Catalyst-free synthesis of diversely substituted 6H-benzo[c]chromenes and 6H-benzo[c]chromen-6-ones in aqueous media under MWI

Article abstract of DOI:10.1039/c2gc36379h

In this paper, a catalyst-free synthesis of diversely substituted 6H-benzo[c]chromenes and 6H-benzo[c]chromen-8-ols via cascade reactions of 2-(2-(allyloxy)phenyl)furan and 2-(2-(prop-2-ynyloxy)phenyl)furan, featured with intramolecular Diels-Alder reactions of furan with unactivated alkene/alkyne in aqueous media under MWI, was developed. In addition, an environmentally friendly oxidation of 6H-benzo[c]chromenes into the corresponding benzo[c]chromen-6-ones using aqueous H₂O₂ as an oxidant, in the absence of any activator/catalyst, was also revealed.

Full text of DOI:10.1039/c2gc36379h

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Green Chemistry
Catalyst-free synthesis of diversely substituted 6H-benzo[c]chromenes
and 6H-benzo[c]chromen-6-ones in aqueous media under MWI
View Online
Yan He, Xinying Zhang*, Liangyan Cui, Jianji Wang, and Xuesen Fan*
Received (in XXX, XXX) Xth XXXXXXXXX 201X, Accepted Xth XXXXXXXXX 201X
First published on the web Xth XXXXXXXXX 201X
DOI: 10.1039/b000000x
5
In this paper, a catalyst-free synthesis of diversely substituted 6H-benzo[c]chromenes and 6H-
benzo[c]chromen-8-ols via cascade reactions of 2-(2-(allyloxy)phenyl)furan and 2-(2-(prop-2-
ynyloxy)phenyl)furan featured with intramolecular Diels-Alder reactions of furan with unactivated
10 alkene/alkyne in aqueous media under MWI was developed. In addition, an environmentally
friendly oxidation of 6H-benzo[c]chromenes into the corresponding benzo[c]chromen-6-ones by
using aqueous H2O2 as an oxidant in the absence of any activator/catalyst was also revealed.
increased pressure may facilitate the envisioned D-A reaction,
Introduction
50 we then tried the reaction in a sealed vessel. To our pleasure,
treatment of 1a in H2O in a sealed vessel at 100 ºC for 24 h
afforded 6H-benzo[c]chromene (2a) in 30% yield. A plausible
pathway for the formation of 2a is shown in Scheme 2.
15 The Diels-Alder reaction is one of the most fundamental
reactions to form carbocyclic and heterocyclic frameworks.1
In particular, intramolecular Diels-Alder reaction with furan
as a diene partner (IMDAF) has been used as a valuable tool
in the synthesis of a plethora of organic compounds.2
20
On the other hand, water is becoming increasingly popular
as a medium for chemical reactions. Apart from economic and
environmental benefits, the aqueous medium also show
favorable effects on many organic transformations.3 Examples
of using water as a solvent for Diels-Alder reaction have been
25 pioneered by Breslow4 and followed by many others.5 In some
cases, D-A reactions in aqueous media exhibit accelerated
rate and even enhanced selectivity compared with those
carried out in hydrocarbon solvents.
55 Scheme 2 Plausible pathway for the formation of 2a from 1a.
As a continuation of our efforts in exploring water as a
30 clean medium for organic transformations,6 we designed and
synthesized 2-(2-(allyloxy)phenyl)furan (1a) as a potential
IMDAF substrate in aqueous medium. Our synthesis of 1a
started with salicylaldehyde and featured with an allenic
ketone intermediate (Scheme 1). The total yield for the four
35 step procedure is 62%.
During our study, we found two papers trying to synthesize
cyclic structures based on a similar concept. Sarkar reported a
Au(III) catalyzed IMDAF of 1-(2-furyl)-1-hex-1-en-5-yn-3-
ols in CH3CN to afford hydroxyphenanthrene in moderate
60 yield (Scheme 3).8 However, when its allyl counterpart, 1-(2-
(furan-2-yl)phenyl)but-3-en-1-ol was tried, the expected
phenanthrene could not be obtained under various conditions
including heating in a sealed tube (Scheme 3).
