Efficient construction of oxa- and aza-[n.2.1] skeletons: Lewis acid catalyzed intramolecular [3+2] cycloaddition of cyclopropane 1,1-diesters with carbonyls and imines

Article abstract of DOI:10.1002/anie.201000563

Chemical equation presented Building bridges: A Lewis acid promoted intramolecular [3+2] cycloaddition of cyclopropane 1,1-diesters with aldehydes, ketones, and imines (see scheme) has been developed to provide a general and efficient strategy for construction of bridged oxa- and aza-[n.2.1] (n = 2,3,4) skeletons. To highlight this method, the core of platensimycin was also constructed.

Full text of DOI:10.1002/anie.201000563

Angewandte
Chemie
DOI: 10.1002/anie.201000563
Cycloaddition
Efficient Construction of Oxa- and Aza-[n.2.1] Skeletons: Lewis Acid
Catalyzed Intramolecular [3+2] Cycloaddition of Cyclopropane
1,1-Diesters with Carbonyls and Imines**
Siyang Xing, Wenyan Pan, Chang Liu, Jun Ren, and Zhongwen Wang*
Highly efficient construction of cyclic skeletons is one of the
most important themes in organic synthesis. The structurally
diverse and interesting family of bridged oxa- and aza-[n.2.1]
(n = 2,3,4) skeletons are well-represented and widely distrib-
uted in nature; they can be found in natural products such as
platensimycin,[1] quinocarcin,[2] bruguierol,[3] and pyrido[3,4-
b]homotropane (PHT; Figure 1),[4] and exhibit broad-ranging
and the regio- and stereoselectivity make intramolecular
cycloadditions[6c,13] an efficient strategy for construction of
complex cyclic skeletons. The representative examples for
Lewis acid catalyzed intermolecular [3+2] cycloadditions of
cyclopropane 1,1-diester are presented by the work of
Johnson and co-workers,[9a–e] and Carson and Kerr.[10d]
Compared to the Lewis acid catalyzed intermolecular
[3+2] cycloadditions of cyclopropane 1,1-diester, the intra-
molecular variant is less prevalent despite showing potential
in the synthesis of natural products.[14,15] The intramolecular
[3+2] cycloaddition can be classified into two categories:
formation of 1) a fused bicyclic [n.3.0] skeleton (type I) and
2) a bridged bicylic [n.2.1] skeleton (type II; Scheme 1).
Figure 1. Several representative natural products.
biological activity. The complex architectures provide the
impetus for the development of new synthetic methodologies.
Additionally, such skeletons are also useful building blocks in
organic synthesis.[5] Therefore, it is not surprising that much
attention has been paid to developing creative strategies for
the construction of such bridged skeletons.
Undoubtedly cycloadditions are one of the most efficient
and direct transformations for the construction of cyclic
skeletons. Although there are many strategically novel
methods developed for the diverse [n.2.1] skeletons,[6] the
development of a more general and efficient strategy to afford
such oxa- and aza-[n.2.1] bicyclic skeletons remains impor-
tant, and continues to attract interest from the organic
synthesis community.
Scheme 1. Two types of intramolecular [3+2] cycloadditions of cyclo-
propane 1,1-diesters.
Snider and co-workers reported a preliminary result on a
type I intramolecular [3+2] cycloaddition of a cyclopropane
[15a]
=
1,1-diester with a C C bond.
Recently, Kerr and co-
workers reported a type I intramolecular [3+2] cycloaddition
=
A cyclopropane is a versatile building block for cyclic
skeletons,[7] and the easily accessible cyclopropane 1,1-diester
has been used in various Lewis acid (LA) promoted
[3+n] cycloadditions.[8–12] The ease of running the reaction
of a cyclopropane 1,1-diester with a C N bond by which aza-
[3.3.0] and aza-[4.3.0] skeletons were constructed; these
reactions were successfully applied to the total synthesis of
FR901483 and allosecurinine.[15b–e] Herein we report our
recent results on the construction of the bridged oxa- and aza-
[n.2.1] (n = 2, 3, 4) skeletons through the type II intramolec-
ular [3+2] cycloaddition of cyclopropane 1,1-diester.
We selected 1a (see Table 1 for structures) as the model
substrate to explore the optimized reaction conditions for an
intramolecular [3+2] cycloaddition to give 2a, and the results
are summarized in the Supporting Information. The best
result (93% yield) was obtained using Sc(OTf)3 as the catalyst
in 1,2-dichloroethane. The structure of 2a was unambiguously
confirmed by X-ray crystal structure analysis[16] (Figure 2).
Several substrates (1b–1p) having different substituents
were subjected to the intramolecular [3+2] cycloaddition
(Table 1). The corresponding [3.2.1] (Table 1, entries 1–8),
[*] Dr. S. Xing, W. Pan, C. Liu, J. Ren, Prof. Z. Wang
State Key Laboratory of Elemento-Organic Chemistry
Institute of Elemento-Organic Chemistry, Nankai University
Tianjin 300071 (P. R. China)
E-mail: wzwrj@nankai.edu.cn
Homepage: http://wzw.nankai.edu.cn
[**] We thank the National Natural Science Foundation of China (No.
20972069), the National Key Project of Scientific and Technical
Supporting Programs (973 Program) (No. 2010CB126106), and the
Ministry of Education of China (RFDF20070055022) for financial
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000563.
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3215

