Enantioselective synthesis of cyclopentadienes by gold(I)- catalyzed cyclization of 1,3-dien-5-ynes

Article abstract of DOI:10.1002/adsc.201300448

An asymmetric synthesis of elusive chiral cyclopentadienes has been developed by gold(I)-catalyzed alkoxycyclization of 1,3-dien-5-ynes. The application of these substrates in completely diastereoselective Diels-Alder cycloaddition reactions, which can be

Full text of DOI:10.1002/adsc.201300448

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DOI: 10.1002/adsc.201300448
Enantioselective Synthesis of Cyclopentadienes by Gold(I)-
Catalyzed Cyclization of 1,3-Dien-5-ynes
Ana M. Sanjuꢀn,a Patricia Garcꢁa-Garcꢁa,a Manuel A. Fernꢀndez-Rodrꢁguez,a
and Roberto Sanza,*
a
ꢂrea de Quꢁmica Orgꢀnica, Departamento de Quꢁmica, Facultad de Ciencias, Universidad de Burgos, Pza. Misael
BaÇuelos s/n, 09001 Burgos, Spain
Fax : (+34)-947-258-831; e-mail: rsd@ubu.es
Received: May 22, 2013; Revised: May 30, 2013; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300448.
Abstract: An asymmetric synthesis of elusive chiral
cyclopentadienes has been developed by gold(I)-
catalyzed alkoxycyclization of 1,3-dien-5-ynes. The
application of these substrates in completely diaste-
reoselective Diels–Alder cycloaddition reactions,
which can be carried out in one pot from achiral
1,3-dien-5-ynes, allows the preparation of highly
functionalized products bearing five stereogenic
centers with high enantiomeric excesses.
lenes (1,2,4-trienes) lead to substituted cyclopenta-
dienes through a metalla-Nazarov 4p electrocycliza-
tion of pentadienyl cationic complexes [Scheme 1, Eq.
(2)].[8] Very recently, highly substituted cyclopenta-
dienes have been obtained from ynamides and prop-
argylic carboxylates.[9] However, the asymmetric syn-
thesis of these derivatives has not been developed.
Continuing with our interest in gold-catalyzed
transformations,[10] we have recently reported the cy-
cloisomerization of 1,3-dien-5-ynes 1 via tandem 6-
endo cyclization–selective migration providing a regio-
specific method for the synthesis of highly substituted
benzene derivatives 2.[11] In this context, and consider-
ing our previous work in the gold-catalyzed cycliza-
tion of o-(alkynyl)styrenes that gives rise to indene
derivatives,[12] we envisaged that these 1,3-dien-5-ynes
Keywords: cycloisomerization; cyclopentadienes;
Diels–Alder reaction; dienynes; gold
Cyclopentadienes are useful synthetic intermediates might be valuable precursors of substituted cyclopen-
in organic as well as in organometallic chemistry.[1]
For instance, they are reactive diene counterparts in
the Diels–Alder cycloaddition,[2] one of the most
useful synthetic reactions for the construction of the
cyclohexane moiety, with up to four contiguous ste-
reogenic centers being created in a single operation,
usually through an endo-favouring transition state. In
this context, enantiomerically pure cyclopentadienes[3]
have been applied in the kinetic resolution of racemic
dienophiles and as chiral templates.[4] However, these
compounds are not readily available from the chiral
pool and, moreover, the asymmetric synthesis of cy-
clopentadienes from achiral substrates is almost un-
known.[5]
On the other hand, gold-catalyzed enyne cycloiso-
merization reactions have been shown as a versatile
strategy for the construction of a wide variety of
cyclic moieties.[6] In this area, 1,5-enynes with a 1,1-
disubstituted alkene moiety are known to undergo 5-
endo alkoxycyclization leading to functionalized cy-
clopentenes [Scheme 1, Eq. (1)].[7] Related gold(I)- or
Scheme 1. Previous related work and proposal for 5- vs. 6-
platinum(II)-catalyzed cycloisomerizations of vinylal- endo cyclization of 1,1-disubstituted-1,3-hexadien-5-ynes 1.
