Efficient control for the cationic platinum(II)-catalyzed concise synthesis of two types of fused carbocycles with angular oxygen functionality

Article abstract of DOI:10.1002/asia.201100460

Double trouble: An efficient method for the preparation of two types of synthetically useful bicyclo[5.4.0]undecanes with an angular oxygen functionality was realized starting from easily available substrates; this method was based on the [3+2] cycloaddit

Full text of DOI:10.1002/asia.201100460

DOI: 10.1002/asia.201100460
Efficient Control for the Cationic Platinum(II)-Catalyzed Concise Synthesis
of Two Types of Fused Carbocycles with Angular Oxygen Functionality
Hiroyuki Kusama, Eiichi Watanabe, Kento Ishida, and Nobuharu Iwasawa*[a]
Fused carbocyclic structures containing a seven-mem-
bered ring with an angular hydroxy group are frequently
found in natural products (Figure 1),[1] and exploitation of
tives, 1,2-H shift products 2 or alkyl migration products 3,
selectively depending on the presence or absence of the sub-
stituent at the propargylic position (Scheme 1).[5c] By using
this reaction, carbonyl ylides could be generated easily with-
out using diazo compounds, and synthetically useful cyclic
compounds with several oxygen functionalities were readily
obtained starting from ynones and vinyl ethers in the pres-
ence of a catalytic amount of PtCl2. However, the two reac-
tion pathways were dependent on the substrates and inten-
tional control of the two pathways was not realized in spite
of their high potential as a synthetic protocol. In this study,
we report a facile method for the construction of two kinds
of synthetically useful fused seven-membered ring skeletons
bearing an oxygen functionality at the angular position by
using 2-(alka-2-ynyl)cycloalkanones as easily available sub-
strates, where both kinds of products are obtained selective-
ly by the appropriate combination of the vinyl ether and the
cationic platinum(II) catalyst.
Figure 1. Various terpenoids containing a fused seven-membered ring
with an angular oxygen functionality.
an efficient synthetic methodology for the construction of
such carbocycles is of great importance in the field of organ-
ic synthesis.[2] Utilization of carbonyl ylides is one of the
most useful methods for the preparation of this class of
compounds, however, this method suffers from several prob-
lems such as the use of diazo compounds as the precursor of
the ylides and the difficulty in preparing required substrates
for the construction of fused carbocycles.[3,4]
We have already reported that platinum-containing car-
bonyl ylides A could be generated from simple acyclic g,d-
ynones 1 and PtCl2 by attack of the carbonyl oxygen in an
endo manner onto the electrophilically activated alkyne
moiety,[5–7] and subsequently they undergo an intermolecular
[3+2] cycloaddition reaction with vinyl ethers to give cyclo-
adducts as carbene complex intermediates B. These inter-
mediates then further undergo typical reactions of carbene
complexes such as 1,2-H shift or 1,2-alkyl migration to
afford two kinds of oxygen-bridged cycloheptene deriva-
We first examined the reaction of 2-(buta-2-ynyl)cyclo-
hexanone 4, which was easily prepared by alkylation of 2-
ethoxycarbonylcyclohexanone followed by decarboxylation,
with benzyl vinyl ether in the presence of a catalytic amount
of PtCl2 in toluene at room temperature (Table 1, entry 1).[8]
Although this substrate had no propargylic substituent, the
fused carbocycle 5 (corresponding to the 1,2-H shift product
2, Scheme 1) was obtained albeit in low yield (<12%) after
three days. To improve the yield of the product, several
other platinum complexes were examined. Although the use
of cis-[PtCl
product (Table 1, entry 2), by using the cationic complex
(PPh3)2][9] in dichloromethane gave no desired
2ACHTUNGTRENNUNG
[10]
generated in situ from cis-[PtCl2ACHTNUGTRNEG(UN PPh3)2] and AgSbF6 dra-
matically accelerated the reaction rate and improved the
yield of product 5 to 55% with moderate diastereoselectiv-
ity (Table 1, entry 3; trans/cis=25:75).[11] Interestingly, trans-
[PtCl2ACHTUNGTRENUNG(PPh3)2]-AgSbF6 gave no cycloaddition product; this is
probably owing to the steric repulsion between the alkyne
moiety and the two phosphine ligands (Table 1, entry 4).
