Rhi-catalyzed two-component [(5+2)+1] cycloaddition approach toward [5-8-5] ring systems

Article abstract of DOI:10.1002/asia.201000053

Bringing it all together: [{Rh(CO)2Cl}2]-catalyzed two-component [(5+2)+1] cycloaddition of ene-vinylcyclopropanes with various tether types and substituents with CO has been designed for the one-pot synthesis of fused tricyclic cyclooctenones with atom and step economies.

Full text of DOI:10.1002/asia.201000053

DOI: 10.1002/asia.201000053
RhI-Catalyzed Two-Component [(5+2)+1] Cycloaddition Approach toward
[5-8-5] Ring Systems
Feng Huang, Zhong-Ke Yao, Yi Wang, Yuanyuan Wang, Jialing Zhang, and
Zhi-Xiang Yu*[a]
The design and synthesis of complex molecules from
simple starting materials in minimum steps is one of the
most challenging goals in organic synthesis.[1] As such, cyclo-
addition reactions catalyzed by transition-metal complexes
have been of continuing interest to chemists owing to their
efficiency in constructing complicated molecules from much
simpler starting materials in atom- and step-economical
fashions.[2,3] One representative example in this line is the
development of transition-metal-catalyzed cycloadditions
for synthesizing cyclooctane ring systems,[4] observed in sev-
eral classes of natural products, many of which exhibit sig-
nificant and broad-ranging biological activities. The transi-
tion-metal-catalyzed cycloaddition reactions to give cyclooc-
tanes usually take place through many elementary organo-
metallic reaction steps (such as oxidative addition, reductive
elimination) so that the unfavorable factors (such as high
degree of ring strain, unfavorable transannular interactions,
and unfavorable entropy and enthalpy penalties) associated
with the classical synthesis of eight-membered carbocycles
can be partially or totally circumvented. Several groups
have reported elegant transition-metal-catalyzed cycloaddi-
tion approaches to eight-membered carbocycles.[5] Recently,
we have also developed a two-component [(5+2)+1] cyclo-
addition of ene-vinylcyclopropanes (ene-VCPs) and CO that
provides an efficient way to obtain fused [5-8] and [6-8] ring
systems, exemplified by the reactions shown in Scheme 1.[6,7]
This method provides a new opportunity for the total syn-
Scheme 1. RhI-catalyzed two-component [(5+2)+1] cycloaddition devel-
oped in the Yu group.[6]
thesis of natural products containing [5-8] and [6-8] bicyclic
skeletons.[6b–d]
It is found that eight-membered carbocycles in many nat-
ural products are fused or bridge-fused with one or more
five- or six-membered carbocycles. For examples, the diter-
perne paclitaxel taxol (1),[8] fusicoauritone (2),[9] and aleuro-
discal (3)[10] are natural compounds containing [6-8-6] tricy-
clic, [5-8-5] tricyclic, and [5-6-8-5] tetracyclic cores
(Scheme 2), respectively. As a result, one-pot synthesis of
the [5-8-5] tricyclic skeleton or a taxol skeleton would be
highly demanded from a synthetic point of view. Even
though several spectacular strategies applying Cope rear-
rangement or electrocyclization have been developed for
[a] F. Huang, Z.-K. Yao, Y. Wang, Y. Wang, J. Zhang, Prof. Dr. Z.-X. Yu
Beijing National Laboratory for Molecular Sciences (BNLMS)
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of the Ministry of Education
College of Chemistry
Peking University
Beijing 100871 (China)
Fax : (+86)10-6275-1708
E-mail: yuzx@pku.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000053.
Scheme 2. Selected polycyclic natural products containing eight-mem-
bered carbocycles.
Chem. Asian J. 2010, 5, 1555 – 1559
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COMMUNICATION
this purpose,[11] no methods by applying transition-metal-cat-
alyzed cycloadditions to construct a [5-8-5] tricyclic skeleton
or taxol skeleton in one operation have been reported. De-
veloping such a method could facilitate the total synthesis of
the natural products and pharmaceutical compounds with
either a [5-8-5] tricyclic skeleton or a taxol skeleton.[12,13]
We envisioned that if the substrates of the previously de-
veloped [(5+2)+1] cycloaddition can be modified by intro-
ducing a five-membered carbocycle ring fused with the cy-
clopropane rings of ene-VCPs, the resulting new ene-VCPs
could undergo a [(5+2)+1] cycloaddition to give either a [5-
8-5] tricyclic skeleton or a taxol skeleton in one operation,
depending on which bond (a or b) in cyclopropane is
À
cleaved (Scheme 3). Cleavage of the external C C bond of
Figure 1. DFT-computed energy surface of the cleavage of vinylcyclopro-
pane ring (R=Me).
