Regioselective gold-catalyzed allylative ring opening of 1,4-epoxy-1,4-dihydronaphthalenes

Article abstract of DOI:10.1055/s-0030-1258528

In the presence of a gold catalyst, the ring opening of 1,4-epoxy-1,4-dihydronaphthalenes with allyltrimethylsilane affords allylnaphthalenes in high yield. For unsymmetrical substrates, high regioselectivity is observed in many cases. This reaction might proceed via tricyclic tetrahydrofuran intermediates which are formed stereoselectively.

Full text of DOI:10.1055/s-0030-1258528

LETTER
2151
Regioselective Gold-Catalyzed Allylative Ring Opening of 1,4-Epoxy-
1,4-dihydronaphthalenes
Allylative Ring
O
p
o
ening of 1,4
s
-Epoxy-1
h
i
phthale
n
nes ari Sawama,*a Koichi Kawamoto,a Hiroyuki Satake,a Norbert Krause,b Yasuyuki Kita*a
a
College of Pharmaceutical Sciences, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577, Japan
Fax +81(77)5615829; E-mail: kita@ph.ritsumei.ac.jp
b
Organic Chemistry, Dortmund University of Technology, Otto-Hahn-Str. 6, 44227 Dortmund, Germany
Received 17 May 2010
We now suggest that the application of this method to 1,4-
epoxy-1,4-dihydronaphthalenes 1 (which contain a 2-
aryldihydrofuran substructure) might lead to formation of
highly functionalized cyclic compounds by nucleophilic
Abstract: In the presence of a gold catalyst, the ring opening of 1,4-
epoxy-1,4-dihydronaphthalenes with allyltrimethylsilane affords
allylnaphthalenes in high yield. For unsymmetrical substrates, high
regioselectivity is observed in many cases. This reaction might pro-
ceed via tricyclic tetrahydrofuran intermediates which are formed addition to cation intermediate A which should be formed
by cleavage of the epoxy moiety of 1.12 Herein, we report
stereoselectively.
an unprecedented direct conversion of 1 to 2-allyl-
naphthalenes13 without formation of 2-alkylated-1,2-di-
hydro-1-naphthols. This transformation proceeds by gold-
catalyzed ring opening with C–C bond formation under
mild conditions in high yield and additionally with high
regioselectivity for many unsymmetrical substrates.
Key words: allylation, ring opening, regioselectivity, naphthalene,
gold catalyst
Ring-opening carbon–carbon bond formation with 1,4-
epoxy-1,4-dihydronaphthalenes 1 is a valuable synthetic
methodology to create highly functionalized ring systems.
There appears to be no reports on the reaction of carbon
nucleophiles with 1 in the presence of Lewis acids or
Brønsted acids, whereas their transformations to 2-alky-
lated 1,2-dihydro-1-naphthols by ring-opening substitu-
tion by alkyllithium1 and transition-metal reagents2 (Fe,3
Cu,4 Pd,5 Rh,6 Ni7) has been widely developed. Normally,
acid-induced ring opening of 1 affords only 1-naphthols8
by rapid hydride shift8d or deprotonation of the unstable
cation intermediates (cf. A, Scheme 1).
We first examined the ring opening of unsubstituted 1,4-
epoxy-1,4-dihydronaphthalene 1a in the presence of a cat-
alytic amount of HAuCl4·3H2O (5 mol%) in CH2Cl2 at
–40 °C (Table 1). Whereas the use of the gold catalyst
alone gave no reaction (entry 1), the addition of either al-
lylTMS (4 equiv, entry 2) or TMSCl (4 equiv, entry 3) af-
forded just 1-naphthol 2a. Neither the desired allylated
nor chlorinated products were obtained. We suppose that
a rapid hydride shift8d in intermediate A to form 2a is fast-
er than nucleophilic addition.
Recently, we have reported an unprecedented gold-
catalyzed allylative ring opening of 2-aryldihydrofurans
to give acyclic products9 which might proceed via a zwit-
terionic intermediate comprising a benzyl cation substruc-
ture and an anionic aurate moiety (Scheme 1).9–11
Therefore, we changed the substrate to 1-methylated het-
erocycles 1b,c with the purpose of stabilizing the cation
intermediate A by the electron-donating methyl group.
