Gold-catalyzed cycloisomerization of alk-4-yn-1-ones

Article abstract of DOI:10.1039/b819704k

Depending on the substitution pattern and the solvent, the gold-catalyzed cyclization of alk-4-yn-1-ones 1 affords different oxygen heterocycles under mild reaction conditions. Alkynones with one substituent at C-3 undergo a 5-exo-dig cycloisomerization t

Full text of DOI:10.1039/b819704k

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Gold-catalyzed cycloisomerization of alk-4-yn-1-ones†
Volker Belting and Norbert Krause*
Received 5th November 2008, Accepted 17th December 2008
First published as an Advance Article on the web 5th February 2009
DOI: 10.1039/b819704k
Depending on the substitution pattern and the solvent, the gold-catalyzed cyclization of
alk-4-yn-1-ones 1 affords different oxygen heterocycles under mild reaction conditions. Alkynones with
one substituent at C-3 undergo a 5-exo-dig cycloisomerization to substituted furans 2, whereas a
6-endo-dig cyclization to 4H-pyrans 3 is observed with substrates bearing two substituents at C-3. In
alcoholic solvents, alkylidene/benzylidene-substituted tetrahydrofuranyl ethers 4 are formed in a
tandem nucleophilic addition/cycloisomerization.
with alkynylaryl aldehydes,6b whereas cyclization of 2-oxobut-
3-ynoates with nucleophiles provides a general route to 3(2H)-
furanones.6c Pyrroles can be obtained by cycloisomerization of
imines formed in situ from alkynones.6d
Introduction
The development of new methods for the synthesis of heterocycles
is of particular interest in organic chemistry. Due to their atom
efficiency, transition metal-catalyzed cycloisomerizations belong
to the most powerful tools in organic synthesis. Whereas gold
catalysis had received little attention in the past (mainly because
gold was considered to be inert and expensive), the use of gold salts
in homogeneous catalysis has attracted more and more interest in
the last decade because of their ability to activate carbon-carbon
multiple bonds.1 Thus, a variety of nucleophiles can be added
under extremely mild conditions to these activated multiple bonds.
Due to their unique electronic structure and reactivity,2 alkynes
are the substrates of choice in homogeneous gold catalysis.1
They are much more reactive than the corresponding alkenes.
In gold-catalyzed cycloisomerization reactions, various inter-
nal nucleophiles such as nitrogen,3 oxygen,4 and even sulfur5
groups have been used for addition to the activated triple
bond, thereby granting access to a high diversity of hetero-
cyclic products. Of particular interest is the use of carbonyl
groups since they can undergo tandem reactions, consisting of
a nucleophilic addition to this function followed by cyclization.
Various heterocycles have been synthesized using this approach
in homogeneous gold catalysis.6 Godet et al. recently reported
a tandem acetalization/cycloisomerization of quinoline deriva-
tives providing pyranoquinolines with a high conversion rate.6a
1-Alkynyl-1H-isochromenes were synthesized by Li and cowork-
ers in an alkynylation-cyclization sequence of terminal alkynes
Furans play a special role in the field of oxygen heterocycles.
Because of their importance as intermediates in organic synthesis7
and their occurrence in a variety of natural compounds8 and
important pharmaceuticals,8,9 many methods for the synthesis
of furans have been developed.10 Whereas some examples for
the cyclization of pentynones leading to furans under strong
acidic11 or basic12 conditions were reported in literature, only a
few examples of transition metal-catalyzed cycloisomerizations
of alk-4-yn-1-ones to substituted furans a◦re known. Besides Pd
catalysis at elevated temperatures (>60 C),13 Nishizawa and
coworkers reported a Hg(OTf)2-catalyzed cycloisomerization of
pentynones to methylfurans in benzene at room temperature.14
Also gold-catalyzed cycloisomerizations of functionalized alkynes
play an important role: (Z)-alk-2-en-4-yn-1-ols,15 alka-2,3-dien-1-
ones,16 alk-3-yn-1-ones17 and alkynyloxiranes18 can be converted
to substituted furans. Herein we report the results of our study
on the gold-catalyzed cycloisomerization of alk-4-yn-1-ones 1
which, depending on the substitution pattern of the substrate
and the reaction conditions, can afford furans 2, 4H-pyrans 3,
or tetrahydrofuranyl ethers 4 (Scheme 1).