À
C C bond of CP ring is not a minimum. DFT optimization
of this hypothetical intermediate led directly to the starting
complex S1, suggesting that the ring opening of the internal
Scheme 3. Two possible RhI-catalyzed [(5+2)+1] cycloadditions.
À
C C bond is intrinsically disfavored thermodynamically due
to the ring strain in the ring-opened intermediate S1b,
which contains an unfavored bridgehead olefin. DFT calcu-
lations can be used to predict the stereochemistry of the [5-
8-5] product in Scheme 3. Calculations showed that the rela-
tive stereochemistry of substituents in positions II and III in
the final [(5+2)+1] cycloadduct is always in a cis configura-
tion (see Scheme 3 and Figure 1). The stereochemistry of
substituents in positions I and II can have either a cis or a
trans configuration, and this is dependent on the substitution
pattern of the substrates (see later discussion of stereochem-
istry).[6]
To test experimentally whether the reaction in Scheme 3
really takes place via pathway a rather than pathway b, we
synthesized an ene-VCP substrate 4 and ran its [(5+2)+1]
reaction in the presence of balloon-pressured mixed gas of
0.2 atm CO+0.8 atm N2, and 10 mol% [Rh(CO)2Cl]2 cata-
lyst in 1,4-dioxane at 808C for 72 h (Scheme 4). Gratifyingly,
cyclopropyl group (the a bond) via pathway a would furnish
À
a [5-8-5] ring system, whereas cleavage of the internal C C
bond of the VCP (the b bond) via pathway b would give a
bridge-fused [6-8] skeleton, presented in taxol and its family
(Scheme 3). No matter which pathway of this [(5+2)+1] cy-
cloaddition occurs, the resulting [(5+2)+1] product, either a
[5-8-5] ring system or a taxol skeleton, is the privileged skel-
eton present in many natural products and such endeavor to
explore this reaction is worthy considering its potential in
total systhesis.[14]
Before testing our hypothesis experimentally, we comput-
ed the activation energies for the two cleavage pathways of
vinylcyclopropane ring by using the DFT (B3LYP) method
(Figure 1).[15] Calculations on the model reaction of S1, a
complex between VCP and the catalytic species
[Rh(CO)Cl],[16] indicated that
the activation free energy for
À
external C C bond cleavage to
S1a (pathway a, via transition
state TS) is 9.3 kcalmolÀ1. In
contrary, all efforts to locate an
À
internal C C bond cleavage
(pathway b) transition state
proved to be unsuccessful. Cal-
culations indicated that the pro-
posed intermediate S1b gener-
ated by cleaving the internal Scheme 4. Preparation and RhI-catalyzed [(5+2)+1] cycloaddition of ene-vinylcyclopropane 4.
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RhI-Catalyzed [(5+2)+1] Cycloaddition toward [5-8-5] Ring Systems
Table 1. Substrates scope for the RhI-catalyzed [(5+2)+1] cycloaddi-
tion.[a,b]
the [(5+2)+1] reaction occurred to give a [5-8-5] ring cyclo-
octenone 5, as a single diastereoisomer with a cis configura-
tion for all bridgehead hydrogen atoms, in 54% isolated
yield. No compound with a taxol skeleton was observed,
agreeing with the DFT predictions.[17]
AHCTUNGTRENNUNG
Entry
Substrate
Product
Yield
1
54%
With the above computational and experimental supports,
we then studied the scope and limitation of this one-pot syn-
thesis of [5-8-5] ring skeleton (Table 1). Under the optimal
conditions (balloon-pressured mixed gas of 0.2 atm CO and
0.8 atm N2, 10% [Rh(CO)2Cl]2 catalyst in dioxane), the new
RhI-catalyzed [(5+2)+1] cycloaddition of ene-VCPs toler-
ates tethers incorporating geminal diester, sulfonamide, and
ether functionalities. Cycloadducts of 5, 7, 9, 11, and 13 have
a cis configuration for the substituents in positions I, II, and
III. If a methyl group is introduced to the IV position of the
substrates (14, 16 and 18), their corresponding [(5+2)+1] cy-
cloadditions gave mixtures of cycloadducts (15a,b, 17a,b,
and 19a,b), in which the bridgehead hydrogen atoms in II
and III are cis, but the hydrogen atoms in position I of the
cycloadducts have both cis and trans configurations with re-
spect to hydrogen atoms in positions II and III. This is con-
sistent with previous results.[6a,b] Construction of a quaterna-
ry carbon atom is very challenging. The present [(5+2)+1]
can well establish a quaternary carbon atom in the final cy-
cloadducts when a methyl group is introduced in position I
