Construction of fused- and spiro-oxa-[n.2.1] skeletons by a tandem epoxide rearrangement/intramolecular [3+2] cycloaddition of cyclopropanes with carbonyls

Article abstract of DOI:10.1039/c4cc02641a

A Lewis acid promoted tandem reaction of epoxide rearrangement and intramolecular [3+2] cycloaddition reaction of cyclopropanes with carbonyls formed by epoxide rearrangement in situ, which were obtained with difficulty by a general method, is reported. A wide variety of fused- and spiro-oxa-[n.2.1] skeletons could be efficiently constructed. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02641a

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Construction of fused- and spiro-oxa-[n.2.1]
skeletons by a tandem epoxide rearrangement/
intramolecular [3+2] cycloaddition of
cyclopropanes with carbonyls†
Cite this: Chem. Commun., 2014,
50, 8061
Received 10th April 2014,
Accepted 5th June 2014
Lu-Feng Wang, Zi-Fa Shi,* Xiao-Ping Cao,* Bao-Sheng Li and Peng An
DOI: 10.1039/c4cc02641a
www.rsc.org/chemcomm
A Lewis acid promoted tandem reaction of epoxide rearrangement
and intramolecular [3+2] cycloaddition reaction of cyclopropanes
with carbonyls formed by epoxide rearrangement in situ, which
were obtained with difficulty by a general method, is reported.
A wide variety of fused- and spiro-oxa-[n.2.1] skeletons could be
efficiently constructed.
Oxa-bridged carbocyclic skeletons are well-represented and widely
found in natural products.1 Such segments usually combined with
fused- or spiro-structures exist in some high activity molecules.
For example, indicol,2 salviasperanol,3 urechitol A,4 and grayano-
Scheme 1 Several representative natural products.
side D5 (Scheme 1) represent two types of oxa-bridged carbocyclic cycloaddition reaction of cyclopropanes with carbonyls gener-
containing fused- or spiro-systems. Highly efficient construction ated from epoxides rearrangement in situ, which were obtained
of these bridged skeletons is one of the most important themes in with difficulty by a general method, to construct the aliphatic
organic synthesis.6 Several creative strategies such as the Diels– bridged oxa-[n.2.1] skeletons containing fused- or spiro-structures.
Alder reaction, transition-metal-catalyzed reactions, and radical Also the strategy was applicable for the construction of the
reactions have been developed to construct the bridged carbo- aromatic ones.
cyclic skeletons.7 However, there is still an urgent need of a
strategy for the construction of these bridged oxa-[n.2.1] skeletons because the catalytic rearrangement of epoxides to establish useful
containing fused- or spiro-structures.
intermediates in organic synthesis has been widely elucidated,10
In this work, we chose the epoxide as the precursor of carbonyls
Cyclopropane derivatives were treated as important building and the ring opening reactions of epoxides could be catalyzed by a
blocks to build cyclic structures due to their facile preparation Lewis acid which could also be used as a co-catalyst in the
and high reactivity.8 Recently, the intramolecular cross- following cycloaddition reaction.
cycloaddition reaction of D–A cyclopropanes with carbonyls,
Initially, preliminary evaluation of our strategy was carried
imines, alkenes, or allenes was carried out to construct the out by using a,b-epoxy ketone 1a/1a0, which is a mixture of the
bridge-containing fused cyclic structures and applied to the epimers, as the model substrate to screen the optimized reac-
total synthesis of natural products, as reported by Wang and co- tion conditions for the tandem epoxide rearrangement/intra-
workers.9 But in this research study, there was less information molecular [3+2] cycloaddition reaction (Table 1). When 1a/1a0
on the construction the aliphatic bridged oxa-[n.2.1] skeletons. was treated in a boron trifluoride ether complex (BF3ꢀEt2O) in
Inspired by high reaction efficiency and product diversity of DCE at ꢁ10 1C, the desired products 2a and 3a were detected in
cyclopropanes, we explored the tandem intramolecular [3+2] a combined yield of 50% in a ratio of 2 : 1 by isolating individual
2a and 3a, respectively (entry 1). Then different reaction con-
State Key Laboratory of Applied Organic Chemistry & College of Chemistry and
Chemical Engineering, Lanzhou University, Lanzhou, 730000, P. R. China.
