Synthesis of Cyclobutene-Fused Eight-Membered Carbocycles through Gold-Catalyzed Intramolecular Enyne [2+2] Cycloaddition

Article abstract of DOI:10.1002/adsc.201701193

Cationic gold(I) complexes with hollow-shaped triethynylphosphine ligands efficiently catalyzed intramolecular [2+2] cycloaddition of 1,9-enynes to afford cyclobutene-fused eight-membered carbocycles that were difficult to synthesize by other catalytic systems. Various 1,9-enynes with carbon linkers with or without a fused ring underwent efficient [2+2] cycloaddition with 5 mol% of the Au catalyst bearing the triarylmethyl-end-capped triethynylphosphine in CH₂Cl₂ at rt in the presence of MS 4A as an additive. More challenging 1,9-enyne substrates with fully saturated acyclic carbon linkers underwent eight-membered ring formation at 60 °C in ClCH₂CH₂Cl in the absence of MS 4A, forming monocyclic 1,3-dienes as major products. (Figure presented.).

Full text of DOI:10.1002/adsc.201701193

Title: Synthesis of Cyclobutene-Fused Eight-Membered Carbocycles
through Gold-Catalyzed Intramolecular Enyne [2+2]
Cycloaddition
Authors: Tomohiro Iwai, Masahiro Ueno, Hiori Okochi, and Masaya
Sawamura
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201701193
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10.1002/adsc.201701193
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Synthesis of Cyclobutene-Fused Eight-Membered Carbocycles
through Gold-Catalyzed Intramolecular Enyne [2+2]
Cycloaddition
Tomohiro Iwai,a Masahiro Ueno,a Hiori Okochi,a and Masaya Sawamuraa,*
a
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
Fax: (+ 81)-11-706-3749; phone: (+ 81)-11-706-3434; e-mail: sawamura@sci.hokudai.ac.jp
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
producing cyclobutene-fused eight-membered ring
compounds from 1,9-enynes have not been reported yet
(Scheme 1, middle),[9,10] even though such compounds
hold potential as synthetic platforms for accessing
structurally intriguing natural compounds containing
bicyclo[6.2.0]decane frameworks such as Tripartilactam,
Abstract. Cationic gold(I) complexes with hollow-
shaped triethynylphosphine ligands efficiently catalyzed
intramolecular [2+2] cycloaddition of 1,9-enynes to
afford cyclobutene-fused eight-membered carbocycles
that were difficult to synthesize by other catalytic
systems. Various 1,9-enynes with carbon linkers with or
Rumphellaoic acid A, and Rumphellolide F (Figure 1).[11–
without
a
fused ring underwent efficient [2+2]
13]
cycloaddition with 5 mol% of the Au catalyst bearing the
triarylmethyl-end-capped triethynylphosphine in CH2Cl2
at rt in the presence of MS 4A as an additive. More
challenging 1,9-enyne substrates with fully saturated
acyclic carbon linkers underwent eight-membered ring
formation at 60 °C in ClCH2CH2Cl in the absence of MS
4A, forming monocyclic 1,3-dienes as major products.
Keywords: gold; cycloisomerization; enyne; medium-
ring compounds; cyclobutene
The gold-catalyzed cycloisomerization of 1,n-enynes has
been studied broadly and become a straightforward and
powerful strategy for synthesizing complicated cyclic
compounds.[1,2] Among them, cyclobutene-fused ring
compounds, resulting from formal intramolecular [2+2]
cycloaddition between the alkyne and the alkene moieties
of the 1,n-enynes, are intriguing targets due to their high
degrees of ring-strain and the potential for further synthetic
elaboration.[3,4] However, construction of such systems
with medium-ring parent frameworks is highly challenging
due to high entropy costs in the cyclization and/or
transannular interactions within the products.[5] Gagosz and
a co-worker reported the synthesis of cyclobutene-fused
seven-membered carbocycles, which are categorized as
normal rings and hence are much easier to form than
medium-sized rings, through the gold-catalyzed
cycloisomerization of 1,8-enynes (Scheme 1, top).[6] Later,
Inagaki and co-workers reported similar gold catalysis with
a boron-containing “Z-type ligand.”[7] Echavarren and co-
workers reported the macrocyclization of 1,n-enynes (n =
10–16) to provide nine-to-fifteen membered cyclic ethers
annulated with a cyclobutene ring (Scheme 1, bottom).[8]
Despite these achievements, catalytic systems capable of
Scheme 1. Gold-catalyzed cycloisomerization of 1,n-enynes for
construction of cyclobutene-fused ring systems.
