Formal [6+4] cycloaddition of a dicobalt acetylene complex with furan derivatives

Article abstract of DOI:10.1016/j.tetlet.2010.12.063

An efficient method for constructing a 10-membered carbocycle with an oxygen bridge has been developed on the basis of a formal [6+4] cycloaddition reaction. Under the influence of EtAlCl₂, a dicobalt hexacarbonyl acetylene complex possessing a

Full text of DOI:10.1016/j.tetlet.2010.12.063

Tetrahedron Letters 52 (2011) 910–912
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Formal [6+4] cycloaddition of a dicobalt acetylene complex
with furan derivatives
Koichiro Dota a, Tadashi Shimizu b, Shoji Hasegawa a, Masaaki Miyashita c, Keiji Tanino a,
⇑
a Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
b Department of Pharmacy, Hyogo University of Health Sciences, Kobe 650-8530, Japan
c Department of Applied Chemistry, Faculty of Engineering, Kogakuin University, Hachioji 192-0015, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient method for constructing a 10-membered carbocycle with an oxygen bridge has been devel-
oped on the basis of a formal [6+4] cycloaddition reaction. Under the influence of EtAlCl2, a dicobalt hexa-
carbonyl acetylene complex possessing a benzoyloxy group and an allylsilane moiety reacted with furan
to give a 11-oxabicyclo[6.2.1]undec-9-ene derivative. On treatment with iodine, the cycloadduct under-
Received 15 November 2010
Revised 3 December 2010
Accepted 15 December 2010
Available online 21 December 2010
went decomplexation followed by rearrangement of the oxygen bridge to afford
clo[5.3.1]undeca-1,5-diene derivative.
a 11-oxabicy-
Ó 2010 Elsevier Ltd. All rights reserved.
OTIPS
O
The development of a new synthetic method for medium-sized
carbocyclic compounds has been one of the most challenging sub-
jects in organic chemistry.1 While the ring closing metathesis of
various dienes has found wide use for this purpose,2 a cycloaddi-
tion approach which produces two C–C bonds in one-stage is
advantageous from the viewpoint of efficiency.3 Recently, we have
reported a novel method for constructing an eight-membered car-
bocycle via a formal [6+2] cycloaddition reaction using dicobalt
acetylene complex 1 as a six-carbon unit (Scheme 1).4,5
The reaction proceeds through intermolecular addition of an
enol triisopropylsilyl (TIPS) ether to the dicobalt propargyl cation
A giving rise to the silyloxonium ion B which in turn undergoes
an intramolecular addition reaction. In this transformation, the
large bond angles of the dicobalt acetylene complex moiety6 were
effective to avoid the formation of a six-membered ring via an
intramolecular cyclization reaction of the cation intermediate A.
On the other hand, furan derivatives are employed as a four-car-
bon unit in a Diels–Alder reaction7 or a [4+3] cycloaddition reac-
tion with a 2-oxyallyl cation species8 (Scheme 2).
While these cycloaddition reactions of furan may proceed
through the concerted mechanisms,9 we became intrigued with
the possibility of a formal [6+4] cycloaddition reaction with dico-
balt acetylene complex 1 via a stepwise pathway similar to that
shown in Scheme 1.
Initially, the reaction of six-carbon unit 1 with furan was exam-
ined under the influence of EtAlCl2 (Scheme 3). While a rather
complex mixture was formed, the desired cycloadduct 3 having a
TIPSO
Me
TIPSO
EtAlCl2
+
Co(CO)3
Co(CO)3
Me
CH2Cl2
Co(CO)3
Co(CO)3
2
BzO
1
73% (syn : anti = 88 : 12)
OTIPS
OTIPS
TIPSO
TIPSO
Co(CO)3
Co(CO)3
R
Co(CO)3
Co(CO)3
R
A
B
Scheme 1. Formal [6+2] cycloaddition reaction of dicobalt acetylene complex 1
with an enol silyl ether.
COR
COR
[4+2]
[4+3]
O
O
O–
+
O
O
O
1
Lewis acid
[6+4]
O
Co(CO)3
Co(CO)3
⇑
Corresponding author. Tel.: +81 11 706 2705; fax: +81 11 706 4920.
E-mail address: ktanino@sci.hokudai.ac.jp (K. Tanino).
Scheme 2. Cycloaddition reactions of furan.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.12.063