E-mail: caoxplzu@163.com, shizf@lzu.edu.cn; Fax: +86 931-8915557
† Electronic supplementary information (ESI) available: Experimental procedures
and analysis data for new compounds. CCDC 988543 (3c) and 988544 (4). For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc02641a
ditions were explored. With DCE as the steady solvent, various
Lewis acid catalysts were assessed (entries 2–7). Both the boron
trifluoride acetic acid complex (BF3ꢀCH3CO2H) and trimethylsilyl
trifluoromethanesulfonate (TMSOTf) could promote the reaction
to give 2a and 3a in a combined yield of 44% at ꢁ10 1C and the
ratios were 2 : 1 and 1 : 1, respectively (entries 3 and 4). When the
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a
Table 1 Optimization of conditions of the tandem reaction of 1a/1a0
Table 2 Scope of the BF3ꢀBu2O-mediated [3+2] tandem reaction of 1
Entrya Substrateb
Yield [%] 2 : 3d
Product
1
1b/1b0 (1 : 1)
2b
3b
86c
1 : 1
Entryb Catalyst
Solvent Temp. [1C] Time [h] Yieldd [%] 2a : 3ae
1
2
3
4
5
6
7
8
BF3ꢀEt2O
DCE
DCE
ꢁ10
ꢁ10
ꢁ10
ꢁ10
rtc
12
12
12
12
12
3
3
12
3
3
12
3
3
12
3
50
Trace
44
44
30
20
37
53
69
50
60
71
71
60
72
74
Trace
50
2 : 1
2
1c/1c0 (1 : 1)
2c
3c
88c
1 : 1
Sc(OTf)3
BF3ꢀCH3CO2H DCE
2 : 1
1 : 1
5 : 1
5 : 1
5 : 1
2 : 1
1 : 1
1 : 1
2 : 1
1.4 : 1
1.4 : 1
1 : 1
1 : 1
1 : 1
TMSOTf
BF3ꢀTHF
SnCl4
DCE
DCE
DCE
DCE
DCE
DCE
DCE
CHCl3
50c
50c
0
Yb(OTf)3
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀEt2O
BF3ꢀTHF
BF3ꢀTHF
BF3ꢀBu2O
3
4
1d/1d0 (1 : 1)
1e/1e0 (1 : 1)
2d
2e
3d
3e
62c
75c
1 : 9
4 : 1
9
rtc
10
11
12
13
14
15
16
17
18
19
20
50c
0
CHCl3 rtc
CHCl3 50c
DCM
0
DCM rtc
DCM Refluxc
3
THF
0
12
12
12
3
DCM rtc
6 : 1
2 : 1
1 : 1
DCM Refluxc
DCM Refluxc
54
86
5
6
1f/1f0 (1 : 1)
2f
54
43
a
The diastereomeric ratio (d.r.) values were confirmed by 1H NMR
b
spectroscopy. All the reaction were carried out with 1a/1a0 (47.4 mg,
0.15 mmol) in the presence of a Lewis acid (0.30 mmol) in solvent
(2.0 mL) under an argon atmosphere. The Lewis acid was added to the
reaction system at 0 1C, stirred for 5 min at 0 1C and then the
temperature was raised. Total yields of 2a and 3a. The ratio of 2a
and 3a was confirmed by the isolated yield of individual 2a and 3a.
c
1g/1g0 (1.5 : 1)
2g
d
e
boron trifluoride tetrahydrofuran complex (BF3ꢀTHF), SnCl4, and
Yb(OTf)3 were used as catalysts, the reaction did not occur at low
temperature (ꢁ10 1C and 0 1C). When the temperature was
increased, they gave us low product yields but higher isomer
ratios (2a: 3a = 5 : 1, entries 5–7). By considering entries 1 to 7,
BF3ꢀEt2O was used as the optimized catalyst to explore the
solvent and temperature effects. Different solvents such as
DCE, CHCl3, and DCM were used at different temperatures
(entries 8–15). Fortunately, when the reaction was performed
in the refluxing DCM, 2a and 3a were obtained in a combined
yield of 74% (2a: 3a = 1 : 1, entry 16).
