-7.98 (m, 1H), 7.59-7.55 (m, 1H), 7.43-
132.7, 132.1, 129.4, 128.8, 124.8, 124.3, 122.8, 121.9, 117.0, 7.31 (m, 8H), 7.25-7.22 (m, 1H), 7.11-7.08 (m, 1H), 5.75 (d, J =
114.7, 109.1, 108.4, 103.9, 103.8, 101.2, 36.4.
4.8 Hz, 1H), 4.68 (d, J = 5.2 Hz, 1H); 13C NMR (CDCl3, 100
MHz): δ 161.4, 155.6, 152.9, 143.4, 143.1, 136.2, 132.2, 128.8,
128.6, 127.8, 127.4, 126.0, 124.4, 124.3, 122.8, 116.9, 114.4,
2-Phenyl-4-(thiophen-2-yl)-4H,5H-pyrano[3,2-c]chromen-5-
one (3i).17j White solid (279 mg, 78%), mp: 170-172 °C (lit.17j 103.9, 102.8, 36.6. Anal. Calcd For C22H14O3S: C, 73.73; H,
1
mp: 168-169 °C); H NMR (CDCl3, 400 MHz): δ 8.00-7.98 (m, 3.94%; Found: C, 73.78; H, 3.99%.
1H), 7.77-7.75 (m, 2H), 7.58-7.54 (m, 1H), 7.49-7.43 (m, 3H),
7.38-7.33 (m, 2H), 7.20-7.19 (m, 1H), 7.14-7.13 (m, 1H), 6.97- 2,4-Di-p-tolyl-4H,5H-pyrano[3,2-c]chromen-5-one
(3o):17a
1
6.95 (m, 1H), 5.94 (d, J = 5.2 Hz, 1H), 5.05 (d, J = 4.8 Hz, 1H); Light yellow oily (296 mg, 78%); H NMR (CDCl3, 400 MHz): δ
13C NMR (CDCl3, 100 MHz): δ 161.7, 155.6, 152.8, 147.8, 147.7, 8.03-8.00 (m, 1H), 7.63-7.61 (m, 2H), 7.58-7.51 (m, 2H), 7.39-
132.6, 132.3, 129.6, 128.9, 127.2, 125.7, 125.1, 125.0, 124.4, 7.24 (m, 5H), 7.12 (d, J = 8.0 Hz, 2H), 5.78 (d, J = 5.2 Hz, 1H),
122.9, 117.0, 114.7, 103.6, 103.0, 31.3.
4.66 (d, J = 4.8 Hz, 1H), 2.40 (s, 3H), 2.30 (s, 3H); 13C NMR
(CDCl3, 100 MHz): δ 161.7, 155.8, 152.9, 147.0, 143.3, 141.0,
139.4, 137.0, 132.0, 129.8, 129.4, 128.5 (2C), 124.7, 124.2,
4-(Furan-2-yl)-2-phenyl-4H,5H-pyrano[3,2-c]chromen-5-one
1
(3j): White solid (256 mg, 75%), mp: 126-127 °C; H NMR 122.8, 116.9, 114.8, 103.2, 36.3, 21.4, 21.2.
(CDCl3, 400 MHz): δ 8.02-8.00 (m, 1H), 7.75-7.73 (m, 2H), 7.61-
7.57 (m, 1H), 7.48-7.36 (m, 5H), 7.32-7.31 (m, 1H), 6.33-6.31 2-(2-Methoxyphenyl)-4-(p-tolyl)-4H,5H-pyrano[3,2-
(m, 1H), 6.26-6.25 (m, 1H), 5.85 (d, J = 5.2 Hz, 1H), 4.88 (d, J = c]chromen-5-one (3p): White solid (329 mg, 83%), mp: 144-
1
4.8 Hz, 1H); 13C NMR (CDCl3, 100 MHz): δ 161.6, 156.7, 155.1, 145 °C; H NMR (CDCl3, 400 MHz): δ 7.96-7.94 (m, 1H), 7.76-
151.0, 148.0, 142.1, 139.9, 132.3, 129.5, 128.8, 128.3, 124.9, 7.73 (m, 1H), 7.54-7.51 (m, 1H), 7.40-7.37 (3H), 7.35-7.31 (m,
124.4, 122.9, 117.0, 114.7, 110.8, 106.8, 100.9, 30.4. Anal. 2H), 7.15 (d, J = 8.0 Hz, 2H), 7.10-7.06 (m, 1H), 7.00 (d, J = 8.4
Calcd For C22H14O4: C, 77.18; H, 4.12%; Found: C, 77.12; H, Hz, 1H), 6.07 (d, J = 4.8 Hz, 1H), 4.71 (d, J = 5.2 Hz, 1H), 3.87
4.05%;
(s, 3H), 2.32 (s, 3H); 13C NMR (CDCl3, 100 MHz): δ 161.7, 157.3,
156.0, 152.8, 144.4, 141.0, 136.7, 131.8, 130.3, 129.3, 128.5,
2-(4-Methoxyphenyl)-4-phenyl-4H,5H-pyrano[3,2-c]chromen- 128.4, 124.1, 122.9, 122.0, 120.6, 116.8, 114.9, 111.5, 108.5,
5-one (3k).17i White solid (313 mg, 82%), mp: 154-155 °C (lit.17i 103.8, 55.7, 36.3, 21.2. Anal. Calcd For C26H20O4: C, 78.77; H,
1
mp: 156-157 °C); H NMR (CDCl3, 400 MHz): δ 7.86-7.84 (m, 5.09%; Found: C, 78.67; H, 5.02%.