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a stereogenic center at C-5 and, in addition, we dem-
onstrate the usefulness of these functionalized dienes
in the diastereo- and enantioselective preparation of
complex cycloadducts by Diels–Alder reactions.
For the initial experiments we selected model sub-
strate 1a,[15] which was treated at room temperature
with different gold(I) catalysts[16] in a ca. 2:1 CH2Cl2/
MeOH mixture as solvent (Scheme 2). Gratifyingly,
we found that the presence of MeOH seems to sup-
press the formation of benzene derivative 2a, as cyclo-
pentadiene derivatives 3a and 4a were mainly formed
in just 15 min of reaction when 1a was treated with
Ph3PAuNTf2.[17] Changing to a catalyst with an elec-
tron-donating ligand, we found that the use of an N-
heterocyclic carbene (IPr) ligated gold(I) complex
Scheme 2. Initial experiments and proof of concept.
tadienes if a gold-catalyzed 5-endo cyclization was clearly favoured the formation of 3a over triene 4a.[18]
feasible (Scheme 1). Pleasantly, the bulkier and electron-rich catalyst
Whereas we and others have reported the benzan- XPhosAuNTf2 completely favoured the addition of
nulation of 1,3-dien-5-ynes under gold[11,13] or rutheni- MeOH preventing both the elimination reaction and
um catalysis,[14] we surmised that cyclopentadiene de- the cycloaromatization, allowing the isolation of 3a in
rivatives such as 3 could be obtained instead of ben- 87% yield (Scheme 2). We also checked that a de-
zenes 2 if we were able to prevent the alkyl migration crease of the amount of MeOH in the reaction mix-
in the gold carbenoid intermediate initially generated ture led to an increase of the elimination product
from 1,1-disubstituted 1,3-dien-5-ynes. To this purpose 4a.[19]
the presence of an external nucleophile like methanol
Having selected XPhosAuNTf2 as the best catalyst,
could be definitive for trapping the cationic gold in- the extent of the reaction regarding the external nu-
termediate (Scheme 1). Herein we report the applica- cleophile was studied (Table 1, entries 1–5). Dienyne
tion of this hypothesis to the development of an enan- 1a reacted with different alcohols to furnish the
tioselective synthesis of cyclopentadienes bearing alkoxy-functionalized cyclopentadienes 3a and 5a–8a,
Table 1. Au(I)-catalyzed alkoxycyclization of dienynes 1. Synthesis of cyclopentadienes 3 and 5–8.[a]
Entry
1
R1
R2
R3
R4
Product
Yield [%][b]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1b
1c
1d
1e
1f
1g
1h
Ph
Ph
Ph
Ph
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
Me
Et
3a
5a
6a
7a
8a
3b
3c
3d
3e
3f
87
81
78
35[c]
66
74
66
86
81
67
76
58
allyl
i-Pr
[d]
Ph
–
4-MeOC6H4
4-BrC6H4
3-Th
Ph
Ph
Me
Me
Me
Me
Me
Me
Me
9
-(CH2)2OCH2-
-(o-C6H4)(CH2)2-
10
11[e]
12[e]
ACHTUNGTRENNUNG
Ph
3-Th
n-Pr
n-Pr
n-Pr
n-Pr
3g
3h
[a]
Reaction mixtures stirred at room temperature for 15 min using 30 equiv. of the corresponding alcohol (3 equiv. for 1,3-
cyclohexanedione).
Yield of isolated product based on the corresponding starting dienyne 1.
40% of 4a was also isolated.
3-Oxocyclohex-1-en-1-yloxy.
Carried out in MeOH. 3-Th=3-thienyl.