Communications
Table 1: Lewis acid catalyzed intramolecular [3+2] cycloaddition of
cyclopropanes 1.[a]
Entry Cyclopropane 1
Product 2
Yield [%][b]
1
2
1a R1 =H, R2 =H, R3 =Me
(+)-1a, 90% ee
2a
(À)-2a
90
91,
97% ee
90
3
4
5
1b R1 =Me, R2 =H, R3 =Me
2b
1c R1 =vinyl, R2 =H, R3 =Me 2c
1d R1 =phenylethynyl, R2 =H, 2d
R3 =Me
74[c]
91
6
7
1e R1 =H, R2 =Me, R3 =Me
1 f R1 =H, R2 =H, R3 =Et
2e
2 f
92
96
Figure 2. ORTEP drawing for the product 2a. The thermal ellipsoids
drawn at the 30% probability level.
[4.2.1] (Table 1, entries 9–12), and [2.2.1] (Table 1, entries 13
and 14) skeletons were successfully formed. In some cases
Sc(OTf)3 showed poor catalytic efficiency, therefore either a
dilute solution of Yb(OTf)3 (Table 1, entries 4, 13, and 14) or
SnCl4 (Table 1, entries 9–12) was used instead. As in the case
of aldehydes, the reactions of ketones also proceeded nicely
(Table 1, entries 3–5). The aliphatic substrates 1g (Table 1,
entry 8) and 1m (Table 1, entry 14) led to lower product
yields, probably because of their conformational flexibility,
which resulted in intermolecular oligo- or polymerization.
Reaction of substrate 1e, having a quaternary carbon center
in the cyclopropane ring, also took place smoothly and
afforded adduct 2e in an excellent yield (Table 1, entry 6).
Compared to its intermolecular variants, in which an
aldehyde and a cyclopropane bearing a cation-stabilizing
substituent (donor-acceptor cyclopropane) were employ-
ed,[9a–e] the scope of the cyclopropane and carbonyl substitu-
tions has been extended to this intramolecular [3+2] cyclo-
addition.
Since many natural products such as pyrrolo[1,2-a]azepine
and pyrrolo[1,2-a]azocine, contain seven- and eight-mem-
bered heterocyclic skeletons, several heteroatom-containing
substrates were made and successfully reacted to deliver the
desired products (Table 1, entries 10–12 and 15–17). To test
whether the type II intramolecular [3+2] cycloaddition strat-
egy was efficient for construction of chiral bridged [n.2.1]
skeletons, (+)-1a was reacted under the optimized reaction
conditions and the cycloadduct (À)-2a was successfully
obtained with 97% ee in 91% yield (Table 1, entry 2).
After the successful construction of oxabicyclo[n.2.1]
skeletons, we tried to build their aza analogues (Table 2).
We chose the reaction of 1a and aniline 3a for screening the
reaction conditions. The challenge for this kind of reaction
was the formation of its oxa-analogue 2a. In 1,2-dichloro-
ethane, the desired azabicyclo[3.2.1]octane product 4a was
always accompanied by various amounts of 2a. When the
reaction (1a/3a = 1:1.5) was carried out in toluene and in the
presence of 4 ꢀ molecular sieves, the formation of 2a was
completely suppressed and 4a was obtained successfully
(Table 2, entry 1). Under the optimized reaction conditions,
8
1g
2g
47
