Catalytic cascade hydroalkoxylation/isomerization/[4 + 2] cycloaddition using enyne alcohols as latent dienes or dienophiles

Article abstract of DOI:10.1039/c3ob41611a

Enyne alcohols can react as precursors of either dienes or dienophiles with different substrates after hydroxylation and isomerization by gold catalysis. As such, oxa-bridged tricyclo[5.2.2.0^2,6]undec-8-ene-3,5-dione derivatives have been obtained by the Diels-Alder reaction and tetrahydro-1H-furo[3,4-c]pyran derivatives could be accessed by the hetero-Diels-Alder cycloaddition.

Full text of DOI:10.1039/c3ob41611a

Organic &
Biomolecular Chemistry
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Catalytic cascade hydroalkoxylation/isomerization/
[4 + 2] cycloaddition using enyne alcohols as latent
dienes or dienophiles†
Cite this: Org. Biomol. Chem., 2013, 11,
6707
Received 7th August 2013,
Accepted 15th August 2013
Rui Guo, Kang-Nan Li and Liu-Zhu Gong*
DOI: 10.1039/c3ob41611a
www.rsc.org/obc
Enyne alcohols can react as precursors of either dienes or dieno- reactions by virtue of gold catalysis for the synthesis of struc-
philes with different substrates after hydroxylation and isomeriza- turally complex cyclic molecules.5
tion by gold catalysis. As such, oxa-bridged tricyclo[5.2.2.02,6]-
Very recently, we have found a variety of relay catalytic
undec-8-ene-3,5-dione derivatives have been obtained by the cascade reactions,6 including the consecutive hydroamination/
Diels–Alder reaction and tetrahydro-1H-furo[3,4-c]pyran deriva- transfer hydrogenation of alkynes6a and a cyclization of alky-
tives could be accessed by the hetero-Diels–Alder cycloaddition.
nols triggered addition of azlactones6b using a combined cata-
lyst system consisting of a chiral phosphoric acid and a gold
complex. In particular, the Au(I)/Brønsted acid binary catalyst
enables enynes as latent 1,3-silyloxydienes to participate in an
unprecedented alternative to traditional Diels–Alder reactions,
providing an efficient route to highly enantioenriched poly-
cyclic compounds.6d
Because of our continuous interest in this field, we envi-
saged that the 1,3-butadiene derivatives, generated from the
cyclization of enyne alcohols 1 catalyzed by an appropriate
gold complex, would principally participate in the subsequent
cycloaddition reaction, which might presumably be catalyzed
by chiral catalysts to afford enantioenriched bicyclic mole-
cules.4,5 Herein, we will demonstrate that the dienes catalyti-
cally generated from enyne alcohols 1 were able to undergo
either the Diels–Alder or inverse-electron-demand hetero-
Diels–Alder reaction, depending on the dienophiles and reac-
tion conditions (Scheme 2).
Heterocyclic structural skeletons are prevalent in biologically
active natural products, pharmaceuticals, and numerous func-
tional molecules.1 Therefore, the development of new synthetic
methods to directly access heterocycles is of great importance.
Indeed, a large number of synthetic methods have emerged
for the synthesis of heterocycles.2 Cascade or domino reac-
tions, because they are able to build up structurally complex
molecules from relatively simpler substrates, have received
increasing attention.3 In recent years, gold catalysis has
allowed one to design a variety of cascade reactions, some of
which are capable of efficiently preparing structurally complex
heterocyles. For instance, Barluenga and co-workers4
established a cascade cycloisomerisation/Diels–Alder reaction
for the synthesis of polycyclic compounds, wherein the AuCl3
was a catalyst for both the cycloisomerization of functionalized
enynes to generate 1,3-butadiene derivatives and the sub-
sequent Diels–Alder reaction with dienophiles (Scheme 1).
Some other groups have also reported various cascade
At the outset of our study, the N-(4-nitrophenyl)-maleic
diimine 2, which could be activated by a Brønsted acid
through formation of hydrogen bonds,7 was used as a dieno-
phile for the proposed cascade Diels–Alder reaction with
enyne alcohols 1a in the presence of combined catalysts of a
Scheme 1 Au(III)-catalyzed cascade cyclization/Diels–Alder reaction.
Hefei National Laboratory for Physical Sciences at the Microscale and Department of
Chemistry, University of Science and Technology of China, Hefei, 230026, China.
E-mail: gonglz@ustc.edu.cn; Fax: +86-551-3606266
†Electronic supplementary information (ESI) available. CCDC 893862, 893863
and 915445. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3ob41611a
Scheme 2 The reactions investigated in this study.