Figure 1. Natural products containing a cyclobutane-fused eight-
membered carbocyclic scaffold.
Previously, we showed that cationic gold complexes
coordinated with hollow-shaped triethynylphosphine
ligands such as L1 and L2 served as powerful catalysts for
cyclization reactions of various alkynes tethered to
different types of nucleophiles. Even eight-membered ring
formation was possible in the reaction of terminal alkynes
tethered to enol silyl ethers (Figure 2).[14] We proposed that
the confined space created by the gold-bound hollow-
shaped ligand biased the alkyne substrates toward the
1
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10.1002/adsc.201701193
Advanced Synthesis & Catalysis
otherwise difficult cyclization reactions. We then decided
to undertake a study to evaluate our gold catalysts for
Table 1. Cycloisomerization of 1,8-Enyne 1.[a]
applicability
toward
broadly
studied
enyne
cycloisomerization reactions, focusing on the [2+2]
cycloaddition as a straightforward means to synthesize 8-4-
fused medium ring compounds.
Yield [%][b]
(2:3:4)
Entry
Cat. [Au]
Solvent
t [h]
CH2Cl2
(0.02 M)
1[c]
2
[Au(NTf2)(L1)]
[Au(NTf2)(L2)]
0.25 70 (40:45:15)
0.25 91 (73:23:4)
CH2Cl2
(0.02 M)
ClCH2CH2Cl
(0.1 M)
3[d]
[Au(NTf2)(XPhos)]
24
24
48[e]
73[e]
ClCH2CH2Cl
(0.01 M)
4[d] [Au(SbF6)(DPB)]2(cod)
Figure 2. Hollow-shaped triethynylphosphines.
[a] Conditions: 1 (0.1 mmol), [Au] (Au: 2 mol%), rt.
[b] Isolated yields. Ratio of 2, 3 and 4 shown in parentheses.
[c] 88% conv. of 1.
We began our investigation by testing the
triethynylphosphine ligands for performance in the gold-
[d] Data taken from ref 7.
catalyzed
seven-membered-ring-forming
[e] Yields of 2.
cycloisomerization of 1,8-enyne 1, for which XPhos or Z-
type ligand DPB were used by Gagosz’s and Inagaki’s
groups, respectively.[6,7] We confirmed that the catalysts
with L1 and L2, having triarylsilyl and triarylmethyl end
caps, respectively, caused much more rapid reactions of 1
(2 mol% Au, rt in 0.02 M of CH2Cl2) than the previously
reported systems [Table 1, 0.25 h (entries 1 and 2) vs. 24 h
Table 2. Gold-Catalyzed Cycloisomerization of 1,9-Enyne 5a.[a]
(entries
3
and 4)], giving 7-exo-dig-cyclization-
isomerization products 2–4 in good-to-high yields.[15-17]
Encouraged by these results, we next examined the
cationic gold complexes [Au(NTf2)(L)] (L = L1 or L2) for
catalytic activity toward eight-membered-ring-forming
cycloisomerization of a 1,9-enyne (5a, 0.1 mmol, 0.02 M
in CH2Cl2) having an o-phenylene linker. With 5 mol%
Yield [%][b]
Entry Ligand
Additive Conv. [%][b]
6a
42
7a 8a
1
2
–
67
6
8
0
0
0
0
0
2
0
0
0
13
0
L1
L2
catalyst loading at rt over
2
h, the expected
–
>99
19[c]
28[c]
62[c,d]
24[c]
38
61
cycloisomerization occurred with both ligands (Table 2,
entries 1 and 2). Better substrate conversion (>99%) and
yield (61%) for cyclobutene-fused benzocyclooctane 6a
were observed with the triarylmethyl-end-capped
triethynylphosphine L2 than with the triarylsilyl-end-
capped phosphine L1 (67% conv., 42% yield). The [2+2]
cycloaddition affording 6a was accompanied by the minor
formation of benzocyclooctenes, 7a and/or 8a, both
lacking a fused four-membered ring.[16] Conventional
ligands such as Ph3P, (PhO)3P, XPhos and IPr gave no
eight-membered ring products while partial substrate
consumption was observed (entries 3–6).[18]
Next, we tried to improve the chemoselectivity toward
6a by the effect of additives to the Au-L2 catalyst system.