However, 1b,c also did not undergo a nucleophilic addi-
tion by allylTMS but rather were converted to 1-naphthols
2b,c (entries 4, 5). To prevent the hydride shift, the sub-
strate was then modified to 1,4-dimethylated compound
1d which is substituted at both bridgehead positions. Con-
sequently, the reaction of 1d with allylTMS proceeded
smoothly to form the desired 2-allylnaphthalene 3d, re-
sulting from nucleophilic attack of the allyl group and
subsequent aromatization, with 82% yield (entry 6).14 In-
terestingly, chlorination with TMSCl did not proceed;
rather, 2,4-dimethyl-1-naphthol (4) was formed with 80%
yield by migration of the methyl group from C-1 to C-2
(entry 7).15
Our previous work:
R1
4
R1
R1
R2
R2
OH
Au(III)
allylTMS
+
2
R2
O
[Au]–
Ar
O
Ar
Ar
This work:
R2
R3
R2
R3
4
LA
Nu–
O
2
R1
R1
[LA]–
In order to optimize the transformation of 1d to 3d, vari-
ous Lewis acids were tested (Table 2). With
HAuCl4·3H2O in CH2Cl2, raising the reaction temperature
to 0 °C resulted in a slightly lower yield (entry 1). In con-
trast, NaAuCl4·2H2O gave a mixture of 3d and the diaste-
reomerically pure tricyclic tetrahydrofuran 5d16 (Figure 1,
Table 2, entry 2). An improved yield of 3d (93%) was ob-
tained with AuCl3 (entry 3), and lower amounts of AuCl3
R4
R4
O
1
A
Scheme 1 Ring openings via benzyl cation intermediates
SYNLETT 2010, No. 14, pp 2151–2155
x
x
.x
x
.2
0
1
0
Advanced online publication: 30.07.2010
DOI: 10.1055/s-0030-1258528; Art ID: U03810ST
© Georg Thieme Verlag Stuttgart · New York

2152
Y. Sawama et al.
LETTER
Table 1 Gold-Catalyzed Various Ring Opening of 1,4-Epoxy-1,4-dihydronaphthalenes
R1
R1
R2
R2
HAuCl4⋅3 H2O (5 mol%)
O
reagent (4 equiv), CH2Cl2, –40 °C
R3
1a–d
OH
OH
2a–c
3d
4
Entry
Reagent
Starting material
Time (h)
Product
Yield (%)
1
R1
R2
R3
H
1
2
3
4
5
6
7
none
1a
1a
1a
1b
1c
1d
1d
H
H
7
n.r.a
2a
2a
2b
2c
3d
4
allylTMS
TMSCl
H
H
H
1
53
79
82
92
82
80
H
H
H
0.5
1
allylTMS
allylTMS
allylTMS
TMSCl
Me
Me
Me
Me
H
H
Me
H
H
1
Me
Me
2
H
1
a 1-Naphthol 2a was produced above 0 °C.
(2 mol%) and allylTMS (2 equiv) gave 3d in excellent
yield of 98% (entry 4). The reaction with AuCl3 was
strongly influenced by the solvent. In CHCl3, toluene,
THF, or MeOH, no conversion was observed.
OMe
H
H
H
MeO
O
O
O
Table 2 Allylative Ring Opening of 1d in the Presence of Various
TMS
TMS
TMS
Lewis Acidsa
5d
5j
5l
Entry Lewis acid
Temp
(°C)
Time
(h)
Yield (%)
of 3d/4/5d
Figure 1 Tri- or tetracyclic compounds as side product
1
HAuCl4·3H2O
NaAuCl4·2H2O
AuCl3
0
0.5
3
77:0:0
Other gold catalysts (AuBr3, Ph3PAuCl/AgSbF6) were
also effective to obtain 3d, but lower reactivities were ob-
served (entries 5, 6). With traditional Lewis acids, e.g.
BF3·OEt2, Cu(OTf)2, Sc(OTf)3 or In(OTf)3, 3d was
formed in low yield together with variable amounts of 4
and 5d (entries 8–12). In contrast to this, AgSbF6 and
AlBr3 gave no conversion at all (entries 7, 13), and TFA
as Brønsted acid resulted in formation of a complex mix-
ture (entry 14).
