Gold-catalyzed cascade reaction of hydroxy enynes for the synthesis of oxanorbornenes and naphthalene derivatives

Article abstract of DOI:10.1021/ol301322a

An efficient and selective gold-catalyzed cascade reaction for the synthesis of oxanorbornenes and naphthalene derivatives from easily prepared hydroxy enynes has been developed. Divergent products could be obtained from the same substrates by different g

Full text of DOI:10.1021/ol301322a

ORGANIC
LETTERS
2012
Vol. 14, No. 13
3344–3347
Gold-Catalyzed Cascade Reaction of
Hydroxy Enynes for the Synthesis of
Oxanorbornenes and Naphthalene
Derivatives
Xian-Rong Song,† Xiao-Feng Xia,† Qing-Bao Song,‡ Fang Yang,† Ying-Xiu Li,†
Xue-Yuan Liu,† and Yong-Min Liang*,†,§
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou
730000, P. R. China, State Key Laboratory Breeding Base of Green Chemistry-Synthesis
Technology, Zhejiang University of Technology, Hangzhou, 3100014, P. R. China, and
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Science, Lanzhou 730000, P. R. China
liangym@lzu.edu.cn
Received May 12, 2012
ABSTRACT
An efficient and selective gold-catalyzed cascade reaction for the synthesis of oxanorbornenes and naphthalene derivatives from easily prepared
hydroxy enynes has been developed. Divergent products could be obtained from the same substrates by different gold catalytic systems.
Electrophilic metal-catalyzed tandem reactions are the
most powerful tools for the efficient construction of com-
plicated molecular frameworks in a one-pot operation.1 In
contrast to multistep reactions, one-pot processes have
obvious advantages that greatly improve synthetic effi-
ciencyand generate less chemicalwaste. Inrecent years, the
oxycyclization of alkynol coupling with another functional
group is a very important strategy because of the rapid
assembly of different heterocyclic systems. Moreover, in-
corporation of this oxycyclization strategy has been high-
lighted in the synthesis of natural products.3 Thus, the
exploration of a novel, efficient, and atom-economical
tandem reaction in this area still needs to be actively
pursued. In this context, gold complexes or salts as pro-
mising catalysts provide an excellent tool for the construc-
tion of the carbon- and heterocyclic compounds with high
(2) For some selected examples, see: (a) Zhang, L.; Kozmin, S. A.
J. Am. Chem. Soc. 2005, 127, 6962. (b) Belting, V.; Krause, N. N. Org.
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4907. (d) Dieguez-Vazquez, A.; Tzschucke, C. C.; Lam, W. Y.; Ley, S. V.
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Angew. Chem., Int. Ed. 2008, 47, 209. (e) Fananas, F. J.; Fernandez, A.;
Cevic, D.; Rodrıguez, F. J. Org. Chem. 2009, 74, 932. (f) Barluenga, J.;
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Fernandez, A.; Rodrıguez, F.; Fananas, F. J. J. Organomet. Chem. 2009,
694, 546. (g) Barluenga, J.; Fernandez, A.; Rodrıguez, F.; Fananas, F. J.
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Chem.;Eur. J. 2009, 15, 8121. (h) Barluenga, J.; Fernandez, A.;
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11660. (i) Patil, N. T.; Kavthe, R. D.; Raut, V. S.; Shinde, V. S.; Sridhar,
B. J. Org. Chem. 2010, 75, 1277. (j) Patil, N. Y.; Singh, V.; Konala, A.;
Mutyala, A. K. Tetrahedron Lett. 2010, 51, 1493. (k) Patil, N. T.;
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† Lanzhou University.
‡ Zhejiang University of Technology.
§ Lanzhou Institute of Chemical Physics.
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10.1021/ol301322a
Published on Web 06/13/2012
2012 American Chemical Society

