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Gold-Catalyzed Cyclization of Enynes.

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Highlights
DOI: 10.1002/anie.200502999
Homogeneous Catalysis
Gold-Catalyzed Cyclization of Enynes
Shengming Ma,* Shichao Yu, and Zhenhua Gu
Keywords:
alcohols · cycloisomerization · enynes · gold ·
homogeneous catalysis
Gold is a “soft” transition metal that
shows high electrophilic affinity for
alkynes, arenes, allenes, and even alkenes. It can also act simultaneously as a
Lewis acid for the activation of electrophiles.[1a, b] Carbon–gold bonds are labile
to protonolysis but undergo b-hydride
elimination reactions only with difficulty. Reactions catalyzed by gold generally
proceed under mild conditions and can
be performed in the presence of water
or even in pure water.[1] Although there
are some new developments in this
area,[2] this Highlight is concerned with
the gold-catalyzed cycloisomerization of
enynes.
Cycloisomerizations of 1,n-enynes
have emerged as attractive tools for
the synthesis of various types of cyclic
compounds in an easy one-pot process
in which a wide range of transitionmetal complexes can be used, either in a
catalytic or stoichiometric manner.[3]
Pioneering work by Echavarren and
co-workers has shown that Au+ is a
versatile catalyst for the intramolecular
enyne metathesis of substrates with a
terminal carbon–carbon triple bond
(Scheme 1).[3c, 4] The intermediate 4
formed by the nucleophilic attack of
the C=C bond at the terminal C C triple
bond was generally proposed as the key
intermediate, as is common for other
electrophilic transition metals such as
platinum,[5, 6] rhodium,[6, 7] and ruthenium,[7] for this type of reaction. Intermediates such as 8 and 9 can undergo
[*] Prof. Dr. S. Ma, Dr. S. Yu, Z. Gu
State Key Laboratory of Organometallic
Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Lu
Shanghai 200032 (P.R. China)
Fax: (+ 86) 21-6416-7510
E-mail: masm@mail.sioc.ac.cn
200
Scheme 1.
intramolecular cyclopropanation with ing a vinyl- or aryl-substituted carbon–
another C C double bond or trapped carbon triple bond. Treatment of 1,8by the nucleophilic addition of MeOH dien-3-yne 14 or 1-aryl-6-enyne 18 with
(Scheme 1).[5, 7] An asymmetric version 2 mol % of gold complex 20 or 21 and
of such a cyclization with moderate AgSbF6 afforded hydrindane 17 or the
ee values has also been realized by using tricyclic product 19 by an intramolecular
different chiral ligands.[6]
interaction of the Au carbene moiety
However, the reaction of tosylated, with the carbon–carbon double bond
nitrogen-tethered enyne 10 with [Au- and the carbon–carbon bond in the
(PPh3)]SbF6 afforded
methylenecyclohexene
derivative 13 through
the double carbon–carbon bond cleavage of
the cyclopropane rings
in intermediates 11 and
12 (Scheme 2).[4]
Such metallocarbene
intermediates
may also be formed
from substrates bear- Scheme 2. Ts = toluene-4-sulfonyl.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 200 – 203
Angewandte
Chemie
rangement that afforded 3,4-allenols 39
through a b-heteroatom elimination of
intermediate 38, which was formed by
the nucleophilic attack of the vinyl ether
moiety at the coordinated C C triple
bond, and reduction. The chirality can
be completely transferred to the final
product 41 by using the enantiomerically enriched propargyl vinyl ether 40
(Scheme 6).[14]
Bicyclo[3.1.0]hexanone
47
was
formed from propargylic acetate 42 as
shown in Scheme 7. Coordination of the
cationic gold(i) complex to the alkyne
induces an intramolecular nucleophilic
attack of the carbonyl oxygen atom of
the ester group at the carbon–carbon
triple bond to afford a zwitterionic
vinylic gold intermediate 44, which was
converted into a new type of carbene
intermediate 45 by the subsequent
cleavage of the C O bond. Intramolecular cyclopropanation would then lead
to the formation of bicyclic vinylic ester
Scheme 3. Cy = cyclohexyl.