2
–40
34:0:22
93:0:0
3b
4c
5
–40
2
AuCl3
–40 to 0
–40 to –5
4
98:0:0
AuBr3
3
60:0:0
6
Ph3PAuCl/AgSbF6 –40 to 0
3.5
24
2
77:0:0
7
AgSbF6
BF3·OEt2
Cu(OTf)2
Sc(OTf)3
In(OTf)3
Zn(OTf)2
AlBr3
–40 to 0
0
n.r.
Encouraged by the high reactivity observed in the AuCl3-
catalyzed allylative ring opening of 1d, we have applied
the optimized conditions (Table 2, entries 3, 4) to various
unsymmetrical 1,4-epoxy-1,4-dihydronaphthalenes 1e–m
which afforded the desired allylation products 3e–m with
moderate to high yield (Table 3).14,17,18 The regioselectiv-
ity was found to be strongly substrate-dominated.19 Com-
pound 1e containing a methoxy group as electron-
donating substituent in the distal20 position (R1) of the ar-
omatic ring underwent allylation with perfect regioselec-
tivity and high yield (entry 1). Bromo substitution (1f)
also favored the regioselective formation of 3f (entry 2),
whereas substrates 1g,h bearing electron-withdrawing
groups on the aromatic ring gave low regioselectivities
even though the allylation products were obtained in high
yield (entries 3, 4).
8
16:25:32
22:71:0
23:15:8
70:30:0
40:0:0
9
r.t.
1.5
18
6
10
–40 to 0
–40 to 0
–40 to 0
–40 to 0
–40 to 0
11
12
13
14
24
24
18
n.r.
TFA
complex mixture
a Reaction conditions: Lewis acid (5 mol%) and allylTMS (4 equiv) in
CH2Cl2.
b Other solvents (CHCl3, toluene, THF, MeOH) gave no conversion.
c Conditions: 2 mol% of AuCl3 and 2 equiv of allylTMS were used.
Synlett 2010, No. 14, 2151–2155 © Thieme Stuttgart · New York

LETTER
Allylative Ring Opening of 1,4-Epoxy-1,4-dihydronaphthalenes
2153
Table 3 Regioselective Allylative Ring Opening of Unsymmetrical
1,4-Epoxy-1,4-dihydronaphthalenes 1
the reaction of sterically hindered substrate 1l also afford-
ed allylated phenanthrene 3l in perfect regioselectivity
and moderate yield, accompanied by tetracycle 5l
(Figure 1).21 In addition, 1,4-epoxy-1,4-dihydronaphtha-
lene 1m containing two different groups (Ph and Me) in
the bridgehead positions gave only the allylated biaryl
product 3m with high yield and regioselectivity.
Entry
1
Time (h) Product (%, ratio 3/6)
R1
R2
R2
R2
O
R1
R1
AuCl3 (5 mol%)
5d
3d
CH2Cl2, –40°C
1e–k
3e–k + regioisomer 6e–k
75%
1a
1e MeO
1f Br
H
0.5
2.5
4
88 (100:0)
81 (88:12)
85 (31:69)
94 (50:50)
93 (100:0)
93 (100:0)
99 (91:9)
R2
R3
4
1
2b
3b
4c
H
1g MeO2C
1h F3C
1i H
H
2
R1
Au
[Au]–
A
H
4
O
5c
MeO
MeO
Me
4
R2
R3
TMS
6c,d
7c
1j MeO
1k H
1
3
H
TMS
3
R1
+
TMSOH
O
B
[Au]–
R2
R3
8c
4
R2
R3
O
H
H
R1
3l 55e
R1
1l
OTMS
C
Ph
Ph
O
5
TMS
9a
O
0.5
Scheme 2 Proposed reaction mechanism
3m 88
1m
In mechanistic terms, the electronic substituent effects on
the reactivity and regioselectivity clearly point to a benzyl
cation intermediate A formed by regioselective cleavage
of the epoxy moiety in substrates 1 (Scheme 2). Interme-
diate A probably undergoes a regioselective nucleophilic
attack by allylTMS at C-2 to afford b-silylcation interme-
diate B.22 Cleavage of the carbon–silicon bond leads to the
allylnaphthol derivative C and regenerates the gold
catalyst. Finally, aromatization of C by elimination of
TMSOH gives the allylnaphthalene product 3. Exposure
of the tricyclic tetrahydrofuran 5d to AuCl3 shows that
this side product is also a precursor of allylnaphthalene
3d. Thus, trapping of the b-silylcation B23 by the oxygen
atom may lead to the formation of 5 which then undergoes
a ring opening, possibly catalyzed by gold, to afford C.
