Selective synthesis of oxygen-containing heterocycles via tandem reactions of 1,2-allenic ketones with ethyl 4-chloroacetoacetate

Article abstract of DOI:10.1039/c3ra23432k

By tuning the reaction conditions and the substitution patterns of substrates, the selective synthesis of oxepin-3(2H)-ones, benzo[d][1,3]dioxoles, or furan-3(2H)-ones starting from readily available acyclic substrates under mild conditions has been estab

Full text of DOI:10.1039/c3ra23432k

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Selective synthesis of oxygen-containing heterocycles
via tandem reactions of 1,2-allenic ketones with ethyl
4-chloroacetoacetate3
Cite this: RSC Advances, 2013, 3, 4156
Received 20th December 2012,
Accepted 30th January 2013
Qiang Wang,ab Zhouqing Xub and Xuesen Fan*a
DOI: 10.1039/c3ra23432k
www.rsc.org/advances
By tuning the reaction conditions and the substitution patterns of
substrates, the selective synthesis of oxepin-3(2H)-ones, ben-
zo[d][1,3]dioxoles, or furan-3(2H)-ones starting from readily
available acyclic substrates under mild conditions has been
established.
isolaurepinnacin,4 lobatrienetriol,5 (2)-brevenal6 and ciguatoxin.7
In addition, oxepinones (3) are versatile intermediates in organic
synthesis.3,4,6,8 Although a number of efficient and reliable
synthetic methods for the construction of the oxepinone scaffold
have been reported,9 they usually use difficult-to-obtain hetero-
cycles as starting materials, and require delicate control of the
reaction conditions (Scheme 4). Therefore, new synthetic methods
toward oxepinones starting from easily obtainable acyclic sub-
strates and carried out under mild conditions are highly desirable.
Thus, the reaction of 1a and 2 was studied in detail with the
aim of developing it into a novel synthetic protocol toward
oxepinones. Firstly, with K2CO3 as a basic promoter, the effect of
different solvents was examined. Among the solvents studied,
CH3CN turned out to be the most efficient (Table 1, entries 1–6).
With CH3CN as solvent, other bases, such as Na2CO3, KOH, LiOH,
NaH, t-BuOK, Cs2CO3, TBAF?3H2O or Et3N, were tried. They gave
lower yields of 3a than that with K2CO3 (Table 1, entries 7–14).
Next, different molar ratios of 1a with 2 were surveyed. It was
observed that with a ratio of 1 : 1.2 (1a : 2), the yield of 3a
increased to 68% (Table 1, entry 15). A larger amount of 2 did not
improve the reaction obviously (entry 16). Thus, the optimum
conditions for the preparation of 3a are summarized as follows:
treating 1a and 2 (1 : 1.2) with K2CO3 in CH3CN at ambient
temperature for 1 h. Under such conditions, 3a could be obtained
in a yield of 68%.
As part of our recent research interests in exploring the versatile
reactivity of 1,2-allenic ketones,1,2 we have envisioned a new
pathway toward cyclopentenone (I) via a cascade reaction of a 1,2-
allenic ketone with ethyl 4-chloroacetoacetate as shown in
Scheme 1.
Thus, 1-phenylbuta-2,3-dien-1-one (1a) and 4-chloroacetoace-
tate (2) were treated with K2CO3 in acetone at room temperature
for 1 h. To our surprise, the expected cyclopentenone (I) was not
isolated. Instead, three oxygen-containing heterocycles, including
oxepinone (3a), benzo[d][1,3]dioxole (4a), as well as furan-3(2H)-
one (5a) were obtained in yields of 50%, 10% and 8%, respectively
(Scheme 2).
The unexpected formation of 3a, 4a and 5a under mild
conditions turns out to be very attractive since all of these
heterocycles are of interest in both synthetic and medicinal
chemistry. Therefore, we were interested in further study with
regard to exploring the selective synthesis of these heterocyclic
compounds by tuning the reaction conditions and the substitution
patterns of the substrates (Scheme 3).
