Diversified Synthesis of Furans by Coupling between Enols/1,3-Dicarbonyl Compounds and Nitroolefins: Direct Access to Dioxa[5]helicenes

Article abstract of DOI:10.1002/asia.201500710

A versatile method for the diversified synthesis of furans and arenofurans has been developed that proceeds through K₂CO₃-promoted cyclization between enols/1,3-dicarbonyl compounds and nitroolefins at reflux in EtOH. This facile method has been successfully employed in the synthesis of benzotrifuran derivatives, which are useful hole-transporting materials. This procedure also provides direct access to dioxa[5]helicenes. This reaction offers a broad substrate scope, uses an inexpensive base and environmentally benign solvent, and is operationally simple.

Full text of DOI:10.1002/asia.201500710

DOI: 10.1002/asia.201500710
Full Paper
Helicenes
Diversified Synthesis of Furans by Coupling between Enols/1,3-
Dicarbonyl Compounds and Nitroolefins: Direct Access to
Dioxa[5]helicenes
Monoranjan Ghosh, Sougata Santra, Pallab Mondal, Dhiman Kundu, and
Alakananda Hajra*[a]
Abstract: A versatile method for the diversified synthesis of
furans and arenofurans has been developed that proceeds
through K2CO3-promoted cyclization between enols/1,3-di-
carbonyl compounds and nitroolefins at reflux in EtOH. This
facile method has been successfully employed in the synthe-
sis of benzotrifuran derivatives, which are useful hole-trans-
porting materials. This procedure also provides direct access
to dioxa[5]helicenes. This reaction offers a broad substrate
scope, uses an inexpensive base and environmentally
benign solvent, and is operationally simple.
Introduction
Polysubstituted furan derivatives are an important class of het-
erocycles and are widely found in nature.[1] These compounds
serve as useful building blocks for various biologically active
molecules, functional materials, and agrochemicals, as well as
useful intermediates in organic synthesis.[2]
The substituted furan ring is also a key scaffold in various
pharmaceutical drugs (Figure 1).[3]
Owing to their broad range of activities, several methods
have been developed for the synthesis of furan derivatives.
The important classical approaches are the Paal–Knorr[4a,b] and
Feist–Benary syntheses,[4c,d] which have been established for
the assembly of polysubstituted furans from dicarbonyl com-
pounds. Prefunctionalized acyclic ketone derivatives, such as
a-aryloxyketones,[5a] o-alkoxyketones,[5b,c] cyclopropenyl keto-
nes,[5d] allenyl ketones,[5e] and b-acyloxy acetylenic ketones,[5f]
have also been employed as key starting materials for the con-
struction of this scaffold. However, these functionalized precur-
sors are generally synthesized through specialized procedures
that involve multi-step procedures. Transition-metal-catalyzed
synthetic strategies that involve inter-/intramolecular coupling
reactions have also been very effective for the synthesis of
furan derivatives.[6] Arenofurans are another important class of
heterocycles and many strategies have been developed for
their synthesis. Most of the reported methods have involved
Figure 1. Important furan-containing molecules.
[a] M. Ghosh, Dr. S. Santra, Dr. P. Mondal, Dr. D. Kundu, Dr. A. Hajra
Department of Chemistry
transition metals as the catalyst and phenol or its derivatives
as the starting material.[7] However, practical and general meth-
ods for the synthesis of these derivatives remain limited. There-
fore, the development of new strategies for the synthesis of
furan derivatives that use basic chemicals as starting materials
and offer higher efficiencies and substrate scope, operational
simplicity, and economic practicability is highly desirable.
Visva-Bharati (A Central University)
Santiniketan
731235, West Bengal (India)
E-mail: alakananda.hajra@visva-bharati.ac.in
Homepage: http://www.visvabharati.ac.in/AlakanandaHajra.html
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201500710.
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Conjugated nitroalkenes are efficient Michael acceptors,
owing to the high electrophilicity of the double bond. These
compounds are also used as versatile substrates in various or-
ganic transformations.[8] In addition, nitroalkenes are capable
of binucleophilic addition to form various heterocycles and car-
bocycles in a cascade fashion through Michael addition/cycliza-
tion/denitration (Scheme 1).[9]
bonyl compounds/enols with nitroolefins at reflux in dry EtOH
with K2CO3 as a base (Scheme 2b).
Results and Discussion
We commenced our study by taking b-naphthol (1a) and b-
methyl-b-nitrostyrene (3a) as model substrates in the presence
of K2CO3 (1 equiv) as a base in dry EtOH for 3 h under refluxing
conditions (Table 1, entry 1). To our delight, the desired naph-
thofuran derivative (4a) was obtained in 95% yield and no fur-
ther improvement was noted on increasing the reaction time.
Encouraged by this result, we proceeded to optimize the reac-
tion conditions, as summarized in Table 1. No significant de-
crease in yield was observed on lowering the amount of base
to 50 mol% (Table 1, entry 2), whereas only 72% yield was ob-
tained when 40 mol% K2CO3 was used (Table 1, entry 3). How-
ever, other bases, such as DABCO (1,4-diazabicyclo[2.2.2]oc-
tane), DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), Cs2CO3, NaOH,
and KOH, were not as effective as K2CO3 for this reaction
(Table 1, entries 4–8). Various solvents were also screened and
dry EtOH (Table 1, entry 2) was found to be the best solvent
among the common solvents, such as MeOH, DMF, DMSO, ace-
tone, MeCN, and THF (Table 1, entries 9–14). It is notable that
dry EtOH afforded a higher yield than rectified spirit (95%
EtOH; Table 1, entry 15). Water was not a good solvent for this
reaction (Table 1, entry 16); however, the reaction did not pro-
ceed in the absence of base (Table 1, entry 17). Thus, optimum
reaction conditions were obtained by using b-naphthol (1a,
In this context, Chen and co-workers and Namboothiri and
co-workers independently explored reactive nitroallylic ace-
tates (the Morita-Baylis–Hillman acetates of nitroalkenes) for
the synthesis of furan derivatives.[10] Recently, we developed
several methods for the synthesis of various biologically impor-
tant scaffolds, such as furan,[9a,11] imidazole,[12a] and imidazopyri-
dine,[9b,12b] by using undecorated, readily accessible nitroal-
kenes and exploring their bielectrophilicity properties. Our pre-
vious work[9a] (Eq. (1), Scheme 2a) on the indium-triflate-cata-
lyzed synthesis of arenofurans from readily accessible nitroal-
kenes and phenol/naphthols prompted us to look for an
environmentally benign strategy. Furthermore, we have devel-
oped a regioselective synthesis of multisubstituted furans from
simple alkyl ketones and nitroolefins (Eq. (2), Scheme 2a).[11]
Herein, we report an efficient and general method for the syn-
thesis of substituted furans through the coupling of 1,3-dicar-
Table 1. Optimization of the reaction conditions for the synthesis of naph-
thofuran.[a]
Scheme 1. Mode of binucleophilic addition to nitroalkenes.
Entry
Base [mol%]
Solvent
EtOH
T [8C]
reflux (85)
Yield [%][b]
1
2
3
4
5
6
7
8
K2CO3 (100)
K2CO3 (50)
K2CO3 (40)
DABCO (100)
DBU (100)
Cs2CO3(100)
NaOH (100)
KOH (100)
K2CO3 (50)
K2CO3 (50)
K2CO3 (50)
K2CO3 (50)
K2CO3 (50)
K2CO3 (50)
K2CO3 (50)
K2CO3 (50)
–
95
94
72
60
56
72
65
68
75
62
37
22
<10
<10
70[c]
50
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
acetone
DMSO
DMF
MeCN
THF
EtOH
water
EtOH
reflux (85)
reflux (85)
reflux (85)
reflux (85)
reflux (85)
reflux (85)
reflux (85)
reflux (70)
reflux (60)
100
9
10
11
12
13
14
15
16
17
100
reflux (90)
reflux (70)
reflux (85)
100
reflux (85)
n.r.
[a] Reaction conditions: compound 1a (0.5 mmol), compound 3a
(0.5 mmol), solvent (1.5 mL), 3 h; [b] yield of the isolated product; [c] recti-
fied spirit (95% EtOH). DMF=N,N-dimethylformamide, DMSO=dimethyl
sulfoxide, n.r.=no reaction.
Scheme 2. Synthesis of substituted furan derivatives.
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Table 2. Substrate scope for the synthesis of substituted furans from
naphthol derivatives.[a]
Table 2. (Continued)
Entry Naphthol
Nitroolefin
Product
Yield [%]
Entry Naphthol
Nitroolefin
Product
Yield [%]
17
18
1c
1c
3b (R=4-Me)
3d (R=4-Cl)
4q
4r
92[d]
87[d]
1
2
3
4
5
1a
1a
1a
1a
1a
3a (R=H)
4a
4b
4c
4d
4e
94
92
90
87
85
3b (R=4-Me)
3c (R=4-NO2)
3d (R=4-Cl)
3e (R=4-Br)
[a] Reaction conditions: compound
1
(0.5 mmol), compound
3
(0.5 mmol), K2CO3 (50 mol%), dry EtOH (1.5 mL), reflux, 3 h; yields are of
the isolated products. [b] The reaction was performed for 6 h. [c] Reaction
conditions: compound 1a (20 mmol), compound 3m (20 mmol), K2CO3
(50 mol%), dry EtOH (60 mL), reflux, 3 h. [d] The reaction was performed
for 2.5 h.