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(entries 10–12). The use of 5 mol% of the catalyst still provided
a reasonable yield (entry 13).
Table 1 Optimization for gold-catalyzed cascade reaction of enyne alcohols 1a
with N-(4-nitrophenyl)-maleic diimine 2a
ð1Þ
Entry
[Au]
Solvent
Yieldb (%)
With the optimized conditions in hand, the generality for a
scope of enyne alcohols 1 was then explored. As shown in
Scheme 3, a diverse spectrum of alkyl groups were tolerated,
delivering 3 as a single diastereomer in good to high yields
(65–83%). The enyne alcohols 1 bearing a linear alkyl substitu-
ent at C1 (R group) underwent the cascade reaction cleanly,
furnishing the desired products (3b–3c) in high yields
(74–78%). A slightly lower yield was obtained when a branched
alkyl substituent was introduced into C1 (3d). Moreover, the
α-branched cyclopropyl group at C1 of the enyne alcohols was
also allowed to provide product 3e in an 83% yield. In
addition, the presence of a aryl group at C1 of 1 was also toler-
ated to give 3f–3h in good yields. The X-ray diffraction analysis
of 3f confirmed the structure and showed that the endo-selec-
tive products were favourably generated.9
1c
2d
3
4
5
6
7
8
9
10
11
12
13f
PPh3AuTFA
PPh3AuTFA
PPh3AuTFA
AuCl3
PPh3AuCl/AgSbF6
PPh3AuCl/AgBF4
PPh3AuCl/AgOTf
PPh3AuCl/AgOTs
PPh3AuCl/AgTFA
PPh3AuTFA
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
Toluene
THF
60
80
78
e
—
e
—
e
—
e
—
e
—
81
Trace
22
63
68
PPh3AuTFA
PPh3AuTFA
PPh3AuTFA
DCE
CHCl3
a Reaction was run as a 0.1 M solution of 2, TFA = CF3COO. b Isolated
yield after purification by flash chromatography. c 4 (10 mol%) was
added and the reaction was run at rt for 6 days. d 4 (10 mol%) was
added and the reaction was run at 60 °C for 2 days. e A complicated
reaction mixture was observed. f 5 mol% PPh3AuTFA.
However, the extension of the optimal conditions to
the cascade hydroalkoxylation/cyclization reaction using
(E)-dimethyl 4-oxo-pent-2-enedioate 6 as a dienophile with 1f
under gold catalysis failed. Thus, the re-optimization of reac-
tion conditions was performed and found that the presence of
catalytic amount of phosphoric acid 4 led to an unpredicted
hetero-Diels–Alder reaction, to give a bicylic compound 7a, as
indicated by single-crystal X-ray diffraction analysis,10 in a
reasonable yield (Table 2, entry 1). Increasing the loading of
the gold complex was beneficial to the reaction whereas the
presence of more amounts of the phosphoric acid led to a
sacrificed yield (Table 2, entry 2 vs. 3). The use of preformed
Ph3AuTFA to replace that in situ generated from PPh3AuCl and
AgTFA somehow eroded the yield (entry 4).11 Elevating the
reaction temperature also led to an unsatisfactory yield (entry
gold complex and a chiral Brønsted acid.8 However, the reac-
tion proceeded very slowly at room temperature. After the reac-
tion proceeded for six days, the desired tricyclic product 3a
was isolated in 60% yield, but with no stereoselectivity
(Table 1, entry 1). When the temperature was elevated to 60 °C,
the reaction could undergo completely within two days and
the yield was improved to 80%, however, no stereoselection
was observed (Table 1, entry 2). These results suggested that
the chiral Brønsted acid might not act as an activator of the
dienophile 2. Indeed, a control experiment to investigate
the Diels–Alder reaction between diene 5a and 2 found that
the diene 5a generated from the gold-catalyzed cyclization of
enyne 1a was able to smoothly undergo the [4 + 2] cyclo-
addition with 2 even in the absence of any catalyst, furnishing
the product 3c in 81% yield (eqn (1)), indicating that the
Diels–Alder step was spontaneous and the Brønsted acid was
therefore hard to control the stereochemistry. Moreover, the
cascade reaction also proceeded cleanly in the presence of the
gold complex alone (Table 1, entry 3). The commercially avail-
able gold trichloride, which was previously applied to catalyze
the cascade cyclization/Diels–Alder reaction,4 however afforded
a complicated reaction, from which no desired product was
observed (Table 1, entry 4). The counterions of gold complexes
were found to exert considerable impact on the catalytic
activity. Among the cationic gold complexes screened (Table 1,
entries 5–9), only gold trifluoroacetate showed good catalytic
activity (entries 3 and 9). The survey of solvents found that the
halogenated solvents were basically better media for this
cascade reaction whereas the reaction did not occur in a non-
Scheme 3 The scope of the cascade hydroalkoxylation/isomerization/Diels–
Alder cycloaddition reaction of enyne alcohols 1 with N-(4-nitrophenyl)-maleic
polar solvent as exemplified by the case conducted in toluene diimine 2.
