Synthesis of cycloalkanone-fused cyclopropanes by Au(I)-catalyzed oxidative ene-yne cyclizations

Article abstract of DOI:10.1016/j.tetlet.2014.10.084

Au(I)-catalyzed oxidative cyclizations that successfully convert 1,5-ene-ynes and a 1,6-ene-yne to the corresponding cycloalkanone-fused cyclopropanes are described. This Au(I)-catalyzed oxidative cyclization can be effectively applied to various substrat

Full text of DOI:10.1016/j.tetlet.2014.10.084

Tetrahedron Letters 55 (2014) 6847–6850
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of cycloalkanone-fused cyclopropanes by Au(I)-catalyzed
oxidative ene-yne cyclizations
⇑
Yuta Uetake, Takashi Niwa, Masahisa Nakada
Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Au(I)-catalyzed oxidative cyclizations that successfully convert 1,5-ene-ynes and a 1,6-ene-yne to the
corresponding cycloalkanone-fused cyclopropanes are described. This Au(I)-catalyzed oxidative cycliza-
tion can be effectively applied to various substrates and is an alternative to the previously used intramo-
lecular cyclopropanation that required potentially explosive diazo compound.
Received 23 August 2014
Revised 4 October 2014
Accepted 14 October 2014
Available online 19 October 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Cycloalkanone
Cyclopropane
Ene-yne cyclization
Oxidative cyclization
Au(I)-catalysis
R3
Cyclopropane shows high reactivity toward various reagents
owing to ring strain and undergoes ring-opening reactions to form
new bonds and stereogenic centers. Hence, cyclopropanes have
R3
CuOTf
R2
R1
SO2Mes
R2
R1
O
O
(10 mol%)
O
O
n
n
N
N
A or B
SO2Mes
N2
(15 mol%)
been utilized as effective synthetic intermediates.
A general
O
O
n= 1, 2
91-98% ee
A
method for cyclopropane preparation is the reaction of an alkene
with a carbene or carbenoid, and the use of a diazo compound in
the presence of a transition metal catalyst is an alternative reliable
method for cyclopropane preparation and has the advantage of
producing a chiral cyclopropane when a chiral catalyst is used.
We have developed a catalytic asymmetric intramolecular cyclo-
propanation (Scheme 1) and successfully utilized it for the total
syntheses of bioactive natural products.1
Bn Bn
CuOTf
(10 mol%)
n
H
H
O
n
O
N
N
A or B
N2
MesO2S
(15 mol%)
96-97% ee
MesO2S
n= 1, 2
B
Scheme 1.
Transition metal-catalyzed ene-yne cyclizations provide an effi-
cient approach to complex carbon frameworks starting from rela-
tively simple substrates.2 The cyclopropanation by transition
metal-catalyzed ene-yne cyclization is superior to the alkene–car-
bene method in terms of atom economy and safety, because the
latter proceeds via a carbene complex generated in situ from a
potentially explosive diazo compound, which requires preparation
and purification prior to use. Herein, we report Au(I)-catalyzed oxi-
dative ene-yne cyclizations that afford cycloalkanone-fused
cyclopropanes.
N
O
IPrAuCl/AgNTf2 (5 mol %)
ClH2CCH2Cl
O
Scheme 2.
reaction because cyclopropanes prepared by intramolecular cyclo-
propanation could be formed by the Au(I)-catalyzed reaction of the
corresponding 1,5-ene-ynes.
For example, cyclopropane 2 has typically been synthesized by
the Cu(I)-catalyzed intramolecular cyclopropanation of diazo com-
pound 1 (Scheme 3), however, it is expected that 2 could also be
prepared by Au(I)-catalyzed oxidative cyclization of 1,5-ene-yne
3. Hence, we began with the synthesis of substrates to examine
the Au(I)-catalyzed oxidative cyclization.
In 2011, Liu et al. reported the Au(I)-catalyzed oxidative ene-
yne cyclization of o-ethynylstyrene to afford cyclopropane-fused
benzocyclopentanone (Scheme 2).3,4 We were interested in this
⇑
Corresponding author. Tel./fax: +81 3 5286 3240.