[a] Dr. H. Kusama, E. Watanabe, Dr. K. Ishida, Prof. Dr. N. Iwasawa
Department of Chemistry
Tokyo Institute of Technology
2-12-1, O-okayama, Meguro-ku, Tokyo 1528551 (Japan)
Fax : (+81)3-5734-2931
The yield was further improved by using cis-[PtCl2(P
ACHTUNGTRENNUNG(m-
E-mail: niwasawa@chem.titech.ac.jp
tol)3)2] or cis-[PtCl2(P(nBu)3)2] as a platinum catalyst, al-
though the diastereoselectivity remained almost the same
AHCTUNGTRENNUNG
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100460.
Chem. Asian J. 2011, 6, 2273 – 2277
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2273

COMMUNICATION
dered vinyl ether (TIPS vinyl
ether) and the small phosphine
ligand (PMe2Ph) mainly provid-
ed the 1,2-H shift product (5/
6=95:5); this suggested that
the reaction pathway was
mainly determined by the vinyl
ether rather than the phosphine
ligand (Table 1, entry 11). Thus,
selective preparation of two
types of synthetically useful,
oxygenated
bicyclo-
AHCTUNGTRENGN[UN 5.4.0]undecane derivatives was
realized starting from the same,
easily available alkynyl cyclo-
hexanone 4 by judicious selec-
tion of the vinyl ether and the
catalyst. It should be noted that
few examples on the use of
mono-cationic
platinum(II)
complexes in alkyne-activation
chemistry have been reported,
and this study is a very rare ex-
ample in which the reaction
pathway is controlled efficiently
by the combination of the vinyl
ether and the phosphine ligand
on the cationic platinum com-
plex.
Scheme 1. Preparation of 8-oxabicyclo
Table 1. Survey of platinum catalysts.
ACHTUNGTNER[NUNG 3.2.1]octane derivatives.
As both types of cycloaddi-
tion products were found to be
obtained selectively by the ap-
propriate choice of the reaction
parameters, we next examined
the reaction of various 2-(buta-
Entry
1[a]
2
3
4
5
6
7
8
Catalysts
PtCl2
R
Time [h]
5 [%]
trans/cis
n.d.[b]
–
25:75
–
22:78
33:67
53:47
88:12
n.d.[b]
n.d.[b]
78:22[d]
6 [%]
trans/cis
Bn
Bn
Bn
Bn
Bn
Bn
TIPS
TIPS
Bn
73
10
3
15
3
6
3
3
3
<12
0
55
0
79
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
cis-[PtCl
cis-[PtCl
2A
2A(PPh3)2], AgSbF6
2A(PPh3)2], AgSbF6
(m-tol)3)2], AgSbF6
(nBu)3)2], AgSbF6
(m-tol)3)2], AgSbF6
(nBu)3)2], AgSbF6
(PMe3)2], AgSbF6
2A(PMe2Ph)2], AgSbF6
2A(PMe2Ph)2], AgSbF6
trans-[PtCl
R
cis-[PtCl2(P
cis-[PtCl2(P
cis-[PtCl2(P
cis-[PtCl2(P
R
R
66
84
83
2-ynyl)cycloalkanones
TIPS vinyl ether using cis-
[PtCl2(P(nBu)3)2] with AgSbF6
with
N
N
9
cis-[PtCl
cis-[PtCl
cis-[PtCl
2A
13[d]
14[d]
78[d]
70[d]
73[e]
trace[d]
81:19
90:10
n.d.[b]
AHCTUNGTRENNUNG
10[c]
11
N
Bn
TIPS
3
4
as the platinum catalyst to
obtain the 1,2-H shift product
N
[a] Toluene was used as solvent. [b] n.d.=not determined. [c] 2 equivalents of vinyl ether were used. [d] Deter-
mined by 1H NMR spectroscopy using 1,1,2,2-tetrachloroethane as an internal standard. [e] Isolated as a
ketone by hydrolysis of 6.
selectively
Table 2).