While organic bases (10 mol%) such as iPr2EtN and 2,6-
lutidine completely inhibited the cycloisomerization, the
use of inorganic bases (100 mg) such as K2CO3 and
Na2CO3 had a significant effect in suppressing the side
reactions (Table 2, entry 2 vs. entries 7 and 8). The most
suitable additive found in our investigation was MS 4A,
which allowed high and selective conversion of 5a to 6a
(entry 9). Reducing the loading of MS 4A (10 mg) led to a
quantitative yield of 6a (entry 10).
3
PPh3
P(OPh)3
XPhos
IPr
–
0
4
–
0
0
5
–
–
0
0
6
0
0
[e]
7
K2CO3
37
1
L2
[e]
8
Na2CO3
92
85
4
L2
9
MS 4A[e]
MS 4A[f]
98
95 (86)
>99
0
L2
10
[a]
>99
0
L2
Conditions: 5a (0.1 mmol), [Au(NTf2)(L)] (5 mol%), CH2Cl2
(5 mL), rt, 2 h.
[b] Determined by 1H NMR spectroscopy. Isolated yield shown in
parentheses.
Trace amounts of unidentified compounds were detected in H
NMR spectroscopy of the crude product.
[d] The alkyne hydration product (38%, NMR) was formed.
[e] Using 100 mg.
[c]
1
[f] Using 10 mg.
To determine the route for formation of the ring-opened
products and the role of MS 4A, the reactivity of 6a was
investigated (Table 3). Exposing 6a to a catalytic amount
of [Au(NTf2)(L2)] at rt induced no reaction (entry 1).
HNTf2 (5 mol%), which may be formed by hydrolysis of
the cationic gold complex,[19] induced the cyclobutene-
ring-opening, giving 7a and 8a in 78% and 21% yields,
2
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10.1002/adsc.201701193
Advanced Synthesis & Catalysis
Table 4. Scope of 1,9-Enynes.[a]
respectively (entry 2).[20] The HNTf2-induced ring-opening
was significantly suppressed by the addition of MS 4A to
the reaction system (entry 3). Thus, the formation of 7a
and 8a was likely caused by Brønsted-acid-catalyzed post
cyclobutene-ring-opening, and MS 4A may work as a
dehydrating agent or a scavenger of the Brønsted acid
formed in the reaction system to inhibit the ring-
opening.[21]
Table 3. Cyclobutene-ring-opening of 6a.[a]
Yield [%][b]
Change from
standard condition
Conv.
[%][b]
Entry
t [h]
7a 8a
1
2
3
[Au(NTf2)(L2)] (5 mol%)
HNTf2 (5 mol%)
15
1
0
0
0
>99 78 21
HNTf2 (5 mol%) + MS 4A (20 mg)
2
2
0
0
[a] Conditions: 6a (0.02 mmol), CD2Cl2 (1 mL), PhCH2CH2Ph
(internal standard), rt.
[b] Determined by 1H NMR spectroscopy.
With [Au(NTf2)(L2)] as a catalyst and MS 4A as an
additive, the scope of 1,9-enynes was explored (Table 4).
The reaction of 5b having a dimethyl malonate unit at the
C6-position gave cyclobutene-fused eight-membered
carbocycle 6b and 1,3-diene 7b in a 93:7 ratio in 87%
combined yield (entry 1). The 1,9-enynes featuring
geminal disubstitution with acetoxymethyl (5c) or
methoxymethyl (5d) groups at the homoallylic carbon
were efficiently converted to 6c and 6d, respectively
(entries 2 and 3). The electron-donating methylenedioxy
group (5e) and the electron-withdrawing fluoro substituent
(5f) were tolerated in the o-phenylene linker (entries 4 and
5). The molecular structure of 6e was unambiguously
confirmed by single-crystal X-ray diffraction (Figure 3).[22]
1,9-Enynes having a cyclopentene (5g) or cyclohexene
(5h) linker instead of the o-phenylene linker in 5a were
also suitable substrates, giving aliphatic tricyclic
compounds 6g and 6h, respectively (entries 6 and 7). 1,9-
Enyne 5i with an acyclic linker gave the cycloaddition
product 6i in high yield and high selectivity (entry 8). The
cyclohexylidene-type substrate 5j underwent efficient and
selective [2+2] cycloaddition, furnishing tetracyclic spiro
compound 6j in 90% yield (entry 9).
