W(CO)5(L)-catalyzed cyclization of α-alkynyl-β- dicarbonyl derivatives: Synthesis of methylenelactones, furans, and methylenecyclopentanes

Article abstract of DOI:10.1055/s-0032-1316536

W(CO)₅(L)-Catalyzed cyclization of α-alkynyl-β-keto acids, keto esters, and diketones provides methylenelactones, furans, and methylenecyclopentanes via 5-exo-dig cyclization. Also, the present approach can be further applied to 5-endo-dig cycl

Full text of DOI:10.1055/s-0032-1316536

1960▌
LETTER
letter
W(CO)5(L)-Catalyzed Cyclization of α-Alkynyl-β-dicarbonyl Derivatives:
Synthesis of Methylenelactones, Furans, and Methylenecyclopentanes
W(CO)5(L)-Catalyzed Cyclization of α-Alkynyl-β-dicarbonyl Derivatives
Xiangjian Meng, Sunggak Kim*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
21 Nanyang Link, Singapore 637371, Singapore
Fax +6567911961; E-mail: sgkim@ntu.edu.sg
Received: 10.04.2012; Accepted after revision: 17.05.2012
Pd,8 Rh,9Au,10 Ag,11 and other metal complexes.1c,12 To
Abstract: W(CO)5(L)-Catalyzed cyclization of α-alkynyl-β-keto
the best our knowledge, tungsten-catalyzed reaction using
acids, keto esters, and diketones provides methylenelactones, fu-
a carboxylic acid as a nucleophile has not been investigat-
rans, and methylenecyclopentanes via 5-exo-dig cyclization. Also,
ed. To find out effective catalysts, we began our study
with several different W and Mo catalysts using a model
compound 1. As shown in Table 1, the cyclization was
sensitive to the catalyst, the solvent, and the reaction con-
ditions. When the reaction was carried out using 20 mol%
W(CO)6 in refluxing THF or in refluxing acetonitrile, the
cyclization did not occur (Table 1, entries 1 and 4), and the
addition of triethylamine did not affect the reaction (Table
1, entries 2 and 3). Gratifyingly, when a THF solution of
1 containing W(CO)6 was irradiated at λ = 350 nm for 16
hours, the cyclization proceeded smoothly to provide
methylenelactone 2 in 93% yield via 5-exo-dig ring clo-
sure (Table 1, entry 5). Furthermore, the addition of trieth-
ylamine shortened the reaction time to six hours (Table 1,
entry 6). It is noteworthy that our attempts using several
Mo catalysts under thermal conditions failed (Table 1, en-
tries 7–9). The remaining cyclizations were carried out
with several structurally different alkynoic acids using 20
mol% W(CO)6 and one equivalent of triethylamine in
THF at λ = 350 nm for 12 hours.
the present approach can be further applied to 5-endo-dig cycliza-
tion.
Key words: tungsten, cyclization, methylenelactones, furans,
methylenecyclopentanes
During the last century transition-metal-catalyzed nucleo-
philic addition onto unactivated C–C multiple bonds has
been extensively studied and proved to be a highly effi-
cient strategy for the construction of carbocyclic and het-
erocyclic compounds.1 Among various transition metals,
Group VI metal carbonyl complexes such as M(CO)5(L)
(M = Cr, Mo, W; L = THF, Et3N, etc.) played a very im-
portant role in the cyclization reaction due to their unique
characteristics in catalytic activations of alkynyl groups.2
Especially for the past two decades, McDonald extensive-
ly utilized Mo(CO)5(Et3N)3 and W(CO)5(Et3N or
DABCO)4 to promote the efficient cyclizations, whereas
Iwasawa employed W(CO)5(THF) as catalyst to achieve
several important cyclizations.5,6
Recently, we reported tungsten and molybdenium carbon-
yl mediated cyclization of N-propargyl amides to oxazo-
line and oxazine derivatives.7 As the extension of our
previous results, we have been interested in the nature of
nucleophiles along with 5-exo versus 6-endo ring-closure
issue due to their importance on the cyclization (Scheme
1) and wish to report the W(CO)5(L)-catalyzed cycliza-
tion of α-alkynyl-β-dicarbonyl derivatives to provide
methylenelactones, furans, and cyclopentenes under mild
conditions.
Table 1 Optimization of Reaction Conditions for the Cyclization of
Acetylenic Acid 1
O
O
OMe
OMe
catalyst (20 mol%), additive
O
O
OH
O
2
1
Entry Catalyst
Additive Solvent Conditions Time Yield
(h) (%)
1
2
3
4
5
6
7
8
9
W(CO)6
W(CO)6
W(CO)6
W(CO)6
W(CO)6
W(CO)6
–
THF
THF
80 °C
80 °C
12 trace
12 trace
12 trace
12 trace
16 93
and/or
X
X
X
X
Et3N
Et3N
–
•
W(CO)5
(OC)5W
5-exo-dig
6-endo-dig
MeCN 80 °C
MeCN 80 °C
X = nucleophile
Scheme 1 W(CO)5(L)-catalyzed cyclization
–
THF
THF
350 nm
350 nm
Transition-metal-catalyzed cyclization of alkynoic acids
to alkylidene lactones has been extensively studied using
Et3N
6
16
16
16
95
0
Mo(CO)3·(MeCN)3 –
MeCN 80 °C
MeCN 80 °C
SYNLETT 2012, 23, 1960–1964
Advanced online publication: 04.07.2012
Mo(CO)3·(DMF)3
Mo(CO)6
–
–
0
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
THF
80 °C
0
DOI: 10.1055/s-0032-1316536; Art ID: ST-2012-D0313-L
© Georg Thieme Verlag Stuttgart · New York

