Copper-catalyzed regioselective synthesis of furan via tandem cycloaddition of ketone with an unsaturated carboxylic acid under air

Article abstract of DOI:10.1039/c4ob01320d

A catalytic decarboxylative annulation has been developed for the regioselective synthesis of trisubstituted furans by the cycloaddition of ketones with α,β-unsaturated carboxylic acids under ambient air. A library of furan derivatives were obtained in good yields from the readily available substrates in the combination of a catalytic amount of Cu-salt and a stoichiometric amount of water. Water plays a crucial role in this catalytic transformation. This journal is

Full text of DOI:10.1039/c4ob01320d

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Copper-catalyzed regioselective synthesis of furan
via tandem cycloaddition of ketone with an
unsaturated carboxylic acid under air†
Cite this: DOI: 10.1039/c4ob01320d
Monoranjan Ghosh, Subhajit Mishra, Kamarul Monir and Alakananda Hajra*
A catalytic decarboxylative annulation has been developed for the regioselective synthesis of trisubstituted
furans by the cycloaddition of ketones with α,β-unsaturated carboxylic acids under ambient air. A library
of furan derivatives were obtained in good yields from the readily available substrates in the combination
of a catalytic amount of Cu-salt and a stoichiometric amount of water. Water plays a crucial role in this
catalytic transformation.
Received 25th June 2014,
Accepted 20th October 2014
DOI: 10.1039/c4ob01320d
www.rsc.org/obc
nes4f etc. However, the synthetic potential of these strategies is
limited by the requirement of functionalized precursors,
Introduction
Polysubstituted furans represent an important class of five- which can only be obtained through specialized protocols
membered heterocycles ubiquitous in natural products, pharma- involving multi-step procedures. As a consequence, the devel-
ceuticals, and agrochemicals as well as act as useful inter- opment of a new synthetic route to synthesize furan derivatives
mediates in organic synthesis.1 The multisubstituted furan is from simple and commercially available starting materials is
a basic scaffold of various biologically active alkaloids and still in demand.
pharmaceutical drugs, such as dantrolene and (+)-(Z)-deoxy-
Transition metal-catalyzed decarboxylative coupling has
pukalide (Fig. 1).
emerged as a powerful tool to form carbon–carbon or carbon–
Various methods have been reported for the synthesis of heteroatom bonds starting from carboxylic acids.5 Use of carb-
furan derivatives due to their important biological activities oxylic acid has several advantages due to its ready availability,
and usefulness.2 Classical approaches such as Paal–Knorr easy handling as well as environmentally benign properties.
synthesis3a,b and Feist–Benary synthesis3c–f have already been Moreover, it produces nontoxic CO2 as the byproduct. From
established for the assembly of polysubstituted furans from the point of view of atom and process-economical chemistry,
dicarbonyl compounds. In addition, several methodologies the most concise route with reduced waste and fewer steps is
have been reported from the prefunctionalized acyclic ketone needed.
derivatives such as cyclopropenyl ketones,4a β-acyloxy acetylenic
To the best of our knowledge, no catalytic method has been
ketones,4b allenyl ketones,4c o-alkoxyketones,4d,e α-aryloxyketo- reported for the synthesis of furans via decarboxylative annula-
tion reaction. Recently, Zhang et al. developed an elegant pro-
tocol for the synthesis of furan derivatives through
a
decarboxylative cyclization between ketones and α,β-unsatu-
rated carboxylic acids.6 However, stoichiometric amounts of
two different copper salts are required for this reaction. Cata-
lytic decarboxylative methods are well-known and have been
used in various chemical transformations.7 Based on our
experience in copper catalysis,8 we envisaged that the synthesis
of furans might be possible by the annulation of ketones with
α,β-unsaturated carboxylic acids via decarboxylative cyclo-
addition using a catalytic amount of copper salt in the pres-
ence of a suitable oxidant. Generally, oxidants such as PIDA,
K2S2O8, TBHP, O2 etc. are used for copper-catalyzed oxidative
coupling reactions.9 Among them molecular O2 is considered
to be the most ideal oxidant due to its high abundance, low
cost, and environment friendly nature and therefore shows a
great demand in the industrial prospects.10 Moreover, water is
Fig. 1 Some biologically active furan containing drugs.