three-membered ring in an intermediate
of type 15 (Scheme 3).[4b, 8]
Zhang and Kozmin observed that a
series of carbon–carbon bond cleavages
induced by the lone pair of electrons on
the oxygen atom in cyclopropyl gold
carbene 23 led to the formation of 1,4cyclohexadiene 27. The silyl group is
believed to stabilize the oxocarbenium
intermediate 24. Treatment of the enyne
28 without this silyl group with 20 mol %
of AuCl in CH2Cl2 gave 5-methyl-3propylbicyclo[3.1.0]hex-2-ene
(29)
through a 1,2-hydrogen shift of the
metallocarbene
intermediate
30
(Scheme 4).[9] Similar reaction pathways
of metallocarbenes were also observed
by F=rstner et al. as well as Gagosz,[10]
and Toste et al.[11]
Dankwardt found that gold-mediated cyclization of o-alkynylphenylvinyl
silyl ether 31 constitutes an efficient
method for the construction of substituted naphthalene 35 through the nucleophilic addition of the enol silyl ether to
the Au-coordinated triple bond[11, 12] to
form the vinylic gold intermediate 33
(Scheme 5).[13]
Sherry and Toste observed that
treatment of propargylic vinyl ether 36
with a catalytic amount of [{Au(PPh3)}3O]BF4 effected a skeletal rearAngew. Chem. Int. Ed. 2006, 45, 200 – 203
Scheme 4. TIPS = triisopropylsilyl.
Scheme 5.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
201
Highlights
46.[10, 15] This mild and efficient method
has been applied as a key step for the
diastereoselective total synthesis of the
natural product sesquicarene (48).[16]
Scheme 6.
The Au-catalyzed skeletal rearrangement proceeds stereospecifically, and
translates the configuration of the reacting alkene into the stereochemistry
of the emerging cyclopropane
unit.
Similarly,
2-cyclopentenones and bicyclic 2-cyclopentenones were produced in the
presence of 2–5 mol % of [Au(PPh3)]OTf in CH3CN at room
temperature from the cycloisomerization of 1-ethynyl-2-propenyl pivaloates in 45–85 %
yields.[17] The reaction proceeded by a subsequent intramolecular Nazarov-type cyclization
of similarly structured vinylic
gold(i) species, elimination of
cationic gold(i), and hydrolysis.
Excellent chirality transfer was
observed when enantioenriched propargyl pivaloates
were treated with 5 mol % of
Scheme 7.
[1] For recent reviews on Au-catalyzed
oxidation, carbonylation, aldol reactions, hydroarylation, hydration, hydroamination, and intramolecular cyclizations see: a) A. S. K. Hashmi, Gold Bull.
2004, 37, 51; b) A. Arcadi, S. Di Giuseppe, Curr. Org. Chem. 2004, 8, 795;
c) G. Dyker, Angew. Chem. 2000, 112,
4407; Angew. Chem. Int. Ed. 2000, 39,
4237; d) D. E. De Vos, B. F. Sels, Angew.
Chem. 2005, 117, 30; Angew. Chem. Int.
Ed. 2005, 44, 30; e) A. S. K. Hashmi,
Gold Bull. 2003, 36, 3; f) A. HoffmannRHder, N. Krause, Org. Biomol. Chem.
2005, 3, 387; for general surveys of
organogold chemistry, see: g) R. J. Puddephatt in Comprehensive Organometallic Chemistry, Vol. 2, 1st ed. (Eds.: G.
Wilkinson, F. G. A. Stone, E. W. Abel),
Pergamon, Oxford, 1982, p. 765; h) R. J.
Puddephatt, The Chemistry of Gold,
Elsevier, Amsterdam, 1978.
[2] A. S. K. Hashmi, Angew. Chem. 2005,
117, 7150; Angew. Chem. Int. Ed. 2005,
44, 6990.
Scheme 8.