1H), 7.53-7.50 (m, 2H), 7.42-7.38 (m, 1H), 7.30-7.28 (m, 2H),
7.23-7.16 (m, 4H), 7.11-7.08 (m, 1H), 6.84-6.81 (m, 2H), 5.55 4-(2-Chlorophenyl)-2-(4-methoxyphenyl)-4H,5H-pyrano[3,2-
(d, J = 4.8 Hz, 1H), 4.52 (d, J = 4.8 Hz, 1H), 3.70 (s, 3H); 13C NMR c]chromen-5-one (3q): White solid (341 mg, 82%), mp: 166-
1
(CDCl3, 100 MHz): δ 161.5, 160.4, 155.7, 152.7, 146.6, 143.8, 167 °C; H NMR (CDCl3, 400 MHz): δ 8.04-8.02 (m, 1H), 7.64-
7.61 (m, 3H), 7.42-7.37 (m, 3H), 7.20-7.16 (m, 3H), 6.96-6.93
12 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7GC01158J
ARTICLE
(m, 2H), 5.74 (d, J = 4.4 Hz, 1H), 5.21 (d, J = 4.4 Hz, 1H), 3.85 128.2, 127.9, 127.8, 125.8, 124.9, 122.4, 116.8, 114.4, 103.4,
(s, 3H); 13C NMR (CDCl3, 100 MHz): δ 160.6, 157.3, 153.1, 36.2, 21.2. Anal. Calcd For C25H17ClO3: C, 74.91; H, 4.27%;
146.8, 141.0, 133.1, 132.3, 131.1, 130.0, 129.6, 128.3, 127.9, Found: C, 74.85; H, 4.21%.
127.5, 126.3, 124.3, 122.8, 117.1, 114.5, 114.1, 102.2, 100.5,
55.5, 34.0. Anal. Calcd For C25H17ClO4: C, 72.03; H, 4.11%; 4-(Benzo[d][1,3]dioxol-5-yl)-9-methyl-2-phenyl-4H,5H-
Found: C, 71.96; H, 4.04%.
pyrano[3,2-c]chromen-5-one (3w). White solid (348 mg, 85%),
1
mp: 205-206 °C; H NMR (CDCl3, 400 MHz): δ 7.76-7.72 (m,
4-(4-Fluorophenyl)-2-(4-methoxyphenyl)-4H,5H-pyrano[3,2-
3H), 7.49-7.36 (m, 4H), 7.24 (d, J = 8.4 Hz, 1H), 6.90-6.87 (m,
c]chromen-5-one (3r): White solid (312 mg, 78%), mp: 155-157 2H), 6.76-6.74 (m, 1H), 5.91 (s, 2H), 5.81 (d, J = 4.8 Hz, 1H),
1
°C; H NMR (CDCl3, 400 MHz): δ 8.01-7.99 (m, 1H), 7.67-7.65 4.62 (d, J = 4.8 Hz, 1H), 2.49 (s, 3H); 13C NMR (CDCl3, 100
(m, 2H), 7.58-7.55 (m, 1H), 7.40-7.32 (m, 4H), 7.01-6.96 (m, MHz): δ 161.8, 155.8, 151.1, 148.0, 147.0, 146.8, 138.0, 134.1,
4H), 5.67 (d, J = 4.8 Hz, 1H), 4.67 (d, J = 4.8 Hz, 1H), 3.86 (s, 133.2, 132.9, 129.4, 128.8, 124.9, 122.4, 121.9, 116.7, 114.3,
1
3H); 13C NMR (CDCl3, 100 MHz): δ 162.0 (d, JC-F = 245 Hz), 109.1, 108.4, 104.0, 103.7, 101.2, 36.4, 21.2. Anal. Calcd For
3
161.6, 160.6, 155.8, 152.8, 147.0, 139.7 (d, JC-F = 3 Hz), 132.1, C26H18O5: C, 76.09; H, 4.42%; Found: C, 76.02; H, 4.34%.