[b]
[c]
[d]
[e]
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Enantioselective Synthesis of Cyclopentadienes by Gold(I)-Catalyzed Cyclization of 1,3-Dien-5-ynes
with variable yields depending on the nature of the al-
cohol.[20] Thus, unhindered primary alcohols (en-
tries 1–3) afforded expected cyclopentadienes 3a, 5a
and 6a in high yields with minor amounts of triene 4a,
whereas the use of isopropyl alcohol gave rise to
higher amounts of the elimination product 4a allow-
ing the isolation of 7a in only moderate yield
(entry 4). We also found that 1,3-cyclohexanedione
could be employed as oxygen-centered nucleophile in
this transformation (entry 5). Our results show that
the role of the used alcohol as co-solvent seems to
have a crucial influence on the reaction pathway,
more than a mere external nucleophile.[21] Then, we
prepared various 1,1-dimethyl-1,3-dien-5-ynes 1b–h
bearing different substituents at 3-, 4-, and 5-positions
Scheme 3. Proposed mechanism for the synthesis of cyclo-
pentadienes.
to assess the scope of this catalytic process using
MeOH as nucleophile, as depicted in Table 1 (en-
tries 6–12). This reaction is applicable to substrates
1b–d with a cyclohexenyl moiety at R2–R3 and pos- goal: the synthesis of the elusive enantioenriched cy-
sessing different substituents at R1, including elec- clopentadienes by controlling the absolute stereo-
tron-rich and electron-withdrawing aromatics as well chemistry of C-5. Based on our previously reported
as heteroaromatic ones (entries 6–8).[22] In addition, enantioselective synthesis of indenes,[12a] as well as re-
regarding the substitution at the central double bond lated gold-catalyzed enantioselective reactions involv-
(R2, R3), the reaction tolerates cyclic aliphatic groups, ing alkyne activation,[25] and after several experiments
including functionalized ones (entries 6–9), as well as on the enantioselective cycloisomerization of 1a using
aromatic substituents (entry 10). However, when dinuclear chiral gold(I) catalysts, we found that,
using dienyne 1g, bearing linear subtituents at R2 and among the commonly used chiral biphosphine ligands
R3 positions, under standard conditions we obtained with biphenyl skeletons, (S)-DM-MeO-BIPHEP gave
a mixture of 3g, the corresponding triene 4g, and ben- the best results for enantio- and chemoselectivity.[26]
zene derivative 2g. After some optimization we found The screening of silver salts was performed in di-
that the desired cyclopentadienes 3g and 3h could be chloromethane (DCM) solution at room temperature
prepared in high yield by performing the reaction in revealing that AgSbF6 was the better co-catalyst and,
pure MeOH as solvent (entries 11 and 12).
as expected, by lowering the reaction temperature
A catalytic cycle[23] that accounts for the formation better ee values were observed. Thus, under the opti-
of cyclopentadienes 3 and 5–8 is shown in Scheme 3. mized conditions developed, cyclopentadiene 3a was
Coordination of the gold complex to the triple bond obtained with a remarkable 92% ee in good yield and
of the starting dienyne 1 would give an intermediate reasonable reaction time (Table 2, entry 1).
