W(CO)5(L)-catalyzed tandem intramolecular cyclopropanation/cope rearrangement for the stereoselective construction of bicyclo[5.3.0]decane framework

Article abstract of DOI:10.1021/ja0671924

Utilizing the biscarbene character of electrophilically activated alkynes, a novel tandem intramolecular cyclopropanation/Cope rearrangement of 3-siloxy-1,3,9-trien-7-ynes catalyzed by W(CO)5(L) for the stereoselective construction of bicyclo[5.3.0]decane framework is achieved. When 3-siloxy-1,3,9-trien-7-ynes were treated with a catalytic amount of W(CO)₆ under photoirradiation, bicyclo[5.3.0]decanes were obtained in good yield stereoselectively. In this reaction, the Cope rearrangement of the divinylcyclopropane intermediates, generated by the intramolecular cyclopropanation of 3-siloxy-1,3,9-trien-7-ynes based on the W(CO)₅(L)-catalyzed electrophilic activation of alkynes, occurs to give synthetically useful functionalized bicyclo[5.3.0]decane derivatives stereoselectively. Copyright

Full text of DOI:10.1021/ja0671924

Published on Web 12/09/2006
W(CO)5(L)-Catalyzed Tandem Intramolecular Cyclopropanation/Cope
Rearrangement for the Stereoselective Construction of Bicyclo[5.3.0]decane
Framework
Hiroyuki Kusama, Yuji Onizawa, and Nobuharu Iwasawa*
Department of Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan
Received October 7, 2006; E-mail: niwasawa@chem.titech.ac.jp
Scheme 1
In this paper, we report a facile method for the stereoselective
construction of the bicyclo[5.3.0]decane skeleton from 3-siloxy-
1,3,9-trien-7-ynes through divinylcyclopropane intermediates based
on the W(CO)5(L)-catalyzed electrophilic activation of alkynes as
a biscarbene equivalent.
During the study on the W(CO)5(L)-catalyzed cyclization of
ω-acetylenic silyl enol ethers,1 we analyzed the structure of the
Scheme 2
zwitterionic addition intermediate A by calculation and found that
cyclopropyl carbene structure B is favorable in the gas phase
(Scheme 1).2,3 In this reaction, protonation occurs through the
zwitterionic intermediate A to give the cyclized product;1b however,
it is expected that cyclopropyl carbene intermediate B could be
utilized for further reaction provided that a process with a low
activation energy is feasible through this intermediate. It is well-
Table 1. Reaction of Dienol Silyl Ether 1a
known that the Cope rearrangement of divinylcyclopropanes occurs
rapidly at room temperature to form a seven-membered ring
system.4 We then expected that treatment of 3-siloxy-1,3,9-trien-
7-ynes with W(CO)5(L) would give zwitterionic intermediates C
and/or divinylcyclopropane carbene complex intermediates D
(Scheme 2), and that the Cope rearrangement of the latter would
occur smoothly to give synthetically useful functionalized bicyclo-
[5.3.0]decane derivatives stereoselectively.
We first examined the reaction of enyne 1a with 10 mol % of
preformed W(CO)5(thf) in THF in the presence of MS 4Å (Table
1, entry 1). The reaction proceeded as expected, and the desired
seven-membered ring product 2a was obtained in 78% yield as a
time
yield
(%)
single stereoisomer after 13 h at 60 °C. The structure of 2a was
confirmed by X-ray analysis, and its relative stereochemistry can
be explained by the transition-state model of the Cope rearrange-
ment of divinylcyclopropanes as shown in X.4 When the reaction
was carried out in toluene under photoirradiation, only 5 mol % of
W(CO)6 was sufficient to complete the reaction within 2 h at room
temperature, and 2a was obtained in 66% yield (entry 2).5 In
addition, by carrying out the same reaction in the presence of 10
mol % of NEt3, the yield of 2a was further improved to 83% (entry
3).5 Some other transition-metal catalysts, such as Re, Pt, and Au,6
were also examined (entries 4-7), but W(CO)5(L) was found to
be the most suitable catalyst for this reaction.
Having established that W(CO)5(L) efficiently catalyzed the
seven-membered ring formation, the reaction was examined em-
ploying several types of substrates with the results being sum-
marized in Table 2. Vinylic (1b) and monosubstituted enynes (1c,
1d) were cyclized to afford the corresponding bicyclic enol silyl
ethers in good yield as a single stereoisomer (entries 1-3). It should
be noted that the reaction of 1c having a (Z)-propenyl moiety at
the alkyne terminus afforded the bicyclic product 2c, the relative
stereochemistry of which was different from that of 2a derived from
(E)-propenyl derivative 1a (Table 1, 1a vs Table 2, entry 2, 1c).
