Asymmetric Annulation of Donor-Acceptor Cyclopropanes with Dienes

Article abstract of DOI:10.1021/jacs.5b04429

An efficient [4 + 3] cycloaddition reaction of D-A cyclopropanes with dienes has been successfully developed. The reaction proceeds well with various dienolsilyl ethers in the presence of Lewis acid, delivering a variety of cycloheptenes and [n,5,0]carbobicycles with excellent stereoselectivity. The asymmetric version of this reaction is also realized using a newly designed chiral Cy-TOX ligand, providing a new approach to access optically active cycloheptenes and [n,5,0]carbobicycles. Mechanisic study reveals that the reaction involves a stepwise pathway, which undergoes an unusual ring opening of five-membered [3 + 2] intermediate and sequential intramolecular cyclization to afford the thermodynamically stable [4 + 3] annulation product.

Full text of DOI:10.1021/jacs.5b04429

Communication
pubs.acs.org/JACS
Asymmetric Annulation of Donor−Acceptor Cyclopropanes with
Dienes
Hao Xu,‡,† Jiang-Lin Hu,‡,† Lijia Wang,‡ Saihu Liao,‡ and Yong Tang*,‡,§
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Lu, Shanghai 200032, China
§Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, China
S
* Supporting Information
a
Table 1. Reaction Optimization
ABSTRACT: An efficient [4 + 3] cycloaddition reaction
of D−A cyclopropanes with dienes has been successfully
developed. The reaction proceeds well with various
dienolsilyl ethers in the presence of Lewis acid, delivering
a variety of cycloheptenes and [n,5,0]carbobicycles with
excellent stereoselectivity. The asymmetric version of this
reaction is also realized using a newly designed chiral Cy-
TOX ligand, providing a new approach to access optically
active cycloheptenes and [n,5,0]carbobicycles. Mechanisic
study reveals that the reaction involves a stepwise pathway,
which undergoes an unusual ring opening of five-
membered [3 + 2] intermediate and sequential intra-
molecular cyclization to afford the thermodynamically
stable [4 + 3] annulation product.
b
b
c
entry
2
time (h)
conv. (%)
3/4
yield (%)
1
2
3
4
5
6
2b
2c
2d
2e
2f
1
>99
>99
91
−
−
<5
<5
38
56
77
87
95
1
53
1
60/40
>99/1
>99/1
95/5
>99
92
31
64
8
2a
2a
>99
>99
ycloheptenes and [n,5,0]carbobicycles are widely present
Cas key structures in a large number of natural products
and biologically active molecules such as salvicnol, palustrol,
and dolastanes.1 Accordingly, the establishment of efficient and
reliable methodologies for the highly enantioselective con-
struction of this structural motif from simple materials is highly
desirable.2 Herein, we envision that a [4 + 3] annulation
protocol of donor−acceptor (D−A) cyclopropanes with
dienolsilyl ethers may serve as a new approach for the
aforementioned subunits. However, reactions of D−A cyclo-
propanes with common dienolsilyl ethers usually result in ring-
opening products or [3 + 2] cycloaddition products, which
suggests this transformation to be challenging.3−7 In our
continuing effort on studies of D−A cyclopropanes in organic
synthesis, we developed a novel and efficient [4 + 3] reaction of
cyclopropane 1,1-dicarboxylates with both acyclic and cyclic
dienolsilyl ethers, which provided a new and concise approach
to the synthesis of cycloheptenes and [n,5,0]carbobicycles.
Importantly, the catalytic asymmetric version of this trans-
formation is also realized with high enantioselectivity by
employing a newly designed chiral trisoxazoline (Cy-TOX).8,9
Herein, we report the preliminary results.
d
7
>99/1
a
1a (0.30 mmol), 2 (0.20 mmol), Cu(ClO4)2·6H2O (0.020 mmol), L-
rac (0.022 mmol), CH2Cl2 (2.0 mL), 40 °C, 4 Å MS (100 mg), argon.
b
Conversion and the ratio of 3/4 were determined by 1H NMR
c
d
spectroscopy. Isolated yield. Cu(SbF6)2 was used. Ad = 2-adamantyl.