4
130.2 (d, JC-F = 8 Hz), 126.2, 125.2, 124.3, 122.8, 116.9, 115.4
2
(d, JC-F = 21 Hz), 114.6, 114.2, 103.7, 101.7, 55.5, 36.0. Anal. 8-Hydroxy-2,4-diphenyl-4H,5H-pyrano[3,2-c]chromen-5-one
1
Calcd For C25H17FO4: C, 74.99; H, 4.28%; Found: C, 74.91; H, (3x). White solid (287 mg, 78%), mp: 233-235 °C; H NMR
4.34%.
(DMSO-d6, 400 MHz): δ 10.68 (s, 1H), 7.96 (d, J = 8.8 Hz, 1H),
7.85-7.82 (m, 2H), 7.50-7.43 (m, 3H), 7.32-7.31 (m, 4H), 7.23-
7.20 (m, 1H), 6.95-6.92 (m, 1H), 6.76 (d, J = 2.4 Hz, 1H), 6.13 (d,
9-Methyl-2,4-diphenyl-4H,5H-pyrano[3,2-c]chromen-5-one
(3s).17f White solid (289 mg, 79%), mp: 215-216 °C (lit.17f mp: J = 4.8 Hz, 1H), 4.58 (d, J = 5.2 Hz, 1H); 13C NMR (DMSO-d6, 100
1
216-218 °C); H NMR (CDCl3, 400 MHz): δ 7.78-7.73 (m, 3H), MHz): δ 161.7, 161.0, 156.1, 154.2, 145.4, 144.2, 132.1, 129.2,
7.47-7.36 (m, 6H), 7.33-7.29 (m, 2H), 7.24-7.22 (m, 2H), 5.84 128.8, 128.6, 128.0, 126.8, 124.4, 124.3, 113.5, 105.7, 104.1,
(d, J = 4.8 Hz, 1H), 4.71 (d, J = 4.8 Hz, 1H), 2.50 (s, 3H); 13C NMR 102.3, 99.3, 35.8. Anal. Calcd For C24H16O4: C, 78.25; H, 4.38%;
(CDCl3, 100 MHz): δ 161.9, 156.0, 151.1, 147.1, 143.8, 134.1, Found: C, 78.20; H, 4.29%.
133.2, 132.9, 129.9, 129.4, 128.8, 128.7, 128.6, 128.3, 127.3,
124.9, 122.4, 116.7, 104.0, 36.8, 21.2.
8-Hydroxy-4-phenyl-2-(p-tolyl)-4H,5H-pyrano[3,2-c]chromen-
1
5-one (3y). White solid (305 mg, 80%), mp: 244-245 °C; H
9-Methyl-4-phenyl-2-(p-tolyl)-4H,5H-pyrano[3,2-c]chromen-
NMR (DMSO-d6, 400 MHz): δ 10.69 (s, 1H), 7.94 (d, J = 8.8 Hz,
1
5-one (3t). White solid (312 mg, 82%), mp: 197-198 °C; H 1H), 7.71 (d, J = 8.4 Hz, 2H), 7.31-7.26 (m, 7H), 6.95-6.92 (m,
NMR (CDCl3, 400 MHz): δ 7.86-7.83 (m, 1H), 7.76 (s, 1H), 7.63- 1H), 6.76 (d, J = 2.0 Hz, 1H), 6.05 (d, J = 5.2 Hz, 1H), 4.56 (d, J =
7.61 (m, 2H), 7.42-7.37 (m, 3H), 7.31-7.28 (m, 3H), 7.24-7.22 4.8 Hz, 1H), 2.34 (s, 3H); 13C NMR (DMSO-d6, 100 MHz): δ
(m, 2H), 5.78 (d, J = 4.8 Hz, 1H), 4.69 (d, J = 4.8 Hz, 1H), 2.49 (s, 161.6, 160.9, 156.1, 154.0, 145.3, 144.2, 138.7, 129.3, 128.5,
3H), 2.41 (s, 3H); 13C NMR (CDCl3, 100 MHz): δ 161.9, 155.9, 127.9, 126.7, 124.2(2C), 113.4, 105.7, 103.1, 102.2, 99.2, 71.1,
151.1, 147.2, 143.9, 139.4, 134.0, 133.1, 129.5, 129.4, 128.7, 35.7, 20.8. Anal. Calcd For C25H18O4: C, 78.52; H, 4.74%; Found:
128.6, 128.4, 127.6, 127.3, 124.8, 122.4, 116.7, 103.1, 36.8, C, 78.58; H, 4.64%.