E-mail address: mnakada@waseda.jp (M. Nakada).
http://dx.doi.org/10.1016/j.tetlet.2014.10.084
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

6848
Y. Uetake et al. / Tetrahedron Letters 55 (2014) 6847–6850
1) LDA, TMSC≡ CCH2Br
1) Li, t-BuOH, THF
CO2H
CO2Me
—78 to 0 °C, 86%
NH3, —78 °C
O
N2
cyclopropanation
2) LiAlH4, Et2O, 0 °C, 82%
2) H2SO4, MeOH
benzene, reflux
99% (2 steps)
Z
1
3
4
O
H
H
1) K2CO3, MeOH, rt
88%
Z
TMS
HO
TBDPSO
oxidative
2
ene-yne cyclization
2) TBDPSCl, imdazole
CH2Cl2, rt, 99%
Z
5
6
Scheme 3.
Scheme 4. Synthesis of alkyne.
Table 1
Au(I)-catalyzed oxidative cyclization of 1,5-ene-yne 6
TBDPSO
H
TBDPSO
CHO
Ph3PAuCl (10 mol %)
AgNTf2 (10 mol %)
TBDPSO
TBDPSO
O
H
H
oxidant (3.0 equiv)
H
H
6
7
8
9
Entry
Catalyst
Oxidant
Solvent
Temp (°C)
Time (h)
Yield (%)a
7
8
9
1
2
3
4
Ph3PAuCl + AgNTf2
Ph3PAuCl + AgNTf2
Ph3PAuCl + AgNTf2
Ph3PAuCl + AgNTf2
Ph3PAuCl + AgNTf2
Ph3PAuCl + AgNTf2
Ph3PAuCl + AgNTf2
Ph3PAuCl + AgNTf2
Ph3PAuCl + AgNTf2
Ph3PAuCl + AgNTf2
Ph3PAuCl + AgOTf
Ph3PAuCl + AgPF6
Ph3PAuCl + AgBF4
Ph3PAuCl + AgSbF6
NMO
PhIO
I
I
I
II
III
None
IV
V
I
I
I
I
I
I
I
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
THF
50
rt
50
rt
rt
50
rt
rt
rt
rt
50
50
50
50
50
50
50
50
48
24
19
48
36
96
12
12
24
24
19
19
19
19
19
19
19
19
NRb
Decomposition
0
Complex mixture
Complex mixture
10
0
Complex mixture
0
NR
0
0
0
0
0
0
47
0
5
CH3NO2
6c
7
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
11
19
0
62
8
9
Trace
0
10
11
12
13
14
15
16
17
18
45
34
33
56
62
48
45
59
0
0
0
0
0
0
0
0
(4-MeOC6H4)3PAuCl + AgSbF6
cHexJohnPhosAuCl + AgSbF6
XPhosAuCl + AgSbF6
0
0
IPrAuCl + AgSbF6
I
a
b
c
Isolated yields.
NR: no reaction.
76% of starting material was recovered.
The Au(I)-catalyzed cyclization of 1,5-ene-yne 6 was examined
in the presence of a catalytic amount of readily available Ph3PAuCl
and AgNTf2 with a variety of oxidants (Table 1). The use of com-
mercially available NMO (entry 1) or PhIO (entry 2) resulted in
no reaction or the decomposition of 6, respectively. The reaction
of 6 with 8-methylquinoline N-oxide (I) did not proceed at room
temperature but afforded the desired compound 7 in 47% at
50 °C (entry 3), so that 1,2-dichloroethane was used as solvent.
The yield was not improved by the use of an excess amount of I
(5.0 equiv) and/or in the presence of MS 4 Å.