PMP = para-methoxyphenyl
the initial exploration, the ester group10 was found to
significantly influence the reaction. When the isopropyl ester
was used, 38% yield of the desired product was achieved with a
60/40 ratio of 3/4 (entry 3). To our delight, the [4 + 3]
cycloadduct could be highly selectively obtained albeit in
moderate yield when using benzyl ester (entry 4). Further
study showed that 87% yield of 3 was obtained when di-2-
adamantyl (2-Ad) cyclopropane 1,1-dicarboxylate 2a was
employed (entry 6). Further screening11 of Lewis acids and
solvents led to the optimal reaction condition under which the
[4 + 3] product rac-3a was furnished exclusively in 95% yield in
dicloromethane in the presence of 10 mol % L-rac/Cu(SbF6)2
(Table 1, entry 7).
The substrate scope of this process was examined next. A
variety of electron-rich phenyl cyclopropanes 2 reacted
smoothly with conjugated enol silyl ethers 1 to afford the [4
+ 3] cycloadducts in good to excellent yields (Table 2, entries
1−3). Cyclopropanes with heteroaryl or alkenyl substituents
Initially, the reaction of dienolsilyl ether 1a with both
dimethyl and diethyl cyclopropane 1,1-dicarboxylate (2b, 2c)
were tried by employing 10 mol % of bisoxazoline (BOX) L-
rac/Cu(ClO4)2·6H2O. There are two possible cycloadducts, [4
+ 3] cycloadduct 3 and [3 + 2] cycloadduct 4. However, no
product was observed due to the decomposition of the
cyclopropanes and side reactions (Table 1, entries 1−2). In
Received: April 29, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b04429
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
Table 2. Generality of the Reaction
Scheme 1. Ligand Effects on Asymmetric [4 + 3]
Annulations
time
(h)
yield
b
entry
R1
R2
R3
product
(%)
1
2
3
Me(1a) 4-MeOC6H4 (2a)
Me(1a) 4-BnOC6H4 (2g)
2-Ad
2-Ad
2-Ad
8
rac-3a
rac-3b
rac-3c
95
88
68
56
34
Me(1a) (3,4-MeO)2C6H3
(2h)
4
5
Me(1a) N-Boc-Indolyl (2i) 2-Ad
30
15
7
rac-3d
rac-3e
rac-3f
rac-3g
80
80
72
84
Me(1a) CHCHPh (2j)
2-Ad
2-Ad
c
6
Me(1a) 2-thiophenyl (2k)
c
7
Me(1a) 5-Me-2-thiophenyl 2-Ad
(2l)
12
d
To further improve the selectivity and gain a deeper
understanding of the inter-relationship between [4 + 3] and
8
Me(1a) Ph (2m)
Bn
14
24
10
8
rac-3h
rac-3i
rac-3j
rac-3k
rac-3l
rac-3m
rac-3n
rac-3o
rac-3p
rac-3q
rac-3r
rac-3s
rac-3t
79
90
84
94
86
75
73
75
93
52
96
94
75
d
9
Me(1a) 2-MeC6H4 (2n)
Me(1a) 3-MeC6H4 (2o)
Me(1a) 4-MeC6H4 (2p)
Me(1a) 4-F-C6H4 (2q)
Me(1a) 4-Cl-C6H4 (2r)
Me(1a) 4-Br-C6H4 (2s)
Me(1a) 4-I-C6H4 (2t)
Me(1a) 4-tBu-C6H4 (2u)
Me(1a) vinyl (2v)
Bn
1
d
[3 + 2] products, H NMR was used to monitor the reaction.