Department of Chemistry, Visva-Bharati University, Santiniketan-731235, India.
E-mail: alakananda.hajra@visva-bharati.ac.in
†Electronic supplementary information (ESI) available: All 1H and 13C NMR
spectra. See DOI: 10.1039/c4ob01320d
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Table 1 Optimization of the reaction conditionsa
Scheme 1 Catalytic decarboxylative synthesis of furans.
Additive
(equiv.)
Yieldb
1a : 2a (%)
Entry Catalyst (mol%)
Solvent
produced as the only byproduct using molecular O2 as an
oxidant. So, the construction of heterocycles from simple and
readily accessible substrates using molecular O2 is compatible
with the increasing requirements for green chemistry and an
efficient process.
Recently, we have reported the synthesis of arenofuran
derivatives by the coupling of phenols/naphthols with nitro-
alkenes via tandem Michael addition/denitration.11 In continu-
ation of our research interest in furan synthesis as well as in
coupling between nitroalkenes with various nucleophiles,12
herein we report an efficient synthetic method for the synthesis
of 2,3,5-trisubstituted furans via decarboxylative annulation
employing a catalytic amount of copper salt in the presence of a
stoichiometric amount of water under ambient air (Scheme 1).
1.
2.
3.
4.
5.
6.
7.
8.
Cu(OAc)2·H2O (10)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (20)
Cu(OTf)2 (20)
—
—
DMA
DMA
DMA
DMA
DMA
DMA
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
2 : 1
1 : 2
1 : 1
n.d.c
5
33
40
<5
8
15
22
44
55
67
45
20
37
22
25
32
60
43
45
n.d.
n.d.
AcOH (1)
TFA (1)
HCl (1)
p-TSA (1)
NaOAc (1) DMA
KOAc (1)
Water (1)
Water (2)
Water (3)
Water (3)
Water (3)
Water (3)
Water (3)
Water (3)
Water (3)
Water (3)
Water (3)
DMA
DMA
DMA
DMA
DMA
DMA
NMP
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
Toluene 1 : 1
Xylene
DMSO
DMF
DMA
DMA
DMA
DMA
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
Cu(ClO4)2·6H2O (20) Water (3)
Results and discussion
CuCl2 (20)
CuBr2 (20)
Water (3)
Water (3)
We commenced our study by choosing propiophenone and
cinnamic acid (1 : 1) as the model substrates to find out the
optimized reaction conditions using various copper salts, addi-
tives and solvents. The results are summarized in Table 1.
Initially, the reaction was carried out employing 10 mol%
Cu(OAc)2·H2O in DMA at 120 °C under an air atmosphere.
The reaction did not proceed at all even after 24 h (Table 1,
entry 1). On increasing the amount of Cu(OAc)2·H2O (20 mol%),
the reaction proceeded in very low conversion (Table 1, entry 2).
Encouraged by this result, we turned our attention to check
the role of an acid and a base in this transformation. In the
presence of various acids (1 equiv.) the desired products were
obtained in low to moderate yields (Table 1, entries 3–6).
Similar effects were observed in the case of bases (Table 1,
entries 7 and 8). On the other hand, in the presence of 1
equiv. of water, the reaction proceeded with 44% yield
(Table 1, entry 9). On increasing the amount of H2O (3 equiv.),
the desired product was obtained in 67% yield (Table 1,
entry 11). The yield of the product was decreased by changing
the molar ratios of the reactants (Table 1, entries 12 and 13).
Different solvents like NMP, toluene, xylene, DMSO and DMF
(Table 1, entries 14–18) were also screened, among them DMA
gave the best result compared to others (Table 1, entry 11). Use of
other Cu-salts such as Cu(OTf)2, Cu(ClO4)2·6H2O resulted in
lower yields (Table 1, entries 19 and 20) whereas CuCl2 and
CuBr2 were almost ineffective for this reaction (Table 1, entries
21 and 22). Finally, the optimized reaction conditions were
achieved using the combination of propiophenone and cinnamic
a Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), solvent: 2 mL,
120 °C, 7 h, air. b Isolated yield. c n.d. = not determined. DMA = N,N-
dimethylacetamide. DMF
=
N,N-dimethylformamide. DMSO
=
dimethylsulfoxide. NMP = N-methyl-2-pyrrolidone.