NMe2
OMe
N
O
Br
N
O
Br
N
O
N
O
N
O
I
II
III
IV
V
Cy
Cy
Cy
Cy
Cy
Cy
P
P
P
iPr
O
N
N
iPr
iPr
O
The reaction using I in THF (entry 4) or nitromethane (entry 5)
resulted in the formation of a complex mixture including a trace
amount of 7. Based on the results in entry 3, a bulky N-oxide,
2,6-lutidine N-oxide II,5 was used as the oxidant (entry 6), but
the reaction proceeded sluggishly and required heating to afford
7 in 11% yield, along with by-product 8 in 10% yield, and predom-
inantly unreacted 6 (76%). The formation of 8 could be explained
by attack of the terminal carbon by oxidant II. The reaction using
a hindered N-oxide, 2,6-dibromopyridine N-oxide III,6 afforded
compound 7 in 19% yield (entry 7). Interestingly, compound 9
cHexJohnPhos
XPhos
SPhos
IPr
First, 1,5-ene-yne 6 possessing a terminal alkyne, was selected as a
substrate for the Au(I)-catalyzed reaction. The synthesis of 6 was
commenced with the Birch reduction of benzoic acid, followed
by esterification to afford compound 4 (Scheme 4); subsequent
alkylation and reduction with LiAlH4 afforded alcohol 5. The TMS
group of alcohol 5 was removed, followed by the formation of
the TBDPS ether to afford compound 6.

Y. Uetake et al. / Tetrahedron Letters 55 (2014) 6847–6850
6849
Table 2
ethoxycarbonylation. The Au(I)-catalyzed reaction of 10a under
the similar conditions as those in entry 14 of Table 1 afforded
11a in 48% yield (entry 1). The reaction proceeded regioselectively;
that is, without affording products derived from the regioisomeric
addition of I, such as compound 8 observed in entry 6 of Table 1.
This selectivity would be attributed to the inherent nature of yno-
ate toward the Au(I)-catalyzed reaction.8 The reaction did not
require heating, indicating that the reactivity of 10a is better than
6 in this reaction. The use of 4 Å improved the yield to 53%; hence,
the reactions in entries 3 and 4 were carried out in the presence of
MS 4 Å. The use of cHexJohnPhos (entry 4) and IPr (entry 6)
improved the yield to 63% and 55%, respectively, but (4-MeOC6-
H4)3P (entry 3) and XPhos (entry 5) gave almost the same results
as Ph3P (entry 1).
Au(I)-catalyzed oxidative cyclization of 1,5-ene-yne 10a
TBDPSO
H
TBDPSO
catalyst (10 mol %), MS 4 Å
CH2Cl2, rt
O
I
(3.0 equiv)
H
CO2Et
N
O
CO2Et
10a
11a
Entry
Catalyst
Time (h)
Yield (%)a
1
2
3
4
5
6
Ph3PAuCl + AgSbF6
Ph3PAuCl + AgSbF6
(4-MeOC6H4)3PAuCl + AgSbF6
cHexJohnPhosAuCl + AgSbF6
XPhosAuCl + AgSbF6
12
12
24
12
48
12
48b
53
51
63
50
55
IPrAuCl + AgSbF6
The Au(I)-catalyzed reaction of 1,5-ene-ynes 10b–g (Table 3)
possessing different hydroxyl and carboxyl protecting groups was
examined. The ester moiety (R5) of 10a was changed from ethyl
to isobutyl (entry 2), isopropyl (entry 3), and tert-butyl (entry 4),
and the reaction of tert-butyl ester 10d afforded 11d in 69% yield.
a
Isolated yields.
MS 4 Å was not used.
b
was formed in 62% yield though the reason is not clear at
this stage. Compound was thought to be formed via
Interestingly, 13% of c-lactone 12 was formed, suggesting that the
reaction proceeded via cationic intermediates.9 The protecting
group of the hydroxyl (R4) was changed from TBDPS to MOM
(entry 5) and Bn (entry 6) to examine the effect of the alkyl ether
oxygen on the Au(I)-catalyzed reaction; however, both protecting
groups did not affect the results. Finally, the reaction of a deoxy-
genated derivative 10g (R4 = H, entry 7) was examined and 11g
was obtained in 75% yield.
9
a
cycloisomerization pathway that proceeds without oxidant III.
However, the reaction in the absence of the oxidant resulted in
the formation of a complex mixture (entry 8). The reactions with
more nucleophilic oxidants, 4-methoxypyridine N-oxide IV7 (entry
9), or DMAP N-oxide V7 (entry 10), resulted in the formation of a
trace amount of 7 or no reaction.