With the optimized conditions in hand, the scope and
generality of the above reaction toward oxepinones was investi-
gated. It turned out that both 1-aryl (Table 2, entries 1–17) and
1-benzyl (entry 18) or 1-phenylethyl (entry 19) substituted 1,2-
allenic ketones underwent this tandem reaction smoothly to
In a first aspect, seven-membered oxacycles are attractive
synthetic targets due to their wide range of biological activities and
frequent occurrence in natural products such as zoapatanol,3
aSchool of Chemistry and Chemical Engineering, Key Laboratory of Green Chemical
Media and Reactions, Ministry of Education, Henan Key Laboratory for
Environmental Pollution Control, Henan Normal University, Xinxiang, Henan
453007, P. R. China. E-mail: xuesen.fan@htu.cn; xuesen.fan@yahoo.com;
Fax: +86 373 3326336
bSchool of Physics–Chemistry, Henan Polytechnic University, Jiaozuo, Henan,
454003, P. R. China
Electronic supplementary information (ESI) available: Experimental information,
3
procedures and spectroscopic data, copies of 1H and 13C NMR spectra and X-ray
crystal structures. CCDC reference numbers 857572, 909448 and 913070 correspond
to 3f, 5c and 4aa respectively. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3ra23432k
Scheme 1 Envisioned synthesis of cyclopentenone (I) via the reaction of a 1,2-
allenic ketone with 4-chloroacetoacetate.
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Table 1 Optimization studies for the formation of 3aa
Scheme 2 Unexpected formation of oxygen-containing heterocycles.
Yield [100%]b
Entry Base
Solvent Molar ratio (1a : 2) 3a 4a
5a
1
2
3
4
5
6
7
8
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
KOH
Acetone 1 : 1
50
23
10
8
THF
DCM
1 : 1
1 : 1
10 Trace
12 Trace Trace
10 13 Trace
60 Trace
66 Trace
12 Trace Trace
23 Trace Trace
14 Trace Trace
23 Trace Trace
25 Trace Trace
57 Trace Trace
42 Trace Trace
Toluene 1 : 1
DMSO 1 : 1
9
8
CH3CN 1 : 1
CH3CN 1 : 1
CH3CN 1 : 1
CH3CN 1 : 1
CH3CN 1 : 1
CH3CN 1 : 1
CH3CN 1 : 1
9
LiOH
NaH
10
11
12
13
14
15
16
t-BuOK
Cs2CO3
TBAF?3H2O CH3CN 1 : 1
Et3N
K2CO3
K2CO3
Scheme 3 Envisioned selective synthesis of 3, 4, or 5.
CH3CN 1 : 1
CH3CN 1 : 1.2
CH3CN 1 : 2
40
11 Trace
68 Trace
67 Trace
7
8
afford oxepinones in moderate to good yields. Functional groups
such as F, Cl, Br, CH3O and CF3 are well tolerated. In addition, the
electronic nature of the substituents does not show an obvious
effect on the yield of 3 (entries 1–17). It is noted herein that the
structure of 3f was unambiguously confirmed by its spectroscopic
data together with X-ray diffraction analysis. An ORTEP drawing of
a
Reaction conditions: 1a (1.0 mmol), base (1.0 mmol), solvent (5.0
b
mL), rt, 1 h. Isolated yields.
5c was confirmed by its spectroscopic data together with X-ray
diffraction analysis (Fig. 2).
3
3f is shown in Fig. 1.
Having established a protocol for the preparation of oxepi-
3
Next, the reaction was extended to substrates with a methyl
group on the internal position, or an aryl group on the terminal
position of the allene moiety (Scheme 5). Interestingly, in these
cases, substituted furan-3(2H)-ones (5) were obtained in moderate
yields. The corresponding oxepinones (3) were formed only in
trace amounts. It has been reported that many furan-3(2H)-ones
exhibit potential biological activities and the furanone scaffold is
found in a number of naturally occurring antitumor agents.10 As a
result, efficient and rapid synthesis of furan-3(2H)-ones is of
significant importance. In this sense, the reaction of 2- or
4-substituted allenic ketones with 4-chloroacetoacetate leading to
furan-3(2H)-ones under mild conditions is a promising finding
and may be used as an alternative method for the preparation of
substituted furan-3(2H)-ones. It is also noted that the structure of
nones (3) and discovered the structural features of substrates
Table 2 Synthesis of substituted oxepin-3(2H)-ones (3)a
Entry
R1
Products
Yield [100%]b
1
2
3
4
5
6
7
8
C6H5
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
68
60
58
60
76
65
56
56
55
59
68
62
54
57
72
80
71
50
57
p-MeC6H4
p-MeOC6H4
p-FC6H4
p-ClC6H4
p-BrC6H4
p-CF3C6H4
m-FC6H4
m-ClC6H4
m-BrC6H4
m-MeC6H4
3,4-Di-MeOC6H3
o-MeOC6H4
o-FC6H4
9
10
11
12
13
14
15
16
17
18
19
o-ClC6H4
o-BrC6H4
1-Naphthyl
C6H5CH2
C6H5CH2CH2
3s
a
Reaction conditions: 1 (1.0 mmol), 2 (1.2 mmol), K2CO3 (1.0
b
Scheme 4 Literature synthetic methods for the preparation of oxepinones.