[a] Conditions: 5 (0.1 mmol), [Au(NTf2)(L2)] (5 mol%), MS 4A
(100 mg, not optimized), CH2Cl2 (5 mL), rt, 2–6 h.
[b] Isolated yield. Product ratio (6:7:8) shown in parentheses.
[c] Isolated 6b contains small amounts of structurally unidentified
compounds.
[d] Using 10 mg of MS 4A.
[e] 1H NMR yield of 6e in the crude product. The purified
material of 6e by silica gel column chromatography contaminated
with inseparable compounds. Further purification by
recrystallization gave analytically pure 6e in 45% yield.
[f] Using 5 mg of MS 4A.
Figure 3. ORTEP drawing of 6e.
3
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10.1002/adsc.201701193
Advanced Synthesis & Catalysis
The Au-L2 catalyst allowed cycloisomerization of even
Dorel, A. M. Echavarren, Chem. Rev. 2015, 115, 9028–
9072.
more challenging 1,9-enyne substrates (5k and 5l) with
fully saturated acyclic carbon linkers in 1,2-dichloroethane
at 60 °C in the absence of MS 4A,[23] giving conjugated
diene isomers 7k and 7l, respectively, as major products
(eq 1).[24]
[2] Selected reviews on catalytic cycloisomerization of 1,n-
eynes with other metals: a) B. M. Trost, M. J. Krische,
Synlett 1998, 1998, 1–16; b) B. M. Trost, F. D. Toste, A. B.
Pinkerton, Chem. Rev. 2001, 101, 2067–2096; c) C. Aubert,
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d) V. Michelet, P. Y. Toullec, J.-P. Genêt, Angew. Chem.
2008, 120, 4338–4386; Angew. Chem. Int. Ed. 2008, 47,
4268–4315.
[3] Selected reviews on cyclobuta(e)ne derivatives in organic
synthesis: a) E. Lee-Ruff, G. Mladenova, Chem. Rev. 2003,
103, 1449–1484; b) J. C. Namyslo, D. E. Kaufmann, Chem.
Rev. 2003, 103, 1485–1538; c) T. Seiser, T. Saget, D. N.
Tran, N. Cramer, Angew. Chem. 2011, 123, 7884–7896;
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L. Conner, M. K. Brown, Angew. Chem. 2015, 127, 12086–
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A. Misale, S. Niyomchon, N. Maulide, Acc. Chem. Res.
2016, 49, 2444–2458.
[4] Gold(I)-catalyzed Intermolecular [2+2] cycloaddition
reactions between alkynes and alkenes to provide
cyclobutenes were reported: a) V. López-Carrillo, A. M.
Echavarren, J. Am. Chem. Soc. 2010, 132, 9292–9294; b)
M. E. de Orbe, L. Amenós, M. S. Kirillova, Y. Wang, V.
López-Carrillo, F. Maseras, A. M. Echavarren, J. Am.
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[5] Selected reviews on ring strain: a) M. A. Winnik, Chem.
Rev. 1981, 81, 491–524; b) C. Galli, L. Mandolini, Eur. J.
Org. Chem. 2000, 2000, 3117–3125.
[6] Y. Odabachian, F. Gagosz, Adv. Synth. Catal. 2009, 351,
379–386.
[7] F. Inagaki, C. Matsumoto, Y. Okada, N. Maruyama, C.
Mukai, Angew. Chem. 2015, 127, 832–836; Angew Chem.
Int. Ed. 2015, 54, 818–822.
[8] a) C. Obradors, D. Leboeuf, J. Aydin, A. M. Echavarren,
Org. Lett. 2013, 15, 1576–1579. See also: b) A. Homs, C.
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A. M. Echavarren, Org. Lett. 2016, 18, 1614–1617.