6
7
1a
1a
83
80
0.5 mmol) and b-methyl-b-nitrostyrene (2a, 0.5 mmol) in the
presence of 50 mol% K2CO3 in dry EtOH (1.5 mL) at reflux for
3 h (Table 1, entry 2).
To explore the scope and limitations of this method, various
nitroolefins were subjected to the reaction conditions (Table 2).
Both b-methyl-b-nitrostyrenes and b-nitrostyrenes reacted well
with naphthol and both electron-donating and electron-with-
drawing substituents on the phenyl moiety of the nitroolefins
reacted efficiently. Nitroolefins that contained electron-donat-
ing groups, such as ÀMe and ÀOMe groups, showed good effi-
ciencies (4b, 4l, 4o, and 4n). Nitrostyrene, which contained
a strongly electron-withdrawing group (ÀNO2) reacted very
well, thereby affording the product in 90% yield (4c). Chloro-
and bromo-substituted nitroolefins also smoothly afforded the
corresponding products (4d, 4m, and 4e). The dioxole part of
nitroolefin did not affect the reaction outcome (4 f). A high
yield was also obtained with a nitroolefin that contained a het-
eroaryl moiety (4g). Intriguingly, the unsaturated cinnamyl-
substituted nitroolefin gave the desired product (4h) in good
yield under the same reaction conditions. In addition, aliphatic
nitroalkenes were also effective, thereby affording the desired
products in good yields (4i and 4j). b-Methyl-b-nitrostyrene
and b-ethyl-b-nitrostyrene smoothly underwent the reaction
(4k). It is notable that this procedure was also effective for a-
naphthol (1b), which produced the corresponding naphthofur-
an derivatives (4l, 4m, and 4n). In addition, unsubstituted b-
nitroolefin furnished the corresponding furan (4o) in good
yield. The reaction also proceeded efficiently on a gram scale
(20 mmol). 1-Naphthyl-substituted nitrostyrene, which con-
tained polyaromatic rings, produced the corresponding naph-
thofuran (4p) in 90% yield. However, 2,7-dihydroxynaphtha-
lene (1c) only afforded mono-furan derivatives, even when
two equivalents of the nitroolefins (4q and 4r) were added
under the reaction conditions.
8
9
1a
1a
72
69
10
11
1a
1a
86[b]
90
12
13
1b
1b
3b (R=4-Me)
3d (R=4-Cl)
4l
4m
75
70
14
1b
72
15
1a
86, (84)[c]
We continued our study by exploring the reactivity of
phenol derivatives (1d) with various nitroolefins (3) under the
reaction conditions (Table 3). Substituted phenols that con-
tained ÀMe, ÀBr, or dioxole groups (sesamol) reacted smoothly
with various nitroolefins and produced the corresponding ben-
zofurans in good yields (6a–6 f).
16
1a
90
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Table 3. Substrate scope for the synthesis of substituted furans from
phenol derivatives.[a]
Scheme 4. Synthesis of substituted furans from 1,3-dicarbonyl compounds.
tives (5) were obtained in high yields (Table 4). Cyclic dike-
tones, such as 5,5-dimethyl-1,3-cyclohexadione and 1,3-cyclo-
hexadione, reacted smoothly with nitroolefins to furnish the
desired furans. Nitroolefins containing ÀMe and ÀOMe group
on the aryl ring gave the desired tetrasubstituted furan deriva-
tives (5b, 5c, 5l, 5m, and 5t) in excellent yields. The chloro-
and bromo-substituted nitrostyrene also reacted very well (5 f,
5g, 5o, 5p, 5u, 5h, 5i, and 5q). A notable advantage of this
method is its efficiency for the synthesis of furan derivatives
from both cyclic and acyclic 1,3-diketones. Acyclic diketones,
such as acetylacetone and ethylacetoacetate, produced the de-
sired products (5s, 5t, 5u, and 5v) in good yields.
We were delighted to find that alicyclic nitroolefins 3s and
3t reacted well under the reaction conditions to produce
large-sized-ring-fused substituted furan derivatives 5w and 5x,
respectively, which could be further converted into biologically
active compounds, such as dibenzofurans and 4-pyrone skele-
tons (Scheme 5).[3g]
[a] Reaction conditions: compound 1d (0.5 mmol), compound
(0.5 mmol), K2CO3 (50 mol%) in dry EtOH (1.5 mL), reflux, 6 h; yields are
of the isolated products. [b] The reaction was performed for 9 h.
3
The most-important achievement of this procedure is the
one-pot synthesis of dioxa[5]helicenes in good yields. Heli-
cenes have shown interesting biological activities, such as mu-
tagenesis of bacterial cells,[15] high tumor-initiating activity,[15]
chiral recognition[16] and selectivity for DNA binding and inter-
calation,[17] and enantioselectivity in telomerase inhibition.[18]
Consequently, much attention has been devoted to the syn-
thesis of helicenes over last few years.[19] To extend the scope
of this method, 6-aryl-substituted dioxa[5]helicenes (8a–8d)
were synthesized by the reaction of b-naphthol and 3-nitro-
2H-chromenes under the reaction conditions (Scheme 6). Nota-
bly, this strategy uniquely allows the synthesis of this type of
helicene derivatives from easily accessible starting materials in
one pot.
Figure 2. ORTEP of 7-(4-methoxyphenyl)-6-methyl-[1,3]dioxolo[4,5-f]benzo-
furan (6e); thermal ellipsoids are set at 30% probability.
X-ray crystallographic analysis was performed to confirm the
structure of compound 6e (Table 3), as shown in Figure 2.[13]
This method was also applicable to phloroglucinol, which re-
acted with three equivalents of the nitroolefins in one pot to
afford the corresponding benzotrifurans (Scheme 3). These
compounds are very useful hole-transporting materials (HTM)
in multilayer organic light-emitting diodes (OLEDs).[3j,14]
Next, we planned to prepare more functionalized arenofuran
derivatives from the as-synthesized products (Scheme 7). The
functionalization of synthesized naphthofurans (4) at the 2-po-
sition was performed by Pd-catalyzed arylation[20] to construct
densely substituted furan derivatives. Substituted naphthofur-
ans 10a and 10b were obtained
This strategy was also extended to the synthesis of highly
substituted furans from 1,3-dicarbonyl compounds (Scheme 4).
To our delight, when 1,3-dicarbonyl compounds were react-
ed with various nitroolefins, a diverse range of furan deriva-
in high yields. Furthermore, the
aromatization of furan 5k was
achieved (to afford compounds
11 a and 11 b) by treatment with
iodine in MeOH or EtOH under
refluxing conditions.[21]
A plausible reaction mecha-
nism is shown in Scheme 8 and
is based on literature re-
ports[10d,22] and our previous
Scheme 3. Synthesis of hole-transporting materials.
work.[9b] Initially, K2CO3-promoted
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tion of water and HNO. In the case of dimedone (2a), the first
step is the formation of Michael adduct D by the base-promot-
ed addition of enol C to the nitroolefin (3a). Finally, the desired
furan (5a) is formed through sequential intramolecular cycliza-
tion and denitration.
Table 4. Substrate scope for the synthesis of substituted furans from 1,3-
dicarbonyl compounds.[a]
Entry 1,3-Dicarbonyl com-
pound
Nitroolefin
Product
Yield
[%]
Conclusion
In summary, we have developed a versatile, one-pot, base-pro-
moted synthesis of polysubstituted furans and arenofurans
from the reaction between readily available phenols or 1,3-di-
carbonyl compounds and nitroolefins at reflux in EtOH. This
method has been applied to the synthesis of benzotrifuran de-
rivatives, which are useful hole-transporting materials. The
most-important achievement of this procedure is the direct
synthesis of dioxa[5]helicenes in good yields. Furthermore, bio-
logically significant large-sized-ring-fused substituted furans
were also obtained by employing this simple strategy. The use
of environmentally benign solvent and an inexpensive base,
wide functional-group tolerance, general applicability, and op-
erational simplicity are the key advantages of this method.
These advantages render this environmentally benign proce-
dure facile and suitable for creating a library of diversified
furan derivatives in academia and in industry.
1
2
3
4
5
6
7
8
9
2a
2a
2a
2a
2a
2a
2a
2a
2a
3a (R=H)
3b (R=4-Me)
3o (R=4-OMe)
3p (R=3-NO2)
3c (R=4-NO2)
3q (R=2-Cl)
3d (R=4-Cl)
3r (R=2-Br)
3e (R=4-Br)
5a
5b
5c
5d
5e
5 f
5g
5h
5i
87
86
85
86
87
86
89
87
90
10
2a
85
11
12
13
14
15
16
17
2b
2b
2b
2b
2b
2b
2b
3a (R=H)
3b (R=4-Me)
3o (R=4-OMe)
3p (R=3-NO2)
3q (R=2-Cl)
3d (R=4-Cl)
3e (R=4-Br)
5k
5l
85
83
81
78
84
86
88
5m
5n
5o
5p
5q
Experimental Section
Typical Procedure for the Synthesis of Naphthofurans 4
A mixture of the naphthol (1, 0.5 mmol) and the nitroolefin (3,
0.5 mmol) was stirred at reflux (80–858C) in dry EtOH (1.5 mL) in
the presence of K2CO3 (35 mg, 50 mol%) until the reaction was
complete (as determined by TLC). Then, the solvent was evaporat-
ed under reduced pressure and the crude residue was extracted
with water/EtOAc (10 mL, 1:1). The organic layer was separated
and dried over Na2SO4. After evaporation of the solvent, the crude
product was purified by column chromatography on a short pad
of silica gel (petroleum ether).