atom-economy and mild conditions in modern organic
chemistry.4 Recently, an intermolecular cascade cyclo-
isomerization/DielsÀAlder reaction catalyzed by gold
to form polycyclic compounds has been reported by
Barluenga.5 The sequences went through the exo- or
endo-cycloisomerization and finally generated the poly-
cyclic products by DielsÀAlder reaction.
best of our knowledge, such cascade reactions have rarely
been reported in the literature.9,10 Herein, we report a
successful realization of this hypothesis and the details of
our discovery.
Scheme 1. Proposed Tandem Catalytic 5-Exo-Dig Cycloisome-
rization/DielsÀAlder Reaction
Inspired by this intriguing study and in continuation of
our work on the synthesis of heterocyclic compounds,6 we
envisioned that the treatment of the easily prepared hydroxy
enynes with gold catalyst would generate interesting
oxanorbornenes (scheme 1), which are very valuable syn-
thetic intermediates in a huge number of ring-opening
reactions to the synthesis of biologically active com-
pounds.7 This process would involve an initial 5-exo-dig
cyclization to form an enol ether intermediate in a highly
regioselective manner,8 which may be in equilibrium with
isobenzofuran,9 and thenfollowedaDielsÀAlderreaction.
To our surprise, besides the desired oxanorbornene com-
pound, a naphthalene compound was also formed. To the
Initially, optimization studies of this transformation
started with hydroxy enyne 1a as the model substrate
(Table 1 and Table S1 in the Supporting Information).
To our delight, 1a in the presence of 5 mol % of PicAuCl2
in 1,4-dioxane at 100 °C gave an oxanorbornene product
epoxybenzo[f]isoquinoline 2a in 65% yield after 2 h (entry
1). A variety of gold catalysts were then screened, and
[BMIM][AuCl4]11 was proven to be the most efficient
(entries 2À5). When [BMIM]AuCl4 loading was decreased
from 5 to 2.5 mol %, the yield was decreased (entry 6).
Other solvents such as THF, MeCN, and toluene were
found to be less effective (see the Supporting Information).
When we decreased the temperature to room temperature,
a mainly 5-exo-dig cyclization product 4a was obtained
(entry 7). [BMIM]AuCl4/AgSbF6 was also applied to
the reaction, but no better result was obtained and a
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Rev. 2012, 41, 2448.
trace amount of 3a was detected by H NMR spectros-
copy (entry 8). Subsequently, the stepwise addition of
[BMIM]AuCl4 and AgSbF6 to the reaction mixture was
investigated (entry 9). Unexpectedly, 2a was not detected,
but a naphthalene product tetrahydrobenzo[f]isoquinoline
3a was isolated in 81% yield after 4 h. Neither changing Ag
sources nor temperature could obviously improve the yield
of 3a (see the Supporting Information). Compared with
the Lewis acid system, Brønsted acid p-TsOH was less
reactive and afforded lower yields of the product 3a (entry
11). Thus, the use of [BMIM]AuCl4 (5 mol %) in 1,4-
dioxane at 100 °C was considered to be an optimal reac-
tion condition to form 2a (conditions A), and the step-
wise addition of [BMIM]AuCl4 (5 mol %) and AgSbF6
(20 mol %) in 1,4-dioxane at 100 °C was found to be the
most efficient and was selected as the standard set of
conditions for 3a (conditions B).
~
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F. J. Chem.;Eur. J. 2010, 16, 7110.
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To define the scope of the tandem reaction, we next
turned our attention to the construction of oxanor-
bornenes under the optimal conditions A. Various
N-tethered, O-tethered, and C-tethered hydroxy enynes were
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Bacchi, A.; Costa, M.; Ca, N. D.; Fabbricatore, M.; Fazio, A.; Gabriele,
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Inorg. Chem. 1999, 38, 5637.
Org. Lett., Vol. 14, No. 13, 2012
3345