LETTER
W(CO)5(L)-Catalyzed Cyclization of α-Alkynyl-β-dicarbonyl Derivatives
1961
Bispropargylic substrate 3a was cyclized smoothly to the clization proceeded via vinylidene carbene intermediates,
corresponding lactone 4a in 92% yield (Table 2, entry 1). and no further additional studies have been reported.17
Substrate 3c bearing a homopropargylic group afforded
In order to determine the scope and limitation of this cy-
4c in 95% yield, indicating the preference of 5-exo over 6-
clization, we began our study with a model compound 8a.
exo cyclization (Table 2, entry 3). Propargyl malonic half
When a THF solution of 8a with W(CO)6 (0.2 equiv) and
ester 3d was almost quantitatively cyclized to the desired
Et3N (1 equiv) was irradiated at λ = 350 nm at 40 °C for
lactone (Table 2, entry 4).
18 hours, a 76:19 mixture of dihydrofuran 9a and furan
10a was isolated without yielding 11, indicating the cycli-
zation did not proceed via the previously reported vinyli-
dene carbene intermediate (Scheme 3). In addition, it is
noteworthy that the use of 0.2 equivalents of Et3N afford-
ed 9a (15%) along with recovery of 8a (80%).17,18
Table 2 W(CO)5(Et3N)-Catalyzed Cyclization of Acetylenic Acids
R2
R2
R1
R1
W(CO)6 (0.2 equiv), Et3N (1 equiv),
THF, 350 nm, 40 °C, 12 h
n
n
O
O
OH
O
3
4
O
W(CO)6 (0.2 equiv), Et3N (1 equiv),
Entry Acid
n
R1
R2
Yield (%)
THF, 350 nm, 40 °C, 18 h
O
OMe
1
2
3
4
5
6a
3a
3b
3c
3d
3e
3f
1
1
1
1
1
2
CO2Me
CO2Me
CO2Me
CO2Me
COPh
CH≡CCH2
92
91
Bn
8a
CH≡CCH2CH2 95
MeO2C
MeO2C
+
MeO2C
+
H
H
H
93
96
87
O
O
O
9a (76%)
10a (19%)
11 (0%)
CO2Et
Scheme 3 W(CO)5(Et3N)-catalyzed cyclization of keto ester 8a
a Reaction was carried out for 36 h.
Additional experimental results are summarized in Table
3. The cyclization of 1,3-dimethylketone 8c gave dihydro-
furan 9c exclusively (Table 3, entry 2), while the cycliza-
tion of 1,3-phenyl methyl ketone 8d was regiospecific but
gave a mixture of two products (Table 3, entry 3). Appar-
ently, the initially formed dihydrofuran derivatives were
partially isomerized to the corresponding furans during
the reaction.
Similarly, carboxy phenyl ketone 3e led to 4e in 96%
yield (Table 2, entry 5). As we have seen in 3f, the 6-exo-
dig cyclization of 3f proceeded slowly, requiring 36 hours
for completion (Table 2, entry 6).
The cyclization of alkyl keto acid 5 was somewhat differ-
ent from that of 3e (Scheme 2). Under the standard cycli-
zation conditions, 5a underwent facile decarboxylation to
give oct-7-yn-4-one in 93% yield. However, when the re-
action was carried out in the absence of triethylamine, the
decarboxylation was reduced to 35%, and a mixture of 6a
and 7a was isolated in 50% and 10%, resulting from com-
petitive addition of the carboxyl and the ketone group to
the activated alkynyl tungsten complex. A similar result
was also observed with 5b.
Table 3 W(CO)5(Et3N)-Catalyzed Cyclization of α-Alkynyl-β-di-
carbonyl Compounds
R3
R2
W(CO)6 (0.2 equiv),
Et3N (1 equiv),
THF, 350 nm, 40 °C, 18 h
R2
R2
R1
R1
R1
O
O
O
8
9
10
Entry Ketone R1
R2
R3
Yield of 9 Yield of 10
(%)
78
94
66
0
(%)
O
O
O
R
HO
R
W(CO)6 (0.2 equiv),
THF, 350 nm, r.t., 15 h
1
2
3
4
8b
8c
8d
8e
n-C3H7 CO2Et
H
17
HO
O
+
Me
Me
Me
COMe
COPh
CO2Et
H
0
21
0
O
R
O
O
H
5a: R = n-Pr
5b: R = Me
6a (50%)
6b (55%)
7a (10%)
7b (11%)
Bn
Scheme 2 W(CO)5(THF)-catalyzed cyclization of keto acid 5
Furthermore, it was known that dihydrofuran derivatives
could be readily isomerized into the corresponding furans
by treatment with Amberlist or DBU.19 Fully substituted
keto ester 8e failed to undergo cyclization under the stan-
dard conditions (Table 3, entry 4).
Since furan skeletons are abundant in various bioactive
natural products and drugs,13 various synthetic methods
have been reported.14 Among them, the recently devel-
oped approaches involve the nucleophilic attack of the
carbonyl oxygen to the transition metals,15 especially the
gold(I)-coordinated alkynyl group.16 In the case of Group
VI metal carbonyl complexes, it was reported that the cy-
The reason for the failure in the cyclization of ketoester A
(R = Bn) may be arisen from an unfavorable equilibrium
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1960–1964