202
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[Au(PPh3)]SbF6 in CH3CN at 20 8C,
which strongly suggests the formation of
vinylic gold(i) species as intermediates
instead of gold–carbene species.[17]
Kozmin and co-workers observed
that the treatment of a 1,5-enyne with
catalytic amounts of AuCl3 triggered the
double cyclization of 49 to produce
intermediate 50,[12l, 18] which underwent
subsequent protonolysis to afford the
heterobicyclic product 51. The remarkable diastereoselective nature of this
double cyclization is decipted in
Scheme 8: Compound 52, which contains an E-alkene moiety, afforded the
trans-oxabicyclic product 53 whereas
compound 54, with a Z alkene, produced the cis-oxabicyclic product 55
exclusively. The tosyl amine moiety
behaved similarly in the presence of
the [Au(PPh3)]ClO4 catalyst.[19]
Gold catalysis represents a new
frontier in catalysis which has substantial benefits to organic synthesis. The
wide scope and high efficiency demonstrated by the gold-catalyzed reaction of
enynes under mild conditions to access
complex or hitherto inaccessible products are particularly appealing. There is
still an urgent need to characterize
reaction intermediates so that plausible
mechanisms and catalytic cycles can be
proposed. It is hoped that this Highlight
will provide the basis for the development of new reactions.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 200 – 203
Angewandte
Chemie
[3] For recent reviews on cycloisomerizations, see: a) C. Aubert, O. Buisine, M.
Malacria, Chem. Rev. 2002, 102, 813;
b) B. M. Trost, F. D. Toste, A. B. Pinkerton, Chem. Rev. 2001, 101, 2067; c) C.
Bruneau, Angew. Chem. 2005, 117, 2380;
Angew. Chem. Int. Ed. 2005, 44, 2328.
[4] a) C. Nieto-Oberhuber, M. P. MuKoz, E.
BuKuel, C. Nevado, D. J. CLrdenas,
A. M. Echavarren, Angew. Chem. 2004,
116, 2363; Angew. Chem. Int. Ed. 2004,
43, 2402; b) N. MMzailles, L. Ricard, F.
Gagosz, Org. Lett. 2005, 7, 4133.
[5] a) M. MMndez, M. P. MuKoz, A. M.
Echavarren, J. Am. Chem. Soc. 2000,
122, 11 549; b) M. MMndez, M. P. MuKoz,
C. Nevado, D. J. CLrdenas, A. M. Echavarren, J. Am. Chem. Soc. 2001, 123,
10 511; c) A. F=rstner, H. Szillat, F.
Stelzer, J. Am. Chem. Soc. 2000, 122,
6785; d) A. F=rstner, F. Stelzer, H.
Szillat, J. Am. Chem. Soc. 2001, 123,
11 863; e) C. Nevado, C. Ferrer, A. M.
Echavarren, Org. Lett. 2004, 6, 3191;
f) L. Charruault, V. Michelet, R. Taras,
S. Gladiali, J.-P. GenÞt, Chem. Commun.
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[6] M. P. MuKoz, J. Adrio, J. C. Carretero,
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[7] N. Chatani, K. Kataoka, S. Murai, N.
Furukawa, Y. Seki, J. Am. Chem. Soc.
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[8] C. Nieto-Oberhuber, S. LPpez, A. M.
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[10] V. Mamane, T. Gress, H. Krause, A.
F=rstner, J. Am. Chem. Soc. 2004, 126,
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[11] M. R. Luzung, J. P. Markham, F. D.
Toste, J. Am. Chem. Soc. 2004, 126,
10 858.
[12] For some of the most typical examples
of Au-catalyzed addition of nucleophiles
to alkynes, see: a) Y. Fukada, K. Utimoto, H. Nozaki, Heterocycles 1987, 25,
297; b) Y. Fukuda, K. Utimoto, J. Org.
Chem. 1991, 56, 3729; c) Y. Fukuda, K.
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A. J. Williams, J. Org. Chem. 1996, 61,
3289; f) J. H. Teles, S. Brode, M. Chabanas, Angew. Chem. 1998, 110, 1475;
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g) T. E. Muller, A.-K. Pleier, J. Chem.
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Mizushima, K. Sato, T. Hayashi, M.
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i) R. Casado, M. Contel, M. Laguna, P.
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[13]
[14]
[15]
[16]
[17]
[18]
[19]
Staben, F. D. Toste, J. Am. Chem. Soc.
2005, 127, 9708; k) S. T. Staben, J. J.
Kennedy-Smith, F. D. Toste, Angew.
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A. F=rstner, P. Hannen, Chem. Commun. 2004, 2546.
X. Shi, D. J. Gorin, F. D. Toste, J. Am.
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
203
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