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Table 2 Optimization of reaction conditions for gold/Brønsted acid-catalyzed
cascade reaction of enyne alcohols 1f with (E)-dimethyl 4-oxopent-2-enedioate
6a
Entry
x/y
1f/6
Solvent
Yieldb (%)
1
2
5/10
2/1
2/1
2/1
2/1
2/1
4/1
4/1
4/1
4/1
4/1
4/1
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
THF
52
62
46
48
44
68
63
—
41
22
—
10/10
10/20
—
10/10
10/10
10/10
10/10
10/10
10/10
10/10
3
4c
5d
6
7
8
9
10
11
DCE
Toluene
Et2O
Scheme 4 The scope of the cascade hydroalkoxylation/isomerisation/Diels–
Alder cycloaddition/hydrolysis/lactonization reaction of enyne alcohols 1 with
(E)-dimethyl 4-oxopent-2-enedioate 6.
a Reaction was run as a 0.05 M solution of 6. b Isolated yield after
purification by flash chromatography. c PPh3AuTFA was used instead
of PPh3AuCl/AgTFA. d Reaction was run at 35 °C.
carbonyl compounds.14 Thus, we initially investigated the
asymmetric cascade reaction using a chiral copper complex as
the co-catalyst (Table 3). To our delight, the chiral cyclo-
addition product was formed in 42% yield and 87% ee using
10 mol% of the chiral copper complex prepared in situ from
Cu(OTf)2 and bis-(oxazoline) 8a (entry 1). Interestingly, the
5). Tuning the ratio of 1f to (E)-dimethyl 4-oxo-pent-2-enedio-
ate (6) from 2/1 to 4/1 gave a much higher yield (68%, entry 6).
A screening of solvents, including chloroform, 1,2-dichloro-
ethane (DCE), and toluene, revealed that dichloromethane is
the best media for this cascade reaction (entries 7, 9, and 10
vs. 5). However, no desired product was isolated when the reac-
tion was conducted in THF and ether, indicating that the reac-
tion was unable to occur in a solvent with coordinating
heteroatoms (entries 8 and 11).12
Table 3 Optimization for the asymmetric gold/Lewis acid catalyzed cascade
reaction of enyne alcohols 1f with (E)-dimethyl 4-oxopent-2-enedioate 6a
The generality for 3-methylene-5-arylpent-4-yn-1-ols bearing
an aryl substituent bonded to the carbon–carbon triple bond
was then investigated (Scheme 4).13 A variety of aryl substitu-
ents on the enyne alcohols 1 were tolerated. Basically, electro-
nically rich aryl substituents at C1 proceeded cleanly to give
the desired products 7 in good yields ranging from 64% to
75% (7b–d and 7g). The variation of the para-substituent on
the phenyl group at C1 of the enyne alcohols 1 showed that
the less sterically hindered substituents were beneficial to the
formation of the products (7b–7d). However, the introduction
of an electron-withdrawing substituent such as 4-F or 3-OMe
on the phenyl group at C1 would diminish the yields of bi-
cyclic compounds 7e and 7h. The substrates with weakly
electron-withdrawing substituents such as halides on the
benzene ring at C1 also participated in a good reaction,
affording the desired products in good yields, as exemplified
by 7f and 7i (71–79%).
Entry
(Box)MXn
Solvent
Yieldb (%)
eec (%)
1
(8a)Cu(OTf)2
(8a)Cu(SbF6)2
(8a)Cu(SbF6)2
(8a)Cu(ClO4)2
(8b)Cu(SbF6)2
(8c)Cu(SbF6)2
(8a)Mg(OTf)2
(8a)Zn(OTf)2
(8a)Ni(ClO4)2
(8a)In(OTf)3
(8a)Cu(SbF6)2
(8a)Cu(SbF6)2
(8a)Cu(SbF6)2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
42
58
60
50
27
87
−92
−82
−92
−87
n.d. f
n.d. f
78
35
28
64
45
2
3d
4
5
6
7
8
e
—
e
—
35
26
64
23
15
49
9
10
11
12
13
Toluene
CHCl3
75
Although the presence of BINOL-based phosphoric acids
and phosphoramides could accelerate the reaction, the
enantioselectivity was disappointing (up to 18% ee, see ESI†).
