Lewis acid catalyzed intramolecular [3+2] cross-cycloaddition of donor-acceptor cyclopropanes with carbonyls: A general strategy for the construction of acetal[n.2.1] skeletons

Article abstract of DOI:10.1002/anie.201106368

Build a bridge: The first catalytic intramolecular [3+2] cycloaddition of monodonor-monoacceptor cyclopropanes (see scheme) provides a general and efficient strategy for construction of structurally diverse acetal[n.2.1] and 1,4-dioxygen-substituted cyclic skeletons, which are widely distributed in biologically important natural products. Copyright

Full text of DOI:10.1002/anie.201106368

DOI: 10.1002/anie.201106368
Cycloaddition
Lewis Acid Catalyzed Intramolecular [3+2] Cross-Cycloaddition of
Donor–Acceptor Cyclopropanes with Carbonyls: A General Strategy
for the Construction of Acetal[n.2.1] Skeletons**
Siyang Xing, Yan Li, Zhen Li, Chang Liu, Jun Ren, and Zhongwen Wang*
Dedicated to Professor Zhengming Li on the occasion of his 80th birthday
The structurally diverse, complex, and interesting family of
bridged oxa[n.2.1] skeletons (n = 2–4) with an additional
bridgehead oxygen (acetal[n.2.1] skeletons) is widely distrib-
uted in nature and exhibits a broad range of important
biological activities (Scheme 1). One of the prominent
structural features of such core skeletons is an embedded
Scheme 2. Strategy to 1,4-difunctionalized six- to eight-membered
rings.
developed for the construction of such bridged skeletons,
including the Diels–Alder reaction, 1,3-dipolar cycloaddition
Scheme 1. Several representative natural products.
of carbonyl ylides, transition-metal-catalyzed cyclization, and
radical cyclization,[3,4] the development of general, efficient,
and conceptually new strategies to afford such structurally
diverse skeletons still remains important and challenging.
Cyclopropanol-derived monodonor–monoacceptor cyclo-
propanes (D-A CP),[5] form an important subclass of the
donor–acceptor cyclopropane family,[6] and are versatile
building blocks in organic synthesis. D-A CP can be employed
for the construction of five-membered rings by Lewis acid
(LA) promoted [3+2] cycloadditions. Despite the importance
1,4-dioxygen acetal moiety, which can be conveniently trans-
formed into the important class of 1,4-difunctionalized six- to
eight-membered rings (Scheme 2).[1,2,3e,h,4e] Thus, it is not
surprising that great attention has been paid to the develop-
ment of creative strategies for the efficient construction of
such bridged skeletons.
Cycloadditions and domino reactions involve the simul-
taneous formation of more than two covalent bonds in one
step, and therefore they are the most efficient and direct
transformations for the synthesis of cyclic skeletons. Although
various cycloaddition and domino strategies have been
of the tetrahydrofuran skeleton, it is surprising that in
[7–9]
=
comparison to examples using C C, alkynes, and nitriles,
[3+2] cycloadditions of D-A CP with carbonyls seemed to be
less successful.[10,11] Reissig et al reported the reaction of
siloxyl D-A CP with carbonyls under the catalysis of a
stoichiometic amount of TiCl4, and an Aldol adduct was
obtained instead of the [3+2] cycloaddition product.[10]
Pagenkopf and Yu reported a similar result from the reaction
of a glucal-derived alkoxyl D-A CP with an aldehyde.[5a,11]
Inspired by our recently reported type II intramolecular
[3+2] cross-cycloaddition ([3+2] IMCC) of cyclopropane 1,1-
diester,[12] we expected this strategy could be applied to D-
A CP (Scheme 3). We now report our recent results for the
construction of the acetal[n.2.1] skeletons by the [3+2] IMCC
of D-A CP. To the best of our knowledge, this is the first
catalytic cycloaddition with carbon–heteroatom multiple
bonds, and the first intramolecular cycloadditon of D-A CP.
[*] Dr. S. Xing, Y. Li, Z. Li, 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, the
National Key Project of Scientific and Technical Supporting
Programs (973 Program) (No. 2010CB126106), and the “111”
Project of Ministry of Education of China (No. B06005) for financial
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106368.
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Table 1: Construction of 8-oxa[3.2.1] and 9-oxa[4.2.1] skeletons.[a]
Cyclopropane 1
Product 2
Yield [%][b] exo/
endo[c]
1a R1 =H, R2 =Me
1b R1 =Me, R2 =Me
1c R1 =vinyl, R2 =Me
1d R1 =phenylethynyl,
R2 =Me
2a
2b
2c
2d
78 (72)[d]
80
94
73
84:16
>95:5
90:10
>95:5
Scheme 3. LA-catalyzed type II [3+2] IMCC of D-A CP.
