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-
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
Chem. Asian J. 2011, 6, 2273 – 2277
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