[9] Related reports on construction of cyclobutene- or
cyclobutane-fused systems containing seven- and medium-
In conclusion, we showed that hollow-shaped
triethynylphosphine ligands enabled the otherwise difficult
synthesis of various cyclobutene-fused eight-membered
carbocycles through gold-catalyzed intramolecular [2+2]
cycloaddition of 1,9-enynes. The effect of the hollow-
shaped ligands in the eight-membered carbocycle
formation was critical since other ligands produced no
eight-membered ring products. Further studies exploring
the applicability of hollow-shaped ligands are in progress
in our laboratory.
Experimental Section
A Typical Procedure of Au(I)-catalyzed Cycloisomerization of
1,9-Enynes: In a nitrogen-filled glove box, 5a (33.1 mg, 0.10
mmol), MS 4A (100 mg), [Au(NTf2)(L2)] (13.0 mg, 0.0050
mmol, 5 mol%) and CH2Cl2 (5 mL) were subsequently placed in
a 19 mL glass tube containing a magnetic stirring bar. The tube
was sealed with a screw cap and removed from the glove box.
The reaction mixture was stirred at rt for 2 h, and filtered through
a short silica gel pad (eluting with CH2Cl2). The solvent was
removed under reduced pressure. An internal standard (1,1,2,2-
tetrachloroethane) was added to the residue. The yields of the
products 6a, 7a, and 8a were determined by 1H NMR
spectroscopy (95%, 0% and 0% yields, respectively). The crude
material was then purified by silica gel column chromatography
(hexane/EtOAc 100:0 to 99:1) followed by GPC to give 6a (28.6
mg, 0.088 mmol, 86% yield).
ring
compounds
through
gold(I)-catalyzed
cycloisomerization, see: a) S. Nayak, N. Ghosh, A. K.
Sahoo, Org. Lett. 2014, 16, 2996–2999; b) D.-Y. Li, Y.
Wei, I. Marek, X.-Y. Tang, M. Shi, Chem. Sci. 2015, 6,
5519–5525.
Acknowledgements
This work was supported by JSPS KAKENHI Grant Number
JP16K13988 in Challenging Exploratory Research to T.I., and by
JST ACT-C Grant Number JPMJCR12YN, JST CREST, and
MEXT KAKENHI Grant Number JP15H05801 in Precisely
Designed Catalysts with Customized Scaffolding to M.S. The
authors also thank Mr. Masato Ueno and Mr. Yuto Goto for
experimental support.
[10] Selected reports on synthesis of medium-sized ring
compounds through the gold-catalyzed cyclization of
alkynes, see: a) C. Ferrer, A. M. Echavarren, Angew. Chem.
2006, 118, 1123–1127; Angew. Chem. Int. Ed. 2006, 45,
1105–1109; b) E. Comer, E. Rohan, L. Deng, J. A. Porco,
Org. Lett. 2007, 9, 2123–2126; c) I. D. G. Watson, S.
Ritter, F. D. Toste, J. Am. Chem. Soc. 2009, 131, 2056–
2057; d) Y. Odabachian, X. F. Le Goff, F. Gagosz, Chem.
Eur. J. 2009, 15, 8966–8970; e) K. Wittstein, K. Kumar, H.
Waldmann, Angew. Chem. 2011, 123, 9242–9246; Angew.
Chem. Int. Ed. 2011, 50, 9076–9080.
References
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Kim, D.-G. Kim, K.-B. Oh, J. Shin, D.-C. Oh, Org. Lett.
2012, 14, 1258–1261.
4
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10.1002/adsc.201701193
Advanced Synthesis & Catalysis
[12] Rumphellaoic acid A: a) H.-M. Chung, W.-H. Wang, T.-L.
Hwang, L.-S. Fang, Z.-H. Wen, J.-J. Chen, Y.-C. Wu, P.-J.
Sung, Mar. Drugs 2014, 12, 5856–5863; See also: b) J. C.
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Hitchcock, J. R. Hanson, I. G. Collado, J. Org. Chem. 2000,
65, 7786–7791.
[13] Rumphellolide F: a) P.-J. Sung, L.-F. Chuang, J. Kuo, J.-J.
Chen, T.-Y. Fan, J.-J. Li, L.-S. Fang, W.-H. Wang, Chem.
Pharm. Bull. 2007, 55, 1296–1301; See also: b) S. F. R.