These results strongly suggest that this reaction is a concerted
reaction and is really proceeding through divinylcyclopropane
entry
conditions
(h)
1
2
3a
4
5
6
7
10 mol % of W(CO)5(thf)/THF, 60 °C
5 mol % of W(CO)6, hν/toluene, rt
5 mol % of W(CO)6, hν/toluene, rt
5 mol % of ReCl(CO)5, hν/toluene, rt
5 mol % of PtCl2/toluene, 70 °C
5 mol % of AuCl/toluene, 70 °C
5 mol % of Ph3PAu(SbF6)/DCE, 70 °C
13
2
2
34
17
3
78
66
83
4
12
54
0
6
a In the presence of 10 mol % of NEt3.
intermediates (Scheme 2). Furthermore, the reaction of disubstituted
enyne (1e), an enyne bearing an acetal moiety (1f), and alkyl-
substituted dienes (1g, 1h) also underwent seven-membered ring
formation smoothly in good yields using only a catalytic amount
of W(CO)6 (entries 4-7).
We next applied this reaction to a concise synthesis of tricyclic
compounds possessing a seven-membered ring moiety. The reaction
of dienol silyl ethers having a cyclohexenyl group at the alkyne
terminus (3) or as part of the diene moiety (5) with a catalytic
amount of W(CO)6 and 10 mol % of NEt3 in toluene under
photoirradiation gave the tricyclic compounds 4 and 6 in good yields
stereoselectively (eqs 1 and 2).
Recently, we reported related tungsten(0) or rhenium(I)-catalyzed
tandem cyclization of 3-siloxy-1,3-dien-7-ynes to give bicyclo[3.3.0]-
octane derivatives as a mixture of diastereomers, which are thought
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J. AM. CHEM. SOC. 2006, 128, 16500-16501
10.1021/ja0671924 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 2. W(CO)5(L)-Catalyzed Reaction of Dienol Silyl Ethers
1b-1h
Scheme 3
derivatives can be synthesized in a single step from easily available
alkynes without the use of diazo compounds.11
In summary, we have developed W(CO)5(L)-catalyzed tandem
intramolecular cyclopropanation/Cope rearrangement of 3-siloxy-
1,3,9-trien-7-ynes. This reaction provides a new, concise approach
for the stereoselective synthesis of synthetically useful function-
alized bicyclo[5.3.0]decane derivatives, which constitute the basic
carbon skeleton of many natural products.
Acknowledgment. This research was partly supported by a
Grant-in-Aid for Scientific Research from Ministry of Education,
Culture, Sports, Science and Technology of Japan. We thank Central
Glass Co., Ltd., for the generous gift of trifluoromethanesulfonic
acid. We also thank Dr. Hidehiro Uekusa and Ms. Sachiyo Kubo
for performing X-ray analysis.
a In the absence of NEt3. b The product was obtained as a 6:1 mixture
of diastereomers.
Supporting Information Available: Preparative methods and
spectral and analytical data of compounds 1-6 (PDF) and X-ray data
for 2a, cis-2f, and 4 (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
References
(1) (a) Maeyama, K.; Iwasawa, N. J. Am. Chem. Soc. 1998, 120, 1928. (b)
Iwasawa, N.; Maeyama, K.; Kusama, H. Org. Lett. 2001, 3, 3871. (c)
Kusama, H.; Yamabe, H.; Iwasawa, N. Org. Lett. 2002, 4, 2569. (d)
Iwasawa, N.; Miura, T.; Kiyota, K.; Kusama, H.; Lee, K.; Lee, P. H.