1e R1 =H, R2 =Et
2e
56
69:31
In an initial exploration of the [3+2] IMCC a single
diastereoisomer of 1a was used as the substrate (see the
Supporting Information for the optimization of the reaction
conditions). To our delight, reactions promoted by 1.2 equiv-
alent of TiCl4 and trimethylsilyl trifluoromethanesulfonate
(TMSOTf), as Lewis acids, in MeNO2, as the solvent, were
both successful, and the reaction using TMSOTf gave a better
yield and diastereoselectivity with exo-2a as the major
isomer. This result implied that the properties of the LA,
especially the steric bulk, might have a great influence on the
stereochemistry. The investigation was then focused on
changing the silyl group. Reactions promoted by triethylsilyl
trifluoromethanesulfonate (TESOTf) and triisopropylsilyl
trifluoromethanesulfonate (TIPSOTf) both gave excellent
diastereoselectivities, as well as better yields. EtNO2 was also
proven to be a suitable solvent. Surprisingly, we found that a
catalytic amount of TESOTf also gave similar results with
regard to the yield and diastereoselectivity. This is the first
catalytic example reported for reactions of this type of
D-A CP with carbon–heteroatom multiple bonds.
1 f n=1
1g n=2
2 f
2g
50
28
<5:95
<5:95
1h R=H
1i R=Me
2h
2i
78 (71)[d]
79
40:60
52:48
1j[e] R=H
2j
2k
78
85
77:23
86:14
1k[e] R=Me
1l R=H
1m R=Me
2l
2m
59
77
>95:5
89:11
With the optimized reaction conditions established, we
began to examine the generality of this method (Table 1).
Substrates 1 were used either in a single diastereoisomeric
form (in most cases) or as a mixture of the two inseparable
isomers. As shown in Table 1, the construction of the 8-
oxabicyclo[3.2.1]octane and 9-oxabicyclo[4.2.1]nonane skel-
etons were successfully carried out. In most cases, the exo
cycloadducts 2 were obtained as the major diastereoisomers
and in moderate to good yields. Exceptionally, substrates 1 f
and 1g with fused rings gave endo cycloadducts 2 f and 2g
with excellent diastereoselectivity (see the Supporting Infor-
mation). The structures of exo-2k and exo-2m[13] were
unambiguously confirmed by X-ray crystal structure analysis.
The following experiments confirmed that the reaction results
were not influenced by the relative stereochemistry of
substrates 1: each of the two isomers of 1a gave the same
result with regard to yield and diastereoselectivity and 1h
behaved the same. For substrates 1j and 1k, a mixture of the
two inseparable isomers was used in the [3+2] IMCC, and
cycloadducts 2j and 2k were afforded in good yields and with
the exo isomer as the major isomer. It should be noted a
diverse range of substrates could be used, thus demonstrating
the generality of the method; aromatic and aliphatic alde-
hydes/ketones, and substrates with fused rings or long alkyl
chains all successfully underwent the [3+2] IMCC.
[a] Reaction conditions: one diastereomer was used, 0.4 mmol scale,
20 mol% of TESOTf, 10.0 mL of MeNO2, À208C, 2 h, Ar. [b] Yields of the
isolated products. [c] The exo/endo ratios were determined by the yields
of the isolated products or by NMR analysis. [d] The yield when the other
diastereomer was used [e] The mixture of two diastereomers was used.
Bn=benzyl.
When we used the [3+2] IMCC to build 7-oxabicyclo-
[2.2.1]heptane skeletons, ring-opening products
3 were
obtained, probably as a result of the hydrolysis of the
presumed [3+2] IMCC cycloadducts 2 (Scheme 4). Com-
pound 3n was obtained from the aliphatic substrate 1n in
50% yield and with excellent diastereoselectivity (> 95:5).
Under TESOTf catalysis an aromatized naphthalene deriva-
tive 4o was obtained together with the ring-opening product
3o. TIPSOTf gave a better yield of 3o (62%) with moderate
diastereoselectivity (exo/endo = 67:33). The reaction of
ketone 1p gave exclusively the ring-opening product 3p in
95% yield and with a moderate diastereoselectivity (exo/
endo = 67:33). This supplies an effective method for the
construction of skeletons with a 1,4-dioxygen-substituted
cyclohexane.
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Scheme 6. TiCl4-promoted reductive ring-opening reaction.
Terpenes containing a 6-7-6 tricyclic core skeleton are a
large family of natural products, a typical example of which is
the icetexane diterpenoids, for example, coulterone and
komaroviquinone.[14] To further demonstrate the potential
of this strategy, a modular approach to the synthesis of this
family of compounds was accomplished (Scheme 7). D-A CP
9 was prepared from 6 and 7. Surprisingly, deprotection of
Scheme 4. Construction of skeletons with a 1,4-dioxygen-substituted
cyclohexane.