Oxidant I was found to be most effective for the Au(I)-catalyzed
oxidative cyclization of 6 (entry 3). This could be attributed to the
suitably hindered structure of I. That is, the reaction with oxidant I
proceeds and the corresponding by-product 8-methylquinoline
does not inhibit the reaction because the coordination of 8-meth-
ylquinoline to Au(I) would be suppressed by the 8-methyl group.
The effect of silver salt (AgNTf2, AgOTf, AgPF6, and AgSbF6) on
the Au(I)-catalyzed reaction was surveyed (entries 11–14), and
the reaction with AgSbF6 was found to afford the best yield
(56%). The ligand of Au(I) was also examined (entries 15–18),
and the use of (4-MeOC6H4)3P (entry 15) and IPr (entry 18)
improved the yield to 62% and 59%, respectively, while cHexJohn-
Phos (entry 16) and XPhos (entry 17) were found to be less effec-
tive than Ph3P (entry 14).
To examine the applicability of the Au(I)-catalyzed cyclization,
the reaction was applied to other substrates (Scheme 5). Cycliza-
tion of 1,6-ene-yne 13, which is a one-carbon homologated deriv-
ative of 10d, was carried out under the conditions optimized in
Table 3, and the desired product 14 was formed in 38% yield. Fur-
thermore, acyclic 1,5-ene-ynes 15 and 18 were converted under
the same conditions to the desired cyclopropanes 16 and 19,
respectively. The yields of 14, 16, and 19 were not so high, how-
ever, further optimization for each substrate may improve the
yield. The reaction of 15 also afforded by-product
as previously observed for 10d (cf. Table 3, entry 4). Interestingly,
the formation of -lactone 20 was not observed in the reaction of
c-lactone 17,
c
18. This may be attributed to the stability of the cationic
intermediate.
To the best of our knowledge, the Au(I)-catalyzed reaction of
1,5-ene-ynes possessing an ester group at the terminal alkyne car-
bon has never been reported, so the reaction of 10a was examined
(Table 2). Compound 10a was prepared from 6 by lithiation and
In summary, we have developed Au(I)-catalyzed oxidative
cyclizations that successfully convert 1,5-ene-ynes and a 1,6-ene-
yne to the corresponding cycloalkanone-fused cyclopropanes in
moderate to good yields. We have shown that this Au(I)-catalyzed
Table 3
Au(I)-catalyzed oxidative cyclization of 1,5-ene-ynes 10a–g
R4
H
R4
cHexJohnPhosAuCl (10 mol %)
AgSbF6 (10 mol %)
O
O
H
MS 4Å, CH2Cl2, rt, 12 h
TBDPSO
O
O
H
CO2R5
CO2R5
I (3.0 equiv)
N
10a-g
O
11a-g
12
Entry
Substrate
R4
R5
Yield (%)a
1
2
3
4
5
6
7
10a
10b
10c
10d
10e
10f
OTBDPS
Et
63 (11a)
57 (11b)
62 (11c)
69 (11d)b
72 (11e)
70 (11f)
75 (11g)
OTBDPS
OTBDPS
OTBDPS
OMOM
OBn
iBu
iPr
tBu
tBu
tBu
tBu
10g
H
a
Isolated yields.
13% of 12 was formed.
b

6850
Y. Uetake et al. / Tetrahedron Letters 55 (2014) 6847–6850
TBDPSO
2860–2861; (b) Honma, M.; Nakada, M. Tetrahedron Lett. 2003, 44, 9007–9011;
TBDPSO
cHexJohnPhosAuCl (10 mol %)
AgSbF6 (10 mol %)
(c) Takano, M.; Umino, A.; Nakada, M. Org. Lett. 2004, 6, 4897–4900; (d) Sawada,
T.; Nakada, M. Adv. Synth. Catal. 2005, 347, 1527–1532; (e) Miyamoto, H.;
Iwamoto, M.; Nakada, M. Heterocycles 2005, 66, 61–68; (f) Takeda, H.; Watanabe,
H.; Nakada, M. Tetrahedron 2006, 62, 8054–8063; (g) Abe, M.; Nakada, M.