Table 2 Decarboxylative coupling with propiophenonea
a Reaction conditions: 1a (0.5 mmol), 2 (0.5 mmol), water (0.27 mL,
3 equiv.), DMA (2 mL), 120 °C, 7 h, air. b 5 h; isolated yields.
acid (1 : 1), 20 mol% Cu(OAc)2·H2O in the presence of 3 equiv. of investigated. At first, various cinnamic acid derivatives were
water at 120 °C for 7 h under ambient air (Table 1, entry 11).
To explore the general applicability, scope and limitation
examined (Table 2). Electron-donating substituents (–Me,
–OMe) in cinnamic acid produced the corresponding furans
of this method, various cinnamic acids and ketones were in moderate to good yields (3ab and 3ac). Moreover, halogen
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substituents in the aryl moiety also produced the furan deriva-
tives with moderate yields (3ae–3ai). Substituents at the
different positions in the aryl moiety slightly affected the
yields of this reaction. Interestingly, in all cases only one
regioisomeric product was obtained. Heteroaryl unsaturated
carboxylic acids were also examined and were found to be
effective for the synthesis of furan derivatives (3aj and 3ak).
Furthermore, various commercially available ketones were
also tested to explore the scope of the reaction (Table 3). Aryl
alkyl ketones, such as butyrophenone (3ba, 3bd and 3bf) and
2-phenyl acetophenone (3cb and 3cf) reacted smoothly to afford
the desired products. Cyclic ketones such as α-tetralone, cyclo-
hexanone and cyclododecanone also produced the corresponding
furan derivatives (3da, 3ea and 3fa) in moderate yields. However,
no reaction occurred in the case of acyclic ketones (ethylmethyl
ketone and isobutylmethyl ketone) and crotonic acid.
Some controlled experiments were performed to evaluate
the mechanism of this reaction (Scheme 2). When the reaction
was carried out in the absence of copper, the reaction did not
proceed at all (eqn (1)), which signifies that the presence of
copper as a catalyst is necessary in this reaction. Only a trace
amount of product was detected under an argon atmosphere
(eqn (2)), which confirms that molecular O2 (air) is essential
for the completion of the catalytic cycle. It is noteworthy to
mention that styrene did not produce the desired product
under the optimized reaction conditions.6 Moreover, we did
not observe the formation of styrene during this reaction. This
result indicates that the reaction did not proceed through the
formation of a styrene intermediate. Moreover, methyl cinna-
mate was unreacted under the present reaction conditions
(eqn (3)). Furthermore the reaction probably does not follow a
radical pathway6,13 as the radical scavengers like quinone
and 2,6-di-tert-butyl-4-methylphenol (BHT) (1.5 equiv.) did not
inhibit the reaction (eqn (4) and (5)).
Scheme 2 Some controlled experiments.
Table 3 Coupling of various ketones with carboxylic acidsa
Scheme 3 Probable reaction mechanism.
On the basis of results obtained from these controlled
experiments, a probable mechanism of the reaction is depicted
in Scheme 3. Nucleophilic addition of the hydroxyl group in
a Reaction conditions: 1 (0.5 mmol), 2 (0.5 mmol), water (0.27 mL,
3 equiv.), DMA (2 mL), 120 °C, 7 h, air; isolated yields.
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enol A, in situ generated from ketones, to cinnamic acids in
5-(4-Methyl-5-phenylfuran-2-yl)-benzo[1,3]dioxole (3ad).6
the presence of Cu(II) salt affords alkyl–Cu(II) intermediate C. White solid (78 mg, 54% yield). Mp 110–112 °C (lit. 112–113 °C);
The intermediate C reacts with enol to lead the intermediate D 1H NMR (400 MHz, CDCl3): δ 7.60 (d, J = 8.4 Hz, 2H), 7.34 (t,
which is oxidized to give the C–Cu(III)–C intermediate E.14 The J = 8.0 Hz, 2H), 7.19–7.10 (m, 3H), 6.75 (d, J = 8.4 Hz, 1H), 6.38
intermediate F is formed from E via reductive elimination (s, 1H), 5.89 (s, 2H), 2.23 (s, 3H); 13C NMR (100 MHz, CDCl3):
and subsequently oxidative aromatization occurs to convert δ 151.6, 148.1, 147.7, 147.0, 131.9, 128.6, 126.6, 125.4, 125.2,
the furan intermediate G. Finally the intermediate G affords 118.7, 117.7, 109.8, 108.7, 104.5, 101.2, 12.2.
the desired furan derivative via decarboxylation.15 Although the
specific role of water is not clear at the present stage, it probably solid (74 mg, 59% yield). Mp 85–86 °C (lit. 87–88 °C); H NMR
5-(4-Fluorophenyl)-3-methyl-2-phenylfuran (3ae).6 White
1
facilitates the formation of enol.