9
1h X=CH2, R4 =H
1i X=O, R4 =H
1j X=O, R4 =Me
1k X=S, R4 =H
2h
2i
2j
68[d]
75[d]
35[d]
85[d]
10
11
12
2k
13
14
15
1l
2l
85[c]
27[c]
87
1m
1n
2m
2n
16
17
1o
1p
2o
2p
90
42[d]
[a] Reaction conditions: 0.29 mmol scale, 20 mol% of Sc(OTf)3, 4.0 mL
of DCE, RT, 2 h, Ar. [b] Yields of isolated products. [c] 32 mL of DCE and
10 mol% of Yb(OTf)3 were used, 358C, 1 day. [d] 4.0 mL of DCE and
20 mol% of SnCl4 were used, 608C, 2 h. DCE=1,2-dichloroethane.
the reactions of 1a with amines 3 were carried out. It was
found that both aryl amines 3a–3d (Table 2, entries 1–4) and
3216
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Chemie
Table 2: Construction of 8-azabicyclo[3.2.1]octane skeletons.[a]
Entry
Amine 3
Product 4
Yield [%][b]
1
2
3
4
5
6
3a: R=4-BrC6H4
3b: R=4-MeOC6H4
3c: R=4-MeC6H4
3d: R=Ph
3e: R=Bn
3 f: R=tBu
4a
4b
4c
4d
4e
4 f
80
82
75
84
79
81[c]
Scheme 3. Formal synthesis of platensimycin: a) tBuLi, diethyl ether,
methallyl chloride; b) 1m HCl, THF, 56% over two steps; c) CH2(CO2Me)2,
piperidine, reflux, 84%; d) Me3SOI, NaH, DMSO, 90%; e) OsO4, NaIO4,
THF/H2O=2:1, 91%; f) Sc(OTf)3 (20 mol%), DCE, 87%; g) LiCl, wet
DMSO, 1608C, 79%. DMSO=dimethylsulfoxide.
[a] Reaction conditions: a mixture of 1a (0.29 mmol) and 3 (0.44 mmol)
in toluene (4.0 mL) was stirred in the presence of 4 ꢀ molecular sieves at
RT for 2 h under Ar. Then 10 mol% of Sc(OTf)3 was added, and the
mixture was stirred for an additional 2 h. [b] Yields of isolated products.
[c] 3 f (1.45 mmol, 5 equiv) was used. Bn=benzyl.
and efficient strategy for construction of bridged oxa- and
aza-[n.2.1] (n = 2,3,4) skeletons. As an example for demon-
strating its potential, this strategy was used to access the
compact core of platensimycin. Additional investigation on
this strategy, including synthesis of several closely related
natural products, is being carried out in our laboratory.
alkyl amines 3e and 3 f (Table 2, entries 5 and 6) could react
with aldehyde 1a successfully to give the corresponding
cycloadducts 4a–4 f in good yields.
By using similar reaction conditions (SnCl4), we inves-
tigated the cycloaddition of 1h with 3c and found that the
corresponding 9-azabicyclo[4.2.1]nonane 4g was obtained in
moderate yield (Scheme 2). Additionally, reaction of 1o and
3c successfully afforded the diaza[3.2.1]octane product 4h in
61% yield (Scheme 2). However, the reaction of aldehyde 1l
with aniline 3c was complex and no aza[2.2.1]heptane cyclo-
adduct was isolated.
Received: January 30, 2010
Published online: March 25, 2010
Keywords: cycloaddition · cyclopropane · Lewis acids ·
.
natural products · polycycles
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Angew. Chem. Int. Ed. 2010, 49, 3215 –3218
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Communications
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