Following the successful construction of oxabicyclo[n.2.1]
skeletons with an “external acetal” moiety, we then turned
our attention to the construction of a 2,8-dioxabicyclo-
[3.2.1]octane skeleton with an “internal acetal” moiety,
which is a core skeleton that exists in natural products
(Scheme 5). However, under the above reaction conditions
[3+2] IMCC of aldehyde 1q did not happen and 1q was
Scheme 7. Synthesis of the core skeleton of komaroviquinone: a) Mg,
CuI,THF/Et2O, 76%; b) (CH2OH)2, PTSA, toluene, reflux, 81%;
c) [PdCl2(PPh3)2], tributyl(1-ethoxyvinyl)stannane, toluene, reflux;
d) N2CHCO2Me, [Rh2(OAc)4], Et2O, 59% over two steps; e) [PdCl2-
(MeCN)2] (5 mol%), acetone, 81%, exo/endo=91:9; f) tBuOK, tolu-
ene, 69%; g) LiCl, wet DMSO, 1608C, 81%; h) PPh3MeBr, tBuOK,
THF, 98%; i) ZnEt2, CH2I2, toluene, 608C, 46% (63%, brsm); j) H2,
PtO2, 1m HBr/MeOH/EtOAc, run twice, 34%. See the Supporting
Information for reaction details. DMSO=dimethyl sulfoxide. PTSA=p-
toluenesulfonic acid. THF=tetrahydrofuran.
Scheme 5. Construction of 2,8-dioxabicyclic[3.2.1] skeletons.
recovered completely. To our delight, when the reaction
mixture was heated to 458C for 2 h, cycloadduct 2q was
furnished in a good yield (74%) and with a moderate
diastereoselectivity (exo/endo = 67:33). For ketone 1r, under
the same reaction conditions, cycloadduct 2r was also
obtained in a good yield (85%) and with the same diastereo-
selectivity as that for aldehyde 1q.
Cycloheptane is a core skeleton that widely exists in
biologically important natural and unnatural products. A
convenient transformation of the acetal moiety of the
constructed acetal[3.2.1] skeleton might provide a method
for the synthesis of these structures. We selected exo-2a as an
example to test this method. Exo-2a was used in a LA-
promoted reductive ring-opening process.[2g] Thus, after being
treated with 3 equivalents of TiCl4 and 8 equivalents of
Et3SiH at À788C in dichloromethane, ring-opening product
5 was successfully obtained in good yield (Scheme 6).
acetal 9, to give the ketone, catalyzed by [PdCl2(MeCN)2] in
acetone led directly to the [3+2] IMCC cycloadduct 11 in
81% yield with a excellent diastereoselectivity (exo/endo =
91:9). The [PdCl2(MeCN)2] probably acted as a LA to
promote the [3+2] IMCC of the possible ketone intermediate
10.[15] A Dieckmann condensation of the major cycloadduct
exo-11, which was the diastereoisomer with the desired
stereochemistry, afforded b-ketoester 12 in 69% yield. The
structure of 12 was confirmed by X-ray crystal structure
analysis.[13] b-Ketoester 12 was then converted into ketone 13
by dealkoxycarbonylation. Olefination of ketone 13 followed
by a Simmons–Smith cyclopropanation furnished 14. Finally,
hydrogenation and hydrolysis of 14 occurred simultaneously
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Communications
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to afford ketone 15, which existed in equilibrium with the
hemiacetal form 16, as revealed by the 1H and 13C NMR
spectra.
In summary, we have successfully developed the first
intramolecular cycloadditon of D-A CP. This is also the first
catalytic [3+2] cycloaddition of D-A CP with carbon–hetero-
atom multiple bonds. This [3+2] IMCC provides a general
and efficient strategy for the construction of structurally
diverse acetal[n.2.1] skeletons and 1,4-dioxygen-substituted
cyclic skeletons. Features of this strategy include the mild
reaction conditions, large range of substrates (a variety of
substituted cyclopropanes and carbonyls), and, in most cases,
moderate to excellent diastereoselectivities. The conversion
of an acetal[3.2.1] skeleton into a functionalized cycloheptane
was carried out by the reductive ring opening of the acetal. A
modular synthesis of icetexane diterpenoids was also accom-
plished, and during this synthesis a novel domino Pd-
catalyzed deprotection/[3+2] IMCC reaction was discovered.
Further investigation will focus on the application of the
[3+2] IMCC strategy to the synthesis of natural products
containing acetal[n.2.1] and six- to eight-membered cyclic
skeletons.
Experimental Section
General procedure for the [3+2] IMCC of D-A CP 1: Under an argon
atmosphere, D-A CP 1 (0.3 mmol) was dissolved in 4.5 mL of MeNO2
and cooled to À208C, and to this mixture was added slowly TESOTf
(0.06 mmol, 3 mL, 0.02m in MeNO2). The solution was stirred at
À208C for 2 h, quenched with a vigorously stirred solution of
saturated aqueous NaHCO3 (30 mL), and extracted with diethyl
ether (3 ꢀ 45 mL). The combined organic phases were dried over
Na2SO4 and concentrated under reduced pressure. The residue was
purified by flash column chromatography on silica gel (ethyl acetate/
petroleum ether 1:50 to 1:30) to afford adduct 2 as a mixture of
diastereoisomers.
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Received: September 8, 2011
Published online: November 7, 2011
Keywords: cycloaddition · cyclopropane · Lewis acids ·
.
medium-sized rings · natural products
=
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