10
Bn
d
As shown in Figure 1, at the initial stage of the reaction (1 h),
11
Bn
d
12
Bn
10
14
13
10
10
10
32
24
12
d
13
Bn
d
14
Bn
d
15
Bn
d
16
Bn
d
17
Bn
18
19
Et (1b) 4-MeOC6H4 (2a)
Ph (1c) 4-MeOC6H4 (2a)
2-Ad
2-Ad
2-Ad
e
20
H (1d)
4-MeOC6H4 (2a)
a
1 (0.30 mmol), 2 (0.20 mmol), Cu(SbF6)2 (0.020 mmol), L-rac
(0.022 mmol), CH2Cl2 (2.0 mL), 40 °C, 4 Å MS (100 mg), argon. No
b
c
[3 + 2] cycloadducts were observed. Isolated yield. DCE (2.0 mL),
at 60 °C. With 20 mol % of catalyst. DCE (1.0 mL), at 80 °C.
d
e
delivered high yields of the desired products. (entries 4−7). For
less active cyclopropanes, the benzyl ester group was
demonstrated to be more favored to ensure a high yield
(entries 8−17). The reaction was found to be insensitive to the
ortho-, meta-, or para-substituents at the aromatic rings (entries
9−11). Notably, reactions of vinyl-substituted cyclopropane
also proceeded smoothly to give the desired product in 52%
yield (entry 17). Furthermore, enol silyl ethers with different
substituents (R1 = Me, Et, Ph, H) were all suitable substrates
(entries 1, 18−20).
Figure 1. Monitoring the Cu(SbF6)2/L-rac-catalyzed reaction between
1a and 2e in CH2Cl2 by H NMR.
1
The presence of the scaffold in enantiopure form in many
natural products encouraged us to develop the asymmetric
version of this reaction. As BOX L-rac is highly efficient for the
racemic reaction, we first tried the asymmetric reaction with
chiral BOX L1 instead of L-rac (Scheme 1). Unfortunately, we
found that BOX L1 slowed down the reaction dramatically,
resulting in 34% yield of 3a due to poor regioselectivity and
incomplete conversion. SaBOX L2 was also employed, which
proved to be very efficient in the asymmetric [3 + 2] annulation
reaction of D−A cyclopropanes with enol silyl ether.3c The
yield of 3a was improved to 73% in 93% ee with a 81/19 ratio
of 3a/4a. Further examination11 of Lewis acids, solvents, ester
groups, SaBOX, and TOX ligands as well as reaction
temperature showed that a mixture of [4 + 3] and [3 + 2]
annulation products were obtained. The best result was
achieved by employing TOX L3 which gave 86% yield of 3a
in 92% ee, with a 92/8 ratio of 3a/4a.
the [3 + 2] annulation product rac-4e (unique peaks at 6.0
ppm) was produced as the major products with a 35/65 ratio of
rac-3e/rac-4e. After 5 h, the peaks of cyclopropane 2j
disappeared, and the [4 + 3] product rac-3e became
predominant. Eight hours later, rac-4e was converted into
rac-3e completely. This observation suggests the [3 + 2]
annulation is a kinetically controlled process, and the [4 + 3]
product rac-3e is thermodynamically favored. In addition, the
intermediate cyclopetane rac-4e was isolated and was subjected
to the optimized reaction conditions (Scheme 2). Significantly,
it was found that rac-4e was readily converted to the
corresponding cycloheptene rac-3e in 82% yield after 10 h.