(Scheme 2
and
The reaction of cyclohexa-
none derivatives 7a with
a
(Table 1, entries 5 and 6).[10d] More importantly, it was found
that triisopropylsilyl (TIPS) vinyl ether could be employed
instead of benzyl vinyl ether to give the 1,2-H shift product
5 (R=TIPS) in good yield, and high trans diastereoselectiv-
methyl substituent at the 6-position gave the corresponding
product 8a in reasonable yield. A cyclohexanone derivative
7b with a siloxy group also gave a highly oxygenated prod-
uct 8b in high yield as a mixture of diastereoisomers
(Scheme 2). The reaction of 2-methyl derivative 7c gave the
trans product[11] as a single diastereoisomer, which has the
same stereochemistry of the methyl and the oxygen func-
tional group to that of some terpenoids such as isoamijiol
and indicol (Table 2, entry 1).[1] The reaction of cyclohexa-
none derivatives containing an acetal moiety 7d also provid-
ed cycloaddition product 8d (Table 2, entry 2). We further
examined the reaction of cyclohexanone derivatives with an
oxygen functionality at the terminal propargylic position, as
ity was achieved by using the cis-[PtCl2(P
catalyst (83%, trans/cis=88:12).
ACHTUGNTRNE(UNNG nBu)3)2]-AgSbF6
Surprisingly, when cis-[PtCl
2ACHTUNGTRENN(NUG PMe3)2] or cis-[PtCl2
ACHTUNGTRENNUNG(PMe2Ph)2] was employed in combination with AgSbF6, the
other type of cycloaddition product, alkyl migration product
6, was obtained as a major product (Table 1, entries 9 and
10). High diastereoselectivity of the product (trans/cis=
90:10) was also achieved in this reaction.[11] It was also
noted that the reaction with the combination of the hin-
2274
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Chem. Asian J. 2011, 6, 2273 – 2277

Fused Carbocycles with Angular Oxygen Functionality
on the cyclohexanone ring 7a,
7b and substrates with various
substituents at the alkyne ter-
minus 7h, 7i, 7j were used and
the corresponding products
were obtained in good yield
with high diastereoselectivity
(Table 3, entries 1–5).[11] Prop-
argyl ether derivatives 7e, 7 f
Scheme 2. The reaction of cyclohexanone derivatives with a substituent at the 6-position. TBS=tert-butyldime-
thylsilyl.
Table 2. Reaction of various cyclohexanones using cis-[PtCl2(P
G
Table 3. Reaction of various cyclohexanones using cis-[PtCl2ACTHUNGTERNNU(G PMe2Ph)2]
as a platinum complex.
as a platinum complex.
R1
R2
Yield [%]
trans/cis
Entry R1
R2
X
Yield [%] trans/cis
Entry
1[a]
2[a]
3
4
5
Me
OTBS
H
H
H
Me
Me
nBu
7a
7b
7h
7i
7j
7e
7 f
83 (9a)
60 (9b)
82 (9h)
76 (9i)
72 (9j)
49 (9e)
63 (9 f)
88:12
89:11
93:7
1
Me Me
-CH2-
7c 73 (8c)
>95:5
70:30
88:12
>95:5
89:11
90:10
89:11
88:12
2[a]
3
H
H
H
H
H
H
H
Me
CACHTUNGRETNNU(G OCH2CH2O) 7d 73 (8d)
CH2OTBS
CH2OTr
vinyl
-CH2-
-CH2-
-CH2-
-CH2-
-CH2-
7e 92 (8e)
7 f 70 (8 f)
7g 92 (8g)
7h 93 (8h)
4[b]
5
6
7
8
A
91:9
N
85:15
75:25
85:15
6
7
H
H
CH2OTBS
CH2OTr
nBu
A
7i
88 (8i)
[a] The stereochemistry between the R1 and buta-2-ynyl substituent was
the same as shown in Scheme 2.