9, which would undergo intramolecular attack of the
With these reaction conditions in hand, the sub-
alkene moiety leading to cationic intermediate 10. strate scope for this highly interesting enantioselective
This could be represented as the contribution of sev- synthesis of 3 was examined. As depicted in Table 2
eral resonance structures (10’, 10’’…), delocalizing the (entries 1–5), 5-methoxyalkylcyclopentadienes 3a–g,
positive charge along different positions of the mole- previously prepared in
a racemic manner (see
cule. In the presence of an external nucleophile such Table 1) were now obtained as optically active prod-
as methanol, direct trapping of carbocation 10’’ or nu- ucts with high enantiomeric excess for dienynes 1a–d,
cleophilic attack on 10 with subsequent ring opening, bearing a cycloalkyl moiety at the R2, R3 positions
would lead to vinyl gold intermediate 11.[24] Further and with moderate ee in the case of 1g with linear
protodeauration affords the corresponding cyclopen- substituents at these positions.[27] In addition, other 5-
tadiene 3, 5–8 regenerating the catalytic gold species. alkoxyalkyl-substituted cyclopentadienes 5a–8a were
It is interesting that whereas in the absence of metha- also prepared with high enantioselectivity (Table 2,
nol a Wagner–Meerwein rearrangement exclusively entries 6–9). Although in some cases chemical yields
takes place in 10’ leading to benzene derivatives 2,[11] for the cyclopentadienes were only moderate due to
this pathway is mostly suppressed in the presence of the concurrent generation of the corresponding tri-
the alcohol. This change in product distribution shows enes 4, whose formation resulted to be more competi-
how important alcohols are for controlling the selec- tive at low temperature, their separation was very
tivity of the reaction.
easy by column chromatography. Thus, new highly
Once we had developed an efficient method for the functionalized cyclopentadienes bearing a stereogenic
synthesis of cyclopentadienes we turned to our main center at C-5 could be prepared in an enantioselective
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Table 2. Au(I)-catalyzed enantioselective synthesis of alkoxy-functionalized cyclopentadienes 3 and 5–8.
Entry
1
R1
R2
R3
R4
Product
Yield [%][a]
ee [%][b]
1
1a
1b
1c
1d
1g
1a
1a
1a
1a
Ph
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
Me
Me
Me
Me
Me
Et
3a
3b
3c
3d
3g
5a
6a
7a
8a
74
40[d]
45
71
56
68
80
31
48
92
92
84
88
40[f]
93
89
84
93
2[c]
3[c]
4
4-MeOC6H4
4-BrC6H4
3-Th
Ph
Ph
Ph
5[e]
6
7
n-Pr
n-Pr
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
allyl
i-Pr
8
Ph
Ph
9[c]
–
[g]
[a]
Yield of isolated product based on the corresponding starting dienyne 1. Variable amounts of the corresponding triene 4
were obtained, see the Supporting Information for details.
Determined by HPLC on a chiral stationary phase, see the Supporting Information for details.
Carried out at À258C.
[b]
[c]
[d]
[e]
[f]
An unidentified product was also obtained along with 3b.
Carried out in MeOH at À158C.
The ee was determined from cycloadduct 13ga.
3-Oxocyclohex-1-en-1-yloxy.
[g]
way. The absolute configuration of the synthesized
dienes was determined to be S,[28] and interestingly, it
resulted to be the same that we had previously ob-
served in the enantioselective synthesis of indenes
from o-(alkynyl)styrenes using the same chiral gold
complex.[12a]
The reactivity of some of the synthesized cyclopen-
tadienes 3 in Diels–Alder cycloadditions was also ex-
amined. So, model cyclopentadiene 3a was reacted
with a variety of dienophiles 12 at room temperature
allowing the isolation of cycloadducts 13aa–ag as
single diastereoisomers (Scheme 4). Highly reactive
dienophiles such as maleimides 12a–d, maleic anhy-
dride 12e, or 4-phenylurazole 12f underwent efficient
cycloaddition to give the corresponding adducts 13aa–
af in high yields with only an endo isomer detected by
1H and 13C NMR analyses of the reaction mixture.
The use of plane-non-symmetrical dienes like 3a also
implies the issue of facial (syn-anti) selectivity.[29] No-
tably, in all the reactions performed, only the anti-
endo cycloadduct was observed. Moreover, when
Scheme 4. Diels–Alder reactions of cyclopentadiene 3a.