Further experiments to elucidate this intriguing mechanis-
tic proposal are in progress.
a AuCl3 (2 mol%), allylTMS (2 equiv) were used.
b AuCl3 (5 mol%), allylTMS (4 equiv) were used.
c AuCl3 (5 mol%), AgSbF6 (15 mol%), allylTMS (4 equiv) were used.
d Only AuCl3 (5 mol%), allylTMS (4 equiv) without AgSbF6 afforded
32% of 3j and 32% of 5j.
e Tetracyclic compound 5l was also obtained with 36% yield as single
diastereomer.
In addition to these electronic effects, a large steric influ-
ence of the proximal20 substituent R2 on the reactivity and
regioselectivity was observed. Substrates containing a
substituent in this position are less reactive towards AuCl3
and require the addition of AgSbF6 to afford the desired
allylation products in high yield (entries 5–8). The reac-
tivity difference is particularly pronounced for the sub-
strates 1e and 1i bearing a single methoxy group in the
distal or proximal position (entry 5 vs. 1). Dimethoxy sub-
stitution (1j) also gave high yield and regioselectivity (en-
try 6). In the absence of AgSbF6, this reaction did not go
to completion, and the tricyclic compound 5j (Figure 1)
was obtained as side product with perfect regio- and ste-
reoselectivity.16 It is particularly noteworthy that substrate
1k having a weak electron-donating methyl group in the
proximal position underwent a high regioselective allyla-
tive ring opening in excellent yield (entry 7). As expected,
In conclusion, we have achieved an unprecedented gold-
catalyzed C–C bond formation of 1,4-disubstituted-1,4-
epoxy-1,4-dihydronaphthalenes 1 with allyltrimethylsi-
lane which affords allylnaphthalenes 3 with excellent
yield. This allylative ring opening requires the presence of
substituents at both bridgeheads of the starting material
and therefore enables the construction of highly substitut-
ed naphthalene derivatives. Moreover, the allylation takes
place with high regioselectivity when unsymmetrical sub-
Synlett 2010, No. 14, 2151–2155 © Thieme Stuttgart · New York

2154
Y. Sawama et al.
LETTER
(7) (a) Feng, C.-C.; Nandi, M.; Sambaiah, T.; Cheng, C.-H.
strates containing electron-donating groups on the aro-
matic ring, or an aryl group at the bridgehead are used.
This regioselectivity is difficult to achieve by reported
procedures.18 The gold-catalyzed allylative ring opening
of 1,4-epoxy-1,4-dihydronaphthalenes 1 also proceeds via
tricyclic tetrahydrofurans 5 which are formed in a highly
stereoselective manner. We continue to expand the scope
of Lewis acid catalyzed nucleophilic ring-opening reac-
tions for regio- and stereoselective construction of com-
plex target molecules.
J. Org. Chem. 1999, 64, 3538. (b) Rayabarapu, D. K.;
Chiou, C.-F.; Cheng, C.-H. Org. Lett. 2002, 4, 1679.
(c) Wu, M.-S.; Rayabarapu, D. K.; Cheng, C.-H. J. Org.
Chem. 2004, 69, 8407. (d) Wu, M.-S.; Jeganmohan, M.;
Cheng, C.-H. J. Org. Chem. 2005, 70, 9545.
(8) (a) Wittig, G.; Pohmer, L. Chem. Ber. 1956, 89, 1334.
(b) Wolthuis, E.; Bossenbroek, B.; DeWall, G.; Geels, E.;
Leegwater, A. J. Org. Chem. 1963, 28, 148. (c) Fetizon,
M.; Anh, N. T. Bull. Soc. Chim. Fr. 1965, 3208. (d) Cooke,
M. D.; Dransfield, T. A.; Vernon, J. M. J. Chem. Soc., Perkin
Trans. 2 1984, 1377.