ACHTUNGTRENNUNG(CH2)2OBn -CH2-
7j 81 (8j)
[a] 20 h. [b] 08C. Trace amounts of isomers were also obtained.
this would be a useful element for cleaving the oxygen
provided the desired products 9e, 9 f in a somewhat lower
yield because of the decomposition of the substrates
(Table 3, entries 6 and 7).
bridge to give 1-hydroxybicycloACTHUNTGRNEUNG[5.4.0]undecane deriva-
tives.[12] As a result, highly oxygenated cycloheptene deriva-
tives 8e and 8 f were obtained without any difficulty
(Table 2, entries 3 and 4). Fur-
ther examination revealed that
various substituents (R3 =vinyl,
nBu, (CH2)2Ph, (CH2)2OBn)
could be employed as the
alkyne substituent and 1,2-H
shift products were obtained in
high yield with mostly 90:10
diastereoselectivity
entries 5–8).
(Table 2,
We next examined the gener-
ality of the reaction for the
preparation of alkyl migration
products by the reaction of var-
ious cyclohexanone derivatives
7 and benzyl vinyl ether using a
catalytic amount of cis-[PtCl2
ACHTUNGTRENNUNG
action preferentially gave the
alkyl migration products 9 in
good yield with reasonable dia-
stereoselectivity.
Cyclohexa-
none derivatives containing a
methyl or a siloxy substituent Scheme 3. The control of two reaction pathways depending on the catalyst and the vinyl ether.
Chem. Asian J. 2011, 6, 2273 – 2277
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
2275

N. Iwasawa et al.
COMMUNICATION
Discrimination of the two reaction pathways, that is, 1,2-
H shift or 1,2-alkyl migration from the carbene complex in-
termediate 12, could be explained as follows (Scheme 3):[5c]
From the experimental results, the carbene complex inter-
mediate 11 derived from vinyl ether containing a large sub-
stituent (R=TIPS) and a large phosphine ligand (P=P-
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Xiao, R.-T. Li, S.-X. Huang, J.-X. Pu, H.-D. Sun, Nat. Prod. Rep.
2008, 25, 871–891.
[2] For reviews of construction of carbocycles containing a seven-mem-
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[6] For other examples of cycloaddition of metal-containing zwitterionic
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8132.
ACHTUNGTRENNUNG(nBu)3, cone angle=1368) undergoes 1,2-H shift preferen-
tially to provide 5 directly,[14] while that with a small sub-
stituent (R=Bn) and a small ligand (P=PMe2Ph, cone
angle=1228) gives alkyl-migration product 6 selectively.[14]
It is likely that due to the steric repulsion of the platinum
complex and the TIPS group, formation of the intermediate
12 and/or 13 with a large substituent (R=TIPS) and a large
ligand (P=PACHTUNGTRENNUNG(nBu)3) is not favorable, and 11 preferentially
undergoes a 1,2-H shift to afford 5. On the contrary, inter-
mediate 11 with a small substituent (R=Bn) and a small
ligand (P=PMe2Ph) undergoes 1,2-alkyl migration to gener-
ate intermediate 12.[15] Further alkyl migration to the oxoni-
um moiety of 12 produces another nonstabilized carbene in-
termediate 13, which undergoes 1,2-H shift to give the alkyl
migration product 6.[16]
In conclusion, an efficient method for the preparation of
bicycloACHTUNGTRENNUNG[5.4.0]undecanes with an angular oxygen functionality
was developed starting from easily available cyclic ketones
and vinyl ethers based on the [3+2] cycloaddition reaction
of the platinum-containing carbonyl ylide. Two types of syn-
thetically useful products were selectively obtained depend-
ing on the substituent of the vinyl ether and the phosphine
ligand of the cationic platinum(II). This is a unique example
of controlling two catalytic reaction pathways simply by
choosing appropriate reaction parameters.