We then performed the cycloisomerization/Diels–
using diethyl fumarate 12g as dienophile two anti ad- Alder sequence in one pot, starting from dienynes
ducts are possible, each of them bearing an endo ester 1 and just adding different maleimides 12a–c when
group at the R1 or R2 position, respectively. Remarka- consumption of 1 was detected by TLC, without pu-
bly, only one anti isomer (13ag), whose structure was rification of the corresponding cyclopentadiene 3 or 6.
confirmed by X-ray analysis,[30] was obtained, locating This approach also produced the cycloadducts 13,
the endo substituent at R1 (Scheme 4).
when using methanol, or 14 when allyl alcohol re-
placed methanol, in usually good yields for the two
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Enantioselective Synthesis of Cyclopentadienes by Gold(I)-Catalyzed Cyclization of 1,3-Dien-5-ynes
Table 3. One-pot two-steps approach to the preparation of cycloadducts 13 and 14 from dienynes 1.
Entry
1
R1
R2
R3
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
-(CH2)4-
R4
R5
Product
Yield [%][a]
1
2
3
4
5
6
7
8
9
1a
1a
1a
1b
1c
1d
1a
1f
1g
Ph
Ph
Ph
Me
Me
Me
Me
Me
Me
allyl
Me
Me
Ph
Bn
Me
Ph
Ph
Ph
Ph
Ph
Ph
13aa
13ab
13ac
13ba
13ca
13da
14aa
13fa
13ga
69
71
73
63
57
60
53
59
74
4-MeOC6H4
4-BrC6H4
3-Th
Ph
Ph
Ph
-(o-C6H4)
ACHTUNGTRENNUNG
n-Pr
[a]
Yield of isolated products 13 and 14 based on the starting dienyne 1. A ca. 3–15% of the corresponding cycloadducts de-
rived from loss of methanol were also formed, see the Supporting Information for details.
steps (Table 3).[31] With this approach three C C and had previously been used as precursors of benzene
one C O bonds, as well as five stereogenic centers, derivatives and this work shows the utility of these
À
À
have been created in a single operation with good easily available starting materials as precursors of
overall yields and complete diastereoselectivity.[32]
(enantioenriched) cyclopentadienes. The synthesized
An exciting advance in asymmetric Diels–Alder dienes are useful partners for Diels–Alder cycloaddi-
chemistry is the use of chiral dienes as stereodirecting tion reactions with selected dienophiles allowing the
elements to render cycloadducts with high enantio- synthesis of functionalized cycloadducts with five ste-
meric and diastereomeric excesses. So, selected cyclo-
adducts 13 and 14 previously obtained as racemic
mixtures (see Scheme 4) were also enantioselectively
prepared by using the combined catalytic system we
had previously optimized, consisting of a gold com-
plex with the (S)-DM-MeO-BIPHEP ligand and
AgSbF6. In this case, the one-pot, two-step approach
was employed, that is, addition of the corresponding
dienophile 12 after cyclopentadiene formation and re-
moval of the silver salt, in order to avoid further elim-
ination of methanol in the alkoxy-functionalized cy-
cloadduct. Using MeOH as solvent for the cycloaddi-
tion reaction allows easy isolation by simple filtration
of the final products that are obtained in high com-
bined yields over two steps (referred to starting dien-
ynes 1) and with high enantioselectivity that could be
further improved after simple recrystallization
(Scheme 5). It is worthy of note that the Diels–Alder
reactions proceeded with complete conservation of
enantiomeric purity of the chiral diene and so cyclo-
adducts 13 and 14, which possess five contiguous ste-
reocenters including two quaternary ones, have been
created from achiral starting materials in a completely
diastereoselective
manner.
and
highly
enantioselective
In conclusion, we have developed an asymmetric
gold-catalyzed synthesis of cyclopentadienes by
Scheme 5. Sequential enantioselective cycloisomerization/
alkoxyACHTUNGTRENNUNGcyclization of 1,3-dien-5-ynes. These substrates Diels–Alder reaction of dienynes 1.