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mmol), CH3CN (5.0 mL), rt, 1 h. Isolated yields.
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Table 3 Optimization studies for the formation of 4aa
Fig. 1 ORTEP drawing of 3f.
Entry Base (equiv.)
Additive
Solvent
Yield [100%]b
1
2
3
4
5
6
7
8
K2CO3 (1.0)
K2CO3 (2.5)
Ba(OH)2?8H2O (2.5) None
None
None
Toluene 13
Toluene 16
Toluene 15
Toluene 13
Toluene 10
Toluene 14
Toluene 62
LiOH (2.5)
None
None
None
TBAB
Na2CO3 (2.5)
Cs2CO3 (2.5)
K2CO3 (2.5)
K2CO3 (2.5)
K2CO3 (2.5)
K2CO3 (2.5)
K2CO3 (2.5)
K2CO3 (2.5)
TBAB (0.1) Toluene 62
9
TBAC
TBAI
TBAB
TBAB
Toluene 50
Toluene 56
10
11
12
CH3CN
Acetone
Trace
Trace
Scheme 5 Synthesis of substituted furan-3(2H)-ones (5).
a
Reaction conditions: 1a (2.0 mmol), 2 (1.0 mmol), phase transfer
b
catalyst (0.05 mmol), solvent (5.0 mL), rt, 1 h. Isolated yields.
leading to furan-3(2H)-ones (5), our attention then moved to the
selective synthesis of benzo[d][1,3]dioxoles (4). It is well known
that benzo[d][1,3]dioxole units occur in numerous natural
products with a broad spectrum of biological properties11a–11d
and have been used as a 5-HT2A receptor antagonist11e and CB1R
inverse agonists.11f In addition, some benzo[d][1,3]dioxole units
are indispensable building blocks in functional materials,11g such
as molecular electronics,11h organic electronic devices,11i etc. In
spite of their importance, efficient synthetic methods for the
preparation of benzo[d][1,3]dioxoles are still limited and the
literature methods usually use o-dihydroxybenzenes rather than
the readily available acyclic compounds as the starting materials.11
Based on our previous observation that the reaction of 1a and 2
could afford 4a in higher yield than that of 3a only when it was run
in toluene (Table 1, entry 4), we then chose toluene as a model
solvent to explore the reaction conditions suitable for the
formation of 4a (Table 3). Firstly, increasing the amount of
K2CO3 from 1 to 2.5 equiv. improved the reaction slightly (Table 3,
entry 2). Using other bases such as Ba(OH)2?8H2O, LiOH, Na2CO3
and Cs2CO3 did not give fruitful results (entries 3–6). In a further
aspect, considering that tetrabutylammonium bromide (TBAB) is
often used as an additive in K2CO3-promoted reactions, we then
tried the reaction of 1a and 2 in the presence of TBAB. To our
delight, under such conditions, the yield of 4a increased to 62%
(entry 7). With other quaternary ammonium salts such as
tetrabutylammonium chloride (TBAC) or tetrabutylammonium
iodide (TBAI) as an additive, the yields of 4a are lower (entries 9–
10). It was also found that when toluene was replaced by CH3CN
or acetone, 4a was formed only in trace amount even in the
presence of TBAB (Table 3, entries 11–12).
In an attempt to probe the generality of the synthetic procedure
toward 4, the reaction of 2 with a variety of 1,2-allenic ketones (1)
was conducted under the optimized conditions (Table 4). While
both 1-aryl or 1-aliphatic substituted allenic ketones are suitable
substrates for this reaction (Table 4, entries 1–14), the type of
substituents on the aromatic ring of R1 affected the yield of 4
obviously. Substrates with an electron-withdrawing group gave
better yields (entries 1–9) than those with an electron-donating
Table 4 Synthesis of substituted benzo[d][1,3]dioxoles (4)a
Entry
R1
R2
R3
Products
Yield [100%]b
1
2
3
4
5
6
7
8
C6H5
p-FC6H4
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C6H5
H
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
—
—
62
65
62
59
60
76
71
59
57
46
36
38
22
63
—
—
p-ClC6H4
p-BrC6H4
p-CF3C6H4
m-FC6H4
m-ClC6H4
m-BrC6H4
m-MeC6H4
p-MeOC6H4
o-MeOC6H4
o-ClC6H4
o-FC6H4
9
10
11
12
13
14
15
16
C6H5CH2CH2
C6H5
C6H5
a
Reaction conditions: 1 (2.0 mmol), 2 (1.0 mmol), K2CO3 (2.5
b
Fig. 2 ORTEP drawing of 5c.
mmol), TBAB (0.05 mmol), toluene (5.0 mL), rt, 1 h. Isolated yields.