(9) Sawama, Y.; Sawama, Y.; Krause, N. Org. Lett. 2009, 11,
5034.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
(10) Deutsch, C.; Gockel, B.; Hoffmann-Röder, A.; Krause, N.
Synlett 2007, 1790.
(11) Epimerization reports by other groups: (a) Epimerization/
rearrangement of alkynyl and aryl C-glycosides with AuCl3:
Yeager, A. R.; Min, G. K.; Porco, J. A. Jr.; Schaus, S. E. Org.
Lett. 2006, 8, 5065. (b) Epimerization of a 2,5-dihydrofuran
during the gold-catalyzed cycloisomerization of an allen-
amide: Hyland, C. J. T.; Hegedus, L. S. J. Org. Chem. 2006,
71, 8658.
Acknowledgment
This work was partially supported by Grant-in-Aid for Scientific
Research (A) from the Japan Society for the Promotion of Science
(JSPS) and Ritsumeikan Global Innovation Research Organization
(R-GIRO) project.
(12) Gold-catalyzed bisallylation of oxabicyclic benzene
derivatives: Hsu, Y.-C.; Datta, S.; Ting, C.-M.; Liu, R.-S.
Org. Lett. 2008, 10, 521.
References and Notes
(13) For a two-step synthesis of 2-alkyl-1-naphthols by Pd-
catalyzed ring opening of 1,4-epoxy-1,4-dihydro-
naphthalenes and subsequent oxidation, see ref. 5j.
(14) General Procedure for the Allylative Ring Opening to
Synthesize 3d–m
(1) Caple, R.; Chen, G. M.-S.; Nelson, J. D. J. Org. Chem. 1971,
36, 2874.
(2) Review: Lautens, M.; Fagnou, K.; Hiebert, S. Acc. Chem.
Res. 2003, 36, 48.
(3) Nakamura, M.; Matsuo, K.; Inoue, T.; Nakamura, E. Org.
Lett. 2003, 5, 1373.
Procedure A
To a solution of the substrate (0.2 mmol) in CH2Cl2 (2 mL)
were added allylTMS (0.8 mmol) and AuCl3 (0.01 mmol) at
–40 °C under nitrogen. After being stirred until reaction was
completed, the reaction mixture was concentrated under
vacuum. The residue was purified by column chromatog-
raphy using hexane–EtOAc (50:1) to give the pure allylation
product.
(4) (a) Bertozzi, F.; Pineschi, M.; Macchia, F.; Arnold, L. A.;
Minnaard, A. J.; Feringa, B. L. Org. Lett. 2002, 4, 2703.
(b) Arrayás, R. G.; Cabrera, S.; Carretero, J. C. Org. Lett.
2003, 5, 1333. (c) Arrayás, R. G.; Cabrera, S.; Carretero,
J. C. Synthesis 2006, 1205. (d) Zhang, W.; Wang, L.-X.;
Shi, W.-J.; Zhou, Q.-L. J. Org. Chem. 2005, 70, 3734.
(e) Millet, R.; Bernardez, T.; Palais, L.; Alexakis, A.
Tetrahedron Lett. 2009, 50, 3474. (f) Millet, R.; Gremaud,
L.; Bernardez, T.; Palais, L.; Alexakis, A. Synthesis 2009,
2101.
(5) For selected references, see: (a) Duan, J.-P.; Cheng, C.-H.
Organometallics 1995, 14, 1608. (b) Fugami, K.; Hagiwara,
S.; Oda, H.; Kosugi, M. Synlett 1998, 477. (c) Lautens, M.;
Renaud, J.-L.; Hiebert, S. J. Am. Chem. Soc. 2000, 122,
1804. (d) Lautens, M.; Hiebert, S.; Renaud, J.-L. J. Am.
Chem. Soc. 2001, 123, 6834. (e) Priego, J.; Mancheňo, O.
G.; Cabrera, S.; Arrayás, R. G.; Llamas, T.; Carretero, J. C.
Chem. Commun. 2002, 2512. (f) Lautens, M.; Hiebert, S.
J. Am. Chem. Soc. 2004, 126, 1437. (g) Li, M.; Yan, X.-X.;
Hong, W.; Zhu, X.-Z.; Cao, B.-X.; Sun, J.; Hou, X.-L. Org.