(400 MHz, CDCl3): δ 7.63–7.57 (m, 4H), 7.35 (t, J = 8.0 Hz, 2H),
7.21–7.17 (m, 1H), 7.02–6.98 (m, 2H), 6.46 (s, 1H), 2.24 (s, 3H);
1
13C NMR (100 MHz, CDCl3): δ 162.2 (d, JC–F = 246 Hz), 150.9,
2
Conclusions
148.3, 131.8, 128.7, 127.2, 126.8, 125.5 (d, JC–F = 8 Hz), 118.8,
115.8 (d, 3JC–F = 22 Hz), 110.6, 12.2.
5-(4-Chlorophenyl)-3-methyl-2-phenylfuran
In summary, we have developed a copper-catalyzed regioselec-
tive synthesis of 2,3,5-trisubstituted furans from the readily
available ketones and α,β-unsaturated carboxylic acids via de-
carboxylative annulation. Water plays a crucial role in this trans-
formation. Operational simplicity, broad substrates scope, low
cost of the catalyst, and use of green oxidant make this proto-
col a practical one in diversity-oriented synthesis.
(3af).6 White
1
solid (82 mg, 61% yield). Mp 81–82 °C (lit. 82–83 °C); H NMR
(400 MHz, CDCl3): δ 7.74 (d, J = 7.6 Hz, 2H), 7.65 (d, J = 8.8 Hz,
2H), 7.48 (t, J = 8.0 Hz, 2H), 7.39–7.37 (m, 2H), 7.35–7.33 (m,
1H), 6.60 (s, 1H), 2.35 (s, 3H); 13C NMR (100 MHz, CDCl3): δ
150.7, 148.6, 132.8, 131.6, 129.3, 128.9, 128.7, 126.9, 125.3,
124.9, 118.8, 111.3, 12.1.
5-(4-Bromophenyl)-3-methyl-2-phenylfuran (3ag). White
solid (99 mg, 63% yield). Mp 84–85 °C; 1H NMR (400 MHz,
CDCl3): δ 7.73 (d, J = 8.4 Hz, 2H), 7.65 (d, J = 8.4 Hz, 2H),
7.48–7.45 (m, 2H), 7.39–7.36 (m, 1H), 7.34–7.28 (m, 1H), 6.61
(s, 1H), 2.35 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 150.7,
Experimental section
Typical experimental procedure for the synthesis of 3-methyl-
2,5-diphenylfuran product (3aa)
A mixture of propiophenone (1a, 0.5 mmol, 67 mg), cinnamic 148.6, 132.8, 131.6, 129.3, 129.0, 128.7, 126.9, 125.3, 124.9,
acid (2a, 0.5 mmol, 74 mg), Cu(OAc)2·H2O (20 mol%, 19 mg) 123.7, 118.8, 111.3, 12.2. Anal. Calcd for C17H13BrO: C, 65.19;
and H2O (0.27 mL, 3 equiv.) in DMA (2 mL) was taken in a H, 4.18%; Found: C, 65.17; H, 4.21%.
reaction vessel and stirred at 120 °C for 7 h under an open
5-(2-Chlorophenyl)-3-methyl-2-phenylfuran (3ah).6 Gummy
1
atmosphere. After completion of the reaction (TLC), the reac- liquid (73 mg, 54% yield); H NMR (400 MHz, CDCl3): δ 7.86
tion mixture was cooled to room temperature, and extracted (dd, J1 = 8.4 Hz, J2 = 2.0 Hz, 1H), 7.63 (dd, J1 = 8.0 Hz, J2 = 1.2
with ethyl acetate. The organic phase was dried over anhydrous Hz, 2H), 7.37–7.33 (m, 3H), 7.25–7.18 (m, 2H), 7.10–7.06
Na2SO4. The crude residue was obtained after evaporating the (m, 1H), 7.01 (s, 1H), 2.26 (s, 3H); 13C NMR (100 MHz, CDCl3):
solvent in vacuum and was purified by column chromato- δ 148.3, 147.9, 131.6, 130.8, 130.0, 129.2, 128.7, 127.8, 127.7,
graphy on silica gel (60–120 mesh) using petroleum ether as 127.0, 126.9, 125.5, 118.7, 116.8, 12.2.