7
1h/1h0 (1 : 1)
NR
8
9
1i/1i0 (1 : 1)
1j/1j0 (2 : 1)
2i
2j
43
92
At the same time, the reaction was attempted using BF3ꢀEt2O
or BF3ꢀTHF as promoters in THF or DCM (entries 17–19), but
satisfactory results were not obtained. In the end, when the
reaction was performed with BF3ꢀBu2O in the refluxing DCM
(entry 20), the combined yield of 2a and 3a was up to 86% with
the same ratio as given in entry 16 (1 : 1). Hence the use of
2 equiv. of BF3ꢀBu2O in the refluxing DCM was determined to
be the optimized reaction condition based on the yield.
Under the optimized reaction conditions, the substrate scope
and limitation of the tandem reaction were explored by using 1 as
substrates in the presence of BF3ꢀBu2O (2 equiv.) in the refluxing
DCM (Table 2). When the two methyl groups in the cyclopropane
1,1-diester moiety were substituted by two ethyl groups (entry 1),
or two germinal methyl groups were introduced into the six-
membered ring of the substrate (entry 2), the reaction proceeded
10
1k/1k0 (3 : 1)
2k/2k0 (2 : 1)b
87
11
12
1l/1l0 (1 : 1)
2l
41
87
1m/1m0 (3 : 1)
2m
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Table 2 (continued)
Entrya Substrateb
Yield [%] 2 : 3d
Product
13
1n/1n0 (1.5 : 1)
2n
83
a
All the reactions were carried out with 1 (0.15 mmol) in the presence
of BF3ꢀBu2O (0.21 mL, 0.30 mmol) in DCM (5.0 mL) for 3 h under an
b
argon atmosphere, as given in Table 1. The diastereomeric ratio (d.r.)
values were confirmed by 1H NMR spectroscopy. Total yields of 2
c
d
and 3. The ratio of 2 and 3 was confirmed by the isolated yield of
individual 2 and 3.
Scheme 2 Proposed reaction mechanism.
smoothly to give the fused-bridged product 2 and spiro-bridged
product 3 in similar yields (2 :3 = 1 : 1, which was confirmed by the
isolated yield of individual 2 and 3). The reaction of substrate
1d/1d0, including a quaternary carbon center in the cyclopropane
ring (entry 3), also occurred smoothly giving the product in a
medium yield. It was supposed that the steric hindrance between
the methyl group of cyclopropane and the carbonyl group of
cyclohexanone reduced the reactivity of the intramolecular [3+2]
cycloaddition reaction, which led to the ratio of 2d to 3d as 1 : 9.
The reaction of substrate 1e/1e0 (entry 4), having a a,b-epoxy
cycloheptanone structure, also afforded adducts 2e and 3e in
75% yield (2e :3e = 4 :1). Substrates 1f/1f0 and 1g/1g0 with a
quaternary carbon center in the epoxide ring gave only fused
cyclic products in moderate yields (entries 5 and 6). Substrate 1h/
1h0 being activated by one ester group was less active and its
reaction did not proceed to give the desired product (entry 7).
Substrate 1i/1i0 with one more methylene was examined and gave
only product 2i in medium yield along with the loss of one formyl
group (entry 8). We suspected that the low efficiency of 2i
formation is probably due to the conformational flexibility of
the eight-membered ring structure in 2i.
A series of aromatic substrates (entries 9–13) were con-
structed to assess the [3+2] cycloaddition reaction of phenyla-
cetaldehydes and propiophenone intermediates, which were
difficult to prepare by a common reaction, with cyclopropanes.