18
2b
82
19[b]
20[b]
21[b]
22[b]
2c (R1, R2 =Me)
3a (R3 =H)
3b (R3 =4-Me)
3d (R3 =4-Cl)
3a (R3 =H)
5s
5t
5u
5v
82
83
84
86
2c
2c
2d (R1 =Me, R2 =OEt)
2-Methyl-1-phenylnaphtho[2,1-b]furan (4a)[9a]
[a] Reaction conditions: compound
2
(0.5 mmol), compound
3
(0.5 mmol), K2CO3 (50 mol%), dry EtOH (1.5 mL), reflux, 5 h; yields are of
the isolated products. [b] The reactions were performed for 3 h.
1
Colorless oil (121 mg, 94%); H NMR (400 MHz, CDCl3): d=7.77 (d,
J=8.0 Hz, 1H), 7.67 (d, J=8.4 Hz, 1H), 7.56–7.49 (m, 2H), 7.39–7.36
(m, 4H), 7.34–7.32 (m, 1H), 7.27–7.22 (m, 1H), 7.17–7.13 (m, 1H),
2.29 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=151.3, 134.3, 130.8,
130.6, 128.9, 128.7, 128.0, 127.6, 125.7, 124.6, 124.1, 123.3, 122.4,
119.1, 112.1, 12.4 ppm.
2-Methyl-1-p-tolylnaphtho[2,1-b]furan (4b)[9a]
Colorless oil (125 mg, 92%); 1H NMR (400 MHz, CDCl3): d=7.84–
7.80 (m, 2H), 7.59–7.58 (m, 2H), 7.34–7.29 (m, 3H), 7.25 (d, J=
8.4 Hz, 3H), 2.40 (s, 3H), 2.359 ppm (s, 3H); 13C NMR (100 MHz,
CDCl3): d=151.1, 151.1, 137.1, 131.0, 130.7, 130.2, 129.2, 128.7,
127.9, 125.4, 124.38, 123.8, 123.1, 122.3, 118.8, 112.0, 21.2,
12.2 ppm.
Scheme 5. Synthesis of precursors to biologically active compounds.
Michael addition of b-naphthol to nitroolefin 3a occurs to
form intermediate A. Next, intramolecular cyclization of inter-
mediate A leads to the formation of cyclic intermediate B and,
subsequently, the final product (4a) is formed by the elimina-
2-Methyl-1-(4-nitrophenyl)naphtho[2,1-b]furan (4c)[9a]
1
Colorless oil (136 mg, 90%); H NMR (400 MHz, CDCl3): d=8.25 (d,
J=8.8 Hz, 2H), 7.81 (d, J=8.0 Hz, 1H), 7.62–7.52 (m, 5H), 7.32–7.28
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(m, 1H), 7.23–7.19 (m, 1H),
2.32 ppm
(s,
3H);
13C NMR
(100 MHz, CDCl3): d=151.7, 151.5,
147.2, 141.5, 131.2, 130.8, 129.0,
127.3, 125.9, 125.2, 124.2, 123.8,
122.6,
121.3,
117.2,
112.0,
12.3 ppm.
1-(4-Chlorophenyl)-2-methyl-
naphtho[2,1-b]furan (4d)[9a]
Colorless oil (127 mg, 87%);
1H NMR (400 MHz, CDCl3): d=8.87
(d, J=8.0 Hz, 1H), 7.12 (d, J=
8.0 Hz, 1H), 7.45(d, J=8.8 Hz, 1H),
7.59 (d. J=8.8 Hz, 1H), 7.46 (d, J=
Scheme 6. Synthesis of dioxa[5]helicenes.
8.4 Hz, 2H), 7.38 (d, J=8.4 Hz, 2H),
7.37–7.34 (m, 1H), 7.30–7.25 (m,
1H), 2.37 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=151.3, 151.2, 133.5, 132.6, 131.7, 130.7, 128.8,
127.6, 125.72, 124.7, 124.0, 122.9, 121.9, 117.8, 112.0, 12.2 ppm.
1-(4-Bromophenyl)-2-methylnaphtho[2,1-b]furan (4e)[9a]
1
Colorless oil (143 mg, 85%); H NMR (400 MHz, CDCl3): d=7.79 (d,
J=8.0 Hz, 1H), 7.63 (d, J=8.4 Hz, 1H), 7.57 (d, J=8.8 Hz, 1H), 7.52
(d, J=8.4 Hz, 3H), 7.49–7.46 (m, 1H), 7.29 (d, J=8.0 Hz, 1H), 7.26–
7.22 (m, 1H), 7.19 (d, J=8.4 Hz, 1H), 2.28 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=151.2, 151.2, 133.1, 132.1, 131.7, 130.6, 128.8,
127.6, 125.7, 124.7, 124.0, 122.9, 121.8, 121.6, 117.7, 112.0,
12.2 ppm; elemental analysis calcd (%) for C19H13BrO: C 67.67,
H 3.89%; found: C 67.57, H 3.98%.
1-(Benzo[d][1,3]dioxol-5-yl)-2-methylnaphtho[2,1-b]furan
(4 f)[9a]
Scheme 7. Synthesis of densely substituted arenofurans.
Colorless oil (125 mg, 83%); 1H NMR (400 MHz, CDCl3): d=7.78–
7.73 (m, 2H), 7.55–7.48 (m, 2H), 7.28–7.19 (m, 2H), 6.84–6.80 (m,
3H), 5.92 (d, J=11.6 Hz, 2H), 2.29 ppm (s, 3H); 13C NMR (100 MHz,
CDCl3): d=151.3, 151.0, 147.7,
147.0, 130.6, 128.7, 127.7, 127.5,
125.6, 124.4, 123.9, 123.8, 123.1,
122.3, 118.5, 111.9, 110.8, 108.5,
101.1, 12.2 ppm; elemental analy-
sis calcd (%) for C20H14O3: C 79.46,
H 4.67%; found: C 79.39, H 4.76%.
1-(Furan-2-yl)-2-methylnaph-
tho[2,1-b]furan (4g)[9a]
Light-brown oil (99 mg, 80%);
1H NMR (400 MHz, CDCl3): d=8.08
(d, J=7.6 Hz, 1H), 8.02 (d, J=
8.4 Hz, 1H), 7.79 (d, J=8.4 Hz, 2H),
7.72 (d, J=8.8 Hz, 1H), 7.59–7.52
(m, 2H), 6.72–6.71 (m, 1H), 6.62–
6.62 (m, 1H), 2.63 ppm (s, 3H);
13C NMR (100 MHz, CDCl3): d=
153.8, 151.3, 146.8, 142.4, 130.7,
128.6, 127.4, 126.0, 124.9, 124.2,
123.5, 122.0, 111.8, 111.1, 110.1,
109.2, 12.7 ppm.
Scheme 8. Proposed reaction mechanism for the synthesis of furan derivatives.
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(E)-2-Methyl-1-styrylnaphtho[2,1-b]furan (4h)[9a]
124.6, 124.2, 122.9, 121.2, 119.8, 118.4, 116.9, 114.2, 55.3, 20.3,
13.3 ppm; elemental analysis calcd (%) for C21H18O2 : C 83.42,
H 6.00%; found: C 83.29, H 5.88%.
1
Light-yellow oil (102 mg, 72%); H NMR (400 MHz, CDCl3): d=8.26
(d, J=8.4 Hz, 1H), 7.81 (d, J=8.0 Hz, 1H), 7.54 (d, J=8.0 Hz, 1H),
7.48–7.46 (m, 2H), 7.41–7.38 (m, 2H), 7.35–7.29 (m, 4H), 7.23–7.21
(m, 1H), 6.71–6.67 (d, J=16 Hz, 1H), 2.45 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=151.1, 137.3, 133.2, 130.7, 130.4, 128.8, 128.7,
128.6, 128.2, 127.7, 127.5, 126.3, 126.0, 124.5, 124.0, 123.5, 122.0,
120.4, 116.5, 111.9, 13.1 ppm.
1-p-Tolyl-naphtho[2,1-b]furan (4o)[9a]
1
Colorless oil (111 mg, 86%); H NMR (400 MHz, CDCl3): d=7.77 (d,
J=8.0 Hz, 1H), 7.55 (q, J=8.8 Hz, 2H), 7.48 (d, J=7.6 Hz, 1H), 7.39
(d, J=8.4 Hz, 1H), 7.35–7.31 (m, 2H), 7.30–7.24 (m, 3H), 7.17 (d, J=
8.4 Hz, 1H), 2.26 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=151.6,
151.2, 135.3, 133.0, 132.5, 130.6, 129.8, 129.4, 128.6, 127.8, 126.9,
125.8, 124.5, 124.0, 122.7, 122.44, 116.1, 112.0, 12.4 ppm.