Tetrahedron Lett. 2006, 47, 6347–6351; (h) Takeda, H.; Nakada, M. Tetrahedron:
Asymmetry 2006, 17, 2896–2906; (i) Watanabe, H.; Takano, M.; Umino, A.; Ito,
T.; Ishikawa, H.; Nakada, M. Org. Lett. 2007, 9, 359–362; (j) Honma, M.; Nakada,
M. Tetrahedron Lett. 2007, 48, 1541–1544; (k) Takeda, H.; Honma, M.; Ida, R.;
Sawada, T.; Nakada, M. Synlett 2007, 579–582; (l) Ida, R.; Nakada, M. Tetrahedron
Lett. 2007, 48, 4855–4859; (m) Abe, M.; Nakada, M. Tetrahedron Lett. 2007, 48,
4873–4877; (n) Watanabe, H.; Nakada, M. Tetrahedron Lett. 2008, 49, 1518–
1522; (o) Watanabe, H.; Nakada, M. J. Am. Chem. Soc. 2008, 130, 1150–1151; (p)
Watanabe, H.; Takano, M.; Nakada, M. J. Synth. Org. Chem. 2008, 66, 1116–1125;
(q) Honma, M.; Takeda, H.; Takano, M.; Nakada, M. Synlett 2009, 1695–1712; (r)
Abe, M.; Saito, A.; Nakada, M. Tetrahedron Lett. 2010, 51, 1298–1302; (s) Hirai, S.;
Nakada, M. Tetrahedron Lett. 2010, 51, 5076–5079; (t) Hirai, S.; Nakada, M.
Tetrahedron 2011, 67, 518–530; (u) Sawada, T.; Nakada, M. Tetrahedron:
Asymmetry 2012, 23, 350–356; (v) Uwamori, M.; Saito, A.; Nakada, M. J. Org.
Chem. 2012, 77, 5098–5107; (w) Sawada, T.; Nakada, M. Org. Lett. 2013, 15,
1004–1007; (x) Uwamori, M.; Nakada, M. Nat. Prod. Commun. 2013, 8, 955–959;
(y) Nakada, M. Chem. Rec. 2014, 14, 641–662.
MS 4Å, CH2Cl2, rt, 6 h
O
H
I (3.0 equiv)
H
CO2tBu
N
O
CO2tBu
13
14
38%
cHexJohnPhosAuCl (10 mol %)
AgSbF6 (10 mol %)
MS 4Å, CH2Cl2, rt, 2.5 h
O
O
H
CO2tBu
CO2tBu
O
O
I (3.0 equiv)
H
H
N
O
15
16
17
17%
24%
cHexJohnPhosAuCl (10 mol %)
AgSbF6 (10 mol %)
O
O
CO2tBu
H
CO2tBu
MS 4Å, CH2Cl2, rt, 4 h
O
O
Ph
20
I (3.0 equiv)
Ph
Ph
N
O
18
19
52%
n.d.
2. Selected reviews: (a) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed.
2006, 45, 7896–7936; (b) Fürstner, A.; Davies, P. W. Angew. Chem., Int. Ed. 2007,
46, 3410–3449; (c) Gorin, D. J.; Sherry, B. D.; Toste, F. D. Chem. Rev. 2008, 108,
3351–3378; (d) Patil, N. T.; Yamamoto, Y. Chem. Rev. 2008, 108, 3395–3442; (e)
Abu Sohel, S. M.; Liu, R.-S. Chem. Soc. Rev. 2009, 38, 2269–2281.
Scheme 5. Au(I)-catalyzed oxidative cyclization of 1,6-ene-yne 13, acyclic 1,5-ene-
ynes 15, and 18.
3. Vasu, D.; Hung, H.-H.; Bhunia, S.; Gawade, S. A.; Das, A.; Liu, R.-S. Angew. Chem.,
Int. Ed. 2011, 50, 6911–6914.
oxidative cyclization can be applied to various substrates and
can be an alternative to the previously used intramolecular
cyclopropanation (IMCP) that required potentially explosive diazo
compound. Further optimization studies, as well as investigations
into the asymmetric catalysis of the Au(I)-catalyzed oxidative
cyclization, are now underway.