Org. Lett. 2002, 4, 4463.
to be obtained in a stepwise tandem-cyclization manner (see Scheme
3).7 On the contrary, we believe that the present reaction proceeds
through a divinylcyclopropane intermediate based on the following
experimental results: (1) While bicyclo[3.3.0]octanes were prepared
as a mixture of diastereomers, bicyclo[5.3.0]decanes were obtained
as a single stereoisomer stereospecifically. It is unlikely that the
second cyclization step of dienyltungsten species (see C in Scheme
2) onto an R,â-unsaturated silyloxonium moiety occurs with perfect
stereoselectivity. (2) Reactions in the presence of a proton source
afforded different results (Scheme 3). In the reaction of 7 with 1
equiv of W(CO)6 and 15 equiv of MeOH in toluene under
photoirradiation, monocyclic ketone 9, the protonated product of
the first cyclization intermediate 8, was obtained in 66% yield as
a major product. Under the same conditions, bicyclic silyl enol ether
2b was obtained in 56% yield without formation of monocyclic
derivative 11 in the reaction of enyne 1b. (3) The reaction of 1,3,9-
trien-7-ynes 1 is much faster than that of 1,3-dien-7-ynes 7. For
example, the reaction of enyne 1a was completed in 2 h (Table 1,
entry 2), while the reaction of alkyne 7 was completed in 7 h under
the same conditions.8 All of these results strongly suggest that the
five-membered ring formation proceeds in a stepwise manner, while
the seven-membered ring formation proceeds in a different,
concerted manner.
(2) Unpublished result.
(3) For recent reviews of the reactions through cyclopropyl carbene complex
intermediates generated from enynes, see: (a) Nieto-Oberhuber, C.; Lo´pez,
S.; Jime´nez-Nu´n˜ez, E.; Echavarren, A. M. Chem.sEur. J. 2006, 12, 5916.
(b) Bruneau, C. Angew. Chem., Int. Ed. 2005, 44, 2328. (c) Ma, S.; Yu,
S.; Gu, Z. Angew. Chem., Int. Ed. 2006, 45, 200.
(4) For reviews on the Cope rearrangement of divinylcyclopropanes to form
a seven-membered ring system, see: (a) Wong, H. N. C.; Hon, M.-Y.;
Tse, C.-W.; Yip, Y. C.; Tanko, J.; Hudlicky, T. Chem. ReV. 1989, 89,
165. (b) Rhoads, S. J.; Raulinus, N. R. Org. React. 1975, 22, 1. (c)
Hudlicky, T.; Fan, R.; Reed, J. W.; Gadamasetti, K. G. Org. React. 1992,
41, 1. (d) Piers, E. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Ed.; Pergamon Press: Oxford, 1991; Vol. 5, pp 971-998.
(5) A small amount of bicyclo[3.3.0]octane derivative was obtained in 3%
yield (entry 2) and 4% yield (entry 3).
(6) In the case of cationic gold complex (entry 7), protonated product of the
first cyclization intermediate C was obtained in 75% yield.
(7) Kusama, H.; Yamabe, H.; Onizawa, Y; Hoshino, T.; Iwasawa, N. Angew.
Chem., Int. Ed. 2005, 44, 468.
(8) In the presence of 10 mol % of NEt3 (the conditions of Table 1, entry 3),
the reaction of alkyne 7 was not completed in 30 h.
(9) (a) Davies, H. M. L.; McAfee, M. J.; Oldenbrug, C. E. J. Org. Chem.
1989, 54, 930. (b) Davies, H. M. L.; Doan, B. D. J. Org. Chem. 1999,
64, 8501. (c) Davies, H. M. L.; Stafford, D. G.; Doan, B. D.; Houser, J.
H. J. Am. Chem. Soc. 1998, 120, 3326. (d) Deng, L.; Giessert, A. J.;
Gerlitz, O. O.; Dai, X.; Diver, S. T.; Davies, H. M. L. J. Am. Chem. Soc.
2005, 127, 1342.
(10) For other examples of the synthesis of bicyclo[5.3.0]decane derivatives
through the Cope rearrangement of divinylcyclopropane using diazo
compounds, see: (a) Ni, Y.; Montgomery, J. J. Am. Chem. Soc. 2006,
128, 2609. (b) Sarpong, R.; Su, J. T.; Stoltz, B. M. J. Am. Chem. Soc.
2003, 125, 13624. (c) Using carbene complexes, see: Harvey, D. F.; Lund,
K. P. J. Am. Chem. Soc. 1991, 113, 5066.
In almost all of the previous cases, the divinylcyclopropanes have
been prepared by the Rh-catalyzed cyclopropanation of dienes with
unsaturated diazo compounds, which are not necessarily easy to
handle.9,10 In the present reaction, alkynes behave as a biscarbene
equivalent by transition-metal activation, and bicyclo[5.3.0]decane
(11) For generation of divinylcyclopropane by cyclopropanation of dienes using
propargylic carboxylates and ruthenium catalyst, see: Miki, K.; Ohe, K.;
Uemura, S. J. Org. Chem. 2003, 68, 8505.
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