Results and discussion
The starting materials are readily available either by FeCl3-
catalyzed nucleophilic substitution of propargyl acetates with
enoxysilanes10a or by reaction of an enolate with propargyl
bromide.6d Different aryl groups were introduced into terminal
alk-4-yn-1-ones by Sonogashira coupling.19 Initial cycloisomer-
ization experiments were performed with ketone 1a as model
substrate using different gold precatalysts (2 mol%) and catalytic
Dortmund University of Technology, Organic Chemistry, Otto-Hahn-Straße
6, D-44227, Dortmund, Germany. E-mail: norbert.krause@tu-dortmund.de;
Fax: +49(231)7553884
† Electronic supplementary information (ESI) available: Experimental
details and NMR spectra. See DOI: 10.1039/b819704k
Scheme 1 Gold-catalyzed cycloisomerization of alk-4-yn-1-ones 1.
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Table 1 Optimization of reaction conditions for the gold-catalyzed cycloisomerization of alkynone 1a to furan 2a
Entry
Solvent
Precatalyst
Additive
Time
Yield/%
1a
2
3
4
5
6
7
8
9
10
11
12
13
14
15
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CH2Cl2
Et2O
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
AuCl3
AgOTf
AgOTf
AgSbF6
AgBF4
—
—
—
1 h
80
91
85
57
20 min
20 min
20 min
5 h
5 h
3 h
4 h
3 h
5 h
5 h
tracesb
tracesc
63
AuCl
HAuCl4
Au(OAc)3
AuBr3
—
—
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
—
—
35d
57
—
—
82
70
47
44
AgOTf
—
AgOTf
AgOTf
AgOTf
AgOTf
20 min
20 min
3 h
THF
MeCN
3 h
a In the absence of p-TsOH · H2O. b 84% of starting material was reisolated. c 78% of starting material was reisolated. d 50% of starting material was
reisolated.
amounts of p-TsOH · H2O (5 mol%) in various solvents at room
temperature (Table 1).
inactive under the reaction conditions (entries 10, 11). Besides
toluene, other weakly coordinating solvents (dichloromethane,
diethyl ether; entries 12, 13) can be used as well, whereas low
reaction rates and yields were observed in THF or acetonitrile
(entries 14, 15).
Since the gold-catalyzed cycloisomerization of alk-4-yn-1-ones
can be performed in the presence or absence of the Brønsted acid
p-TsOH, both stable and acid-labile substrates can be converted to
substituted furans under the appropriate reaction conditions. The
generality of the method was proven by applying the optimized
reaction conditions to various alk-4-yn-1-ones 1 (Table 2). Both
In contrast to the cyclization of alk-4-ynones with p-TsOH
which requires refluxing in toluene,10a the furan 2a was obtained
with 80% yield at room temperature in the presence of just 2 mol%
of the cationic gold precatalyst Ph3PAuOTf generated in situ from
Ph3PAuCl and AgOTf (entry 1). Combining gold and acid catalysis
by addition of 5 mol% of p-TsOH · H2O led to a slightly increased
reaction rate and chemical yield (entry 2). In contrast to this,
other cationic (entries 3, 4) or neutral gold precatalysts (entries
5–9) gave inferior results. AgOTf and p-TsOH · H2O alone are
Table 2 Gold catalyzed cycloisomerization of alk-4-yn-1-ones 1 to substituted furans 2a
Entry
1
R1
R2
R3
R4
Time/min
2 (Yield/%)
1
2
3
4
5
6
7
8
9
1b
1c
1d
1e
1f
1g
1h
1i
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
Ph
H
H
Ph
20 (30)
20
120 (180)
60
60
45
45
60
300
60
30 (40)
90
2b 87 (82)
2c 77
4-MeOC6H4
i-Bu
4-MeOC6H4
Ph
i-Pr
CH2C CH
H
H
2d 76 (74)
2e 64
∫
4-BrC6H4
4-MeO2CC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
4-MeOC6H4
Ph
2f 84
4-MeOC6H4
2g 57
CH2CHMeCH2CH2
2h 72
Me
i-Pr
Me
H
Et
i-Bu
Et
Ph
i-Pr
2i 81
1j
1k
1l
2j 69
10
11
12
2k 71
2l 63 (67)
2m 75
1m
Ph
Me
a Reaction time and yield given in parentheses refer to cyclizations performed in the absence of p-TsOH · H2O.