1-Isopropyl-2-methyl-naphtho[2,1-b]furan (4i)[9a]
Light-yellow oil (77 mg, 69%); 1H NMR (400 MHz, CDCl3): d=8.39
(d, J=8.4 Hz, 1H), 7.90 (d, J=8.4 Hz, 1H), 7.61 (d, J=8.8 Hz, 1H),
7.55–7.50 (m, 2H), 7.43–7.39 (m, 1H), 3.77–3.70 (m, 1H), 2.55 (s,
3H), 1.49 (d, J=6.8 Hz, 3H), 1.44 ppm (d, J=6.8 Hz, 3H); 13C NMR
(100 MHz, CDCl3): d=151.2, 148.8, 130.9, 129.2, 128.3, 125.7, 124.4,
123.7, 123.5, 122.4, 122.3, 112.1, 25.6, 22.4, 14.3 ppm.
2-Methyl-1-(naphthalen-1-yl)naphtho[2,1-b]furan (4p)[9a]
Colorless oil (138 mg, 90%); 1H NMR (400 MHz, CDCl3): d=8.09–
8.06 (m, 2H), 7.99 (d, J=8.4 Hz, 1H), 7.86–7.81 (m, 3H), 7.74–7.00
(m, 2H), 7.61–7.58 (m, 1H), 7.44–7.38 (m, 2H), 7.36–7.33 (m, 1H),
7.17–7.13 (m, 1H), 2.46 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=
151.9, 151.3, 133.7, 132.9, 131.5, 130.6, 128.5, 128.4, 128.3, 128.2,
127.8, 126.3, 126.1, 126.0, 125.6, 125.6,124.5, 123.9, 123.4, 123.2,
116.5, 112.0, 12.3 ppm.
8,9,10,11-Tetrahydronaphtho[2,1-b]benzofuran (4j)
Colorless oil (95 mg, 86%); 1H NMR (400 MHz, CDCl3): d=8.27 (d,
J=8.0 Hz, 1H), 7.97 (d, J=8.4 Hz, 1H), 7.68–7.62 (m, 2H), 7.59–7.55
(m, 1H), 7.50–7.46 (m, 1H), 314–3.11 (m, 2H), 2.89–2.86 (m, 2H),
2.03–1.96 ppm (m, 4H); 13C NMR (100 MHz, CDCl3): d=153.3, 151.5,
130.6, 128.8, 128.5, 125.7, 123.9, 123.7, 123.6, 122.6, 114.4, 112.3,
23.8, 23.2, 23.2, 22.7 ppm; elemental analysis calcd (%) for C16H14O:
C 86.45, H 6.35%; found: 86.59, H 6.47%.
2-Methyl-1-p-tolylnaphtho[2,1-b]furan-8-ol (4q)
1
Yellow oil (132 mg, 92%); H NMR (400 MHz, CDCl3): d=7.69 (d, J=
8.8 Hz, 1H), 7.50 (d, J=8.8 Hz, 1H), 7.38 (d, J=8.8 Hz, 1H), 7.28 (d,
J=7.6 Hz, 2H), 7.20(d, J=8.0 Hz, 2H), 7.02 (s, 1H), 6.88 (dd, J1 =
8.8 Hz, J2 =2.8 Hz, 1H), 5.12 (Br, s, 1H), 2.35 (s, 3H), 2.30 ppm (s.
3H); 13C NMR (100 MHz, CDCl3): d=153.3, 151.7, 150.7, 137.1,
130.6, 130.3, 129.3, 129.1, 126.9, 125.8, 124.3, 121.2, 118.6, 115.1,
109.7, 106.0, 21.3, 12.2 ppm; elemental analysis calcd (%) for
C20H16O2: C 83.31, H 5.59%; found: C 83.19, H 5.71%.
2-Ethyl-1-phenyl-naphtho[2,1-b]furan (4k)[9a]
1
Light-yellow oil (122 mg, 90%); H NMR (400 MHz, CDCl3): d=7.80
(d, J=8.4 Hz, 1H), 7.63 (d, J=8.0 Hz, 1H), 7.60–7.56 (m, 2H), 7.45–
7.34 (m, 5H), 7.27 (t, J=7.6 Hz, 1H), 7.17 (t, J=8.0 Hz, 1H), 2.65 (q,
J=7.6 Hz, 2H), 1.21 ppm (t, J=7.6 Hz, 3H); 13C NMR (100 MHz,
CDCl3): d=156.2, 151.1, 134.1, 130.6, 130.5, 128.7, 128.5, 127.9,
127.5, 125.5, 124.4, 123.9, 123.1, 122.3, 118.1, 112.1, 19.9, 13.2 ppm.
1-(4-Chlorophenyl)-2-methylnaphtho[2,1-b]furan-8-ol (4r)
1
Light-yellow oil (134 mg, 87%); H NMR (400 MHz, CDCl3): d=7.72
(d, J=8.8 Hz, 1H), 7.53 (d, J=8.8 Hz, 1H), 7.42–7.38 (m, 3H), 7.36–
7.34 (m, 2H), 6.95–6.90 (m, 2H), 5.13 (Br, s, 1H), 2.31 ppm (s, 3H);
13C NMR (100 MHz, CDCl3): d=153.5, 151.8, 150.9, 133.5, 132.6,
131.8, 130.9, 130.8, 129.4, 128.9, 125.8, 124.7, 120.8, 117.6, 115.3,
109.7, 105.8, 12.2 ppm; elemental analysis calcd (%) for C19H13ClO2:
C 73.91, H 4.24%; found: C 74.09, H 4.38%.
2-Methyl-3-p-tolylnaphtho[1,2-b]furan (4l)
1
Colorless oil (102 mg, 75%); H NMR (400 MHz, CDCl3): d=8.20 (d,
J=8.0 Hz, 1H), 7.80 (d, J=8.0 Hz, 1H), 7.54 (q, J=8.4 Hz, 2H), 7.46
(t, J=7.6 Hz, 1H), 7.36–7.33 (m, 3H), 7.20 (d, J=7.6 Hz, 2H), 2.52 (s,
3H), 2.33 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=150.3, 149.1,
136.6, 131.0, 129.9, 129.4, 128.8, 128.2, 126.1, 124.6, 124.0, 123.0,
121.1, 119.7, 118.3, 117.9, 21.2, 12.9 ppm; elemental analysis calcd
(%) for C20H16O: C 88.20, H 5.92%; found: C 88.09, H 6.05%.
Typical Procedure for the Synthesis of Benzofurans 6
A mixture of the phenol (1d, 0.5 mmol) and the nitroolefin (3,
0.5 mmol) was stirred at reflux (80–858C) in dry EtOH (1.5 mL) in
the presence of K2CO3 (35 mg, 50 mol%) until the reaction was
complete (as determined by TLC). Then, the solvent was evaporat-
ed and the crude residue was extracted with water/EtOAc (10 mL,
1:1). The organic layer was separated and dried over Na2SO4. After
evaporation of the solvent, the crude product was purified by
column chromatography on a short pad of silica gel (petroleum
ether).
3-(4-Chloro-phenyl)-2-methylnaphtho[1,2-b]furan (4m)
1
Colorless oil (102 mg, 70%); H NMR (400 MHz, CDCl3): d=8.23 (d,
J=8.0 Hz, 1H), 7.85 (d, J=8.4 Hz, 1H), 7.58 (d, J=8.8 Hz, 1H),
7.54–7.49 (m, 2H), 7.42–7.38 (m, 5H), 2.55 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=150.6, 149.2, 132.8, 131.4, 131.1, 130.2, 129.0,
128.3, 126.3, 124.8, 123.6, 123.3, 121.1, 119.8, 117.9, 117.1,
12.9 ppm; elemental analysis calcd (%) for C19H13ClO: C 77.95,
H 4.48%; found: C 77.83, H 4.59%.
3-(4-Chlorophenyl)-2,5-dimethylbenzofuran (6a)
2-Ethyl-3-(4-methoxyphenyl)-naphtho[1,2-b]furan (4n)
1
Colorless oil (86 mg, 67%); H NMR (400 MHz, CDCl3): d=7.43–7.38
1
Colorless oil (108 mg, 72%); H NMR (400 MHz, CDCl3): d=8.24 (d,
(m, 4H), 7.31–7.28 (m, 2H), 7.05 (d, J=6.8 Hz, 1H), 2.46 (s, 3H),
2.40 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=152.6, 151.7, 132.9,
132.4, 131.7, 130.4, 129.1, 128.7, 125.1, 119.1, 115.9, 110.5, 21.6,
13.0 ppm; elemental analysis calcd (%) for C16H13ClO: C 74.85,
H 5.10%; found: C 74.99, H 5.25%.
J=8.0 Hz, 1H), 7.83 (d, J=8.0 Hz, 1H), 7.55–7.54 (m, 2H), 7.51–7.47
(m, 1H), 7.39–7.35 (m, 3H), 6.96 (d, J=9.2 Hz, 2H), 3.79 (s, 3H),
2.88 (q, J=7.6 Hz, 2H), 1.33 ppm (t, J=7.6 Hz, 3H); 13C NMR
(100 MHz, CDCl3): d=158.6, 155.1, 131.0, 130.1, 128.2, 126.0, 125.2,
Chem. Asian J. 2015, 10, 2525 – 2536
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5-Bromo-2-methyl-3-phenylbenzofuran (6b)[9a]
132.9, 129.7, 128.1, 126.8, 115.7, 109.8, 12.9 ppm; elemental analy-
sis calcd (%) for C33H24O3: C 84.59, H 5.16%; found: C 84.75,
H 5.29%.