1962
X. Meng, S. Kim
LETTER
to intermediate B and the difficulty of depronation in in- ters 13a and 13b underwent 5-endo-dig cyclization clean-
termediate B to provide tetrahydrofuran C (Scheme 4). ly to afford 14a and 14b, respectively (Table 4, entry 1
Thus, the formation of enol ester E may be requisite for and 2). Although long reaction times were required, the
the cyclization. Furthermore, a facile deprotonation in B reaction proceeded cleanly in all cases.
due to the presence of the ester group would shift the un-
Table 4 W(CO)5(DABCO)-Catalyzed Cyclization of α-Alkynyl-β-
dicarbonyl Compounds
favorable equilibrium to the formation of dihydrofuran D.
O
O
O
O
W(CO)6 (0.2 equiv),
DABCO (1 equiv),
toluene, 350 nm, 40 °C
O
R2
R2
R2
MeO
R1
O
MeO
R1
R1
–
R
OMe
R
a
n
W(CO)5
+
O
O
R
O
C
Et3N:W(CO)5
R = Bn
13
14
15
B
n = 1, 2
W(CO)5
A
Entry Ketone R1
R2
n
Time Product Yield
R = H
O
O
(h)
(%)
b
R = H
MeO
MeO
O
1
13a
13b
13c
13d
13d
Me
Pr
CO2Me
CO2Et
CO2Me
CO2Et
CO2Et
1
1
2
2
2
36
72
36
48
48
14a
14b
15c
15d
15d
94
82
91
84
–
H+
W(CO)5
+
HO
OMe
O
2
O
– W(CO)5
H
D
F
3
Me
Pr
E
4
Scheme 4 Plausible catalytic cycle
5a
Pr
44
(45)b
We next studied the Conia-ene type carbocyclization us-
ing α-alkynyl-β-keto ester 11 under the similar conditions.
The Conia-ene type reactions have been well studied and
achieved using various transition-metal catalysts.1a,20 Us-
ing Group VI metal catalysts, a similar endo carbocycliza-
tion approach was reported, in which stable enolate anions
were used for the cyclization.21 Irradiation of a solution of
11 with 0.2 equivalents of W(CO)6 and one equivalent of
triethylamine in THF at λ = 350 nm for 48 hours afforded
12 in 43% yield together with recovery of the starting ma-
terial (40%) via 5-exo-dig cyclization, and there was no
indication of the product from 6-endo-dig cyclization
(Scheme 5).
6
13e
13e
Me
Me
COPh
COPh
2
2
72
48
15e
15e
87
7a
15
(80)b
a The reaction was carried out in the presence of Et3N (1 equiv).
b Recovered starting material.
However, the present method has limitations with substi-
tuted alkynes 16 and 17, which failed to give the cycliza-
tion products under the similar conditions. Furthermore,
6-exo-dig cyclization using 18 also failed (Figure 1).
O
O
O
CO2Et
CO2Et
CO2Me
O
catalyst (20 mol%), 350 nm
O
CO2Et
16
17
18
CO2Et
12
11
W(CO)5(Et3N), THF
W(CO)5(DABCO), toluene
43%
85%
Figure 1
Scheme 5 W(CO)5(L)-catalyzed cyclization of keto ester 11
In conclusion, we have developed a new and mild method
for the synthesis of methylenelactones, furans, and meth-
ylenecyclopentanes from α-alkynyl-β-dicarbonyl deriva-
tives using W(CO)5(Et3N or DABCO) catalyst. The
cyclizations proceeded via a 5-exo-dig manner, and the 5-
endo-dig cyclization was also possible, depending on the
structure of the substrates. Further studies and applica-
tions of W(CO)5(L)-catalyzed cyclizations are currently
under way.23
As shown in Table 4, similar observations were realized
(Table 4, entries 5 and 7). After much experimentation, it
was found that the use of DABCO in toluene improved the
reaction considerably.22 When the reaction was repeated
using one equivalent of DABCO in toluene under the
same conditions, the reaction was faster and 12 was isolat-
ed in 85% yield. Several structurally different substrates
underwent 5-exo-dig cyclization to give methylenecyclo-
pentane derivatives 15 in high yields (Table 4, entries 3,
4, and 6). Furthermore, the present method could be fur-
ther applied to the 5-endo-dig cyclization by reducing one
carbon at the alkynyl chain. α-Alkynyl-β-dicarbonyl es-
Acknowledgment
We thank the Nanyang Technological University (NTU) for finan-
cial support and Miss J. S. K. Lim for her experimental assistance.
Synlett 2012, 23, 1960–1964
© Georg Thieme Verlag Stuttgart · New York