Moreover, the optically active 4e (88% ee) could also be
transformed into 3e (88% ee) in the presence of 10 mol %
BOX L-rac/Cu(SbF6)2 without loss of enantiomeric excess.11
These results clearly demonstrate that the reaction mainly
B
DOI: 10.1021/jacs.5b04429
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Scheme 2. Conversion of [3 + 2] Cycloadduct to [4 + 3]
Product
Table 3. Asymmetric [4 + 3] Annulation of 1 with 2
undergoes a stepwise mechanism in which the [3 + 2]
annulation first takes place kinetically, followed by the ring
opening of the [3 + 2] intermediate and intramolecular
cyclization to afford the thermodynamically stable [4 + 3]
annulation product. The stepwise mechanism of our method-
ology distinguishes it from the concerted cycloaddition reaction
of 1,3-diphenylisobenzofuran with cyclopropanes.12
Based on these observations, we continued to optimize the
reaction conditions by means of several typical methods that
would favor a thermodynamically controlled process, including
elevating the reaction temperature and prolonging the reaction
time. However, no further improvement on increasing the ratio
of 3a/4a was made. In order to minimize the undesired [3 + 2]
products and to increase the catalytic efficiency, we designed
i
new ligands that can speed up this transformation. Since Pr-
TOX L3 was found to favor the [4 + 3] product, we envisioned
that TOX ligands possessing a similar chiral environment, but
bearing sterically rigid cyclohexyl backbones, might be
beneficial for the transformation of 4a to 3a. As expected, we
found that Cy-TOX L4 could promote the reaction very
efficiently, affording 3a exclusively in 94% yield with 94% ee
(Scheme 1).
Under the optimized conditions, 1a reacted with 2a in the
presence of Cu(ClO4)2·6H2O and L4 in dichloromethane
providing 3a in 94% yield with 94% ee (Table 3). Good to
excellent levels of enantioselectivity were obtained in the
reaction of electron-rich phenyl-substituted cyclopropanes with
88−95% ee (3a−3c, 3r). Moreover, cyclopropanes with
heteroaryl and alkenyl substitutents were also compatible for
the reaction with high enantioselectivity (3e, 3f). Of note is the
cyclic enol silyl ethers, bearing five-, six- and seven-membered
rings, could also react with electron-rich phenyl- and aryl-
substituted cyclopropanes smoothly in good yields with
excellent enantioselectivities, even under an elevated temper-
ature in DCE (3u−3w, 3y).13 The obtained optically active
[n,5,0]carbobicyclic structural motif is a key intermediate in a
plenty of biologically active and natural products.1
Reaction conditions: Cu(ClO4)2·6H2O (0.020 mmol), L4 (0.022
mmol), 1 (0.30 mmol) and 2 (0.20 mmol) in 2.0 mL of CH2Cl2, 40
a
°C, the ratio of [4 + 3] and [3 + 2] cycloadducts is >99/1. 60 °C, in
b
c
DCE. Using Cu(SbF6)2. 20 mol % Cu(ClO4)2·6H2O was used.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterizations, and analytical data
of products, spectra of NMR, and HPLC. The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/jacs.5b04429.
In conclusion, an efficient Cu(II)/TOX catalyzed [4 + 3]
annulation of D−A cyclopropanes with dienes has been
developed. By employing a newly designed chiral Cy-TOX
instead of BOX ligand, asymmetric version of the current
reaction can be realized with excellent enantioselectivity,
providing an efficient and new access to a variety of optically
active cycloheptenes and [n,5,0]carbobicycles. To the best of
our knowledge, these reactions represent the first examples of
catalytic asymmetric [4 + 3] annulation reactions of enol silyl
ethers with D−A cyclopropanes. Preliminary study reveals that
a stepwise mechanism is mainly involved in the reaction, which
undergoes an unusual five-membered ring opening of the [3 +
2] intermediate, followed by an intramolecular cyclization to
afford the thermodynamically stable [4 + 3] annulation
product.
AUTHOR INFORMATION
Corresponding Author
*tangy@sioc.ac.cn
■
Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank financial support from the National Natural Sciences
Foundation of China (21432011, 21421091 and 21272250),
the Major State Basic Research Development Program (grant
no. 2015CB856600) and the Chinese Academy of Sciences.
C
DOI: 10.1021/jacs.5b04429
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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(12) For reaction of cyclopropanes with 1, 3-diphenylisobenzofuran
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(13) The ee of 3u and 3w was determined after desilylation using
HF·Py. The absolute configuration of 3u was assigned by X-ray
crystallography of the desilylated product 5u. For details, please see
the Supporting Information.
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DOI: 10.1021/jacs.5b04429
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