1
White solid (93 mg, 65%); m.p. 98–998C. H NMR (400 MHz, CDCl3):
d=7.59 (d, J=2 Hz, 1H), 7.42–7.35 (m, 4H), 7.31–7.26 (m, 1H),
7.25–7.21 (m, 2H), 2.43 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=
152.7, 152.7, 132.0, 130.7 128.8, 108.8, 127.2, 126.3, 122.0, 116.6,
115.7, 112.1, 12.8 ppm.
2,5,8-Trimethyl-3,6,9-tri-p-tolylbenzo[1,2-b:3,4-b’:5,6-b’’]trifur-
an (7b)
1
White solid (145 mg, 57%), m.p.>2008C. H NMR (400 MHz, CDCl3):
6-Methyl-7-phenyl-[1,3]dioxolo[4,5-f]benzofuran (6c)
d=7.53 (d, J=8.0 Hz, 6H), 7.34 (d, J=8.0 Hz, 6H), 2.44 (s, 9H),
2.38 ppm (s, 9H); 13C NMR (100 MHz, CDCl3): d=149.2, 144.2, 136.4,
129.9, 129.6, 128.8, 115.5, 109.8, 21.3, 12.9 ppm; elemental analysis
calcd (%) for C36H30O3: C 84.68, H 5.92%; found: C 84.79, H 6.09%.
White solid (93 mg, 74%), m.p. 102–1048C. 1H NMR (400 MHz,
CDCl3): d=7.52–7.38 (m, 4H), 7.37–7.28 (m, 1H), 6.94 (s, 2H), 5.93
(s, 2H), 2.46 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=150.6,
149.0, 145.5, 144.5, 133.0, 128.8, 127.0, 122.0, 1173, 101.2, 98.3,
93.3, 12.9 ppm; elemental analysis calcd (%) for C16H12O3: C 76.18,
H 4.79%; found: C 76.35, H 4.95%.
Typical Procedure for the Synthesis of Compounds 5
A mixture of 1,3-dicarbonyl compound (2, 0.5 mmol) and nitroole-
fin (3, 0.5 mmol) was stirred at reflux (80–858C) in dry EtOH
(1.5 mL) in the presence of K2CO3 (35 mg, 50 mol%) until the reac-
tion was complete (as determined by TLC). Then, the solvent was
evaporated and the crude residue was extracted with water/EtOAc
(10 mL, 1:1 v/v). The organic layer was separated and dried over
Na2SO4. After evaporation of the solvent, the crude product was
purified by column chromatography on a short pad of silica gel
(petroleum ether/EtOAc, 20:1 v/v).
6-Methyl-7-p-tolylbenzofuro[5,6-d][1,3]dioxole (6d)
White solid (97 mg, 73%); m.p. 117–1188C. 1H NMR (400 MHz,
CDCl3): d=7.23 (d, J=8.0 Hz, 2H), 7.16 (d, J=8.0 Hz, 2H), 6.83 (s,
2H), 5.83 (s, 2H), 2.35 (s, 3H), 2.30 ppm (s, 3H); 13C NMR (100 MHz,
CDCl3): d=150.2, 148.8, 145.2, 144.2, 136.5, 129.7, 129.4, 128.5,
122.0, 117.0, 101.0, 98.2, 93.1, 21.1, 12.7 ppm; elemental analysis
calcd (%) for C17H14O3: C 76.68, H 5.30%; found: C 76.52, H 5.49%.
7-(4-Methoxyphenyl)-6-methyl-[1,3]dioxolo[4,5-f]benzofuran
2,6,6-Trimethyl-3-phenyl-6,7-dihydrobenzofuran-4(5H)-one
(6e)
(5a)
White solid (105 mg, 75%); m.p. 111–1138C. 1H NMR (400 MHz,
CDCl3): d=7.22 (d, J=8.8 Hz, 2H), 6.87 (d, J=8.8 Hz, 2H), 6.80 (d,
J=8.8 Hz, 2H), 5.80 (s, 2H), 3.71 (s, 3H), 2.31 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=158.5, 150.0, 148.7, 145.2, 144.2, 129.7, 125.0,
122.0, 116.7, 114.1, 101.0, 98.1, 93.1, 55.1, 12.6 ppm; elemental
analysis calcd (%) for C17H14O4: C 72.33, H 5.00%; found: C 72.21,
H 5.19%.
1
Yellow oil (110 mg, 87%); H NMR (400 MHz, CDCl3): d=7.39–7.35
(m, 4H), 7.30–7.28 (m, 1H), 2.74 (s, 2H), 2.37 (s, 2H), 2.31 (s, 3H),
1.15 ppm (s, 6H); 13C NMR (100 MHz, CDCl3): d=193.6, 165.0, 149.1,
131.7, 129.9, 127.9, 127.1, 119.1, 118.6, 53.1, 37.7, 35.0, 28.6,
12.1 ppm; elemental analysis calcd (%) for C17H18O2: C 80.28,
H 7.13%; found: C 80.05, H 7.39%.
2,6,6-Trimethyl-3-p-tolyl-6,7-dihydro-5H-benzofuran-4-one
(5b)
6-Ethyl-7-phenyl-[1,3]dioxolo[4,5-f]benzofuran (6 f)
1
Colorless oil (98 mg, 74%); H NMR (400 MHz, CDCl3): d=7.40–7.34
1
Yellow oil (115 mg, 86%); H NMR (400 MHz, CDCl3): d=7.20 (d, J=
(m, 4H), 7.29–7.25 (m, 1H), 6.89 (s, 1H), 6.85 (s, 1H), 5.88 (s, 2H),
2.74 (q, J=7.6 Hz, 2H), 1.24 ppm (t, J=7.6 Hz, 3H); 13C NMR
(100 MHz, CDCl3): d=155.6, 148.9, 145.4, 144.3, 132.8, 128.8, 128.7,
126.9, 121.9, 116.5, 101.1, 98.3, 93.2, 20.3, 13.0 ppm; elemental
analysis calcd (%) for C17H14O3: C 76.68, H 5.30%; found: C 76.40,
H 5.48%.
8.0 Hz, 2H), 7.10 (d, J=8.0 Hz, 2H), 2.65 (s, 2H), 2.28 (s, 5H), 2.22
(s, 3H), 1.07 ppm (s, 6H); 13C NMR (100 MHz, CDCl3): d=193.4,
164.7, 148.7, 136.6, 129.6, 128.5, 118.9, 118.5, 53.0, 37.6, 34.8, 28.5,
21.2, 12.0 ppm; elemental analysis calcd (%) for C18H20O2: C 80.56,
H 7.51%; found: C 80.74, H 7.69%.
3-(4-Methoxyphenyl)-2,6,6-trimethyl-6,7-dihydrobenzofuran-
4(5H)-one (5c)
Typical Procedure for the Synthesis of Benzotrifurans 7
A mixture of phloroglucinol (1e, 0.5 mmol) and the nitroolefin (3,
1.5 mmol) was stirred at reflux (80–858C) in dry EtOH (1.5 mL) in
the presence of K2CO3 (35 mg, 50 mol%) until the reaction was
complete (as determined by TLC). Then, the solvent was evaporat-
ed and the crude residue was extracted with water/EtOAc (10 mL,
1:1 v/v). The organic layer was separated and dried over Na2SO4.
After evaporation of the solvent, the crude product was purified
by column chromatography on a short pad of silica gel (petroleum
ether).
1
Yellow oil (120 mg, 85%); H NMR (400 MHz, CDCl3): d=7.25–7.24
(d, J=8.8 Hz, 2H), 6.84 (d, J=9.2 Hz, 2H,), 3.74 (s, 3H), 2.66 (s, 2H),
2.28 (s, 2H), 2.22 (s, 3H), 1.07 ppm (s, 6H); 13C NMR (100 MHz,
CDCl3): d=193.6, 164.8, 158.5, 148.5, 130.8, 123.8, 118.5, 118.4,
113.3, 52.2, 52.9, 37.6, 34.8, 28.5, 12.0 ppm; elemental analysis
calcd (%) for C18H20O3: C 76.03, H 7.09%: Found: C 76.24, H 7.20%.
2,6,6-Trimethyl-3-(3-nitrophenyl)-6,7-dihydrobenzofuran-
4(5H)-one (5d)
2,5,8-Trimethyl-3,6,9-triphenylbenzo[1,2-b:3,4-b’:5,6-b’’]trifur-
an (7a)
Yellow oil (128 mg, 86%); 1H NMR (400 MHz, CDCl3): d=8.16 (s,
1H), 8.09–8.06 (m, 1H), 7.68 (d, J=8.0 Hz, 1H), 7.46 (t, J=8.0 Hz,
1H), 2.69 (s, 2H), 2.31 (s, 2H), 2.27 (s, 3H), 1.08 ppm (d, J=4.5 Hz,
6H); 13C NMR (100 MHz, CDCl3): d=193.5, 165.3, 149.9, 147.9,
136.0, 133.5, 128.6, 124.4, 121.9, 118.0, 117.2, 52.7, 37.4, 34.9, 28.4,
1
White solid (128 mg, 55%), mp>2008C. H NMR (400 MHz, CDCl3):
d=7.64 (d, J=8.0 Hz, 6H), 7.43 (t, J=7.6 Hz, 6H), 7.32 (t, J=7.6 Hz,
3H), 2.44 ppm (s, 9H); 13C NMR (100 MHz, CDCl3): d=149.5, 144.2,
Chem. Asian J. 2015, 10, 2525 – 2536
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12.0 ppm; elemental analysis calcd (%) for C17H17NO4: C 68.21,
H 5.72; N, 4.68%; found: C 68.39, H 5.59; N, 4.81%.
analysis calcd (%) for C18H20O2: C 80.56, H 7.51%; found: C 80.40,
H 7.35%.