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reogenic centers in a completely diastereo- and highly General Procedure for the Gold(I)-Catalyzed
enantioselective way.
Enantioselective Synthesis of Cyclopentadienes 3a–d
and 5a–8a
AgSbF6 (5 mol%, 5.1 mg) was added to a solution of (S)-
DM-MeO-BIPHEPACHTNUTRGNEUNG(AuCl)2 (2.5 mol%, 8.7 mg), in dry
Experimental Section
CH2Cl2 (0.3 mL) and the resulting reaction mixture was
stirred for 10–15 min under N2. The appropriate nucleophile
[(30 equiv., 9 mmol) or (3 equiv., 0.9 mmol, 100 mg for 1,3-
cyclohexanedione)] was added and the mixture was cooled
to À308C, À258C or À158C (see Table 2). Then a solution
of the corresponding 1,3-dien-5-yne 1 (0.3 mmol) in dry
CH2Cl2 (0.3 mL) was added and the reaction mixture was
stirred until complete disappearance of starting material, as
monitored by TLC (16 h). The mixture was diluted with
a 9:1 mixture of hexane/EtOAc, and filtered through a pad
of celite. The solvent was removed under reduced pressure
and the remaining residue was purified by flash chromatog-
raphy on silica gel using mixtures of hexane/EtOAc as elu-
ents. The corresponding enantioenriched cyclopentadienes
3a–d or 5a–8a were isolated in the yields and ee reported in
Table 2. HPLC traces for the prepared compounds are pre-
sented in the Supporting Information.
General Remarks
All reactions involving air-sensitive compounds were carried
out under an N2 atmosphere (99.99%). All glassware was
oven-dried (1208C), evacuated and purged with nitrogen.
All common reagents and solvents were obtained from com-
mercial suppliers (VWR, Alfa and Aldrich) and used with-
out any further purification. Solvents were dried by standard
methods. Hexane and ethyl acetate were purchased as ex-
trapure grade reagents and used as received. Gold and silver
catalysts were purchased from Aldrich or Strem. Chiral
gold(I) complexes were prepared according to the methods
described in the literature.[25a] For the preparation of starting
dienynes see the Supporting Information. TLC was per-
formed on aluminum-backed plates coated with silica gel 60
with F254 indicator; the chromatograms were visualized
under ultraviolet light and/or by staining with a Ce/Mo re-
agent and subsequent heating. Rf values are reported on
silica gel. Flash column chromatography was carried out on
silica gel 60, 230–240 mesh. NMR spectra were measured on
Varian Mercury-Plus 300 MHz and Varian Inova-400 MHz
spectrometers. High resolution mass spectra (HR-MS) were
recorded on a Micromass Autospec spectrometer using EI
at 70 eV. Melting points were measured on a Gallenkamp
apparatous using open capillary tubes and are uncorrected.
GC-MS and low resolution mass spectra (LR-MS) measure-
ments were recorded on an Agilent 6890N/5973 Network
GC System, equipped with an HP-5 MS column. An Agilent
HPLC chromatograph equipped with V-UV diode-array de-
tectors was used for the determination of the enantiomeric
ratio; Chiralcel-OD-H and Chiralpack-AD-H were em-
ployed as chiral columns.
Acknowledgements
We gratefully acknowledge the Ministerio de Ciencia e Inno-
vaciꢀn (MICINN) and FEDER (CTQ2010-15358 and
CTQ2009-09949/BQU) and Junta de Castilla
y Leꢀn
(BU021A09 and GR-172) for financial support. A.M.S.
thanks Junta de Castilla y Leꢀn (Consejerꢁa de Educaciꢀn)
and Fondo Social Europeo for a PIRTU contract. P.G.-G.
and M.A.F.-R. thank MICINN for “Juan de la Cierva” and
“Ramꢀn y Cajal” contracts.