investigated (Table 2). In most cases, the desired products
were generated in moderate to good yields. When R1 were
aryl groups, the electron-withdrawing groups gave results
superior to those with an electron-donating aryl groups
in this cascade reaction to generate the desired product 2n
in 32% yield (entry 17).
The role of the gold catalyst in the DielsÀAlder cycload-
dition step was also investigated. The 5-exo-dig cyclization
product 4a was isolated in the presence of 5 mol %
[BMIM]AuCl4 at room temperature. We observed that
the treatment of 4a in the absence of gold catalyst at 100 °C
(the same condition) resulted only a small amounts of the
oxanorbornene 2a (10% yield). The result demonstrated
that gold catalyst could accelerate the DielsÀAlder
reaction.
Table 1. Optimization of Reaction Conditionsa
Table 2. Synthesis of Oxanorbornenes and Naphthalene
Derivatives via Gold Catalyst from Hydroxy Enynesa
yieldb (%)
entry
catalyst (mmol %)
solvent
time (h) 2a
3a
1
2
3
4
5
6
PicAuCl2 (5)c
PPh3AuCl (5)
AuCl3 (5)
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
2
2
2
2
2
2
2
2
4
4
4
65
60
69
75
50
46f
25
HAuCl4.2H2O (5)
LAuCl4(5)d
LAuCl4 (2.5)
7e LAuCl4 (5)
8g LAuCl4(5)/AgSbF6 (20) 1,4-dioxane
9h LAuCl4 (5)/AgSbF6 (20) 1,4-dioxane
10h LAuCl4 (5)/AgSbF6 (10) 1,4-dioxane
11h LAuCl4 (5) /p-TsOH (20) 1,4-dioxane
<5
81
70
45
<5
a Unless otherwise noted, all reactions were performed with 0.1 mmol
of 1a in solvent (1.0 mL) at 100 °C. b Isolated yields. c Pic = picolinate.
d L = BMIM, BMIM =1-butyl-3-methylimidazolium. e At room tem-
perature. f The yield of the 5-exo-dig cyclization product 4a. g A solution
of LAuCl4 and AgSbF6 in 1,4-dioxane was stirred at room temperature
for 10 min. h The reaction was stirred at 100 °C for 2 h with
[BMIM]AuCl4 until most of the 1a was converted to product 2a, and
then AgX (X = SbF6, BF4, OTf) or p-TsOH was added and the mixture
was heated to 100 °C.
(entries 2À7). In addition, the corresponding products
were not separated except for 1g, and unexpectedly, the
corresponding naphthalene compounds were only ob-
tained in low yield when N-tethered and O-tethered
hydroxy enynes containing a methoxy group were on the
para-position of rgw aryl group (R1) (entries 5, 19, and 20),
which might be due to the lower stability of the corre-
sponding oxacyclic structure.12 When R1 was an alkyl
group or H, the desired products were also obtained in
high yields (entries 13À16). The vinyl and phenylethynyl in
R1 also gave good yields of 2k and 2l, respectively (entries
11 and 12). The structure of 2l was confirmed by X-ray
crystal structure analysis (see the Supporting Information).
Under the standard reaction conditions, the substituted
parent phenyl rings with F or Cl functionalities in R2 were
compatible for this transformation in good yields (entries 8
and 9), but when the substituent was a methyl, the desired
product was not examined (entry 10). Hydroxy enyne 1n
without the fused aromatic rings successfully participated
a Unless otherwise noted, all reactions were performed with 0.1 mmol
of 2. Conditions A: 5 mol % of [BMIM]AuCl4, 2.0 h. Conditions B:
5 mol % of [BMIM]AuCl4, 2.0 h, then AgSbF6 (20 mol %), 2.0 h.
b Isolated yield. c The yield stands for the corresponding product 3. d For
2.5 h. e No detected.
We next examined the scope of thisnew approach for the
synthesis of naphthalene derivatives 3 (Table 2). Starting
materials hydroxy enynes 1aÀu were also investigated
under the standard conditions B. We found that 1aÀi,k,
mÀu underwent cascade processes efficiently and formed
the corresponding products 3aÀi,k,mÀu in moderate to
good yields (entries 1À9, 11, and 13À21). The structure of
3h was also confirmed by X-ray crystal struture analysis
(see the Supporting Information). In contrast to product 2,
when R1 was a straight-chain alkyl group or H, the
corresponding yields were not good (entries 13À15), which
(12) (a) Padwa, A.; Brodney, M. A.; Dimitroff, M. J. Org. Chem.
1998, 63, 5304. (b) Ginn, J. D.; Lynch, S. M.; Padwa, A. Tetrahedron
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3346
Org. Lett., Vol. 14, No. 13, 2012