4. Au(I)-catalyzed reaction of alkynes using pyridine N-oxide: (a) Ye, L.; Cui, L.;
Zhang, G.; Zhang, L. J. Am. Chem. Soc. 2010, 132, 3258–3259; (b) Lu, B.; Li, C.;
Zhang, L. J. Am. Chem. Soc. 2010, 132, 14070–14072; (c) He, W.; Li, C.; Zhang, L. J.
Am. Chem. Soc. 2011, 133, 8482–8485; (d) Ye, L.; He, W.; Zhang, L. Angew. Chem.,
Int. Ed. 2011, 50, 3236–3239; (e) Luo, Y.; Ji, K.; Li, Y.; Zhang, L. J. Am. Chem. Soc.
2012, 134, 17412–17415; (f) Wang, Y.; Ji, K.; Lan, S.; Zhang, L. Angew. Chem., Int.
Ed. 2012, 51, 1915–1918; (g) Bhunia, S.; Ghorpade, S.; Huple, D. B.; Liu, R.-S.
Angew. Chem., Int. Ed. 2012, 51, 2939–2942; (h) Dateer, R. B.; Pati, K.; Liu, R.-S.
Chem. Commun. 2012, 48, 7200–7202; (i) He, W.; Xie, L.; Xu, Y.; Xiang, J.; Zhang,
L. Org. Biomol. Chem. 2012, 10, 3168–3171; (j) Ghorpade, S.; Su, M.-D.; Liu, R.-S.
Angew. Chem., Int. Ed. 2013, 52, 4229–4234; (k) Henrion, G.; Chavas, T. E. J.; Goff,
X. L.; Gagosz, F. Angew. Chem., Int. Ed. 2013, 52, 6277–6282; (l) Ji, K.; Zhao, Y.;
Zhang, L. Angew. Chem., Int. Ed. 2013, 52, 6508–6512; (m) Shi, S.; Wang, T.; Yang,
W.; Rudolph, M.; Hashmi, A. S. K. Chem. Eur. J. 2013, 19, 6576–6580; (n) Pawar, S.
K.; Wang, S. K.; Bbunia, S.; Jadhav, A. M.; Jadhav, R.-S. Angew. Chem., Int. Ed. 2013,
52, 7559–7563; (o) Huple, D. B.; Ghorpade, S.; Liu, R.-S. Chem. Eur. J. 2013, 19,
12965–12969; (p) Nösel, P.; Dos Santos Compride, L. N.; Lauterbach, T.; Rudolph,
M.; Rominger, F.; Hashmi, A. S. K. J. Am. Chem. Soc. 2013, 135, 15662–15666; (q)
Kawade, R. K.; Liu, R.-S. Org. Lett. 2013, 15, 4094–4097.
Acknowledgments
This work was financially supported in part by the Grant-in-Aid
for Scientific Research on Innovative Areas (2105) and for Scientific
Research (B) (25293003) by MEXT, and a Waseda University Grant
for Special Research Projects.
Supplementary data
5. Koktla, H. P.; Thomson, P. F.; Bae, S.; Doddi, V. R.; Lakshman, M. K. J. Org. Chem.
2011, 76, 7842–7848.
6. Chavas, T. E. J.; Goff, X. L.; Gagosz, F. Angew. Chem., Int. Ed. 2013, 52, 6277–6282.
7. Commercially available.
8. The same regioselectivity can be found in the Au(I)-catalyzed hydroarylation of
enoates: Reetz, M. T.; Sommer, K. Eur. J. Org. Chem. 2003, 3485–3496.
9. A related reaction: Newhouse, T. R.; Kaib, P. S. J.; Gross, A. W.; Corey, E. J. Org.
Lett. 2013, 15, 1591–1593.
Supplementary data (full characterization of new compounds)
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2014.10.084.
References and notes
1. (a) Honma, M.; Sawada, T.; Fujisawa, Y.; Utsugi, M.; Watanabe, H.; Umino, A.;
Matsumura, T.; Hagihara, T.; Takano, M.; Nakada, M. J. Am. Chem. Soc. 2003, 125,