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terminal (entries 1–4) and internal alkynes (entries 5–12) can be
cyclized with similar efficiency to afford the furans 2 with 57–87%
yield. The presence of sterically demanding groups (entries 3, 9,
12) led to slightly increased reaction times, but full conversion was
reached within 1.5–5 h. The method also tolerates the presence
of electron-rich (entries 2, 4, 6, 10) or electron-deficient aryl
groups (entries 5–9), of additional triple bonds (entry 4) as well
as ester groups (entries 6, 10). It should be noted that even the
tetrasubstituted furan 2m is accessible with good yield (75%) by
our method. Performing the cycloisomerization in the absence
of p-toluenesulfonic acid resulted in slightly slower reactions, but
very similar yields (entries 1, 3, 11).
On the basis of the accelerating affect of the Brønsted acid p-
TsOH, we propose the reaction mechanism shown in Scheme 2.
The catalytic cycle is initiated by coordination of the gold
catalyst to the triple bond of enol B to afford the p-complex
C which is transformed into the zwitterionic intermediate D
by 5-exo-dig attack of the oxygen at the activated triple bond.
Protodemetalation of D releases the gold catalyst into the catalytic
cycle and affords the alkylidene derivative E which undergoes a
rapid isomerization to the substituted furan F. The accelerating
effect of the Brønsted acid p-TsOH may be twofold: it catalyzes
the equilibrium between the starting alkynone A and its enol
form B, and it might also be involved in the protodemetalation
of intermediate D.
Interestingly, the formation of a six-membered ring G by
6-endo-dig cyclization of intermediate C was not observed with
the alk-4-yn-1-ones used so far. However, this pathway becomes
accessible if the substrate bears two substituents at C-3 which
prevent formation of a furan 2. Treatment of the alkynones 1n–q
with 2 mol% of Ph3PAuCl and AgOTf afforded the corresponding
4H-pyrans 3 with 53–67% yield after 20–45 min reaction time at
room temperature (Table 3). With AgOTf alone, no conversion
of substrate 1n was observed (entry 1). The butyl-substituted
alkynone 1q (entry 4) was found to cyclize faster than the phenyl-
substituted counterparts 1n–o (entries 1–3) and gave the highest
yield. In contrast to the formation of furans 2, addition of
p-TsOH · H2O does not affect the rate of the 6-endo-dig cyclization.
A third pathway for the gold-catalyzed cyclization of alk-4-yn-
1-ones was opened by changing the solvent from toluene to an
alcohol. This acts not only as solvent, but also as nucleophile and
induces formation of alkylidene/benzylidene-substituted tetrahy-
drofuranyl ethers 4 instead of furans 2 or 4H-pyrans 3. The scope
of this tandem nucleophilic addition/cycloisomerization reaction
is summarized in Table 4.
Whereas treatment of the terminal alkynone 1a with 2 mol%
of Ph3PAuCl and AgOTf in methanol resulted in the formation
of a complex product mixture, various alkynones 1l–t with an
internal triple bond and one (entries 1, 2) or two substituents
at C-3 (entries 3–14) afforded the 5-exo-dig cyclization products
4 with 53–87% yield. The substrate was consumed after 10–
30 min at room temperature when primary alcohols were used
while 60 min were required in the reaction of alkynone 1o with
isopropanol (entry 6). No conversion of ketone 1n or 1r was
observed in the absence of Ph3PAuCl (entries 3, 9). Similar to
the other cyclization modes of alkynones 1, the formation of the
tetrahydrofuranyl ethers 4 is not affected by the substituent pat-
tern. Besides various benzylidene-substituted tetrahydrofuranyl
ethers, the alkylidene-substituted product 4qa was obtained with
53% yield from alkynone 1q and ethanol (entry 8). Even the
cyclohexyl-substituted ketone 1n furnished the corresponding
spiro compounds 4na/4nb with good yield (66/76%, entries 3,
4). In the case of substrates bearing two different substituents at
C-3, the tandem nucleophilic addition/cycloisomerization leads to
the formation of diastereomeric products; the diastereoselectivities
are low and range from 55 : 45 to 80 : 20 (entries 1, 2, 5–8). All
tetrahydrofuranyl ethers were formed as a single isomer with
regard to the excocyclic double bond. The configuration of product
4oa was determined with the aid of an NOE experiment; a strong
Scheme 2 Proposed mechanism for the gold-catalyzed cycloisomeriza-
tion of alk-4-yn-1-ones to furans.