Scheme 1 Reaction conditions: (a) 3-bromoprop-1-ene, K2CO3, DMF,
r.t.; (b) 3-bromoprop-1-yne, Zn, DMF/THF, r.t.; (c) Jones reagent,
acetone, 0 ºC; (d) AgNO3, CH3CN, 80 ºC.
40
With 1a in hand, we began to investigate its reactivity as a
potential IMDAF substrate. Initially, a suspension of 1a in
water was heated at reflux for 48 h, but no reaction was
observed (Table 1, entry 1). In the presence of a Lewis acid,
FeCl3, the reaction still did not occur (entry 2). These results
65 Scheme 3 Reaction conditions: (a) furan-2-ylboronic acid, Pd(PPh3)4,
Et3N, DMF, 100 ºC, argon; (b) 3-bromoprop-1-yne, In, NaI, DMF,
r.t.; (c) AuCl3, CH3CN, 20 ºC, 48%; (d) 3-bromoprop-1-ene, In, NaI,
DMF, r.t.
45 are actually in accord with the observations made by other
researchers that due to its aromaticity furan is a relatively
unreactive diene partner toward unactivated alkenes for D-A
reaction even in intramolecular cases.7 Considering that
In another example, Gundersen et al. prepared 1a through a
70 synthetic process featured with the Suzuki-Miyaura cross-
coupling and found that an intramolecular D-A cycloaddition

Green Chemistry
Page 2 of 6
of 1a could be realized in xylene at 150 ºC. From that reaction,
oxabicycloheptadiene (A), rather than 6H-benzo[c]chromene
(2a), was obtained in a yield of 28% (Scheme 4).9
35
The scope of the reaction leading to 2 was then explored.
Various functional groups, such as methoxy, bromo and nitro
groups, were well tolerated. It was also observed that the
electronic property of the substituents on the aromatic rings
affected the yields obviously and substrates with electron-
View Online
40 donating groups gave better yields (Table 2, entries 2, 7, 8)
than that with electron-withdrawing groups (Table 2, entries
4-6). Moreover, disubstituted furan moiety participated in this
reaction smoothly and gave diversely substituted 6H-benzo[c]
chromenes in moderate yields (entries 9-10). It is noted that
45 substrate with a methyl group on the terminal of alkene (1k)
underwent a Claisen rearrangement to give 2-(but-3-en-2-yl)-
6-(furan-2-yl)phenol (I) under the conditions (entry 11),
5 Scheme 4 Reaction conditions: (a) furan-2-ylboronic acid, Pd(OAc)2,
PPh3, K2CO3, EtOH, argon, 80 ºC, 16 h; (b) 3-bromoprop-1-ene,
K2CO3, CH3CN, r.t. 48 h; c) xylene, 150 ºC, 24 h, 28%.
Meanwhile, literature survey revealed that 6H-benzo
chromenes (2) are an important class of oxygen-containing
10 heterocycle showing interesting biological and photochemical
properties.10 Although synthetic methods toward benzo-
chromenes have been extensively investigated,11 general
protocols without using expensive/toxic reagents and solvents
are still highly desirable.
Table 2 Scope of the reaction leading to 6H-benzo[c]chromenes (2)a
15
Hence, the reaction of 1a in water was studied in detail
with regard to different reaction temperature and time (Table
1). We were pleased to find that the reaction was significantly
improved at elevated tmeperatures and a yield of 71% was
obtained when it was run in water at 150 ºC for 24 h (entry 6).
Entry
1
Substrates
Products
Yields (%)b
77
20 To compare the effect of H2O with routinely used organic
solvents, it was then tried in 1,4-dioxane, toluene, CH3CN or
ethanol, respectively. But none of them gave higher yield of
2a than H2O (entries 7-10). Furthermore, since many organic
reactions progress much faster under microwave irradiation
25 (MWI) than with conventional heating,12 especially for
reactions carried out in solvents with high dielectric constants
such as water and alcohols, the reaction of 1a in water was
then run under MWI. Inspiringly, with the promotion of MWI,
the period for a complete reaction shortened from 24 h to 5 h
30 and the yield of 2a amounted to 77% (entry 12).