References
General Procedure for the Gold(I)-Catalyzed
Synthesis of Cyclopentadienes 3a–f and 5a–8a
[1] See, for instance: a) H. Adams, N. A. Bailey, M. Colley,
P. A. Schofield, C. White, J. Chem. Soc. Dalton Trans.
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A. T. McPhail, J. Organomet. Chem. 1995, 491, 11–18.
[2] a) The Diels–Alder Reaction: Selected Practical Meth-
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VCH, Weinheim, 2002.
[3] E. Winterfeldt, Chem. Rev. 1993, 93, 827–843.
[4] See, for instance: a) C. Borm, D. Meibom, E. Winter-
feldt, Chem. Commun. 1996, 887–894; b) M.-E. Trꢄn-
Huu-Dꢄu, R. Wartchow, E. Winterfeldt, Y.-S. Wong,
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Eur. J. 2003, 9, 3849–3858.
A solution of the corresponding 1,3-dien-5-yne 1 (0.5 mmol)
in dry CH2Cl2 (0.5 mL) was added to a solution of XPho-
sAuNTf2 (5 mol%, 24 mg) and the appropriate nucleophile
[(30 equiv., 15 mmol) or (3 equiv., 1.5 mmol, 168 mg for 1,3-
cyclohexanedione)] in dry CH2Cl2 (0.5 mL). The reaction
mixture was stirred at room temperature until complete dis-
appearance of the dienyne derivative was observed by TLC
(10–15 min). The mixture was diluted with hexane/EtOAc
(9:1), and filtered through celite. Then, the solvent was re-
moved under reduced pressure and the remaining residue
was purified by flash chromatography on silica gel using
mixtures of hexane/EtOAc as eluents. The corresponding cy-
clopentadienes 3a–f or 5a–8a were isolated in the yields re-
ported in Table 1. Characterization data and NMR spectra
are presented in the Supporting Information.
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needed as well as to a solvent effect (78% ee was ob-
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[28] Determined by X-ray analysis of cycloadduct 13ad. The
configurations of the remaining cyclopentadienes were
assigned by analogy. Structural parameters for 13ad
have been deposited with The Cambridge Crystallo-
graphic Data Centre under CCDC 929962. These data
can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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[30] CCDC 929963 (13ag) contains the supplementary crys-
tallographic data for this compound. These data can be
obtained free of charge from The Cambridge Crystallo-
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[15] For the preparation of starting dienynes 1 see ref.[11]
[16] No reaction was observed with other metal complexes
such as PtCl2 or AgSbF6.
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[18] Triene 4a, probably derived from a competitive elimi-
nation pathway, was obtained in variable amounts with
different catalysts. See the Supporting Information for
details about optimization of the reaction conditions.
[19] The use of 1.5 equiv. of MeOH gave rise to a ca. 10:1
mixture of 3a:4a.
[31] Small amounts of the cycloadducts derived from loss of
MeOH were also obtained probably due to the pres-
ence of the gold catalyst.
[32] For other examples of gold-catalyzed cascade cycloiso-
merization/Diels–Alder reactions, see: a) A. S. K.
Hashmi, M. Rudolph, J. P. Weyrauch, M. Wçlfle, W
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Frey, J. W. Bats, Angew. Chem. 2005, 117, 2858–2861;
Angew. Chem. Int. Ed. 2005, 44, 2798–2801; b) J. Bar-
luenga, J. Calleja, A. Mendoza, F. Rodrꢁguez, F. J.
FaÇanꢀs, Chem. Eur. J. 2010, 16, 7110–7112; c) Z.-Y.
Han, D.-F. Chen, Y.-Y. Wang, R. Guo, P.-S. Wang, C.
Wang, L.-Z. Gong, J. Am. Chem. Soc. 2012, 134, 6532–
6535; d) B. Liu, K.-N. Li, S.-W. Luo, J.-Z. Huang, L.-Z.
Gong, J. Am. Chem. Soc. 2013, 135, 3323–3326.
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