2,6,6-Trimethyl-3-(4-nitrophenyl)-6,7-dihydrobenzofuran-
2-Methyl-3-phenyl-6,7-dihydrobenzofuran-4(5H)-one (5k)
4(5H)-one (5e)
1
Colorless oil (96 mg, 85%); H NMR (400 MHz, CDCl3): d=7.30–7.28
1
(m, 4H), 7.24–7.20 (m, 1H) 2.79 (t, J=6.0 Hz, 2H), 2.40 (t, J=6.4 Hz,
2H), 2.22 (s, 3H), 2.11–2.05 ppm (m, 2H); 13C NMR (100 MHz,
CDCl3): d=194.0, 165.7, 148.6, 131.6, 129.7, 127.7, 126.9, 119.5,
119.0, 38.5, 23.6, 22.4, 11.9 ppm; elemental analysis calcd (%) for
C15H14O2: C 79.62, H 6.24%; found: C 79.46, H 6.39%.
Yellow oil (130 mg, 87%); H NMR (400 MHz, CDCl3): d=8.15 (d, J=
8.8 Hz, 2H), 7.48 (d, J=8.8 Hz, 2H), 2.70 (s, 2H), 2.32 (s, 2H), 2.28
(s, 3H), 1.09 ppm (s, 6H); 13C NMR (100 MHz, CDCl3): d=193.5,
165.6, 150.3, 146.7, 138.8, 131.2, 130.5, 123.1, 122.9, 118.1, 117.6,
52.8, 37.5, 35.0, 28.5, 12.2 ppm; elemental analysis calcd (%) for
C17H17NO4: C 68.21, H 5.72; N, 4.68%; found: C 68.37, H 5.58; N,
4.83%.
2-Methyl-3-p-tolyl-6,7-dihydro-5H-benzofuran-4-one (5l)
Light-yellow oil (99 mg, 83%); 1H NMR (400 MHz, CDCl3): d=7.31
(d, J=8.4 Hz, 2H), 7.22 (d, J=8.0 Hz, 2H), 2.89 (t, J=6.4 Hz, 2H),
2.51 (dd, J1 =6.0 Hz, J2 =7.6 Hz, 2H), 2.40 (s, 3H), 2.33 (s, 3H),
2.18 ppm (t, J=6.4 Hz, 2H); 13C NMR (100 MHz, CDCl3): d=193.9,
165.6, 148.3, 136.5, 129.5, 129.5, 128.5, 128.4, 119.6, 118.9, 38.5,
23.5, 22.3, 21.1, 11.8 ppm; elemental analysis calcd (%) for
C16H16O2: C 79.97, H 6.71%; found: C 80.14, H 6.86%.
3-(2-Chlorophenyl)-2,6,6-trimethyl-6,7-dihydrobenzofuran-
4(5H)-one (5 f)
White solid (124 mg, 86%); mp 145–1478C. 1H NMR (400 MHz,
CDCl3): d=7.37–7.34 (m, 1H), 7.21–7.18 (m, 3H), 2.68 (d, J=4.8 Hz,
2H), 2.321–2.20 (m, 2H), 2.10 (s, 3H), 1.09 (s, 3H), 1.07 ppm (s, 3H);
13C NMR (100 MHz, CDCl3): d=193.1, 164.3, 149.8, 134.3, 131.8,
131.1, 129.3, 128.8, 126.2, 119.4, 116.0, 52.5, 37.5, 35.0, 28.5,
11.9 ppm; elemental analysis calcd (%) for C17H17ClO2: C 70.71,
H 5.93%; found: C 70.55, H 6.16%.
3-(4-Methoxyphenyl)-2-methyl-6,7-dihydrobenzofuran-4(5H)-
one (5m)
1
Colorless oil (103 mg, 81%); H NMR (400 MHz, CDCl3): d=7.22 (d,
3-(4-Chlorophenyl)-2,6,6-trimethyl-6,7-dihydrobenzofuran-
4(5H)-one (5g)
J=8.4 Hz, 2H), 6.83 (d, J=8.8 Hz, 2H), 3.74 (s, 3H), 2.78 (t, J=
6.4 Hz, 2H), 2.39 (t, J=6.4 Hz, 2H), 2.21 (s, 3H), 2.08 ppm (t, J=
6.4 Hz, 2H); 13C NMR (100 MHz, CDCl3): d=194.1, 165.6, 158.5,
148.2, 130.8, 123.9, 119.6, 118.6, 113.2, 55.1, 38.6, 23.6, 22.4,
11.8 ppm; elemental analysis calcd (%) for C16H16O3: C 74.98,
H 6.29%; found: C 75.21, H 6.38%.
Yellow oil (128 mg, 89%); 1H NMR (400 MHz, CDCl3): d=7.25 (s,
4H), 2.66 (s, 2H), 2.29 (s, 2H), 2.22 (s, 3H), 1.07 ppm (s, 6H);
13C NMR (100 MHz, CDCl3): d=193.5, 165.0, 149.1, 132.8, 131.0,
130.1, 128.0, 118.2, 118.0, 52.9, 37.5, 34.8, 28.4, 12.0 ppm; elemental
analysis calcd (%) for C17H17ClO2: C 70.71, H 5.93%; found: C 70.56,
H 6.13%.
2-Methyl-3-(3-nitrophenyl)-6,7-dihydrobenzofuran-4(5H)-one
(5n)
3-(2-Bromophenyl)-2,6,6-trimethyl-6,7-dihydrobenzofuran-
4(5H)-one (5h)
1
Light-yellow oil (105 mg, 78%); H NMR (400 MHz, CDCl3): d=8.23–
8.22 (m,1H), 8.16–8.14 (m, 1H), 7.77(d, J=8.4 Hz, 1H), 7.54 (t, J=
8.0 Hz, 1H), 2.91 (t, J=6.4 Hz, 2H), 2.51 (t, J=6.8 Hz, 2H), 2.34 (s,
3H), 2.20 ppm (t, J=6.4 Hz, 2H); 13C NMR (100 MHz, CDCl3): d=
194.2, 166.3, 149.6, 147.8, 136.0, 133.5, 128.6, 124.4, 121.9, 119.2,
117.3, 38.4, 23.5, 22.4, 11.9 ppm; elemental analysis calcd (%) for
C15H13NO4: C 66.41, H 4.83; N, 5.16%; found: C 66.30, H 4.98; N,
5.01%.
1
Colorless oil (144 mg, 87%); H NMR (400 MHz, CDCl3): d=7.62 (d,
J=8.8 Hz, 1H), 7.31–7.29 (m, 1H), 7.25–7.23 (m, 1H), 7.21–7.18 (m,
1H), 2.75 (d, J=4.4 Hz, 2H), 2.33 (d, J=12.0 Hz, 2H), 2.16 (s, 3H),
1.17 (s, 3H), 1.15 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=193.1,
164.2, 149.6, 133.2, 132.5, 131.7, 129.0, 126.9, 124.8, 119.4, 117.9,
52.4, 37.5, 35.0, 28.6, 28.4, 11.9 ppm; elemental analysis calcd (%)
for C17H17BrO2: C 61.28, H 5.14%; found: C 61.42, H 5.32%.
3-(2-Chlorophenyl)-2-methyl-6,7-dihydrobenzofuran-4(5H)-one
3-(4-Bromophenyl)-2,6,6-trimethyl-6,7-dihydrobenzofuran-
(5o)
4(5H)-one (5i)
1
Yellow oil (109 mg, 84%); H NMR (400 MHz, CDCl3): d=7.44–7.41
1
(m, 1H), 7.27–7.23 (m, 3H), 2.90–2.86 (m, 2H), 2.47–2.42 (m, 2H),
2.17 ppm (s, 5H); 13C NMR (100 MHz, CDCl3): d=193.6, 165.2, 149.4,
134.1, 131.8, 131.0, 129.2, 128.7, 126.1, 120.5, 116.0, 38.1, 23.4, 22.4,
11.8 ppm; elemental analysis calcd (%) for C15H13ClO2: C 69.10,
H 5.03%; found: C 68.95, H 5.18%.
Colorless oil (149 mg, 90%); H NMR (400 MHz, CDCl3): d=7.41 (d,
J=8.8 Hz, 2H), 7.18 (d, J=8.4 Hz, 2H), 2.66 (s, 2H), 2.29 (s, 2H),
2.22 (s, 3H), 1.07 ppm (s, 6H); 13C NMR (100 MHz, CDCl3): d=193.5,
165.0, 149.1, 131.9, 131.3, 130.9, 130.5, 121.0, 118.1, 118.0, 52.8,
37.5, 34.9, 28.4, 12.0 ppm; elemental analysis calcd (%) for
C17H17BrO2: C 61.28, H 5.14%; found: C 61.40, H 5.35%.