2a
1a
2
85
2b
1b
3
4
5
78
68
71
2c
1c
Cl
O
1d
2d
Table 1 Optimization of the reaction conditions for the preparation of 2aa
1e
2e
2f
O2N
6
7
57
81
O
1f
Entry
solvent
T (ºC)
t (h)
Yield (%)b
-
1
2
H2O
H2O
100
100
100
120
150
170
150
150
150
150
150
150
48
48
24
24
24
24
48
24
48
24
24
5
c
-
1g
2g
3
H2O
30d
52d
71d
72d
30d
65d
40d
64d
4
H2O
8
9
83
74
2h
5
H2O
1h
6
H2O
7
1,4-dioxane
toluene
CH3CN
ethanol
H2O
1i
2i
8
9
10
11
45
10
11
12
60
2j
1j
e
-
H2O
77d,e
72
a Reaction conditions: 0.5 mmol of 1a, 5 mL of solvent; b Isolated yields;
I
1k
c With FeCl3 (20 mol %); d Run in a sealed vessel; e Under MWI.
50 a Reaction conditions: 0.5 mmol of 1, 5 mL of H2O, 150 ºC, sealed vessel,
MWI; b Isolated yields.

Page 3 of 6
Green Chemistry
Having explored the intramolecular D-A reaction of 2-(2-
8-ols (4) in moderate to good yields (Table 4). Disubstituted
(allyloxy)phenyl)furan (1) in water, our attention moved to its
propargyl counterpart, 2-(2-(prop-2-ynyloxy)phenyl)furan
(3a). It turned out that 3a could be prepared through similar
synthetic process as that for the preparation of 1a and the
overall yield of 3a is 58% (Scheme 5).
furan moiety proved to be efficient diene partner for this
reaction (entry 9). It is worth to be noted that 2-(2-(but-2-
View Online
35 ynyloxy)phenyl)furan, with a methyl group attached on the
5
terminal of alkyne, underwent this cascade process smoothly
(entry 10), although previous study showed that disubstituted
alkyne failed to take part in Au(III) catalyzed IMDAF.13 In a
further aspect, compared with 6H-benzo[c]chromene (2), 6H-
40 benzo[c]chromen-8-ols (4) have an extra hydroxy group,
which may endow the parent compounds with distinctive
properties and be used as a useful handle for further structual
elaborations.
Scheme 5 Reaction conditions: (a) 3-bromoprop-1-yne, K2CO3,
DMF, r.t.; (b) 3-bromoprop-1-yne, Zn, DMF/THF, r.t.; (c) Jones
Table 4 Scope of the reaction leading to 6H-benzo[c]chromen-8-ols (4)a
10 reagent, acetone, 0 ºC; (d) AgNO3, CH3CN, 80 ºC.
Next, the reactivity of 3a in aqueous medium was studied
(Table 3). It turned out that upon treatment in water in a
sealed vessel at 150 ºC for 24 h, 3a could be transformed into
the expected 6H-benzo[c]chromen-8-ol (4a) notwithstanding
15 the yield was low (Table 3, entry 1). Following optimization
with regard to different solvents revealed that the yield of 4a
was remarkably improved by using ethanol as the reaction
medium (entry 4). Interestingly, when the reaction was carried
out in a mixed solvent of water and ethanol with a ratio of 4:1
20 (v/v), it could afford 4a in 70% yield (entry 11). Furthermore,
it was observed that MWI could accelerate the reaction
significantly. For example, the transformation of 3a in
water/ethanol (4:1) under MWI accomplished in 6 h to give
4a in 72% yield (entry 13).