Then the tandem protocol was applied to the aromatic sub-
strates 1j/1j0 and 1k/1k0 (entries 9 and 10). To our delight, the
desired products were produced in excellent yields by using
these substrates. But substrate 1l/1l0 (entry 11) gave a low
product yield, probably because of its conformational flexibil-
ity. When compounds 1m/1m0 and 1n/1n0 were used as sub-
strates, propiophenone intermediates were first generated
through epoxide rearrangements and the reactions proceeded
smoothly in excellent yields (entries 12 and 13).
diester moiety through Lewis acid binding interactions. Mean-
while, another free carbonyl attacked the asymmetric cyclopro-
pane dipole, which was activated by a Lewis acid-binded diester
motif to generate oxonium intermediate A or B, respectively,
and these CQO groups in the intermediate were located on the
same side of the tetrahydropyrylium, which would control
the nucleophilic cyclization between enolate and oxoniums on
one side. Consequently the single relative configuration of the
cyclization products 2a and 3a were obtained with the bridge
oxygen and carbonyl on the opposite side. Furthermore, the
intermediate 5/50 was isolated when the reaction was adopted at
0 1C, and the isolated 5/50 could also be transformed to 2a and 3a
under the same conditions described in Table 2, which strongly
supported the dicarbonyl-involved process.
To further demonstrate the first step of the tandem reaction
involving the epoxide rearrangement process, we chose com-
pound 1i/1i0 as a substrate in our research because its low
activity would be accessible to trap the reaction intermediate.
When the substrate 1i/1i0 was promoted by BF3ꢀBu2O in DCM at
0 1C, only the epoxide rearrangement product 6/60 was detected
and could also be isolated, subsequently the corresponding
desirable product 2i was obtained in modest yield when the
reaction was further heated to reflux (Scheme 3), which proved
that the first step of the tandem reaction is a carbonyl migration
in the epoxide rearrangement to generate intermediate 6/60.
Then the [3+2] cross-cycloaddition of the cyclopropane with
the carbonyl of the ketone in 6/60 gave product 2i. Also, metal
catalyst Sc(OTf)3 in 1,2-dichloroethane (DCE) was investigated
(Scheme 3), and fused product 7 was detected; the hydrogen
X-ray crystallography of 4 (formed from 2c by air oxidation)
and 3c (ESI,† Fig. S1)11 confirmed the relative configuration of 2
and 3. This suggested that the bridge oxygen and carbonyl in
2a–g and 3a–e are on the different sides of the carbon rings.
A plausible reaction-mechanism was proposed (Scheme 2).7g
The first step involved the epoxide-ring rupture from the
quaternary carbon side followed by carbonyl migration to
generate intermediate 5/50. Then the carbonyl of an aldehyde
or the carbonyl of a ketone could be trapped together with a Scheme 3 Further reactions of 1i.
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In summary, we have successfully developed a tandem epoxide
rearrangement/intramolecular [3+2] cycloaddition reaction of
cyclopropane with carbonyls for the efficient construction of fused-
or spiro-oxa-[n.2.1] skeletons under mild reaction conditions. The
carbonyls for the [3+2] cycloaddition were produced in situ by the
epoxide rearrangement, which were not easily obtained by general
methods. This reaction is applicable for both aliphatic and
aromatic epoxide compounds. Further studies towards the appli-
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natural products are still in progress in our laboratory.
The authors are grateful to the National Basic Research
Program of China (973 Program, Grant No. 2010CB833203), the
National Natural Science Foundation of China (Grant No.
21272098, 21190034, and J1103307), the Program for Changjiang
Scholars and Innovative Research Team in University (PCSIRT:
IRT1138), the Fundamental Research Funds for the Central Uni-
versity (FRFCU: lzujbky-2013-ct02), and the 111 Project for finan-
cial support. We gratefully acknowledge Yong-Liang Shao in the
Lanzhou University for conducting X-ray crystallographic analyses.
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