[7] For reviews of cycloaddition reaction of metal-containing zwitterion-
ic intermediates, see: a) H. Kusama, N. Iwasawa, Chem. Lett. 2006,
35, 1082–1087; b) N. Asao, Synlett 2006, 1645–1656.
Experimental Section
General Procedure
[8] For recent platinum(II)-catalyzed reactions with alkynes, see: a) C.
Nevado, A. M. Echavarren, Synthesis 2005, 167–182; b) C. Bruneau,
Angew. Chem. 2005, 117, 2380–2386; Angew. Chem. Int. Ed. 2005,
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[9] C. H. Oh, J. H. Lee, S. J. Lee, J. I. Kim, S. C. Hong, Angew. Chem.
2008, 120, 7615–7617; Angew. Chem. Int. Ed. 2008, 47, 7505–7507.
[10] For reactions of mono-cationic platinum bisphosphine complexes
with alkynes, see: a) D. Brissy, M. Skander, P. Retailleau, G. Frison,
A. Marinetti, Organometallics 2009, 28, 140–151; b) D. Brissy, M.
Skander, H. Jullien, P. Retailleau, A. Marinetti, Org. Lett. 2009, 11,
A
dichloromethane solution (1.0 mL) of cyclic g,d-ynone (15.0 mg,
0.100 mmol) was added to cis-[PtCl2(P(nBu)3)2] (6.7 mg, 0.010 mmol) and
AHCTUNGTRENNUNG
AgSbF6 (3.4 mg, 0.010 mmol) and the mixture was stirred at room tem-
perature. After one minute, a dichloromethane solution (1.0 mL) of TIPS
vinyl ether (100.2 mg, 0.5 mmol) was added to the mixture and stirred at
room temperature until the starting material disappeared or for approxi-
mately 10 h. The reaction was quenched with triethylamine (0.1 mL) and
the mixture was filtered through a short pad of Celite with ethyl acetate
(30 mL) as an eluent and then the solvent was removed under reduced
pressure to give crude product, which was purified by silica-gel column
chromatography (3% ethyl acetate in hexane) to give alkene 5-trans and
5-cis (29.1 mg, 83%, trans/cis=88:12) as a diastereomeric mixture.
Acknowledgements
2137–2139; c) In the previous work, cis-
ACHTUNGTRENUN[G PtCl2(PAHCUTNGTREN(NUNG m-tol)3)2] provided
This research was supported by a Grant-in-Aid for Scientific Research
from the Ministry of Education, Culture, Sports, Science, and Technology
of Japan. K.I. has been granted a Research Fellowship of the Japan Soci-
ety for the Promotion of Science for Young Scientists.
a better yield than cis-[PtCl2A(PPh3)2]: K. Ishida, H. Kusama, N. Iwa-
G
CHTUNGTRENNUNG
sawa, J. Am. Chem. Soc. 2010, 132, 8842–8843. For reactions with
alkenes, see: d) X. Han, R. A. Widenhoefer, Org. Lett. 2006, 8,
3801–3804; e) M. E. Cucciolito, A. D’Amora, A. Vitagliano, Orga-
nometallics 2005, 24, 3359–3361; f) M. E. Cucciolito, A. D’Amora,
A. Vitagliano, Organometallics 2010, 29, 5878–5884; g) J. A. Fedu-
cia, A. N. Campbell, M. Q. Doherty, M. R. Gagnꢂ, J. Am. Chem.