K. Dota et al. / Tetrahedron Letters 52 (2011) 910–912
911
OTIPS
afforded vinyl bromide 4 which in turn was subjected to the
Kumada coupling reaction12 with Me3SiCH2MgCl. The resulting
propargyl alcohol 5 was converted to the six-carbon unit 7 through
benzoylation followed by treatment with Co2(CO)8 in high overall
yield.
OTIPS
furan (n eq)
EtAlCl2 (2 eq)
Co(CO)3
Co(CO)3
1 (1 eq)
O
Co(CO)3
Co(CO)3
CH2Cl2
–78 ~ –30 °C
BzO
With the new six-carbon unit 7 in hand, the formal [6+4] cyclo-
addition reactions with furan and substituted furans were explored
(Table 1). As expected, allylsilane 7 reacted with furan more
cleanly than enol silyl ether 1, and cycloadduct 8a was obtained
in 77% yield (entry 1). Similarly, the reactions of 7 with 3-methyl-
furan, 3-bromofuran, and 3-(phenylthio)furan afforded the corre-
sponding cycloadducts 8b, 8c, and 8d, respectively (entries 2–4).
Since the formal [6+4] cycloaddition reaction proceeds by a step-
wise mechanism, the regioselective formation of these cycload-
ducts stemmed from initial addition of the dicobalt propargyl
cation at the less hindered C5 position of 3-substituted furan deriv-
atives. In contrast, the reversal of regioselectivity was observed in
the reaction with 3-(triisopropylsilyloxy)furan (entry 5).13 The
exceptional result can be rationalized by the strong electron donat-
ing effect of the silyloxy group to enhance the initial addition at the
C2 position. While 3,4-tetramethylenefuran also underwent the
formal cycloaddition reaction giving rise to tricyclic compound
10 in moderate yield (entry 6), 2-methylfuran failed to give the de-
sired product (entry 7). Formation of 2,5-disubstituted furan 11
having an allylsilane moiety suggests that the cyclization step of
the cationic intermediate suffered from steric hindrance at the
C5 position.
Next, we explored the transformation of the cycloadducts into
the corresponding acetylenes by oxidative decomplexation.14
While the reaction of 8a with ceric ammonium nitrate (CAN)
merely gave a complex mixture, an unexpected product 12 having
a 11-oxabicyclo[5.3.1]-undeca-1,5-diene skeleton15 was obtained
by the use of iodine in methanol (Scheme 5). Treatment of the
resulting iodide 12 with butyllithium induced cleavage of the oxy-
gen bridge, and 10-membered acetylene 13 was produced in high
yield.
O
H
H
furan
yield of 3
O
(1 eq)
(2 eq)
(5 eq)
20%
39%
43%
Co(CO)3
Co(CO)3
3
Scheme 3. Formal [6+4] cycloaddition reaction of dicobalt acetylene complex 1
with furan.
TMS
Br
HC CCH2OH
EtMgBr
cat. CuCl
TMSCH2MgCl
NiCl2(dppp)
Br
THF
92%
THF
Br
4
5
OH
OH
TMS
TMS
BzCl, Et3N
DMAP
Co2(CO)8
Co(CO)3
Co(CO)3
CH2Cl2
CH2Cl2
92%
BzO
92% from 4
6
7
OBz
Scheme 4. Synthesis of new six-carbon unit 7.
11-oxabicyclo[6.2.1]undec-9-ene skeleton10 was isolated in 20%
yield. The use of an excess amount of furan led to an increase in
yield up to 43%, suggesting that furan is not nucleophilic enough
to undergo faster addition with the dicobalt propargyl cation (A
in Scheme 1) than another molecule of 1 possessing an enol silyl
ether moiety.
These results led us to design a new six-carbon unit with
slightly reduced nucleophilicity, and allylsilane derivative 7 was
prepared as shown in Scheme 4. The copper-catalyzed coupling
reaction11 between propargyl alcohol and 2,3-dibromopropene
The formation of compound 12 can be explained by assuming
tricyclic oxonium ion intermediate E.16 Thus, decomplexation of
8a affords strained nine-membered acetylene 14 with a highly
reactive nature toward addition reactions. Subsequently, 14 would
undergo addition with iodine to form epiiodonium ion D that is
converted to tricyclic oxonium ion E through participation of the
Table 1
Formal [6+4] cycloaddition reactions of dicobalt acetylene complex 7 with furan derivativesa
Entry
Furan derivative
Product
Yieldb
1
2
3
4
Furan
3-Methylfuran
3-Bromofuran
3-(Phenylthio)furan
H
H
8a (X = H)
8b (X = Me)
8c (X = Br)
8d (X = PhS
77%
85%
78%
79%
X
O
Co(CO)3
Co(CO)3
H
O
TIPSO
O
5
6
7
9
83%
47%
45%
Co(CO)3
TIPSO
H
O
Co(CO)3
H
O
10
11
Co(CO)3
H
Co(CO)3
TMS
Me
O
2-Methylfuran
Co(CO)3
Co(CO)3
a
Typical reaction conditions: six-carbon unit 7 (0.5 mmol), furan (2.5 mmol), EtAlCl2 (1.1 mmol), CH2Cl2 (2.5 mL), À20 °C to 0 °C.
b
Isolated yield.