3-(4-Chlorophenyl)-2-methyl-6,7-dihydrobenzofuran-4(5H)-one
2-Ethyl-6,6-dimethyl-3-phenyl-6,7-dihydrobenzofuran-4(5H)-
(5p)
one (5j)
1
Yellow oil (112 mg, 86%); H NMR (400 MHz, CDCl3): d=7.27–7.22
1
Pale yellow oil (114 mg, 85%); H NMR (400 MHz, CDCl3): d=7.29–
(m, 4H), 2.80 (t, J=6.4 Hz, 2H), 2.41 (t, J=6.8 Hz, 2H), 2.21 (s, 3H),
2.03 ppm (t, J=6.4 Hz, 2H); 13C NMR (100 MHz, CDCl3): d=194.1,
165.9, 148.8, 132.9, 131.0, 130.2, 128.1, 128.0, 119.4, 118.1, 38.5,
23.6, 22.4, 11.9 ppm; elemental analysis calcd (%) for C15H13ClO2:
C 69.10, H 5.03%; found: C 68.93, H 5.20%.
7.28 (m, 4H), 7.23–7.21 (m, 1H), 2.68 (s, 2H), 2.58 (q, J=7.6 Hz,
2H), 2.29 (s, 2H), 1.15 (t, J=7.6 Hz, 3H), 1.08 ppm (s, 6H); 13C NMR
(100 MHz, CDCl3): d=193.6, 164.9, 154.0, 131.6, 129.7, 127.7,
172.00, 118.3, 118.3, 52.9, 37.6, 34.8, 28.5, 19.4, 12.9 ppm; elemental
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35.2, 28.7, 27.9, 27.7, 25.8, 25.7, 25.7, 21.7 ppm; elemental analysis
calcd (%) for C16H22O2: C 78.01, H 9.00%; found: C 78.20, H 9.16%.
3-(4-Bromophenyl)-2-methyl-6,7-dihydrobenzofuran-4(5H)-one
(5q)
1
Yellow oil (134 mg, 88%); H NMR (400 MHz, CDCl3): d=7.39 (d, J=
8.4 Hz, 2H), 7.16 (d, J=8.4 Hz, 2H), 2.78 (t, J=6.4 Hz, 2H), 2.38 (t,
J=6.4 Hz, 2H), 2.19 (s, 3H), 2.07 ppm (p, J=6.4 Hz, 2H); 13C NMR
(100 MHz, CDCl3): d=194.1, 166.0, 148.8, 131.4, 130.9, 130.6, 121.0,
119.4, 118.1, 38.5, 23.6, 22.4, 11.9 ppm. Anal. calcd. for C15H13BrO2:
C 59.04, H 4.29%; found: C 58.92, H 4.38%.
3,4,6,7,8,9-Hexahydrodibenzo[b,d]furan-1(2H)-one (5x)
1
Colorless oil (105 mg, 73%); H NMR (400 MHz, CDCl3): d=2.74 (t,
J=6.4 Hz, 2H), 2.58–2.54 (m, 2H), 2.48 (t, J=6.4 Hz, 2H), 2.37 (t,
J=6.8 Hz, 2H), 2.09–2.03 (m, 2H), 1.77–1.71 (m, 2H), 1.66–
1.61 ppm (m, 2H); 13C NMR (100 MHz, CDCl3): d=195.7, 165.7,
151.0, 120.5, 115.4, 38.2, 23.6, 22.9, 22.7, 22.7, 21.6 ppm; elemental
analysis calcd (%) for C12H14O2: C 75.76, H 7.42%; found: C 75.62,
H 7.30%.
2-Ethyl-3-phenyl-6,7-dihydrobenzofuran-4(5H)-one (5r)
Yellow oil (98 mg, 82%); 1H NMR (400 MHz, CDCl3): d=7.29–7.26
(m, 4H), 7.23–7.21 (m, 1H), 2.81 (t, J=6.0 Hz, 2H), 2.56 (q, J=
7.6 Hz, 2H), 2.40 (t, J=6.8 Hz, 2H), 2.09 (t, J=6.4 Hz, 2H), 1.15 ppm
(t, J=7.6 Hz, 3H); 13C NMR (100 MHz, CDCl3): d=194.1, 165.8,
153.8, 131.8, 129.8, 128.3, 127.8, 127.0, 126.1, 119.6, 118.5, 38.6,
23.7, 22.5, 19.4, 13.0 ppm; elemental analysis calcd (%) for
C16H16O2: C 79.97, H 6.71%; found: C 79.83, H 6.89%.
Typical Procedure for the Synthesis of Dioxa[5]helicenes 8
A mixture of 2-naphthol (1a, 0.5 mmol) and 3-nitro-2H-chromenes
(3u, 0.5 mmol) was stirred at reflux (80–858C) in dry EtOH (1.5 mL)
in the presence of K2CO3 (35 mg, 50 mol%) until the reaction was
complete (as determined by TLC). Then, the solvent was evaporat-
ed and the crude residue was extracted with water/EtOAc (10 mL,
1:1 v/v). The organic layer was separated and dried over Na2SO4.
After evaporation of the solvent, the crude product was purified
by column chromatography on a short pad of silica gel (petroleum
ether).
1-(2,5-Dimethyl-4-phenyl-furan-3-yl)-ethanone (5s)
1
Colorless oil (87 mg, 82%); H NMR (400 MHz, CDCl3): d=7.32–7.28
(m, 2H), 7.25 (d, J=7.2 Hz, 1H), 7.15 (d, J=8.0 Hz, 2H), 2.44 (s, 3H),
2.06 (s, 3H), 1.83 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=195.9,
156.0, 146.8, 133.6, 129.7, 126.3, 127.2, 122.9, 120.7, 30.5, 14.1,
11.4 ppm; elemental analysis calcd (%) for C14H14O2: C 78.48,
H 6.59%; found: C 78.59, H 6.71%.
6-Phenyl-6H-naphtho[1’,2’:4,5]furo[2,3-c]chromene (8a)
1
Yellow oil (71 mg, 41%); H NMR (400 MHz, CDCl3): d=8.80 (d, J=
1-(2,5-Dimethyl-4-(p-tolyl)furan-3-yl)ethanone (5t)
8.4 Hz, 1H), 8.20 (d, J=9.2 Hz, 1H), 8.02 (d, J=8.0 Hz, 1H), 7.80 (d,
J=8.8 Hz, 1H), 7.72–7.65 (m, 2H), 7.58–7.55 (m, 1H), 7.50–7.47 (m,
2H), 7.42–7.7.39 (m, 3H), 7.25–7.20 (m, 2H), 7.16–7.13 (m, 1H)
6.49 ppm (s, 1H); 13C NMR (100 MHz, CDCl3): d=153.7, 151.8, 151.7,
137.0, 131.2, 129.2, 129.0, 128.7, 128.0, 127.8, 127.6, 126.2, 126.1,
125.2, 124.9, 124.6, 122.5, 121.0, 119.3, 118.1, 113.2, 112.6,
75.8 ppm; elemental analysis calcd (%) for C25H16O2: C 86.19,
H 4.63%; found: C 86.02, H 4.78%.
Light-yellow oil (94 mg, 83%); 1H NMR (400 MHz, CDCl3): d=7.11
(d, J=8.0 Hz, 2H), 7.03 (d, J=8.0 Hz, 2H), 2.42 (s, 3H), 2.28 (s, 3H),
2.05 (s, 3H), 1.84 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=196.0,
155.9, 146.7, 136.9, 130.5, 129.6, 129.0, 122.9, 120.6, 30.6, 21.1, 14.1,
11.4 ppm; elemental analysis calcd (%) for C15H16O2: C 78.92,
H 7.06%; found: C 78.78, H 7.19%.
1-(4-(4-Chlorophenyl)-2,5-dimethylfuran-3-yl)ethanone (5u)
1
6-(p-Tolyl)-6H-naphtho[1’,2’:4,5]furo[2,3-c]chromene (8b)
Yellow oil (104 mg, 84%); H NMR (400 MHz, CDCl3): d=7.29 (d, J=
8.4 Hz, 2H), 7.09 (d, J=8.4 Hz, 2H), 2.44 (s, 3H), 2.06 (s, 3H),
1.89 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=195.4, 156.3, 147.1,
133.2, 132.1, 131.1, 128.5, 122.7, 119.6, 30.7, 14.2, 11.4 ppm; ele-
mental analysis calcd (%) for C14H13ClO2: C 67.61, H 5.27%; found:
C 67.42, H 5.41%.
1
Gummy mass (77 mg, 43%); H NMR (400 MHz, CDCl3): d=8.70 (d,
J=8.0 Hz, 1H), 8.09 (d, J=9.2 Hz, 1H), 7.91 (d, J=8.0 Hz, 1H), 7.69
(d, J=8.8 Hz, 1H), 7.55 (d, J=8.8 Hz, 2H), 7.46 (t, J=8.0 Hz, 1H),
7.26 (d, J=8.0 Hz, 2H), 7.14–7.09 (m, 4H), 7.02 (d, J=9.2 Hz, 1H),
6.35 (s, 1H), 2.27 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=153.6,
151.8, 139.0, 134.0, 131.2, 129.5, 129.4, 129.1, 128.0, 127.7, 127.7,
126.1, 126.1, 125.1,124.9, 124.6, 122.4, 121.1, 119.3, 118.1, 113.2,
112.6, 75.7, 21.2 ppm; elemental analysis calcd (%) for C26H18O2:
C 86.16, H 5.01%; found: C 86.01, H 5.15%.