21.5, 21.2. Anal. Calcd C26H20O3: C, 82.08; H, 5.30%; Found: C,
82.02; H, 5.35%.
4-(Benzo[d][1,3]dioxol-5-yl)-8-hydroxy-2-phenyl-4H,5H-
pyrano[3,2-c]chromen-5-one (3z). White solid (338 mg, 82%),
mp: 215-218 °C; 1H NMR (DMSO-d6, 400 MHz): δ 10.69 (s, 1H),
2-(4-Methoxyphenyl)-9-methyl-4-phenyl-4H,5H-pyrano[3,2-
c]chromen-5-one (3u). White solid (321 mg, 81%), mp: 176- 7.94 (d, J = 8.8 Hz, 1H), 7.83-7.81 (m, 2H), 7.49-7.42 (m, 3H),
177 °C; 1H NMR (CDCl3, 400 MHz): δ 7.76 (s, 1H), 7.68-7.65 (m, 6.94-6.91 (m, 1H), 6.87-6.82 (m, 2H), 6.76-6.74 (m, 2H), 6.08
2H), 7.41-7.20 (m, 7H), 6.99-6.97 (m, 2H), 5.70 (d, J = 4.8 Hz, (d, J = 4.8 Hz, 1H), 5.96 (s, 2H), 4.50 (d, J = 4.8 Hz, 1H); 13C NMR
1H), 4.68 (d, J = 5.2 Hz, 1H), 3.86 (s, 3H), 2.49 (s, 3H); 13C NMR (DMSO-d6, 100 MHz): δ 161.6, 161.0, 156.0, 154.1, 147.3,
(CDCl3, 100 MHz): δ 161.9, 160.5, 155.9, 151.1, 144.0, 134.0, 146.1, 145.2, 138.3, 132.0, 129.2, 128.8, 124.3, 124.2, 121.1,
133.1, 128.7, 128.6, 127.2, 126.3, 125.5, 122.4, 116.7, 114.4, 113.4, 108.5, 108.2, 105.7, 104.1, 102.2, 100.9, 99.3, 35.4.
114.2, 103.8, 102.2, 89.9, 55.6, 36.7, 21.2. Anal. Calcd For Anal. Calcd For C25H16O6: C, 72.81; H, 3.91%; Found: C, 72.71;
Chemical Formula: C26H20O4: C, 78.77; H, 5.09%; Found: C, H, 3.84%.
78.67; H, 5.02%.
Acknowledgements
4-(4-Chlorophenyl)-9-methyl-2-phenyl-4H,5H-pyrano[3,2-
c]chromen-5-one (3v). White solid (332 mg, 83%), mp: 173-
1
174 °C; H NMR (CDCl3, 400 MHz): δ 7.74-7.69 (m, 3H), 7.45-
A. Majee acknowledges the financial support from the DST-RSF
Major Research Project (Ref. No. INT/RUS/RSF/P-08). S. Santra
7.42 (m, 2H), 7.33-7.31 (m, 2H), 7.25-7.22 (m, 3H), 7.16-7.14
and G. V. Zyryanov acknowledge the Russian Science
(m, 1H), 7.11-7.09 (m, 1H), 5.77 (d, J = 4.8 Hz, 1H), 4.71 (d, J =
5.2 Hz, 1H), 2.47 (s, 3H); 13C NMR (CDCl3, 100 MHz): δ 161.8,
Foundation – Russia (Ref. # 16-43-02020) for funding. We are
thankful to DST-FIST and UGC-SAP programme.
156.0, 151.1, 142.3, 134.2, 133.4, 130.0, 129.5, 128.9, 128.4,
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20 Further information can be found in the CIF file. The crystal
data are deposited in the Cambridge Crystallographic Data
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