45
Entry
1
Substrate
Product
Yield (%)b
72
4a
3a
2
3
4
5
6
82
78
62
60
52
3b
3c
4b
4c
25 Table 3 Optimization of the reaction conditions for the preparation of 4aa
3d
3e
3f
4d
4e
4f
Entry
T (ºC)
t (h)
Solvent
Yield (%)b
1
2
150
150
150
150
150
150
150
150
150
150
150
120
150
120
24
36
48
24
24
24
24
24
24
24
24
24
6
H2O
22
51
31
60
53
45
38
23
41
56
70
52
72c
62c
toluene
3
CH3CN
4
ethanol
5
H2O/ethanol (1:4)
H2O/ethanol (1:3)
H2O/ethanol (1:2)
H2O/ethanol (1:1)
H2O/ethanol (2:1)
H2O/ethanol (3:1)
H2O/ethanol (4:1)
H2O/ethanol (4:1)
H2O/ethanol (4:1)
H2O/ethanol (4:1)
7
77
6
3g
4g
7
8
8
9
79
65
9
4h
3h
10
11
12
13
14
4i
3i
10
53
6
a Reaction conditions: 0.5 mmol of 3a, 5 mL of solvent, sealed vessel;
3j
4j
b Isolated yields; c Under MWI.
a Reaction conditions: 0.5 mmol of 3, 4 mL of H2O, 1 mL of C2H5OH,
b
150 oC, sealed vessel, MWI; Isolated yields.
Afterwards, the generality of the above reaction was
30 investigated. It turned out that all the substrates studied could
participate in this reaction and afforded 6H-benzo[c]chromen-
With a series of diversely substituted benzo[c]chromenes in
hand, we were interested in their oxidative transformation into

Green Chemistry
Page 4 of 6
the corresponding benzo[c]chromen-6-ones. It is not only
benzo[c]chromen-6-ones (5) with good efficiency (Table 6).
Table 6 Scope of the reaction leading to benzo[c]chromen-6-ones (5)a
because benzo[c]chromen-6-ones constitute one of the major
classes of natural products,14 but also due to the fact that
View Online
many benzo[c]chromen-6-ones display
a wide range of
5
important biological activities.15 In this regard, Bowman et al.
have developed an efficient procedure for the oxidation of
benzo[c]chromene toward benzo[c]chromen-6-one by using
PCC (pyridinium chlorochromate) as an oxidant.16 While
stoichiometric oxidations based on chromium (VI) are still
Entry
1
Substrate
Product
Yield (%)b
10 frequently used for lab-scale preparations due to their broad
synthetic applicability and mild reaction conditions, they have
the disadvantage of being toxic and potentially dangerous to
human and the environment. Therefore, an environmentally
friendly protocol for the oxidation of benzo[c]chromenes is
15 highly desirable. In recent years, hydrogen peroxide (H2O2) as
an oxidant is attracting more and more interests since it is not
only extremely cheap but also remarkably environmentally
benign as it has the highest atom efficiency and yields H2O as
byproduct.17 Based on the above facts, we determined to
20 study the possibility of using H2O2 as an oxidatant for the
oxidation of benzo[c]chromenes.
For this purpose, 6H-benzo[c]chromene (2a) was firstly
treated with an aqueous solution of H2O2 (30%). But the
reaction was reluctant to proceed even over prolonged
25 reaction period (Table 5, entries 1-3). In order to improve the
reaction by running it in a homogeneous manner, ethanol was
used as a solvent. To our delight, treatment of a solution of 2a
in ethanol with H2O2 at 80 °C for 3 days gave the expected
6H-benzo[c]chromen-6-one (5a) in a yield of 66% (entry 5).
30 More encouragingly, further studies showed that under the
promotion of MWI, the oxidation of 2a could accomplish in 8
h to give 5a in 70% yield (entry 8), thus resulting an efficient
and environmentally benign protocol for the preparation of
benzo[c]chromen-6-ones.
70
72
5a
2a
2
2b
5b
3
4
5
68
66
68
2c
2d
2e
5c
Cl
O
5d
5e
6
75
5g
2g
7
8
70
66
72
2h
5h
5i
2i
35 Table 5 Optimization of the reaction conditions for the preparation of 5aa
9
2j
5j
a Reaction conditions: 0.2 mmol of 2, 1.2 mmol of H2O2 (30%), 2 mL
b
45 of C2H5OH, 80 ºC, MWI; Isolated yields.