an eluent to afford the pure product as a white solid (75 mg,
5-(2-Bromophenyl)-3-methyl-2-phenylfuran (3ai). White
67% yield). Mp 45–46 °C (lit.6 46–47 °C); H NMR (400 MHz, solid (87 mg, 56% yield), Mp 81–82 °C; 1H NMR (400 MHz,
CDCl3): δ 7.62 (d, J = 7.6 Hz, 4H), 7.36–7.27 (m, 4H), 7.18–7.14 CDCl3): δ 7.87 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 1H), 7.64 (dd, J1 =
(m, 2H), 6.51 (s, 1H), 2.23 (s, 3H); 13C NMR (100 MHz, CDCl3): 8.0 Hz, J2 = 1.2 Hz, 2H), 7.38–7.34 (m, 3H), 7.26–7.20 (m, 2H),
δ 151.7, 148.3, 131.8, 130.9, 128.7, 128.6, 127.3, 126.7, 125.3, 7.12–7.08 (m, 1H), 7.02 (s, 1H), 2.27 (s, 3H); 13C NMR
1
123.7, 118.7, 110.9, 12.2.
(100 MHz, CDCl3): δ 148.3, 148.0, 131.6, 130.8, 130.0, 129.2,
3-Methyl-2-phenyl-5-p-tolylfuran (3ab).6 White solid (79 mg, 128.7, 127.8, 127.7, 127.0, 127.0, 125.5, 118.7, 116.8, 12.2.
64% yield). Mp 80–82 °C (lit. 78–79 °C); 1H NMR (400 MHz, Anal. Calcd for C17H13BrO: C, 65.19; H, 4.18%; Found:
CDCl3): δ 7.77 (d, J = 7.6 Hz, 2H), 7.67 (d, J = 8.0 Hz, 2H), 7.48 C, 65.17; H, 4.20%.
(t, J = 8.0 Hz, 2H), 7.32 (t, J = 7.2 Hz, 1H), 7.28–7.24 (m, 2H),
4-Methyl-5-phenyl-[2,2′]bifuranyl (3aj). Colorless oil (63 mg,
1
6.60 (s, 1H), 2.42 (s, 3H), 2.37 (s, 3H); 13C NMR (100 MHz, 54% yield); H NMR (500 MHz, CDCl3): δ 7.67 (d, J = 8.0 Hz,
CDCl3): δ 151.9, 147.7, 137.0, 131.8, 129.3, 128.5, 128.1, 126.5, 2H), 7.41 (t, J = 8.0 Hz, 3H), 7.28–7.25 (m, 2H), 6.59 (d, J =
125.1, 123.6, 118.6, 110.1, 21.2, 12.1.
3.0 Hz, 1H), 6.50 (s, 1H), 6.47–6.46 (m, 1H), 2.31 (s, 3H);
5-(4-Methoxyphenyl)-3-methyl-2-phenylfuran (3ac).6 White 13C NMR (125 MHz, CDCl3): δ 148.1, 146.8, 144.6, 141.9, 131.6,
1
solid (71 mg, 54% yield). Mp 97–98 °C (lit. 98–99 °C); H NMR 131.2, 128.6, 126.9, 125.4, 118.3, 111.5, 110.8, 105.2, 12.1. Anal.
(400 MHz, CDCl3): δ 7.63–7.56 (m, 4H), 7.35 (t, J = 8.0 Hz, 2H), Calcd for C15H12O2: C, 80.34; H, 5.39%; Found: C, 80.35; H, 5.42%.