Table 3 Gold-catalyzed cycloisomerization of alk-4-yn-1-ones 1 to 4H-pyrans 3
Entry
1
R1
R2
R3
R4
Time/min
3 (Yield/%)
1
2
3
4
1n
1o
1p
1q
Ph
Ph
Ph
n-Bu
(CH2)5
Ph
Ph
Me
Ph
45
40
40
20
3n (53)a
3o (58)
3p (64)
3q (67)
Ph
Ph
Ph
Me
Me
Me
a No conversion in the absence of Ph3PAuCl.
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Table 4 Gold-catalyzed tandem nucleophilic addition/cycloisomerization alk-4-yn-1-ones 1 to tetrahydrofuranyl ethers 4
Entry
1
R1
R2
R3
R4
R5
Time/min
4 (yield/%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1l
Ph
Ph
Ph
Ph
Ph
Ph
Ph
n-Bu
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
H
Ph
Ph
Ph
Ph
Ph
Ph
Me
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
Et
Me
Et
Me
i-Pr
Me
Et
Me
Et
Me
Et
n-Pr
Me
20
40
10
30
20
60
20
30
10
30
10
20
20
20
4la (75)a
4lb (67)a
4na (76)b
4nb (66)
4oa (87)c
4ob (59)c
4pa (78)d
4qa (53)e
4ra (62)b
4rb (64)
4sa (76)
4sb (75)
4sc (70)
4ta (59)
1l
1n
1n
1o
1o
1p
1q
1r
1r
1s
1s
1s
1t
(CH2)5
(CH2)5
Ph
Ph
Ph
Ph
Me
Me
Et
Et
Et
Me
Me
Me
Me
Me
Me
Me
Et
Et
Et
Me
4-MeO2CC6H4
a dr = 80 : 20. b No conversion in the absence of Ph3PAuCl. c dr = 57 : 43. d dr = 63 : 37. e dr = 55 : 45.
cross peak between the vinylidene proton and the methyl group at
C-4 of the tetrahydrofuran ring proves the Z-configuration.
Interestingly, the addition of the Brønsted acid p-
toluenesulfonic acid has a pronounced influence on the course
of the gold-catalyzed cyclization of alk-4-yn-1-ones 1 in alcohols.
Whereas treatment of alkynone 1l with catalytic amounts of
Ph3PAuCl, AgOTf, and p-TsOH · H2O in methanol or ethanol
furnished the furan 2l (67/71% yield) instead of the tetrahydro-
furanyl ethers 4la/4lb, the substrate 1o with two substituents at
C-3 gave a 1 : 3-mixture of the 4H-pyran 3o and the ether 4oa,
accompanied by other non-identified products.
Similar to the tandem cycloisomerization-hydroalkoxylation
of homopropargylic alcohols to tetrahydrofuranyl ethers,4e the
formation of the heterocycles 4 may be explained by a hy-
droalkoxylation of 2,3-dihydrofurans E (Scheme 2). However, it
seems difficult to explain why no attack of the alcohol at the
exocyclic double bond of intermediate E is observed. In order to
account for this experimental finding, we assume that the reaction
is initiated by a (gold-catalyzed?) formation of the hemiacetal H
which then undergoes addition to the activated triple bond in
intermediate I (Scheme 3). This leads to the zwitterionic species J,
protodemetalation of which affords the tetrahydrofuranyl ether K
with Z-configuration at the exocyclic double bond.
Scheme 3 Proposed mechanism for the gold-catalyzed tandem nucle-
ophilic addition/cycloisomerization alk-4-yn-1-ones to tetrahydrofuranyl
ethers.
with alkynones 1n–q bearing two substituents at C-3 which
furnish 4H-pyrans 3 by 6-endo-dig cycloisomerization. Finally,
both substrate types afford alkylidene/benzylidene-substituted
tetrahydrofuranyl ethers 4 when the reaction was carried out in
an alcoholic solvent. These transformations probably proceed via
enols or hemiacetals which act as internal nucleophiles for the
attack at the triple bond which is activated by the gold catalyst.
Our results underline the power of gold catalysis for the synthesis
of structurally diverse heterocycles.
Conclusions
In this paper, we describe three efficient pathways for the gold-
catalyzed cycloisomerization of alk-4-yn-1-ones. Substrates 1a–
m with a terminal or internal triple bond undergo a 5-exo-
dig cycloisomerization to the corresponding multisubstituted
furans 2 under very mild conditions by using the cationic gold
complex Ph3PAuOTf in toluene. Addition of catalytic amounts of
p-TsOH · H2O accelerates the reaction, but is not mandatory. Var-
ious functional groups are tolerated in this transformation, which
uses much milder and/or environmentally benign conditions than
previous methods. A change in regioselectivity was observed
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