Entry
solvent
-
T (ºC)
100
100
100
80
t (d or h)
1 d
Yield (%)b
-
Following studies revealed that the oxidation of 6H-benzo
[c]chromen-8-ols (4a) by aqueous H2O2 is much easier than
that of 2a. While the oxidation process of 2a by H2O2 with
conventional heating took several days (Table 5, entry 5), the
50 oxidation of 4a under similar conditions accomplished in 24 h
to give 8-hydroxy-6H-benzo[c]chromen-6-one (6a) in a yield
of 75%. This is most likely attributed to the electron-rich
nature of 4a due to the presence of an electron-donating
hydroxyl group. Furthermore, oxidation of other 6H-benzo[c]
55 chromen-8-ols (4) turned out to be equally successful and the
results were listed in Table 7.
Finally, synthetic elaborations of 6H-benzo[c]chromenes
was explored by using 4a as a model substrate. Thus,
treatment of 4a with BBr3 in CH2Cl2 affords 2'-(bromomethyl)
60 biphenyl-2,4'-diol (7) in 90% yield (Scheme 6).18 Subsequent
treatment of 7 with FeCl3 in benzene gives 2'-benzylbiphenyl-
2,4'-diol (8) in a yield of 89%. This sequence of reactions
provides an attractive alternative to cross-coupling protocols
1
2
-
3 d
trace
trace
22
3
-
5 d
4
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
1 d
5
80
3 d
66
6
80
4 d
69
54c
70c
70c
7
80
6 h
8
80
8 h
9
80
9 h
10
80
10 h
71c
a Reaction conditions: 0.2 mmol of 2a, 1.2 mmol of H2O2 (30%), 2 mL of
solvent; b Isolated yields; c Under MWI.
With the optimized reaction conditions, other 6H-
40 benzo[c]chromenes could be oxidized into the corresponding

Page 5 of 6
Green Chemistry
in some cases for the preparation of the challenging hindered
biaryls with multi-functional groups.
20 components were visualized by observation under UV light (254
and 365 nm). MWI promoted reactions were performed in a
commercial microwave reactor (XH-200A, Beijing Xianghu
Table 7 Scope of synthesis of 8-hydroxy-6H-benzo[c]chromen-6-one (6)a
View Online
Science and Technology Development Co. Ltd, Beijing, China).
1
Experimental details, spectroscopic data, copies of H and 13C
25 NMR spectra for all starting materials and products are available
as electronic supporting information.
A typical procedure for the synthesis of 2a
Entry
1
Substrate
Product
Yield (%)b
75
2-(2-(Allyloxy)phenyl)furan (1a, 0.5 mmol) was put into a 10 mL
reaction vessel equipped with a magnetic stirring bar. To this was
30 added H2O (5 mL) and the vessel was sealed and put into the
cavity of the microwave synthesis apparatus and irradiated at 300
W at 150 °C for 15 × 20 min at intervals. Between two
irradiations, the reaction temperature was allowed to cool to 60
°C. The internal pressure within the vessel was not measured.
35 Upon completion of the reaction, the vessel was cooled to room
temperature and extracted with ethyl acetate (5 mL × 3). The
combined organic phases were dried, filtered and concentrated
under vacuum. The residue was purified by column
chromatography on silica gel eluenting with ethyl acetate/hexane
40 (5-10%) to give 6H-benzo[c]chromene (2a). 2b-2j were obtained
in a similar manner. The synthetic procedure for 4a-4j is similar
except for the solvent used is a mixed solvent of water and
ethanol with a ratio of 4:1 (v/v).