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dominating formation of 5c and 5d over oxepinone (3) for
substrates having a phenyl group on the terminal position of the
allene moiety (Scheme 5) may be explained by the extra stability
resulting from the presence of a phenyl group on the C–C double
bond (Scheme 6).
When the reaction of 1a and 2a is run in a nonpolar
hydrocarbon solvent like toluene, which usually facilitates
C-alkylation, carbanion E is formed and then undergoes an
intramolecular aldol reaction to afford intermediate F. Subsequent
nucleophilic substitution occurs with F to produce oxirane G. Base
promoted ring-opening of oxirane G affords H, from which
o-dihydroxybenzene 6a is formed via dehydration and isomeriza-
tion. Finally, base catalyzed double Michael addition of 6a onto 1a
affords 4a (Scheme 7).
Fig. 3 ORTEP drawing of (E)-4aa.
group (entries 10–11). Notably, the retarding effect of sterics on
this reaction is illustrated in entries 11–13 where ortho substitution
on the aryl ring led to sharply reduced yields. This effect is further
exemplified by the observations that 1,2-allenic ketones bearing a
substituent either on the inner or terminal position of the allenic
unit failed to give the corresponding benzo[d][1,3]dioxole products
(entries 15–16).
To confirm the structure of 4, the crystal structure of the 2,4-
dinitrophenylhydrazone of 4a, named as 4aa, was determined by
X-ray diffraction analysis as shown in Fig. 3.
The plausible pathway for the formation of
4 via a
dihydroxybenzene intermediate was partly confirmed by our
following studies. We found that although 6a could not be
isolated from the reaction of 1a with 2, it can be obtained by BBr3
promoted cleavage of 4a. Subjecting 6a and 1a to the reaction
conditions affords 4a in 86% yield (Scheme 8).
3
The above results also suggest that the tandem reaction of 1
and 2 in toluene can be used as a potential synthetic method
toward o-dihydroxybenzenes (6) directly from simple acyclic
substrates. Literature searching reveals that o-dihydroxybenzenes
are usually prepared through hydrolysis of o-chlorophenol,
demethylation of o-methoxyarenols, and oxidation of phenols or
o-hydroxybenzaldehydes or ketones.13 To our knowledge, direct
construction of o-dihydroxybenzenes from acyclic materials has
not been reported before.
In summary, the selective synthesis of oxepin-3(2H)-ones,
benzo[d][1,3]dioxoles, or furan-3(2H)-ones using readily available
acyclic substrates rather than complicated cyclic compounds as
the starting materials has been achieved by tuning the reaction
conditions and the substitution patterns of substrates. With
advantages such as readily available acyclic substrates, the absence
of expensive catalysts/reagents, and extremely mild reaction
conditions, the synthetic methods developed herein should have
important applications in heterocyclic chemistry and related areas.
Based on the above observations, a plausible mechanistic
pathway for the formation of 3a and 5a is proposed in Scheme 6.
Firstly, base triggers the cascade process by deprotonating 2 to give
an anion, which undergoes a conjugate addition on 1a to afford
anion A, which can isomerise to its tautomers B, C, D. When the
reaction is run in a polar aprotic solvent like CH3CN, which
usually favours O-alkylation,12 B undergoes an intramolecular
O-nucleophilic substitution to afford 3a. Alternatively, when the
O-nucleophilic substitution occurs with intermediate D, 5a would
be formed. The fact that 1,2-allenic ketone substrates having a
substituent on the internal position of the allene moiety
(Scheme 5) afford 5b as a main product rather than the
corresponding oxepinone (3) could be explained by the fact that
the supposed enolate B9 leading to oxepinone (3) is more difficult
to form due to steric effects (Scheme 6). On the other hand, the
Scheme 6 Plausible pathway for the formation of 3a and 5a.
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Scheme 7 Plausible pathway for the formation of 4a.
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Scheme 8 Preparation of 6a from 4a and its reaction with 1a to afford 4a.
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We thank the National Natural Science Foundation of China
(21172057, 21272058), RFDP (20114104110005) and EDHP
(2011A150015) for financial support.
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