Lett. 2004, 6, 2833. (h) Cabrera, S.; Arrayás, R. G.; Alonso,
I.; Carretero, J. C. J. Am. Chem. Soc. 2005, 127, 17938.
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Chem. 2006, 71, 4810. (k) Zhang, T.-K.; Yuan, K.; Hou,
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T.; Kumada, A.; Yoshida, K. Chem. Lett. 2007, 36, 500.
(m) Imamoto, T.; Saitoh, Y.; Koide, A.; Ogura, T.; Yoshida,
K. Angew. Chem. Int. Ed. 2007, 46, 8636.
Procedure B
To a solution of the substrate (0.2 mmol) in CH2Cl2 (2 mL)
were added allylTMS (0.8 mmol), AgSbF6 (0.03 mmol), and
AuCl3 (0.01 mmol) at –40 °C under nitrogen. After being
stirred until reaction was completed, the reaction mixture
was concentrated under vacuum. The residue was purified
by column chromatography using hexane–EtOAc (50:1) to
give the pure allylation product.
(15) For the synthesis of 2,4-dimethyl-1-naphthols by methyl
migration from C-1 to C-2 via ring opening of 1,4-dimethyl-
1,4-epoxy-1,4-dihydronaphthalenes, see ref. 8b,c and: Peng,
F.; Fan, B.; Shao, Z.; Pu, X.; Li, P.; Zhang, H. Synthesis
2008, 3043.
(16) The determination of relative configuration of 5d,j was
accomplished with NOE experiments.
(17) Synthesis of 1-naphthols by regioselective acid-catalyzed
ring opening of unsymmetrical substrates: (a) Giles,
R. G. F.; Hughes, A. B.; Sargent, M. V. J. Chem. Soc.,
Perkin Trans. 1 1991, 1581. (b) Batt, D. G.; Jones, D. G.;
Greca, S. L. J. Org. Chem. 1991, 56, 6704. (c) Schlosser,
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(d) Masson, E.; Schlosser, M. Eur. J. Org. Chem. 2005,
4401. (e) Bailly, F.; Cottet, F.; Schlosser, M. Synthesis 2005,
791.
(6) (a) Lautens, M.; Fagnou, K. J. Am. Chem. Soc. 2001, 123,
7170. (b) Murakami, M.; Igawa, H. Chem. Commun. 2002,
390. (c) Lautens, M.; Dockendorff, C.; Fagnou, K.; Malicki,
A. Org. Lett. 2002, 4, 1311. (d) Lautens, M.; Schmid, G. A.;
Chau, A. J. Org. Chem. 2002, 67, 8043. (e) Lautens, M.;
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(18) Regioselective synthesis of 2-alkylated-1,2-dihydro-1-
naphthols from unsymmetrical substrates could be only
accomplished with Rh catalysts, see ref. 6d.
Synlett 2010, No. 14, 2151–2155 © Thieme Stuttgart · New York

LETTER
Allylative Ring Opening of 1,4-Epoxy-1,4-dihydronaphthalenes
2155
(19) Regiochemistry of 3e–m was determined by NOE
experiments.
(20) In this paper, the same expressions as in ref. 4c are used. The
position of R1 in substrates 1e–k is termed distal in relation
to the dihydrofuran moiety. The position of R2 is termed
proximal.
(21) The relative configuration of 5l could not be assigned.
(22) Reaction of 1d with crotyltrimethylsilane afforded 1,4-
dimethyl-2-(1-methylallyl)naphthalene with 38% yield,
which indicates that the allyl nucleophile directly attacks at
C-2 of A. The formation of 3 by allylation at C-4 and
subsequent Cope rearrangement is disfavored by the steric
hindrance at C-4, and the high activation barrier for the Cope
rearrangement of 1,5-dienes which usually requires strong
heating.
(23) Examples for the trapping of b-silyl cations to give
cycloadducts: (a) Nishitani, K.; Harada, Y.; Nakamura, Y.;
Yokoo, K.; Yamakawa, K. Tetrahedron Lett. 1994, 35,
7809. (b) Sugita, Y.; Kimura, Y.; Yokoe, I. Tetrahedron
Lett. 1999, 40, 5877.
Synlett 2010, No. 14, 2151–2155 © Thieme Stuttgart · New York