7.20–7.18 (m, 1H), 6.85 (d, J = 8.8 Hz, 2H), 6.40 (s, 1H), 3.76
3-Methyl-2-phenyl-5-thiophen-2-yl-furan (3ak).6 Colorless oil
1
(s, 3H), 2.24 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 159.1, (68 mg, 56% yield); H NMR (300 MHz, CDCl3): δ 7.70 (d, J =
151.9, 147.6, 132.0, 128.6, 126.5, 125.2, 125.2, 124.0, 118.7, 7.4 Hz, 2H), 7.44 (t, J = 7.8 Hz, 2H), 7.31–7.28 (m, 2H),
114.3, 109.4, 55.4, 12.2.
7.26–7.22 (m, 1H), 7.07–7.04 (m, 1H), 6.48 (s, 1H), 2.32 (s, 1H);
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13C NMR (75 MHz, CDCl3): δ 147.8, 147.4, 133.8, 131.5, 128.5, 23.2, 23.2, 22.2. Anal. Calcd for C14H14O: C, 84.81; H, 7.12%;
127.6, 126.7, 125.2, 124.0, 122.3, 118.6, 110.7, 12.0.
Found: C, 84.85; H, 7.19%.
3-Ethyl-2,5-diphenylfuran (3ba). Colorless oil (79 mg, 64%
2-Phenyl-4,5,6,7,8,9,10,11,12,13-decahydrocyclododeca[b]-
yield); 1H NMR (400 MHz, CDCl3): δ 7.76–7.71 (m, 4H), furan (3fa). Colorless liquid (48 mg, 34% yield); 1H NMR
7.48–7.39 (m, 4H), 7.33–7.28 (m, 2H), 6.72 (s, 1H), 2.77 (q, J = (400 MHz, CDCl3): δ 7.62–7.60 (m, 2H), 7.35–7.31 (m, 2H),
7.6 Hz, 2H), 1.33 (t, J = 7.6 Hz, 3H); 13C NMR (100 MHz, 7.20–7.16 (m, 1H), 6.46 (s, 1H), 2.66 (t, J = 6.8 Hz, 2H), 2.40
CDCl3): δ 151.9, 147.5, 131.7, 130.8, 128.6, 128.5, 128.4, 127.1, (t, J = 6.8 Hz, 2H), 1.82–1.76 (m, 2H), 1.69–1.62 (m, 2H), 1.36–1.32
126.7, 125.4, 125.3, 125.1, 123.6, 108.6, 19.2, 14.3. Anal. Calcd (m, 8H), 1.27–1.24 (m, 4H); 13C NMR (100 MHz, CDCl3):
for C18H16O: C, 87.06; H, 6.49%; Found: C, 87.08; H, 6.52%.
δ 151.6, 151.3, 131.5, 128.6, 126.6, 123.3, 121.9, 106.6, 27.7,
5-(4-Ethyl-5-phenylfuran-2-yl)-benzo[1,3]dioxole (3bd). 26.3, 24.9, 24.8, 24.5, 24.4, 23.0, 22.4, 22.3, 21.6. Anal. Calcd
Yellow oil (72 mg, 50% yield); 1H NMR (400 MHz, CDCl3): for C20H26O: C, 85.06; H, 9.28%; Found: C, 85.09; H, 9.22%.
δ 7.58 (d, J = 8.0 Hz, 2H), 7.36–7.31 (m, 3H), 7.19–7.11 (m, 3H),
6.75 (d, J = 8.0 Hz, 1H), 6.45 (s, 1H), 5.88 (s, 2H), 2.63 (q, J =
7.6 Hz, 2H), 1.20 (t, J = 7.6 Hz, 3H); 13C NMR (100 MHz,
CDCl3): δ 151.9, 148.1, 147.1, 147.0, 131.8, 129.6, 128.6, 128.2,
126.7, 125.4, 117.6, 108.7, 107.7, 104.5, 101.2, 19.4, 14.5. Anal.
Calcd for C19H16O3: C, 78.06; H, 5.52%; Found: C, 78.09; H, 5.58%.