of the substrates, even though the cycloaddition yields are
lower in some cases compared to others (see entries 2, 4,
and 5 in Table 1). It is important to stress that the present
[(5+2)+1] cycloaddition is also applicable in synthesizing a
[6-8-5] ring system (20 to 21). The A/B ring of 21 has a trans
configuration. This agrees with our previous result, showing
that the [(5+2)+1] cycloadduct of 6/8 prefers a trans config-
uration.[6] The stereochemistry has been determined by 1D
and 2D NMR spectroscopies, three cycloadducts (11, 17a,
and 21) have been further confirmed by X-ray analysis
(Figure 2). From the above results, we can see that the pres-
ent method is efficient to generate [5-8-5] systems with rea-
sonable yields (Table 1, entries 1, 3, 7, and 8). Only when
quaternary carbon centers are introduced, the yields are not
satisfactory (Table 1, entries 4 and 5). Considering the feasi-
bility and facility of the one-pot construction of a [5-8-5]
skeleton, the present method has great potential in the syn-
thesis of natural products and their analogues with [5-8-5]
skeletons.
2
3
4
61%
79%[c]
37%
5
6
44%
62%[d,e]
cis/trans=1:6
67%[d,f]
cis/trans=1:1.3
7
65%[d,f]
cis/trans=1.5:1
8
9
30%
[a] E=CO2Me. Yields of isolated product were reported unless otherwise
indicated. [b] All reactions were conducted under a balloon-pressured
mixed gas of 0.2 atm CO and 0.8 atm N2 with 10 mol% [Rh(CO)2Cl]2 cat-
alyst in 1,4-dioxane at 808C for 72 h unless otherwise indicated. [c] This
yield was based on the recovered starting material. The conversion in
this entry was 72%. [d] Total yields of two stereoisomers. [e] Inseparable
stereoisomers. [f] Seperable stereoisomers.
As described above, the stereochemistry of substituents in
positions II and III of the final [5-8-5] and [6-8-5] cycload-
ducts is always in a cis configuration. This is controlled by
the cyclopropane cleavage step (see Figure 1). The stereo-
chemistry in positions I and II , which is controlled by
alkene insertion step of the catalytic cycle,6a is in either a cis
or trans configuration, depending on whether there is a sub-
stitution in position IV of the substrates (entries 1–5 vs. en-
tries 6–8). This suggests that the stereochemistry of position
III can be transferred to the stereochemistry in position II,
which can be further transferred to position I of the cycload-
ducts. Therefore, an asymmetric [5-8-5] and [6-8-5] skeletons
can be envisioned if a chiral cyclopropanation is used to
control the stereochemistry in position III of the sub-
strates.[18]
In conclusion, we have reported here an efficient and
quick approach for diastereoselective synthesis of [5-8-5]
and [6-8-5] tricyclic systems in one operation from readily
prepared substrates by using atom- and step-economical
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COMMUNICATION
The solution was bubbled with mixed CO (0.2 atm CO+0.8 atm N2) for
5 min. The reaction mixture was stirred at 808C under the balloon-pres-
sured mixed gas of CO (0.2 atm) and N2 (0.8 atm) for 72 h. After being
cooled to room temperature, the mixture was concentrated and the resi-
due was purified by flash column chromatography with silica gel to
afford the cycloaddition products.
Acknowledgements
We are indebted to generous financial support from a new faculty grant
from the 985 Program for Excellent Researches in Peking University, the
National Natural Science Foundation of China (20825205-National Sci-
ence Fund for Distinguished Young Scholars, 20772007, 20672005, and
2010CB833203-National Basic Research Program of China-973 Pro-
gram).
Keywords: cycloaddition · density functional calculations ·
fused-ring systems · homogeneous catalysis · rhodium
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RhI-catalyzed [(5+2)+1] cycloaddition. Quaternary carbon
centers can also be introduced in the cycloadducts. Further
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Experimental Section
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RhI-Catalyzed [(5+2)+1] Cycloaddition toward [5-8-5] Ring Systems
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Received: January 20, 2010
Published online: March 26, 2010
Chem. Asian J. 2010, 5, 1555 – 1559
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