4a
6a
2
78
6b
6c
4b
3
4
5
72
70
68
4c
OH
Cl
O
6d
4d
4e
6e
A typical procedure for the synthesis of 5a
45 To a round-bottom flask equipped with a condenser containing
6H-benzo[c]chromene (2a, 0.2 mmol) was added H2O2 (30%, 1.2
mmol) and ethanol (2 mL). The flask was put into the cavity of
the microwave synthesis apparatus and irradiated at 300 W at 80
°C for 16 × 30 min at intervals. Between two irradiations, the
50 reaction temperature was allowed to cool to 40 °C. Upon
completion, it was cooled to room temperature and extracted with
ethyl acetate (5 mL × 3). The combined organic phases were
dried, filtered and concentrated under vacuum. The residue was
purified by column chromatography on silica gel eluenting with
55 ethyl acetate/hexane (10-15%) to give 6H-benzo[c]chromen-6-
one (5a). 5b-5e and 5g-5j were obtained in a similar manner.
6
7
80
82
4g
6g
6h
4h
5
a Reaction conditions: 0.2 mmol of 4, 1.2 mmol of H2O2 (30%), 2 mL
of C2H5OH, 80 oC; Isolated yields.
b
A typical procedure for the synthesis of 6a
To a round-bottom flask equipped with a condenser containing
6H-benzo[c]chromen-8-ol (4a, 0.2 mmol) was added H2O2 (30%,
60 1.2 mmol) and ethanol (2 mL). The flask was put into an oil bath
and stirred at 80 °C for 24 h. Upon completion, it was cooled to
room temperature and extracted with ethyl acetate (5 mL × 3).
The combined organic phases were dried, filtered and
concentrated under vacuum. The residue was purified by column
65 chromatography on silica gel eluenting with ethyl acetate/hexane
(10-20%) to give 8-hydroxy-6H-benzo[c]chromen-6-one (6a).
6b-6e, 6g and 6h were obtained in a similar manner.
Scheme 6 Synthesis of hindered biaryls from 4a.
Experimental
1
10
The H, 13C NMR spectra were recorded at 400 MHz or 100
MHz, respectively. Chemical shifts were reported in ppm from
tetramethylsilane (TMS) as internal standard in CDCl3 or
DMSO-d6 solutions. Multiplicity was indicated as follows: s
(singlet); d (doublet); t (triplet); m (multiplet); dd (doublet of
Conclusions
15 doublets), etc. and coupling constants were given in Hz. High
resolution mass spectra (HRMS) were performed on a time-of-
flight (microTOF) mass spectrometer. The conversion of starting
materials were monitored by thin layer chromatography (TLC)
using silica gel plates (silica gel 60 F254 0.25 mm) and
In summary, we have developed a catalyst-free synthesis of
70 diversely substituted 6H-benzo[c]chromenes and 6H-benzo[c]-
chromen-8-ols in aqueous media under MWI. Moreover, an
environmentally sustainable oxidation of 6H-benzo[c]chromenes
into the corresponding 6H-benzo[c]chromen-6-ones by using

Green Chemistry
Page 6 of 6
1984, 25, 1239; (d) R. Breslow, Acc. Chem. Res., 1991, 24, 159; (e) R.
Breslow, Acc. Chem. Res., 2004, 37, 471.
aqueous hydrogen peroxide as an oxidant without using any
55
60
65
catalyst was also revealed. Notable features of the above
processes include: 1) efficient and clean procedure carried out in
aqueous media; 2) cascade reaction including cycloaddition of
furan with unactivated alkene/alkyne and ring-opening of the
oxabicycloheptadiene adduct in the absence of any catalyst; 3)
significant rate acceleration under MWI compared with
conventional heating; 4) environmentally friendly oxidation with
hydrogen peroxide in the absence of any activator. With the
10 advantages as described above, the synthetic methods developed
in this paper are expected to be used as valuable alternative
protocols for the preparation of the synthetically and biologically
interesting 6H-benzo[c]chromenes and 6H-benzo[c]chromen-6-
ones.
5
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8
9
15 Acknowledgement
We thank the National Natural Science Foundation of China
(20972042, 21172057), PCSIRT (IRT1061), 2012IRTSTHN006
and RFDP (20114104110005) for financial support.
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