Hinkley, N. B. Perry, R. T. Weavers, Tetrahedron 1997, 53,
7035–7044.
[18] With XPhos, the alkyne hydration product of 5a was
obtained in 38% yield (NMR).
[19] Selected example of reactions between cationic gold
complexes and water, see: a) R. S. Ramón, S. Gaillard, A.
Poater, L. Cavallo, A. M. Z. Slawin, S. P. Nolan, Chem.
Eur. J. 2011, 17, 1238–1246; b) A. Zhdanko, M. Ströbele,
M. E. Maier, Chem. Eur. J. 2012, 18, 14732–14744; c) L.
Adriaenssens, A. Escribano-Cuesta, A. Homs, A. M.
Echavarren, P. Ballester, Eur. J. Org. Chem. 2013, 2013,
1494–1500.
[20] The HNTf2-catalyzed cyclobutene-opening reaction of a
bicyclo[5.2.0]nonen was also reported by Gagosz’s group
(ref 6).
[14] Synthesis
and
applications
of
hollow-shaped
triethynylphosphines: a) A. Ochida, H. Ito, M. Sawamura,
J. Am. Chem. Soc. 2006, 128, 16486–16487; b) T. Iwai, H.
Okochi, H. Ito, M. Sawamura, Angew. Chem. 2013, 125,
4333–4336; Angew. Chem. Int. Ed. 2013, 52, 4239–4242.
See also for an account review: c) T. Iwai, M. Sawamura,
Bull. Chem. Soc. Jpn. 2014, 87, 1147–1160.
[21] Effects of molecular sieves in gold catalysis, see: a) Y.
Tang, B. Yu, RSC Adv. 2012, 2, 12686–12689. b) M.
Kumar, G. B. Hammond, B. Xu, Org. Lett. 2014, 16, 3452–
3455. See also: c) N. Fontes, J. Partridge, P. J. Halling, S.
Barreiros, Biotechnol. Bioeng. 2002, 77, 296–305.
[22] CCDC 1559273 (6e) contains the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge
[15] Cationic gold complexes [Au(NTf2)(L)] used in this paper
were prepared from the corresponding neutral gold
complexes [AuCl(L)] and [AgNTf2] (1.5 equiv) by mixing
in CH2Cl2 followed by filtration through a short Celite pad
and drying under vacuum.
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
[16] Cyclobutenes 2 and 6, and 1,3-dienes 3 and 7 were
characterized by NMR and ESI-MS spectroscopy. The
structure of 1,4-dienes 4 and 8 were tentatively assigned on
the basis of 1H NMR analysis by reference to Gagosz’s
report (ref 6). See Supporting Information for details. A
reaction pathway for the formation of cyclobutenes and 1,3-
diene products through the gold(I)-catalyzed intramolecular
[2+2] cycloaddition of 1,9-enynes is likely the same as that
of 1,8-enynes proposed by Gagosz and co-workers.
[23] The cycloisomerization of 5k and 5l was less efficient in
the presence of MS 4A.
[24] The Au-L2 system catalyzed cycloisomerization of a 1,10-
enyne substrate to form nine-membered carbocyclic
compounds with relatively low yields. More efficient nine-
membered ring formation occurred from a 1,10-enyne
substrate having an ethereal linker in accord with
Echavarren’s reports (ref 8). The [Au(NTf2)(L2)] system
showed better catalytic performance than the previously
reported catalysis using [Au(MeCN)(tBuXPhos)]SbF6 or
[Au(MeCN)(IPr)]BARF. See Supporting Information for
details.
[17] The reaction of 1 with [Au(NTf2)(L2)] (5 mol%) and MS
4A (10 mg) gave a 68:32:0 mixture of 2, 3 and 4 in 97%
yield (NMR).
5
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10.1002/adsc.201701193
Advanced Synthesis & Catalysis
COMMUNICATION
Synthesis of Cyclobutene-Fused Eight-Membered
Carbocycles
through
Gold-Catalyzed
Intramolecular Enyne [2+2] Cycloaddition
Adv. Synth. Catal. Year, Volume, Page – Page
Tomohiro Iwai, Masahiro Ueno, Hiori Okochi, and
Masaya Sawamura*
6
This article is protected by copyright. All rights reserved.