Ethyl-2,5-dimethyl-4-phenylfuran-3-carboxylate (5v)
Yellow oil (105 mg, 86%); 1H NMR (400 MHz, CDCl3): d=7.29–
7.7.25,(m, 2H), 7.23–7.20 (m, 1H), 7.10–7.05 (m, 2H), 4.03 (q, J=
7.2 Hz, 2H), 2.49 (s, 3H), 2.11 (s, 3H), 1.01 ppm (t, J=7.2 Hz, 3H);
13C NMR (100 MHz, CDCl3): d=164.3, 157.3, 147.0, 133.2, 129.9,
127.5, 126.7, 121.3, 113.5, 59.7, 14.0, 13.8, 11.7 ppm; elemental
analysis calcd (%) for C15H16O3: C 73.75, H 6.60%; found: C 73.92,
H 6.79%.
6-(4-Methoxyphenyl)-6H-naphtho[1’,2’:4,5]furo[2,3-c]chromene
(8c)
1
Gummy mass (90 mg, 48%); H NMR (400 MHz, CDCl3): d=8.72 ( d,
J=8.0 Hz, 1H), 8.10 (d, J=9.2 Hz, 1H), 7.92 (d, J=8.0 Hz, 1H), 7.70
(d, J=8.8 Hz, 1H), 7.56 (d, J=9.2 Hz, 2H), 7.47 (t, J=7.8 Hz, 1H),
7.29 (d, J=8.8 Hz, 2H), 7.15–7.12 (m, 2H), 7.01 (d, J=8.8 Hz, 1H),
6.82 (d, J=8.8 Hz, 2H), 6.34 (s, 1H), 3.72 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=160.2, 153.8, 151.8, 131.2, 129.2, 129.2, 129.0,
128.5, 127.7, 126.1, 125.1, 124.9, 124.6, 122.4, 121.1, 119.3, 118.8,
118.1, 114.1, 113.3, 112.6, 75.5, 55.2 ppm; elemental analysis calcd
(%) for C26H18O3: C 82.52, H 4.79%; found: C 82.38, H 4.92%.
3,3-Dimethyl-3,4,6,7,8,9,10,11-octahydrocycloocta[b]benzofur-
an-1(2H)-one (5w)
Colorless oil (93 mg, 76%); 1H NMR (400 MHz, CDCl3): d=2.74 (t,
J=6.4 Hz, 2H), 2.66 (t, J=6.4 Hz, 2H), 2.58 (s, 2H), 2.21 (s, 2H),
1.66–1.29 (m, 4H), 1.44–1.35 (m, 4H), 1.04 ppm (s, 6H); 13C NMR
(100 MHz, CDCl3): d=195.2, 164.0, 152.3, 119.3, 116.8, 52.8, 37.6,
Chem. Asian J. 2015, 10, 2525 – 2536
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6-(4-Chlorophenyl)-6H-naphtho[1’,2’:4,5]furo[2,3-c]chromene
(8d)
4-Methoxy-2-methyl-3-phenylbenzofuran (11a)
1
Colorless oil (85 mg, 72%); H NMR (400 MHz, CDCl3): d=7.40–7.38
Yellow solid (86 mg, 45%); m.p. 120–1228C. 1H NMR (400 MHz,
CDCl3): d=8.68 (d, J=8.4 Hz, 1H), 8.08 (d, J=9.2 Hz, 1H), 7.92 (d,
J=8.0 Hz, 1HJ), 7.71 (d, J=8.8 Hz, 1H), 7.57–7.48 (m, 2H), 7.48–
7.44 (m, 1H), 7.31 (d, J=8.8 Hz, 2H), 7.16–7.12 (m, 2H), 7.04–7.02
(m, 1H), 6.35 ppm (s, 1H); 13C NMR (100 MHz, CDCl3): d=153.7,
151.5, 151.1, 135.25, 134.9, 131.2, 129.2, 129.0, 128.9, 128.6, 128.0,
127.9, 126.4, 126.2, 125.2, 124.8, 124.7, 122.7, 120.9, 119.2, 118.1,
113.3, 112.5, 74.9 ppm; elemental analysis calcd (%) for C25H15ClO2:
C 78.43, H 3.95%; found: C 78.32, H 4.09%.
(m, 2H), 7.35–7.31 (m, 2H), 7.27–7.25 (m, 1H), 7.12–7.08 (m, 1H),
7.02 (d, J=8.8 Hz, 1H), 6.57 (d, J=8.0 Hz, 1H), 3.66 (s, 3H),
2.35 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=155.2, 153.9, 150.3,
133.2, 130.4, 129.8, 128.7, 127.4, 126.5, 124.0, 116.7, 104.0, 103.7,
55.3, 12.4 ppm; elemental analysis calcd (%) for C16H14O2: C 80.65,
H 5.92%; found: C 80.52, H 5.80%.
4-Ethoxy-2-methyl-3-phenylbenzofuran (11 b)
Colorless oil (82 mg, 65%); 1H NMR (400 MHz, CDCl3): d=7.39 (d,
J=8.4 Hz, 2H), 7.31 (d, J=8.4 Hz, 2H), 7.25–7.21 (m, 1H), 7.07 (t,
J=8.0 Hz, 1H), 7.00 (d, J=8.0 Hz, 1H), 6.55 (d, J=8.0 Hz, 1H), 3.88
(q, J=8.4 Hz, 2H), 2.35 (s, 3H), 1.10 ppm (t, J=8.4 Hz, 3H);
13C NMR (100 MHz, CDCl3): d=155.3, 153.2, 150.2, 133.1, 130.6,
127.2, 126.5, 123.9, 117.9, 116.8, 104.8, 104.0, 64.0, 14.4, 12.4 ppm;
elemental analysis calcd (%) for C17H16O2: C 80.93, H 6.39%; found:
C 80.80, H 6.52%.
Typical Procedure for the Functionalization of Furans at the 2-
Position
Pd(OAc)2 (2 mol%), PPh3 (4 mol%), K2CO3 (1.5 equiv), and PivOH
(pivalic acid, 30 mol%) were placed in a sealed tube. Then, 4-iodo-
toluene (0.5 mmol) and furan (4, 0.5 mmol) were added, the tube
was purged with argon, and DMAc (N,N-dimethylacetamide,
1.5 mL) was added. Then, the reaction mixture was stirred at
1208C for 24 h. After cooling the reaction mixture to room temper-
ature the solution was extracted with EtOAc. The organic layer was
evaporated and the crude product was purified by column chro-
matography on silica gel (petroleum ether).
Acknowledgements
A.H. acknowledges financial support from the CSIR, New Delhi
(Grant No. 02(0168)/13/EMR-II). M.G. thanks the UGC (SRF),
India for a research fellowship.
1-Phenyl-2-p-tolylnaphtho[2,1-b]furan (10a)
Keywords: denitration · furans · helicenes · heterocycles ·
synthetic methods
Gummy mass (125 mg, 75%); 1H NMR (400 MHz, CDCl3): d=7.79
(d, J=8.4 Hz, 1H), 7.68–7.61 (m, 1H), 7.59–7.56 (m, 1H), 7.52 (d, J=
8.8 Hz, 1H), 7.48–7.44 (m, 1H), 7.41–7.39 (m, 1H), 7.36–7.33 (m,
1H), 7.28–7.26 (m, 6H), 7.26–7.24 (m, 1H), 7.17–7.15 (m, 1H),
2.31 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=151.2, 141.6, 134.1,
130.7, 130.4, 129.8, 129.3, 128.7, 128.5, 127.8, 127.5, 126.7, 125.9,
125.5, 124.4, 123.9, 123.1, 122.2, 118.9, 112.0, 12.2 ppm; elemental
analysis calcd (%) for C25H18O: C 89.79, H 5.43%; found: C 89.91,
H 5.56%.
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1-(4-Chlorophenyl)-2-p-tolylnaphtho[2,1-b]furan (10b)
Gummy mass (151 mg, 82%); 1H NMR (400 MHz, CDCl3): d=7.81
(d, J=8.4 Hz, 1H), 7.61 (d, J=6.0 Hz, 2H), 7.46–7.43 ( m, 4H), 7.39–
7.37 (m, 2H), 7.33–7.30 (m, 3H), 7.21–7.18 (m, 2H), 7.00 (d, J=
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C 81.29, H 4.50%.
Typical Procedure for the Synthesis of Densely Substituted
Benzofurans by Aromatization
The as-synthesized furan (5k, 0.5 mmol) was stirred in a solution of
I2 (1 equiv) in MeOH (1 mL) at 808C for a certain period of time (as
determined by TLC analysis). Following completion of the reaction,
the mixture was diluted with water/EtOAc (10 mL, 1:1 v/v). The or-
ganic layer was separated and dried over Na2SO4. After evaporation
of the solvent, the crude product was purified by column chroma-
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Manuscript received: July 9, 2015
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Accepted Article published: July 29, 2015
Final Article published: August 28, 2015
Chem. Asian J. 2015, 10, 2525 – 2536
www.chemasianj.org
2536
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim