Pd-catalyzed cycloisomerization-allylation of 4-alkynones: Synthesis of 5-homoallylfuran derivatives

Article abstract of DOI:10.1016/j.tetlet.2011.06.037

In the presence of Pd₂(dba)₃-tBu₃P catalyst and Cs₂CO₃, cycloisomerization-allylation reactions of 4-alkynone derivatives with allyl carbonates in MeCN proceed in 5-exo mode to give highly-substituted furans having homoallyl groups at 5-position.

Full text of DOI:10.1016/j.tetlet.2011.06.037

Tetrahedron Letters 52 (2011) 4299–4302
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Tetrahedron Letters
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Pd-catalyzed cycloisomerization–allylation of 4-alkynones: synthesis
of 5-homoallylfuran derivatives
⇑
⇑
Akio Saito , Yohei Enomoto, Yuji Hanzawa
Laboratory of Organic Reaction Chemistry, Showa Pharmaceutical University, 3-3165 Higashi-Tamagawagakuen, Machida, Tokyo 194-8543, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
In the presence of Pd2(dba)3–tBu3P catalyst and Cs2CO3, cycloisomerization–allylation reactions of
4-alkynone derivatives with allyl carbonates in MeCN proceed in 5-exo mode to give highly-substituted
furans having homoallyl groups at 5-position.
Received 25 May 2011
Revised 7 June 2011
Accepted 8 June 2011
Available online 16 June 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Cascade reaction
Cycloisomerization
Allylation
syn-Oxipalladation
Furan
Alkynone
Since the synthetic1 and pharmacological2 potentials of furan
compounds have been widely recognized, the new and efficient
synthetic method for the construction of furan nucleus has been
attracting extensive interest.3 For the mild construction of furan
nucleus, transition metal-catalyzed cyclizations of alkynyl,4–6 alle-
nyl,7 or cyclopropyl8 ketones, alkynyl oxiranes9 and enynols10,11
have been developed. In particular, Pd-catalyzed reactions of such
substrates with electrophiles in tandem approaches bring about
the formation of furan rings concomitantly with incorporation of
allyl,5a,6d,e,7g,m,9d,10k,11 aryl, vinyl groups,7h,i,9c and so on5f,i,6c,f,7d–f
into the furan rings. Since most of these reactions proceeded
through 5-endo cyclization, the electrophiles were mainly incorpo-
rated into 3-position of furan rings (Scheme 1).
At the outset, in a similar manner to our previous works on the
cycloisomerization–allylation reaction,12f it turned out that 4-alky-
none 1a reacted with allyl ethyl carbonate (1.2 equiv) in MeCN in
the presence of 2.5 mol % Pd2(dba)3, 20 mol % Cy3P, 6 mol % IPrÁHCl
[1,3-bis(2,6-diisopropyl-phenyl)imidazolium chloride] and 3 equiv
Cs2CO3 to give furan 2a in 83% yield at 90 °C for 4 h (Table 1, entry
1). Since the Pd-catalyzed formation of 2a in the presence of
Cs2CO3, regardless of addition of IPrÁHCl, brought about the similar
result to entry 1 (entry 2), our further effort was focused on the
evaluation of phosphine ligands for the reaction of 1a with allyl
ethyl carbonate (entries 4–9). The use of tBu3P suppressed the for-
mation of non-allylated furan 3a, and the yield of 2a was improved
up to 87 % (entry 4). In place of tBu3P, the use of the more air-stable
On the other hand, tandem cycloisomerization-coupling reac-
tion of 4-alkynones with aryl halides in the presence of Pd(0)
catalyst under an atmosphere of an inert gas or CO is a straightfor-
ward access to 2,5-disubstituted or 2,3,5-trisubstituted furans hav-
ing aryl4d,e or arylcarbonyl4c,h groups at 5-position side chain via
oxipalladation in 5-exo manner (Scheme 2). As a part of our study
on the synthesis of heterocyclic compounds,12 we developed the
cycloisomerization–allylation of propargylamides based on
Pd-catalyzed 5-exo cyclization.12f In this Letter, we describe the
Pd-catalyzed formation of the 5-homoallylfuran derivatives from
4-alkynones with allyl carbonates (Scheme 2).13
R2
R2
R2
O
R1
R1
R1
R3
O
R3
O
R3
Pd(0), E-X 5-endo cyclization
Pd
E
E
R2
R1
R2
R1
R3
R3
O
O
⇑
Corresponding authors. Tel./fax: +81 (0)42 721 1570.
E-mail addresses: akio-sai@ac.shoyaku.ac.jp (A. Saito), hanzaway@ac.shoya-
Scheme 1. Synthesis of furans via 5-endo cyclization.
ku.ac.jp (Y. Hanzawa).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.06.037

4300
A. Saito et al. / Tetrahedron Letters 52 (2011) 4299–4302
and 1e proceeded smoothly as well as terminal alkynes (entries 4
and 5). In the case of ketone 1h, however, the -allylated 4h was
obtained as a main product (entry 8). On the other hand, the pres-
R2
a
R2
R1
R2
R1
R1
Pd E
E
Pd(0), E-X
ent Pd-catalytic system could be applied to the formation of dihy-
5-exo-
cyclization
O
O
O
drofuran
6 from 3,3-dimethyl substrate 5 with allyl ethyl
carbonate, and 6 was obtained in 66% yield as a single cis-isomer,
which is determined by the observed NOE (Scheme 3).
E = Ar (Arcadi and Cacchi et al), ArCO (Arcadi et al., Yu et al.)
E = allyl (Present Works)
In the studies on the Pd-catalyzed formation of furans by the
tandem cycloisomerization-coupling reactions of 4-alkynones with
aryl halides, the reactions are proposed to proceed via oxipallada-
tion–reductive elimination domino mechanism (Scheme 2).4c–e,h
The similar mechanism is suggested in the Pd-catalyzed oxidative
cyclization–carbonylation of 4-alkynones.15 Therefore, the forma-
tion of 6 in Scheme 3 would be considered to consist of the oxipal-
Scheme 2. Synthesis of furans via 5-exo cyclization.
14
tBu3PÁHBF4 afforded good result in either case of 10 or 20 mol %
amount (entries 10 and 11).
The scope of the present Pd-catalyzed formation of various
substituted furans is summarized in Table 2, and Schemes 3 and
5. Thus, in the presence of Pd2(dba)3, tBu3PÁHBF4, and Cs2CO3, not
only diketone 1a and 1b but ketoester 1c–e, ketonitrile 1f and
ketosulfone 1g were also successfully reacted with allyl ethyl car-
bonate to give the corresponding 5-homoallylfurans 2a–g in good
yields (Table 2, entries 1–7). In these reactions, inner alkynes 1d
ladation of intermediate A formed by 5 and
p-allyl palladium
complex, which was generated by palladium(0) and allyl ethyl car-
bonate, and the subsequent reductive elimination of palladium(II)
in intermediate B.16 In the competitive reaction between 7 and 8
with allyl ethyl carbonate (7:8:allyl ethyl carbonate = 1:1:1) under
the present Pd-catalyzed conditions, it was found out that 7 having
Table 1
Screening of catalytic system for the formation of 2a
O
O
O
Ph
Pd2(dba)3 (2.5 mol%)
EtO2CO
(1.2 equiv)
Ph
Ph
N
Cl
N
+
+
Me
Ph
ligand, Cs2CO3 / MeCN, 90°C
O
O
Ph
Ph
O
1a
2a
3a
IPr HCl
Entry
Ligand (mol %)
Cs2CO3 (equiv)
Time (h)
2aa (%)
3aa (%)
1aa (%)
1
2
3
4
5
6
7
8
Cy3P (20), IPrÁHCl (6)
Cy3P (20)
Cy3P (20)
3.0
3.0
0
4
4
22
6
83
81
13
14
71
0
11
5
44
33
7
0
0
0
7
0
0
18
0
0
0
21
0
67
0
0
0
8
tBu3P (20)
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
1.0
3.0
87b
77
(2-furyl)3P (20)
(4-F-Ph)3P (20)
Ph3P (20)
[2,4,6-(MeO)3Ph]3P (20)
Dppf (10)
tBu3PÁHBF4 (20)
tBu3PÁHBF4 (10)
tBu3PÁHBF4 (10)
Cy3PÁHBF4 (10)
4
22
22
22
22
6
6
6
6
66
17
11
9c
10
11
12
13
Trace
84b
85b
19
77b
0
a
b
c
Yields were determined by 1H NMR analysis.
Isolated yield.
Other bidentate phosphine ligands (dppe, dppp, dppb) were also inferior.
Table 2
Cycloisomerization–allylation of various 4-alkynones 1 with allyl ethyl carbonatea
Pd2(dba)3 (2.5 mol%)
Bu3P HBF4 (10 mol%)
Cs2CO3 (3 equiv)
R2
t
R2
R1
EtO2CO
(1.2 equiv)
R1
+
MeCN, 90°C
Ph
R3
O
O
2
O
R3
1
4h
Entry
Substrate
R1
R2
R3
Time (h)
Product
Yielda (%)
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
Ph
Me
Me
Me
Ph
Ph
Ph
Ph
Bz
Ac
H
H
H
Ph
Me
H
H
H
6
7
6
6
6
4
4
7
2a
2b
2c
2d
2e
2f
85
67
78
72
73
75
91
68
CO2Et
CO2Et
CO2Et
CN
SO2Ph
H
1g
1h
2g
4h
a
Isolated yield.

A. Saito et al. / Tetrahedron Letters 52 (2011) 4299–4302
4301
Pd2(dba)3 (2.5 mol%)
Bu3P HBF4 (10 mol%)
Cs2CO3(3 equiv)
O
O
O
O
Me
Me Me
t
Me
Me
Me
EtO
Ph
EtO2CO
EtO
+
EtO
MeCN, 90°C, 16 h
PdLn
Pd
Ln
(1.2 equiv)
Ph
O
Ph
O
C
5
A
NOE
O
O
Me
Me
Me
Me
H
H
EtO
EtO
O
O
LnPd
Ph
Ph
B
6 66%
Scheme 3. Cyclization–allylation of 5 with allyl ethyl carbonate.
Pd2(dba)3 (2.5 mol%)
O
O
O
O
t
Bu3P HBF4 (10 mol%)
Cs2CO3(3 equiv)
EtO
Ph
EtO
EtO
Me
Me
EtO2CO
EtO2CO
Me
+
7 (13%) + 8 (86%)
+
+
+
Ph
Ph
MeCN, rt, 16 h
(1.0 equiv)
(26%)
O
O
7
8
9 61%
Scheme 4. Cyclization–allylation of 5 with allyl ethyl carbonate.
Ligand: tBu3P HBF4 (2 h)
Pd2(dba)3 (2.5 mol%)
Ligand (10 mol%)
Cs2CO3(3 equiv)
Ph
O
O
O
O
R1=Ph, R 2=H: 10 66%, 11 24%
R1=H, R2=Ph: 10 63%, 11 27%
Ph
EtO2CO
R2
Ph
Ph
Ph
+
+
MeCN, 90°C, 2 or 3 h
Ph
R1
(1.2 equiv)
O
O
Ligand: Cy3P HBF4 (3 h)
Ph
Ph
R1=Ph, R2=H: 10 87%, 11 4%
R1=H, R2=Ph: 10 88%, 11 4%
10
11
1a
Pd2(dba)3 (2.5 mol%)
Bu3P HBF4 (10 mol%)
Cs2CO3(3 equiv)
Me
Me
O
O
O
O
O
O
t
R2
Ph
Ph
Me
Ph
Ph
Ph
EtO2CO
R3
or
or
+
MeCN, 90°C, 1-19 h
O
O
R1
Ph
Ph
Ph
(1.2 equiv)
1a
12 85%
13 66%
14 74%
Scheme 5. Cycloisomerization–allylation of 1a with various allyl carbonate derivatives.
a
-hydrogen reacted with allyl ethyl carbonate in preference to al-
Pd-catalyzed system and synthetic applications to natural products
are under way.
kyne parts of 8 (Scheme 4).17 Thus, the carbopalladation route18,19
through the intermediate C (Scheme 3) would not take part in the
formation of 6. Therefore, the present formation of 5-homoallyl
furans 2 from 1 would proceed via oxipalladation–reductive elim-
ination domino mechanism and the subsequent isomerization of
intermediate analogous to 6.
Acknowledgment
This work was partially supported by Grants-in-Aid for Young
Scientists (B) 21790024 and for High Technology Research Centre
Project (19-8) from MEXT, Japan.
As shown in Scheme 5, by the present catalytic systems, the
reaction of 1a with ethyl 1-phenylallyl carbonate (R1 = Ph, R2 = H)
or cinnamyl ethyl carbonate (R1 = H, R2 = Ph) gave regioisomeric
mixture of 10 and 11 (10:11 = 66:24 or 63:27). The use of
Cy3PÁHBF4 instead of tBu3PÁHBF4, however, made an improvement
in the ratio of 10:11 to yield 7 in 87–88% along with 8 in 4% yields
in both cases. In the reaction of 1a with other allyl carbonates, the
Pd2(dba)3–tBu3PÁHBF4 systems worked well to give the corre-
sponding furans 12–14, respectively (12: 85%, 13: 66%, 14: 74%).
In conclusion, we demonstrated the facile synthesis of the
5-homoallyl furans by Pd-catalyzed cycloisomerization–allylation
of 4-alkynones with allyl carbonates. From the formation of vinyl-
idenedihydrofuran 6 and the competition experiment between 7
and 8 with allyl ethyl carbonate, we believed that the present furan
formation would proceed through oxipalladation–reductive
elimination domino mechanism. Further studies on the present
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}
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