LETTER
W(CO)5(L)-Catalyzed Cyclization of α-Alkynyl-β-dicarbonyl Derivatives
1963
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Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
m
iotSrat
ungIifoop
r
t
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Typical Procedure for the W(CO)6-Catalyzed
Cyclization of α-Alkynyl-β-dicarbonyl Ester 13b
To a toluene solution (2 mL) of α-alkynyl-β-dicarbonyl ester
13b (42 mg, 0.2 mmol) was added DABCO (22 mg, 0.2
mmol) and W(CO)6 (14 mg, 0.04 mmol) under a nitrogen
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1960–1964

1964
X. Meng, S. Kim
LETTER
atmosphere at r.t., and then the mixture was irradiated at 350
nm. After the completion of the reaction, the mixture was
quenched with H2O (2 mL) and extracted with Et2O (3 × 10
mL). The organic layer was washed with brine, dried with
anhyd MgSO4, and evaporated under reduced pressure. The
crude product was purified by flash column chromatography
(hexane–EtOAc = 20:1) to give the corresponding product
14b (34 mg, 82%). 1H NMR (400 MHz, CDCl3): δ = 6.01 (d,
J = 5.6 Hz, 1 H), 5.94–5.77 (m, 1 H), 4.19 (qd, J = 7.1, 0.9
Hz, 2 H), 2.47–2.37 (m, 6 H), 1.61 (dd, J = 14.6, 7.3 Hz, 2
H), 1.26 (t, J = 7.1 Hz, 3 H), 0.89 (t, J = 7.4 Hz, 3 H). 13
C
NMR (101 MHz, CDCl3): δ = 206.0, 172.0, 136.1, 128.8,
73.4, 61.4, 40.7, 31.9, 30.1, 17.1, 14.1, 13.6. ESI-HRMS:
m/z calcd for C12H19O3 [M + H]+: 211.1334; found:
211.1336.
Synlett 2012, 23, 1960–1964
© Georg Thieme Verlag Stuttgart · New York