5-(4-Chlorophenyl)-3-ethyl-2-phenylfuran (3bf). Gummy
liquid (87 mg, 62% yield); 1H NMR (400 MHz, CDCl3):
δ 7.71–7.65 (m, 4H), 7.46 (t, J = 7.2 Hz, 2H), 7.39–7.36 (m, 2H),
7.34–7.28 (m, 1H), 6.70 (s, 1H), 2.76 (q, J = 7.6 Hz, 2H), 1.32
(t, J = 7.6 Hz, 3H); 13C NMR (100 MHz, CDCl3): δ 151.0, 148.1,
132.8, 131.6, 129.8, 129.0, 128.7, 127.1, 125.6, 125.6, 124.9,
109.2, 19.4, 14.5. Anal. Calcd for C18H15ClO: C, 76.46;
H, 5.35%; Found: C, 76.50; H, 5.36%.
Acknowledgements
A. H. acknowledges the financial support from CSIR, New
Delhi (grant no. 02(0168)/13/EMR-II). We are thankful to
DST-FIST and UGC-SAP. M. G. and S. M. thank UGC and
K. M. thanks CSIR for their fellowships. Authors thank the
reviewers for their valuable suggestions and comments to
improve the quality of the paper.
Notes and references
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2,3-Diphenyl-5-p-tolylfuran (3cb). White solid (87 mg, 57%
yield). Mp 99–100 °C; 1H NMR (400 MHz, CDCl3): δ 7.57 (d, J =
8.0 Hz, 2H), 7.52 (d, J = 8.4 Hz, 2H), 7.40–7.37 (m, 2H),
7.32–7.28 (m, 2H), 7.26–7.20 (m, 3H), 7.17–7.13 (m, 3H), 6.67
(s, 1H), 2.30 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 152.9,
147.6, 137.5, 134.5, 131.3, 129.5, 128.8, 128.7, 128.5, 127.9,
127.5, 127.3, 126.2, 124.6, 123.9, 108.9, 21.4. Anal. Calcd for
C23H18O: C, 89.00; H, 5.85%; Found: C, 89.05; H, 5.92%.
5-(4-Chlorophenyl)-2,3-diphenylfuran (3cf). White solid
1
(98 mg, 60% yield). Mp 93–94 °C; H NMR (400 MHz, CDCl3):
δ 7.71 (d, J = 8.4 Hz, 2H), 7.62 (d, J = 8.0 Hz, 2H), 7.50–7.47
(m, 2H), 7.45–7.38 (m, 4H), 7.38–7.34 (m, 2H), 7.32–7.27
(m, 2H), 6.83 (s, 1H); 13C NMR (100 MHz, CDCl3): δ 151.6,
148.3, 134.2, 133.2, 131.0, 129.1, 129.1, 128.8, 128.8, 128.5,
127.8, 127.5, 126.3, 125.1, 124.7, 110.0. Anal. Calcd for
C22H15ClO: C, 79.88; H, 4.57%; Found: C, 79.85; H, 4.60%.
2-Phenyl-4,5-dihydronaphtho[1,2-b]furan (3da). Colourless
1
liquid (66 mg, 54% yield); H NMR (400 MHz, CDCl3): δ 7.72
(d, J = 8.8 Hz, 2H), 7.56 (d, J = 7.2 Hz, 1H), 7.41–7.37 (m, 2H),
7.27–7.23 (m, 2H), 7.20–7.18 (m, 1H), 7.13–7.10 (m, 1H), 6.62
(s, 1H), 2.99 (t, J = 8.0 Hz, 2H), 2.77 (t, J = 8.0 Hz, 2H);
13C NMR (100 MHz, CDCl3): δ 153.3, 149.8, 134.8, 131.1, 128.8,
128.1, 128.0, 127.2, 126.9, 126.5, 123.7, 121.6, 119.2, 106.6,
29.1, 21.1. Anal. Calcd for C18H14O: C, 87.78; H, 5.73%; Found:
C, 87.75; H, 5.79%.
2-Phenyl-4,5,6,7-tetrahydrobenzofuran (3ea). Yellow liquid
(40 mg, 40% yield); 1H NMR (400 MHz, CDCl3): δ 7.62–7.59
(m, 2H), 7.35–7.31 (m, 2H), 7.20–7.16 (m, 1H), 6.46 (s, 1H),
2.65 (t, J = 7.6 Hz, 2H), 2.45 (t, J = 7.6 Hz, 2H), 1.88–1.83
(m, 2H), 1.77–1.72 (m, 2H); 13C NMR (100 MHz, CDCl3):
δ 151.7, 150.9, 131.5, 128.6, 126.6, 123.3, 119.1, 106.1, 23.4,
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