Soc. 2006, 128, 13290–13297. For reactions of dicationic platinum bi-
Keywords: alkynes
· carbonyl ylide · cycloaddition ·
platinum · ring expansion
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Chem. Asian J. 2011, 6, 2273 – 2277

Fused Carbocycles with Angular Oxygen Functionality
sphosphine complexes with alkynes, see: h) L. Charruault, V. Mi-
chelet, R. Taras, S. Gladiali, J.-P. GenÞt, Chem. Commun. 2004, 850–
851; i) P. Y. Toullec, C.-M. Chao, Q. Chen, S. Gladiali, J.-P. GenÞt, V.
Michelet, Adv. Synth. Catal. 2008, 350, 2401–2408.
factor (cone angle) of phosphine ligands rather than with their elec-
tronic factor (the Tolman electronic parameter). Suitable phosphines
for preparation of the alkyl-migration product: PMe3 (cone angle=
1188, n(CO)=2064.1 cmÀ1) and PMe2Ph (cone angle=1228, n(CO)=
2065.6 cmÀ1). Phosphines which preferentially gave the alkyl-migra-
[11] Trans and cis are defined as follows: the isomer that has the bridged
oxygen atom and the angular hydrogen atom in a trans orientation
of the cyclohexane ring is designated as trans, and the other isomer
as cis. This definition is employed throughout this manuscript. For
the determination of the stereochemistry, see the Supporting Infor-
mation.
tion product: PACTHNUTRGNEUNG
(nBu)3 (cone angle=1368, n(CO)=2060.3 cmÀ1), P-
AHCTUNGTRENNUNG
(m-tol)3 (cone angle=1658, n(CO)=2067.2 cmÀ1), PPh3 (cone
angle=1458, n(CO)=2068.9 cmÀ1). For IR data [n(CO)] of
[Ni(CO)3L] with various phosphines, see: C. A. Tolman, J. Am.
Chem. Soc. 1970, 92, 2953–2956.
[12] The total synthesis of resiniferatoxin was achieved by using 1-
[15] For other recent examples of 1,2-alkyl migration in platinum–car-
bene intermediates, see: a) B. Crone, S. F. Kirsch, Chem. Eur. J.
2008, 14, 3514–3522; b) J. Sun, M. P. Conley, L. Zhang, S. A.
Kozmin, J. Am. Chem. Soc. 2006, 128, 9705–9710; c) H. Funami, H.
Kusama, N. Iwasawa, Angew. Chem. 2007, 119, 927–929; Angew.
Chem. Int. Ed. 2007, 46, 909–911.
[16] In the reaction with substrates, which have a small substituent (R=
Bn) and a small ligand (P=PMe2Ph), equilibrium of 11, 12, and 13
may exist. It is likely that the 1,2-H shift to the electron-deficient
carbene carbon has hydride-shift character and the OBn substituent
in 13 promotes the 1,2-H shift owing to the cation-stabilizing effect
of the substituent.
hydroxybicycloACHTUNGTRENNUNG[5.3.0]undecane derivatives as a key intermediate:
P. A. Wender, C. D. Jesudason, H. Nakahira, N. Tamura, A. L.
Tebbe, Y. Ueno, J. Am. Chem. Soc. 1997, 119, 12976–12977.
[13] In all entries, small amounts of the 1,2-H shift products were ob-
tained. The ratios of the 1,2-alkyl and 1,2-H shift products in the
crude reaction mixtures were as follows (determined by 1H NMR
spectroscopy): 89:11 (entry 1), 78:22 (entry 2), 95:5 (entries 3, 4, and
5), 87:13 (entry 6), 85:15 (entry 7). As it was difficult to separate the
alkyl-migration products as enol ether derivatives and 1,2-H shift
products, the products were isolated as ketones by hydrolyzing the
enol ether moiety.
[14] We examined the reactions with various phosphine ligands. The
trend in product selectivity is correlated mainly with the steric
Received: May 16, 2011
Published online: July 22, 2011
Chem. Asian J. 2011, 6, 2273 – 2277
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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