might be due to the formation of carbocation intermediate
E (Scheme 3).13 Moreover, the results in Table 2 revealed
the fact that Ag sources play an important role in enhanc-
ing the yield of 3, as well as accelerating the conversion of
2 into 3 (entries 1, 5, 19, and 20). Not surprisingly, when R1
was a vinyl group, the corresponding product was not
afforded under condition B (entry 12). It was found that, in
most cases, N-tethered hydroxy enynes afforded better
results than O-tethered and C-tethered hydroxy enynes in
this cascade reaction.
Scheme 3. Proposed Mechanisms of the Gold-Catalyzed
Tandem Reaction
To explore the mechanism of the tandem reactions, we
found that the ring-opening of 2a was induced by different
catalytic systems, the product 3a could only be obtained in
the presence of 5 mol % of [BMIM]AuCl4 and 20 mol % of
AgSbF6 (Scheme 2). The results clearly demonstrated that
the formation of naphthalene 3ashould use [BMIM]AuCl4
with AgSF6 as a cocatalyst in the tandem reactions. More-
over, the addition of AgSF6 was not only intended to act as
a scavenger for chloride, but also the presence of silver can
influence the gold cation reactivity.14
oxanorbornene product 2a goes through ring-opening
induced by gold catalyst to form the species E, which
undergoes protonation and then dehydrates to produce 3a.
In summary, we have developed an efficient gold-cata-
lyzed cascade reaction for the construction of oxanorbor-
nenes and naphthalene derivatives from easily prepared
hydroxy enynes. The sequences involve a highly regio-
selective 5-exo-dig cycloisomerisation, followed by a
DielsÀAlder reaction to afford oxanorbornenes. With the
same hydroxy enynes, we also obtained naphthalene deriv-
atives using a different catalytic system by further aroma-
tization reaction. Gold catalysts are effective to catalyze all
three processes in this tandem reaction. The value of this
cascade reaction is reflected by the applicability of diverse
alcohol substrates and operational simplicity.
Scheme 2. Controlled Experiments
On the basis of the above investigations, a plausible
mechanism for this transformation was proposed in
Scheme 3. Initially, the coordination of the alkynyl moiety
of hydroxy enyne 1a with gold catalyst gives the complex
A, which followed subsequent 5-exo-dig cyclization to
generate species B. B undergoes protonation to generate
intermediate 4a, which might be in equilibrium with iso-
benzofuran C and followed concomitant intramolecular
DielsÀAlder reaction to afford the product 2a. Then, the
Acknowledgment. We thank the National Science
Foundation (NSF-21072080) and the National Basic Re-
search Program of China (973 Program) 2010CB8-33203
for financial support. We also acknowledge support
from the “111” Project and the Program for Changjiang
Scholars and Innovative Research Team in University
(IRT1138).
Supporting Information Available. Detailed experimen-
tal procedure, copies of 1H NMR and 13C NMR spectra
of all compounds, and X-ray data for 2l and 3h (CIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
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Akhmedov, N. G.; Zhang, H.; Liu, X.; Petersen, J. L.; Shi, X. J. Am.
Chem. Soc. 2012, 134, 9012. (b) The experimental procedure of condi-
tions 4: A solution of 5 mol % of [BMIM][AuCl4]/20 mol % of AgSbF6
in 1,4-dioxane was stirred at room temperature for 10 min, and then
AgCl was removed by Celite filtration.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 13, 2012
3347