912
K. Dota et al. / Tetrahedron Letters 52 (2011) 910–912
can be found, in the online version, at doi:10.1016/j.tetlet.2010.
12.063.
H
H
I2 (2 eq)
O
BuLi (3 eq)
OH
8a
MeOH
rt, 4.5 h
THF
–78 °C, 0.5 h
I
References and notes
MeO
MeO
12
90%
13
52%
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H
H
O
H
H
O
O
I
H
H
I
14
D
E
MeOH
Scheme 5. Novel decomplexation reaction of cycloadduct 8a mediated by iodine.
oxygen bridge. While oxonium ion E possesses two reactive allylic
carbon atoms to be attacked by a nucleophile, regioselective intro-
duction of methanol occurs so as to effect maximum relief of ring
strain in the 1-oxabicyclo[3.3.0]octan-2,6-diene substructure.17
In summary, we have developed an efficient method for
constructing a 10-membered carbocycle with an oxygen bridge
via a formal [6+4] cycloaddition reaction. Under the influence of
EtAlCl2, dicobalt acetylene complex 7 possessing an allylsilane moi-
ety reacted with a furan derivative to afford a 11-oxabicy-
clo[6.2.1]undec-9-ene derivative. The product can be transformed
into 10-membered acetylene through a novel decomplexation reac-
tion mediated by iodine. It is noteworthy that a 11-oxabicy-
clo[6.2.1]undecane skeleton is widely found as a substructure of
natural products (e.g., oxygenated germacrane sesquiterpenoids
and cladiellin diterpenoids).18 We are currently exploring the syn-
thetic studies of marine terpenoids on the basis of the formal
[6+4] cycloaddition reaction.
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ether moiety to give the corresponding ketone.
Acknowledgments
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of compound 12 having a p-bromobenzoyloxy group instead of the methoxy
group.
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We thank Professor Tamotsu Inabe (Hokkaido University) for
the X-ray diffraction measurements. This work was partially sup-
ported by the Global COE Program (Project No. B01: Catalysis as
the Basis for Innovation in Materials Science) and Grant-in-Aid
for Scientific Research on Innovative Areas (Project No. 2105: Or-
ganic Synthesis Based on Reaction Integration) from the Ministry
of Education, Culture, Sports, Science and Technology, Japan.
17. Rearrangement of an oxygen bridge via a tricyclic oxonium ion intermediate:
Supplementary data
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Supplementary data (experimental procedures and character-
ization data for the new compounds) associated with this article