Previous findings indicated that chiral Lewis acids, such
as copper complexes of chiral bis-(oxazoline), were excellent
a Reaction was run as a 0.05 M solution of 6. b Isolated yield after
purification by flash chromatography. c Determined by HPLC.
catalysts for the asymmetric hetero-Diels–Alder reactions of d Reaction was run at 35 °C. e Trace product. f Not determined.
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variation of the counter anion of the copper complex from tri-
flate to hexafluoroantimonate was able to switch the stereo-
chemistry and led to further enhanced results of 58% yield
and 92% ee (entry 2 vs. 1). Although the yield was slightly
improved, the enantioselectivity was significantly sacrificed
upon conducting the reaction at an elevated temperature
(entry 3). The use of other chiral Lewis acids with bis-(ox-
azoline) gave unsatisfactory results (entries 4–10). In compari-
son with dichloromethane, other solvents, including
tetrahydrofuran, toluene and chloroform, always gave inferior
results presumably due to the poorer solubility of the [Cu-box]-
(SbF6)2 complexes (entries 11–13). Subsequently, some other
substrates were examined using the combined catalyst system
consisting of the gold complex and the chiral copper complex
(Scheme 5). The presence of either an electron-donating or
withdrawing substituent in the enyne alcohols 1 was appli-
cable to smoothly undergo the asymmetric relay catalytic
cascade reaction, furnishing the products in acceptable yields
and with high enantioselectivities of up to 90% ee. The absol-
Scheme 7 Proposed mechanism for the formation of the deuterated product.
ute and relative configurations were determined by X-ray diffr- hand, the hetero-Diels–Alder reaction between 1f and 6, in the
action analysis.15
presence of deuterium water (D2O) showed that the product
Interestingly, the cascade cyclization/Diels–Alder reaction of 7a′ was formed with deuteration at C1–C3. As shown in
1a with 2 still proceeded in the presence of a non-poisoning Scheme 7, the gold complex first catalyzes the cyclization of
base, 2,6-di-tert-butylpyridine (eqn (2)), which was used to sca- the enyne alcohols 1 to generate active intermediates 10. Sub-
venge the trifluoroacetic acid probably formed from the reac- sequently, the proton formed by deprotonation of 10 was
tion.16 The results suggested that the gold complex, rather exchanged to deuterium ions by reacting with D2O, which par-
than the Brønsted acid, promoted the exo/endo isomerization ticipated in the protodemetalization of 10 to furnish deuter-
of the initially formed enol ether I (Scheme 6). On the other ated intermediates 12 at C1. The re-coordination of the gold
complex to the double bond outside the ring of 12 affords
intermediates 13, which after deprotonation gave intermedi-
ates 14, as the deuteration occurred at both C2 and C3. By
quenching with the deuterium ions, the intermediates 14 were
subsequently transformed into the dienes 5′ and the gold
catalyst was regenerated. According to these controlled experi-
ments and previous reports on related reactions,17 a mechan-
ism was proposed in Scheme 8. The dienes 5′ could undergo
Diels–Alder reaction with the dienophile 2 to yield the tricyclic
compounds 3. On the other hand, the inverse-electron-
demand hetero-Diels–Alder reaction between 5′ and the
(E)-dimethyl 4-oxo-pent-2-enedioate 6 occurred under the cata-
lysis of the Brønsted acid 418 or Lewis acid to give bicyclic com-
pounds 15, which were hydrolyzed to generate 17 promoted by
the gold complex. After the proton transfer and lactonization,
the intermediates 18 were converted to the final products 7.
In summary, we have developed an unprecedented cascade
Scheme 5 Representative examples for the asymmetric transformation.
hydroalkoxylation/isomerization/Diels–Alder
reaction, providing an efficient method to access tricyclo-
[5.2.2.02,6]undec-8-ene-3,5-dione
derivatives and 1-oxo-
cycloaddition
3,3a,4,7a-tetrahydro-1H-furo[3,4-c]pyran derivatives, which are
not easily accessible through traditional organic reactions.
Interestingly, the key 1,3-butadiene intermediates generated
from gold-catalyzed hydroalkoxylation isomerization of the
enyne alcohols were able to function as either dienes or dieno-
philes, depending on the substrates and reaction conditions.
The excellent stereoselectivity was also obtained in the
cascade cycloisomerization/hetero-Diels–Alder reaction using
Scheme 6 Gold-catalyzed cycloisomerization of 1.
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Scheme 8 Proposed mechanism for the formation of 3 and 7.
the combined gold complex and chiral copper Lewis acid (up
to 92% ee).
This project was supported by NSFC (21172207) and the
Ministry of Education.
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